Initiating and Adjusting Invasive Ventilatory Support

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Chapter 44

Initiating and Adjusting Invasive Ventilatory Support

Robert M. Kacmarek

Mechanical ventilation entails the use of sophisticated life-support technology aimed at maintaining tissue oxygenation and removal of carbon dioxide (CO₂). At its most basic level, mechanical ventilation supports or replaces the normal ventilatory pump, moving air into and out of the lungs. The primary function of a mechanical ventilator is simply to ventilate. The main indication for mechanical ventilation is inadequate or absent spontaneous breathing.

Mechanical ventilation is not without risk, and the complications and hazards can be life-threatening. The decision to initiate mechanical ventilatory support is a serious one that requires sound clinical judgment and a clear understanding of the various approaches to ventilatory support. This chapter reviews and describes the initial setup of the ventilator. After ventilator initiation, adjustments in ventilatory support are made on the basis of the patient’s response. Techniques for patient stabilization; methods for optimizing oxygenation, ventilation, synchrony, and acid-base balance; and methods for minimizing harmful side effects are described.

Goals of Mechanical Ventilation

The goals of mechanical ventilatory support are to maintain adequate alveolar ventilation and oxygen (O₂) delivery, restore acid-base balance, and reduce the work of breathing (WOB) with minimum harmful side effects and complications.¹ Mechanical ventilation also may reduce increased myocardial work secondary to hypoxemia and an increased WOB.¹ Other physiologic objectives of mechanical ventilatory support include increasing or maintaining lung volume with positive end expiratory pressure (PEEP) and continuous positive airway pressure (CPAP) for promotion, improvement, or maintenance of lung recruitment.¹

A lung protective ventilatory strategy is an approach to mechanical ventilation that includes the use of small tidal volume (V_T) and appropriate levels of PEEP.² This approach is usually employed in patients with acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS). However, the concept of lung protection should be applied to all patients requiring ventilatory support for acute respiratory failure. Lung injury is primarily caused by an elevated transpulmonary pressure during positive pressure ventilation.³ Transpulmonary pressure is the difference between alveolar pressure and pleural pressure. A safe transpulmonary pressure during mechanical ventilation is not firmly established, but most clinicians would agree that the lower the transpulmonary pressure, the less likely the development of ventilator-induced lung injury.⁴ High transpulmonary pressures are associated with alveolar overdistention and lung injury.³

Plateau pressure (P_plat), the end inspiratory equilibration pressure, measures the mean peak alveolar pressure and is the best bedside clinical reflection of transpulmonary pressure.2,4,5 Although P_plat is not an accurate measurement of transpulmonary pressure, the transpulmonary pressure never exceeds the P_plat.⁴ P_plat provides an excellent bedside assessment of the level of potentially dangerous ventilating pressure. Limiting P_plat reduces the likelihood of ventilator-induced lung injury. Generally, the lower the P_plat, the better the patient outcome.^2,5 Ideally, P_plat should be less than 30 cm H₂O.⁵ However, a P_plat greater than 30 cm H₂O may be applied in patients with a decreased thoracic compliance without resulting in overdistention⁵ because a decrease in chest wall compliance (obesity, massive fluid resuscitation, abdominal distention, elevated bladder pressure) increases the pleural pressure, decreasing the transpulmonary pressure. Generally, the lowest possible P_plat is maintained by selecting a V_T of 4 to 8 ml/kg of ideal body weight (IBW). The higher the P_plat, the smaller the V_T should be. Generally, V_T greater than 10 ml/kg IBW is never indicated in critically ill patients.

Lung injury can also be caused by repetitive opening and closing of unstable lung units.⁶ The application of an appropriate level of PEEP ensures that unstable lung units are maintained in the open position reducing the likelihood of additional lung injury.

Specific clinical objectives of mechanical ventilation include reversal of hypoxemia, hypercapnia, and associated respiratory acidosis and prevention or reversal of ventilatory muscle dysfunction. The general trajectory of pH, PCO₂, and PO₂ during the progression of acute respiratory failure is depicted in Figure 44-1. Mechanical ventilation may be used to allow sedation or paralysis for certain procedures, to decrease myocardial and ventilatory muscle O₂ consumption to maximize O₂ delivery to the tissues, to decrease intracranial pressure acutely in the presence of closed head injury or cerebral edema (by reducing PaCO₂ to 25 to 30 mm Hg for a short period and promoting cerebral vasoconstriction), to prevent or reverse atelectasis, and to stabilize the chest wall in the case of a massive flail or chest wall resection. Table 44-1 lists the most common causes of acute respiratory failure leading to ventilatory support in the United States and Canada. Hazards of mechanical ventilation include decreased venous return and cardiac output, patient-ventilatory asynchrony, and ventilatory muscle dysfunction owing to inappropriate ventilator settings, ventilator-associated pneumonia, and ventilator-induced lung injury.¹ Box 44-1 lists the goals of ventilatory support, and Box 44-2 lists specific objectives of mechanical ventilation.

Box 44-2

Specific Clinical Objectives of Ventilatory Support

• To reverse hypoxemia

• To reverse acute respiratory acidosis

• To relieve respiratory distress

• To prevent or reverse atelectasis

• To reverse ventilatory muscle dysfunction

• To allow sedation and neuromuscular blockade

• To decrease systemic or myocardial O₂ consumption

• To maintain or improve cardiac output

• To reduce intracranial pressure

• To stabilize the chest

From Slutsky AS: Mechanical ventilation. American College of Chest Physicians’ Consensus Conference. Chest 104:1833–1859, 1993.

TABLE 44-1

Most Common Causes of Acute Respiratory Failure Requiring Mechanical Ventilation in the United States and Canada

Condition	Rank	Percentage
Postoperative respiratory failure	1	17
Sepsis	1	17
Other	2	16
Heart failure	3	13
Pneumonia	3	13
Trauma	3	13
ARDS	4	9
Aspiration	5	3

Modified form Esteban A, Anzueto A, Alia I, et al: How is mechanical ventilation employed in the intensive care unit? An international utilization review. Am J Respir Crit Care Med 161:1450, 2000.

FIGURE 44-1 Typical progression of acute respiratory failure. Initially, there is a decline in arterial O₂ tension and saturation. When PaO₂ decreases to approximately 60 mm Hg *(A)*, the patient begins to breathe more, PaCO₂ decreases, and pH increases. Early in the progression, arterial blood gas results show acute alveolar hyperventilation (uncompensated respiratory alkalosis) secondary to hypoxemia. As the patient’s condition worsens, increases in ventilatory workload typically lead to the adoption of a rapid, shallow breathing pattern; although minute ventilation may remain high, effective ventilation decreases, PaCO₂ begins to increase, and pH begins to decrease *(B)*. At point C, arterial blood gas results may show normal PaCO₂ and pH with moderate to severe hypoxemia. If mechanical ventilation is not initiated, the patient’s condition may progress to acute ventilatory failure, severe hypoxemia, and corresponding severe respiratory acidosis *(D)*.

Ventilator Initiation

When the decision to begin mechanical ventilatory support is made, one must choose the mode of ventilation, select an appropriate device, and establish the initial ventilator settings. In the selection of initial ventilator settings, the goal is to optimize the patient’s oxygenation, ventilation, and acid-base balance, while avoiding harmful side effects. This goal is achieved by choosing an appropriate mode of ventilation, fractional inspired oxygen (FiO₂), V_T (volume ventilation) or pressure level (pressure ventilation), rate, peak flow and flow waveform, inspiratory time, and PEEP level. Appropriate trigger sensitivity, pressure limit, alarms, backup ventilation, and humidification must be selected. After initial ventilator setup, adjustments must be made on the basis of the patient’s response and the patient-specific clinical objectives of ventilatory support. Most patients who need mechanical ventilatory support receive invasive positive pressure ventilation; however, an increasing number of patients are being ventilated noninvasively (see Chapter 46). Next, the clinician must choose the mode of ventilation (e.g., volume assist/control [VA/C], pressure assist/control [PA/C], pressure support ventilation [PSV], pressure regulated volume control [PRVC], volume support, adaptive support ventilation, proportional assist ventilation [PAV], or neurally adjusted ventilatory assist [NAVA]) and initial ventilator settings (e.g., rate, V_T or pressure level, FiO₂, PEEP). Finally, the clinician must choose appropriate alarm and apnea settings. Box 44-3 summarizes key decisions that must be made as a part of initial ventilator setup.

Box 44-3 Initial Ventilator Setup

Initial ventilator setup includes the following key decisions:

• Noninvasive vs. invasive ventilation

• Type and method of establishment of an airway

• Partial vs. full ventilatory support

• Choice of ventilator

• Mode of ventilation

• Assist-control ventilation (volume vs. pressure) vs. SIMV (with or without pressure support)

• Pressure support

• Other newer modes and adjuncts to ventilation

Next, the clinician must consider key ventilatory values:

• Trigger method (pressure or flow trigger) and sensitivity

• V_T (volume ventilation) or pressure level (pressure support and PA/C)

• Rate

• Inspiratory flow, inspiratory time, expiratory time, or I : E ratio

• Inspiratory flow waveform

• FiO₂

• PEEP/CPAP

Last, the clinician must choose appropriate alarm and backup values:

• Low-pressure, low PEEP alarms

• High-pressure limit and alarm

• Volume alarms (low V_T/high V_T, high and low minute ventilation)

• High rate and low rate alarms

• Apnea alarm and apnea ventilation values

• High/low O₂ alarm

• High/low temperature alarm

• I : E ratio limit and alarm

Full Ventilatory Support versus Partial Ventilatory Support

Full ventilatory support can be defined as the application of mechanical support such that all or most of the energy necessary for effective alveolar ventilation is provided by the ventilator.⁸ When a ventilator is set to deliver full ventilatory support, the patient is either passive or simply triggers the breath to initiate inspiration allowing the ventilator to perform most of the work of breathing. However, it is very difficult to set the ventilator to assume all of the work of breathing without significantly sedating the patient. In most patient-triggered approaches to ventilatory support, patient-ventilatory synchrony is a major issue, and very careful titration of the ventilator settings is necessary to ensure synchrony and minimize patient WOB. Patient-ventilatory synchrony is discussed in detail later in the chapter.

Partial ventilatory support implies that only a percentage of the WOB is provided by the ventilator.⁸ Normally, when partial ventilatory support is indicated, SIMV, PSV, volume support, PAV, and NAVA are the modes of choice. However, as with full ventilatory support, care in setting the ventilatory is critical to ensure that patient-ventilator synchrony is maximized. Partial ventilatory support strategies minimize the loss of ventilatory muscle function, require less sedation, assist in recruiting and stabilizing alveolar units, and generally move patients closer to ventilator discontinuance than full ventilatory support approaches.

Choice of a Ventilator

After the decision is made to initiate mechanical ventilator support, the clinician must select an appropriate ventilator. This decision should be guided by considering the features, modes available, pressure and flow capabilities, alarms and monitoring systems included, and reliability. However, the most important feature is the clinician’s familiarity with the equipment. Only a ventilator with which the clinician is totally familiar with every feature should ever be used.

Initial Ventilator Settings

Initial ventilator settings are chosen based on the patient’s clinical presentation and the need to provide full or partial ventilator support.

Mini Clini

Selecting Initial Ventilator Settings

Problem

A 52-year-old man, 5 ft 10 in (178 cm) tall and weighing 200 lb (91 kg), is being returned from the operating room after coronary artery bypass surgery. He is being manually (bag-tube) ventilated with supplemental O₂ by the anesthesiologist en route to the ICU. He is apneic at this time. The patient has no history of lung disease and has never smoked cigarettes. Heart rate and blood pressure are stable, and SpO₂ during manual ventilation is 99%. What initial mode, V_T, rate, and FiO₂ should the RT select when starting ventilatory support for this patient?

Solution

The patient is apneic at this time but is likely to resume spontaneous breathing as the anesthetic wears off and sedation is reduced. Because the patient is expected to resume breathing spontaneously, volume ventilation or pressure ventilation in either assist-control or SIMV is appropriate.

Initial V_T and rate should be selected to provide full ventilatory support. Generally, initial V_T of approximately 6 to 8 ml/kg IBW or pressure control setting to establish this V_T with a rate of 12 to 20 breaths/min provides an adequate starting minute ventilation for most adult patients. The formulas for estimating IBW are:

< ?xml:namespace prefix = "mml" />IBW in kilograms(men)=[106+6(H−60)]/2.2

IBW in kilograms(women)=[105+5(H−60)]/2.2

where H is height in inches.

For this patient:

IBW=[106+6(70−60)]/2.2=75.5kg

On the basis of IBW of 75.5 kg, initial V_T can be set at about 600 ml. Initial inspiratory flow should be set at 60 L/min with a decreasing ramp flow waveform to achieve an inspiratory time of approximately 1 second. Initially, rate can be set at 12/min. Because the patient has a normal respiratory system and it is usual to return from the operating room with a below-normal body temperature, a low initial control rate is indicated. Trigger sensitivity (assist-control or SIMV) should be set so that minimal patient effort triggers the ventilator without autocycling.

Initial FiO₂ should be set at 1.0, but because of the patient’s history and the presence of normal lung function, it is expected that it will be reduced rapidly as the patient recovers. Initial PEEP is set at 5 cm H₂O. If the SIMV mode is chosen, PSV should be started at 10 cm H₂O and adjusted as needed when the patient resumes spontaneous breathing.

In summary, appropriate initial ventilator settings for this patient are:

Mode: Assist-control (volume or pressure) or SIMV (volume or pressure) with PSV

V_T: 8 ml/kg, 600 ml

f_mach: 12 breaths/min

PSV: 10 cm H₂O (SIMV mode only)

FiO₂: 1.0 followed by immediate assessment and SpO₂ observation with titration downward as indicated

Inspiratory flow and time: 60 L/min, decreasing ramp, inspiratory time approximately 1 second

Pressure limit: 30 cm H₂O and adjust to 10 to 15 cm H₂O above PIP after patient connection

Humidification: Heated humidifier to achieve temperature >35° C at the airway or an appropriate HME

Choice of Mode

Most modern critical care ventilators include VA/C, PA/C, SIMV, and PSV and many of the newer modes of ventilation. However, some modes of ventilation are found only on a specific type of ventilator, such as PAV PB 840 (Covidien-Nellcor, Boulder, Colorado), adaptive support ventilation Hamilton ventilators (Hamilton Medical, Bonaduz, Switzerland), NAVA Servo-i ventilator (Maquet, Inc, Wayne, New Jersey), and SmartCare Draeger XL (Draeger Medical, Inc, Telford, Pennsylvania).

Assist-Control Ventilation (Patient-Triggered or Time-Triggered Continuous Mandatory Ventilation)

Assist-control ventilation can be delivered in either pressure-targeted or volume-targeted ventilation. Suggested initial settings for assist-control volume ventilation in the care of adults are listed in Box 44-4. Advantages of assist-control volume ventilation include the assurance that a minimum safe level of ventilation is achieved, yet the patient can still set his or her own breathing rate. In the event of sedation or apnea, a minimum safe level of ventilation is guaranteed by the selection of an appropriate backup rate, usually approximately 4 to 6 breaths/min less than the patient’s assist rate but not less than the rate necessary to provide a minimum safe level of ventilation (e.g., a backup rate of at least 12 to 14 breaths/min).⁸

Box 44-4 Typical Values for Ventilator Initiation for Adults Receiving Volume or Pressure Assist-Control Ventilation

• Trigger sensitivity: −0.5 to −1.5 cm H₂O or 2 to 3 L/min set to minimize trigger work without autocycling

• V_T: Volume ventilation 6 to 8 ml/kg IBW; pressure ventilation, pressure level to achieve 6 to 8 ml/kg IBW

• Rate: Backup rate of ≥12 to 14 breaths/min if providing controlled ventilation

• Inspiratory flow: Volume ventilation 60 to 80 L/min to achieve inspiratory time of approximately <1 second and I : E ratio of ≤1 : 2; inspiratory flow ≥80 L/min may be required in the care of some patients to meet or exceed the patient’s spontaneous inspiratory flow demand

• Flow waveform volume ventilation: Decreasing ramp

• Inspiratory time pressure ventilation: <1.0 second

• PEEP: 5 cm H₂O

• Pressure limit: Start at 30 to 40 cm H₂O depending on approach (volume 40 cm H₂O, pressure 30 cm H₂O) and adjust after patient connection to 10 to 15 cm H₂O above PIP

• Humidification: Begin with heated humidifier to provide temperature 35° C at the airway connection or appropriate HME

Because assist-control ventilation usually provides full ventilatory support, it may result in less WOB than partial support modes. However, less WOB should not be assumed just because the patient is in assist-control ventilation. Trigger work may be significant if inappropriate sensitivity settings are selected. In addition, when a breath is triggered, inspiratory muscle activity persists.9,10 If the inspiratory flow rate during volume ventilation does not meet or exceed the patient’s inspiratory demand, or inspiratory time is too lengthy, the patient’s WOB may be greater, equaling or exceeding the work of a spontaneous unassisted breath.^9,10 In pressure ventilation, lengthy inspiratory times, inadequate rise time, and improperly set pressure levels can also cause asynchrony. (See later sections focusing on patient-ventilator synchrony.)

If properly applied and tolerated by the patient, assist-control ventilation may provide ventilatory muscle rest that allows the ventilatory muscles to recover from ventilatory muscle dysfunction. Disadvantages of assist-control mode include an increase in WOB.8,11 Assist control also may be poorly tolerated by awake, nonsedated patients. The patient may fight the ventilator, or asynchronous patient-to-ventilator breathing patterns may develop. Because flow is based on patient demand in pressure-targeted ventilation, synchrony is generally better achieved during PA/C than with VA/C ventilation. Advantages and disadvantages of pressure-controlled ventilation (PCV) are described in Box 44-5.

Box 44-5 Advantages and Disadvantages of Pressure-Controlled Ventilation

Advantages

• Variable flow results in square pressure waveform and improves gas distribution

• Ensures that P_plat cannot exceed set pressure control level

• All alveoli are placed under the same sustained inspiratory pressure, which decreases hyperinflation of more compliant alveoli compared with volume ventilation square wave flow

• Sustained inspiratory pressure may result in more alveolar recruitment

• Improved gas distribution allows for lower V_T

• Lower PIP is achieved compared with that achieved with volume ventilation with a square flow waveform

Disadvantages

• Higher mean airway pressure can decrease venous return and decrease cardiac output if preload is inadequate

• V_T varies depending on lung compliance, resistance, and patient effort

• If V_T or minute ventilation alarms are not set properly, alveolar hypoventilation and acidosis may not be detected

Assist-control volume ventilation is the most common ventilator mode used throughout the world as the primary initial mode of ventilatory support.7,12 Regardless of the indication for ventilatory support or underlying disease, this mode is able when properly adjusted and the patient is properly managed to provide adequate ventilatory support for all indications for ventilatory support.^6,12

Controlled Ventilation (Time-Triggered Continuous Mandatory Ventilation)

Controlled ventilation, pressure, or volume is achieved using the assist-control mode when the patient is apneic because of a medical condition, anesthesia, or use of sedative drugs and paralytic agents. Ventilators in use today do not prevent a patient with sufficient effort from triggering the ventilator, a situation always to be avoided. Controlled ventilation can be achieved only with pharmacologic agents. Advantages of controlled ventilation include eliminating WOB and complete control over the patient’s ventilatory pattern. In cases in which WOB is high, controlled ventilation may allow for ventilatory muscle rest, reduce O₂ consumption of the ventilatory muscles, and “free up” O₂ for delivery to the tissues.⁸

Controlled ventilation is a common initial approach in situations of severe acute respiratory failure especially if the primary problem is hypoxemia. Figure 44-2 depicts the effects of inspiratory time on V_T during controlled ventilation. Disadvantages of controlled ventilation include the need for sedatives and perhaps paralytic drugs. All patients given paralytic drugs must be sedated adequately because paralysis does not alter the patients’ perception of their surroundings. All patients’ senses are active; none are affected by paralysis; only voluntary muscles are paralyzed. In addition, in the care of apneic patients, ventilator malfunction or disconnection can lead to death.

FIGURE 44-2 Flow versus time waveform during PCV. *Curve A* shows a flow pattern during controlled ventilation in which inspiratory time *(I)* is inadequate to ensure maximum V_T has been delivered. During inspiration, flow does not decrease to zero before exhalation occurs, so the preset pressure has not equilibrated to that in the lung. *Curve B* shows an increase in I from *curve A.* Inspiratory flow reaches zero maximizing V_T delivery and allows the preset pressure to equilibrate in the lungs. Exhaled V_T is greater for *curve B* than for *curve A* despite the pressure setting not changing.

Synchronized Intermittent Mandatory Ventilation

Synchronized intermittent mandatory ventilation (SIMV) may be used as a means of providing partial or full ventilatory support.⁸ With SIMV, the machine breath may be volume or pressure targeted; in adults, it is typically a volume-targeted breath. SIMV often is combined with pressure support to overcome the imposed work of breathing (WOB_I) during spontaneous breathing owing to the artificial airway. SIMV allows the clinician to vary the amount of support provided from minimal to full ventilatory support. Disadvantages of SIMV include possible development of respiratory muscle dysfunction, especially in patients with rapid, shallow spontaneous breathing patterns; acute hypoventilation with use of low rates if patients do not continue to do their share of breathing; and an increase in WOB secondary to lack of ventilatory support during spontaneous breaths unless pressure support is applied.⁸ SIMV also delays weaning compared with spontaneous breathing trials or pressure support.^13,14 The advantages and disadvantages of assist-control and SIMV modes are summarized in Table 44-2. Outside of the United States, SIMV is an infrequently used mode of ventilation because of the above-mentioned problems.

TABLE 44-2

Advantages and Disadvantages of Synchronized Intermittent Mandatory Ventilation

Advantages	Disadvantages
Lower mean airway pressure may result than is achieved with assist-control ventilation	SIMV with PSV may increase mean airway pressure
Ventilatory muscle activity, strength, and coordination are maintained	Ventilatory muscle fatigue may occur
Level of support to maintain adequate levels of alveolar ventilation is easy to titrate	Acute hypoventilation may occur, especially with lower machine rates (<8-10 breaths/min)
Weaning protocols are easy to apply	Weaning may be prolonged
Spontaneous breathing, which is physiologic, is incorporated	Addition of pressure support often is required to overcome WOB_I
Patients tend not to hyperventilate and may not fight the ventilator, as they may do with assist mode	Patients may have difficulty adjusting to the ventilator; breath stacking is possible with intermittent mandatory ventilation
Sedation or paralysis is not required, as it is in control mode	Patients may experience or continue a rapid, shallow breathing pattern or continue to make spontaneous breathing efforts during delivery of a “machine breath”
Full or partial ventilatory support and level of support can be titrated according to patient’s need	Patient’s workload increases considerably when SIMV rate decreases to approximately 50% of full ventilatory support value

Pressure Support Ventilation

Pressure support ventilation (PSV) assumes minimal control over the patient’s ventilatory pattern. Specifically, only the level of pressure applied is controlled by the ventilator, and all other aspects of gas delivery are controlled by the patient. However, PSV is very similar to PA/C. The primary difference is that in PSV flow terminates the breath, whereas in PA/C time terminates the breath. Other than this, PA/C has a backup rate, and with pressure support an apnea mode of ventilation is set.¹⁵ PSV can reduce work of breathing and may improve patient-ventilator synchrony by placing more control with the patient.¹⁶ Many clinicians use PSV simply to overcome WOB imposed by the artificial airway.¹⁶ The PSV level needed to overcome WOB_I may be estimated as follows:

PSV=(PIP−Pplat)×V˙IspontaneousV˙ ventilator

where PSV is the pressure support level needed to overcome WOB_I, PIP is the peak inspiratory pressure during a volume-control machine breath, P_plat is the plateau pressure after an inspiratory pause (usually >1 second), is the patient’s spontaneous peak inspiratory flow (L/sec), and ventilator is the ventilator peak inspiratory flow rate (L/sec) with a square wave inspiratory flow waveform. An example of the calculations for PSV needed to overcome WOB_I is presented in Box 44-6.

Box 44-6 Calculation of Pressure Support Ventilation Level Needed to Overcome Imposed Work of Breathing and During Synchronized Intermittent Mandatory Ventilation

• Machine delivered V_T during VA/C: 450 ml

• Machine inspiratory flow rate: 50 L/min (1 L/sec)

• Flow pattern: Square wave

• PIP: 25 cm H₂O

• P_plat: 23 cm H₂O

• Patient’s spontaneous inspiratory flow rate: 30 L/min (0.5 L/sec)

PSV=(PIP−Pplat)×V˙1spontaneousVentilator inspiratory flow=(40−30cm H2O)×0.5L/s1L/s=10cm H2O×0.5L/sL/s=5cm H2O

PSV can and is increasingly being used as a primary mode of ventilation. PSV is essentially the only mode of ventilation used during noninvasive ventilation. It is also an acceptable mode of ventilation for any patients capable of triggering ventilatory support who have an intact ventilatory drive. Many clinicians use this mode in the initial phases of ventilatory support and following the most acute phase of ventilatory failure. The actual PSV level needed is based on the desired V_T. PSV is adjusted to ensure the desired V_T is delivered, and rise time and termination criteria are set to avoid asynchrony. Few clinicians at the present time attempt to calculate PSV level based on the previously listed formula.

High-Frequency Oscillatory Ventilation

High-frequency oscillatory ventilation (HFOV) is the primary approach to high-frequency ventilation used in adults. Respiratory rates range from about 3 Hz (180/min) to about 8 Hz (480/min), and very small V_T, often approaching anatomic dead space, is delivered.¹⁷ Gas transport during HFOV is due to conventional bulk flow, longitudinal (Taylor) dispersion, pendelluft, asymmetric velocity profiles, cardiogenic mixing, or enhanced molecular diffusion.¹⁷ Although high-frequency ventilation has been shown to be safe and effective in maintaining oxygenation and ventilation in various patients,^17–19 HFOV has not been shown to be superior to conventional ventilation. The primary setting where HFOV has been used is in the treatment of ARDS, but randomized trials comparing the two indicate no difference in outcome. Most clinicians consider HFOV and conventional ventilation equivalent in their ability to manage ARDS. Many clinicians wait until the patient reaches a critical level of illness before instituting HFOV; however, if one considers HFOV the better approach to managing ARDS, it should be started early in the course of ARDS instead of waiting and using it only as a rescue technique. High-frequency ventilation may allow for reduction in airway pressure and may have value as part of a lung protective strategy for the management of ARDS.¹⁹

Initial Choice of Mode

Most patients who need mechanical ventilation in the acute care setting initially are managed with volume or pressure ventilation in the assist-control mode or with PSV.7,12 SIMV may also be used, but it has no advantage over these modes, and it has considerable disadvantages. However, there is no evidence suggesting any of the modes are more beneficial in terms of patient outcomes except that weaning is delayed with SIMV.^13,14 Consequently, the choice of initial ventilator mode is primarily one of clinician preference and patient tolerance. Once the patient is stabilized on a ventilator mode, decisions can be made regarding the use of other, newer modes of ventilation, such as PRVC, volume support, adaptive support ventilation, PAV, or NAVA.

Rule of Thumb

For most patients, begin mechanical ventilatory support with VA/C, PA/C, or PSV. When the patient is stabilized, other modes of ventilation can be considered.

Tidal Volume and Rate

V_T and machine rate should be chosen concurrently because these are the two major determinants of minute ventilation. Normal spontaneous V_T for unstressed adults is on average 6.3 ml/kg IBW (approximately 5 to 7 ml/kg IBW) with a respiratory rate of 12 to 18 breaths/min establishing a minute ventilation of approximately 100 ml/kg IBW per minute.²⁰ In the past, the Radford nomogram was used to estimate V_T and rate on the basis of estimated body weight (Figure 44-3). In modern practice, acceptable V_T for mechanical ventilation usually ranges from 4 to 10 ml/kg IBW,^1,5 although V_T larger than 8 ml/kg IBW is harmful in patients with ALI/ARDS^2,21,22 and is mostly harmful to any patient in acute respiratory failure regardless of the cause of the failure.

FIGURE 44-3 Radford nomogram. Although first published in 1955, normal resting V_T can still be accurately predicted using this nomogram. (Modified from Radford EP Jr: Ventilation standards for use in artificial respiration. J Appl Physiol 7:451–460, 1955.)

Generally, regardless of mode, an initial V_T of 6 to 8 ml/kg IBW with a rate of 12 to 16 breaths/min is suggested for patients without acute restrictive disease.5,23 After initiation of ventilation, the P_plat can be assessed, and V_T can be adjusted downward, as needed, for maintenance of a P_plat less than 30 cm H₂O. A smaller initial V_T (4 to 6 ml/kg IBW) is appropriate for patients with ALI/ARDS^2,20,21 with high P_plat and is usually necessary in patients with severe acute asthma. Table 44-3 lists V_T values for men and women according to calculated IBW.

TABLE 44-3

Tidal Volume Based on Ideal Body Weight *

Height (in)	Height (ft)	Weight (lb)	Weight (kg)	6 ml/kg	8 ml/kg	10 ml/kg	12 ml/kg
Men
58	4′10″	94	43	260	340	430	520
59	4′11″	100	45	270	360	450	540
60	5′0″	106	48	290	380	480	580
61	5′1″	112	51	310	410	510	610
62	5′2″	118	54	320	430	540	650
63	5′3″	124	56	340	450	560	670
64	5′4″	130	59	350	470	590	710
65	5′5″	136	62	370	500	620	740
66	5′6″	142	65	390	520	650	780
67	5′7″	148	67	400	540	670	800
68	5′8″	154	70	420	560	700	840
69	5′9″	160	73	440	580	730	880
70	5′10″	166	75	450	600	750	900
71	5′11″	172	78	470	620	780	940
72	6′0″	178	81	490	650	810	970
73	6′1″	184	84	500	670	840	1010
74	6′2″	190	86	520	690	860	1030
75	6′3″	196	89	530	700	890	1070
76	6′4″	202	92	550	740	920	1100
77	6′5″	208	95	570	760	950	1140
Women
55	4′7″	80	36	220	290	360	430
56	4′8″	85	39	230	310	390	470
57	4′9″	90	41	250	330	410	500
58	4′10″	95	43	260	340	430	520
59	4′11″	100	45	270	360	450	540
60	5′0″	105	48	290	380	480	580
61	5′1″	110	50	300	400	500	600
62	5′2″	115	52	310	416	520	620
63	5′3″	120	55	330	440	550	660
64	5′4″	125	57	340	460	570	680
65	5′5″	130	59	350	470	590	710
66	5′6″	135	61	370	490	610	730
67	5′7″	140	64	380	510	640	770
68	5′8″	145	66	400	530	660	790
69	5′9″	150	68	410	540	680	820
70	5′10″	155	70	420	560	700	840
71	5′11″	160	73	440	580	730	876
72	6′0″	165	75	450	600	750	900

^*Ideal body weight (lb): Men, 106 + [6(H − 60)]; women, 105 + [5(H − 60)], where H is height in inches.

Rule of Thumb

When starting ventilatory support for most adult patients, use an initial V_T of 6 to 8 ml/kg (IBW) and a respiratory rate of 12 to 16 breaths/min.

V_T times rate (f) determines minute ventilation () As a rule, for adult patients, the resultant minute ventilation should be approximately 80 to 100 ml/kg IBW per minute.⁸ A 70-kg adult (IBW) would have a minute ventilation of approximately 7000 ml/min. Patients with elevated CO₂ production () or increased physiologic dead space (V_Dphys) need a larger minute ventilation to maintain acceptable PaCO₂. Minute volume should be increased by increasing the rate, not the V_T.

In SIMV, the total minute ventilation is composed of spontaneous tidal volume (V_Tsp), spontaneous rate (f_sp), machine tidal volume (V_Tmach), and machine rate (f_mach). For SIMV, total minute ventilation () is described as follows:

V˙E TOT=V˙Emachine+V˙Espontaneous

and

V˙E=(VTmach×fmach)+(VTsp-average×fsp)

For PA/C or PSV, the delivered V_T depends on the pressure limit, the inspiratory time, and the patient’s lung mechanics. Generally, the pressure limit is increased or decreased to achieve a target V_T while a P_plat of less than 30 cm H₂O is maintained. A good initial pressure setting is to start at 15 cm H₂O (above baseline pressure) and observe the resulting V_T. Pressure is increased or decreased to achieve the desired volume. As with VA/C, minute ventilation with PA/C is simply rate multiplied by . Recommended initial V_T and frequency for various patient types are described in Table 44-4.

TABLE 44-4

Patient Type	Tidal Volume (ml/kg)	Frequency (breaths/min)
Adults
Normal lungs	6-8	12-16
Neuromuscular disease, postoperative period, or with normal pulmonary mechanics in which maintaining lung volume is a concern	6-10	12-16
Acute restrictive disease, ALI/ARDS	4-8*	20-35
Obstructive lung disease (COPD)	6-8	10-12^†
Acute severe asthma exacerbation	4-6	10-12
Children
Age 8-16 yr	6-8	20-30
Age 0-8 yr	6-8	25-35

^*For ALI/ARDS, maintain P_plat at ≤30 cm H₂O. V_T begins at 8 ml/kg and is gradually reduced to 6 ml/kg. V_T of 4 ml/kg may be required in the care of these patients to avoid ventilator-induced lung injury.

^†For patients with obstructive disease, ensure a short inspiratory time and long expiratory time to avoid air trapping and minimize auto-PEEP. Lower V_T and rate may be necessary in acute asthma to avoid further lung overinflation.

Patients with ALI/ARDS may need lower V_T to avoid further lung injury and a higher rate to maintain effective alveolar ventilation while P_plat is maintained less than 30 cm H₂O. Results of multicenter studies suggest V_T of 4 to 8 ml/kg IBW for patients with ARDS.2,20,21 In these studies, patients receiving a V_T of 4 to 8 ml/kg had lower mortality and morbidity than patients receiving a V_T of 10 to 12 ml/kg. For patients with ALI/ARDS, one may begin with V_T of 8 ml/kg and decrease the volume as needed to maintain P_plat less than 30 cm H₂O.² Machine rates of 25 to 35 breaths/min may be needed in patients with ALI/ARDS to maintain adequate minute ventilation. Box 44-7 summarizes the ARDS Clinical Network guidelines for initial ventilator setup.^2,24

Box 44-7

Initial Ventilator Setup and Management of Oxygenation, Plateau Pressure, and pH

1. Calculate predicted (ideal) body weight as follows:

• Men: Weight in kilograms = 50 + 2.3 (height in inches − 60)

• Women: Weight in kilograms = 45.5 + 2.3 (height in inches − 60)

2. Select assist-control mode.

3. Set V_T to 8 ml/kg of predicted body weight.

4. Reduce V_T by 1 ml/kg at intervals of ≤2 hours until V_T is 6 ml/kg.

5. Set initial rate to achieve baseline minute ventilation (). Do not exceed 35 breaths/min.

6. Adjust V_T and rate to achieve pH of 7.30 to 7.45 while maintaining P_plat of ≤30 cm H₂O.

7. Set inspiratory flow rate above patient demand (may be >80 L/min).

8. For oxygenation to achieve PaO₂ of 55 to 80 mm Hg or SpO₂ 88% to 95%, use the following incremental FiO₂/PEEP combinations. Higher PEEP options (lower row) decrease FiO₂ and may be preferred in patients with high FiO₂ who can tolerate higher PEEP (stable blood pressure, no barotrauma). Survival is similar with both PEEP approaches.

FiO₂	0.3	0.4	0.4	0.5	0.5	0.6	0.7	0.7
Low PEEP	5	5	8	8	10	10	10	12
High PEEP	12-14	14	16	16	18-20	20	20	20
FiO₂	0.7	0.8	0.9	0.9	0.9	1.0	1.0	1.0
Low PEEP	14	14	14	16	18	20	22	24
High PEEP	20	20-22	22	22	22	22	22	24

9. Check P_plat, SpO₂, respiratory rate, V_T, and pH (if available) at least every 4 hours and after each change in PEEP or V_T:

• If P_plat is >28 cm H₂O, decrease V_T by 1-ml/kg steps (minimum 4 ml/kg)

• If P_plat is <25 cm H₂O and V_T is <6 ml/kg, increase V_T by 1-ml/kg steps until P_plat is >25 cm H₂O or V_T is 6 ml/kg

• If P_plat is <20 and breath stacking occurs, V_T may be increased in 1-ml/kg increments (maximum 8 ml/kg)

10. The pH goal is 7.30 to 7.45.

For acidosis management (pH < 7.30):

• If pH is 7.15 to 7.30, increase the rate until pH is >7.30 or PaCO₂ is <25 mm Hg (maximum 35); if rate is 35 and PaCO₂ is <25 mm Hg, NaHCO₃ may be given

• If pH is <7.15, increase rate to 35; if pH remains <7.15 and NaHCO₃ is considered, V_T may be increased in 1-ml/kg steps until pH is >7.15 (P_plat target may be exceeded)

For alkalosis management (pH > 7.45), decrease ventilator rate, if possible.

Adapted from National Institutes of Health (NIH) National Heart Lung and Blood Institute (NHLBI) ARDS Clinical Network Mechanical Ventilation Protocol Summary (Mechanical Ventilation Protocol Summary, revised 25 January 2005).

Trigger Sensitivity

Trigger sensitivity for patient-triggered ventilation should be set at the most sensitive level avoiding autotriggering to minimize trigger work and missed triggering. With flow triggering, the trigger should be set at 1 to 2 L/min, and with pressure triggering, the range is generally −0.5 to −1.5 cm H₂O. However, because of pin holes in disposable ventilator circuits, the sensitivity may need to be adjusted to 3 or 4 L/min or −2 cm H₂O to avoid autotriggering. The increased use of ventilator graphics packages has led to the recognition that patients’ inspiratory efforts often are insufficient to trigger the ventilator.⁸ Factors that can prolong ventilator response time include large V_T, low trigger sensitivity, auto-PEEP, high bias circuit flow, and abdominal paradox (Box 44-8). (See later discussion of asynchrony.)

Many ventilators offer the option of a pressure or a flow trigger. With older generation intensive care unit (ICU) ventilators, flow triggering offered slightly lower trigger work than pressure triggering,25–27 although the gain in terms of the patient’s total WOB was slight. Newer ventilators with fast pressure-triggering capabilities are as sensitive as flow-triggered devices.²⁸ However, no triggering mechanism can reduce the WOB that is a result of auto-PEEP. Auto-PEEP must be addressed by other means (see later section).²⁷ Flow-trigger settings vary by ventilator. Generally, for flow triggering, the trigger flow should be set 1 to 2 L/min below baseline or bias flow.

Inspiratory Flow, Time, and Inspiratory-to-Expiratory Ratio for Volume Ventilation

Most modern critical care ventilators allow the clinician to select peak flow, V_T, and rate or inspiratory time (or percentage inspiratory time, V_T, and rate). For most adults, an initial inspiratory time of approximately 0.8 second (range 0.6 to 1.0 second) with a resultant inspiratory-to-expiratory (I : E) ratio of 1 : 2 or lower is a good starting point. This value corresponds to an initial peak flow setting of approximately 60 L/min (range 40 to 80 L/min) and a down ramp or square flow waveform.²⁹ Higher flow (≤100 L/min) may improve gas exchange in patients with chronic obstructive pulmonary disease (COPD), probably because of the resulting better synchrony and increase in expiratory time.^9,29

Inspiratory flow rate should be adjusted to ensure that the flow provided meets or exceeds the patient’s spontaneous inspiratory flow.²⁹ This rate can be achieved by observing the resultant pressure-time curve contour on a graphics monitor. Ideally, the pressure curve should show a linear increase during inspiration.^9,29 A large negative deflection indicates inadequate trigger sensitivity, and excessive scalloping of the waveform indicates inadequate ventilatory flow settings (Figure 44-4).^9,10 A less sensitive trigger level and lower ventilator inspiratory flow tend to increase the patient’s WOB. Common ventilator configurations and related controls that determine inspiratory flow, time, and I : E ratio are described in Figure 44-5.

FIGURE 44-4 Plot of an ideal pressure-time waveform during volume-targeted ventilation *(top).* Plot of actual pressure-time curve *(dotted line)* is superimposed on the ideal curve *(bottom).* The scooped-out actual pressure waveform is evident. *Green area* reflects the work performed by the patient during assisted volume-targeted ventilation. This type of actual pressure waveform is indicative of inadequate peak inspiratory flow or too lengthy an inspiratory time. The peak flow should be increased, normally to between 60 L/min and 90 L/min to minimize patient effort during volume-limited ventilation. (Modified from Marini JJ, Rodriguez M, Lamb V: The inspiratory workload of patient-initiated mechanical ventilation. Am Rev Respir Dis 134:902, 1986.)

FIGURE 44-5 Relationship between V_T, inspiratory flow, inspiratory time, expiratory time, and I : E ratio in various ventilator systems. A, Effects of V_T, flow, and respiratory rate on inspiratory time, expiratory time, and I : E ratio. Some ventilators provide V_T, inspiratory flow, and rate control in volume control (VC) and SIMV modes. B, Effects of volume, inspiratory time, and rate on inspiratory flow, expiratory time, and I : E ratio. Other ventilators provide controls for inspiratory time (or percentage inspiratory time), V_T (or minute ventilation), and rate in the VC and SIMV modes. In the VC mode (controlled ventilation), ventilators with a percentage inspiratory time control maintain a constant I : E ratio with changes in respiratory rate. In the SIMV mode, changes in SIMV rate alter I : E ratio on these machines.

For ventilators with V_T, peak flow, and rate controls, inspiratory time is determined by V_T, peak flow, and flow pattern. To decrease inspiratory time, one may increase peak flow, decrease V_T, or change from a down ramp to a square wave flow pattern. Expiratory time and I : E ratio are determined by inspiratory time and rate. To increase expiratory time (and decrease I : E ratio), one may decrease the inspiratory time as described earlier or increase the expiratory time by decreasing the rate.²⁹

For ventilators with V_T (or minute ventilation), percentage inspiratory time, and rate controls, the inspiratory time and V_T determine the inspiratory flow rate. On these ventilators, one can directly increase or decrease the percentage inspiratory time. At the same rate, as inspiratory time (or percentage inspiratory time) decreases, expiratory time and inspiratory flow rate increase, and I : E ratio decreases. An increase in V_T at the same percentage inspiratory time and rate also increases inspiratory flow rate with no change in I : E ratio. Box 44-9 shows the calculation of inspiratory flow rate based on percentage inspiratory time settings. To alter I : E ratio on these ventilators, one simply adjusts inspiratory percent time. Decreasing rate at the same inspiratory percent time setting does not affect I : E ratio, and both inspiratory time and expiratory time increase owing to a longer respiratory cycle. Changing the inspiratory flow waveform on these ventilators has no effect on inspiratory time, expiratory time, or I : E ratio; however, flow waveform changes affect peak and mean airway pressure.

Box 44-9 Calculation of Inspiratory Flow Rate from Percentage Inspiratory Time

The effect of percentage inspiratory time on inspiratory flow rate can be estimated as follows:

Inspiratory flow rate=Set minute ventilationPercentage inspiratory time×0.01

For example, a patient being treated with a Servo ventilator may have the following ventilator settings:

• Set minute ventilation = 12 L/min

• Set CMV rate = 20 breaths/min

• Resulting V_T = 12 L/20 breaths/min = 0.6 L or 600 ml

• Set time inspiratory percentage = 25%

• Set pause time percentage = 0%

Inspiratory flow rate=12L/min25%×0.01=12L/min0.25=48L/min

CMN, Continuous mechanical ventilation.

Flow Waveform

Flow waveform options on mechanical ventilators vary from a preset square wave to seven adjustable waveforms on older ventilators. Common choices available on current generation ventilators for waveform are square or down ramp (decreasing or “decelerating” waveform). Pressure support and pressure-controlled modes also deliver decreasing flow waveforms, but the decrease is patient-specific and not programmed into the gas delivery. The literature on clinical application of specific waveforms is mixed.³⁰ However, as one moves from an increasing (“accelerating”) flow waveform to a square wave to a decreasing flow waveform, while holding inspiratory time constant, there tends to be a predictable decrease in peak airway pressure and a corresponding increase in mean airway pressure.³⁰ Increases in mean airway pressure may improve oxygenation, while further impeding venous return to the heart.³⁰ At least as far as inspiratory flow patterns are concerned, what is good for the lungs may be bad for the heart. We suggest a decreasing, or down ramp, flow waveform when the goal is optimization of the distribution of inspired air and improvement in oxygenation. A square waveform may be useful in reducing mean airway pressure in patients with severe hypotension or cardiovascular instability. Figure 44-6 compares the effect of ventilator flow waveforms on peak and mean airway pressure. Box 44-10 describes guidelines for selecting flow waveform during volume ventilation.

Box 44-10 Guidelines for Selecting Inspiratory Flow Waveforms during Volume Ventilation

Constant Flow Waveform

• Alternative terms: Square wave, rectangular wave, constant flow generator

• Advantages: High flow provided with a reduced inspiratory time and improved I : E ratio; may decrease mean airway pressure, which may be helpful in terms of venous return and cardiac output in compromised patients

• Disadvantages: Increased PIP may lead to excessive pressure; lower mean airway pressure may affect oxygenation

Decreasing Flow Waveform

• Alternative terms: Down ramp, decelerating flow, descending ramp

• Advantages: Lower PIP and higher mean airway pressure; this flow waveform may improve gas distribution, oxygenation, and patient-ventilator synchrony

• Disadvantages: Increased mean airway pressure may impede venous return and cardiac output in compromised patients; in ventilators that have a peak flow control, the down ramp increases inspiratory time and I : E ratio and decreases expiratory time

FIGURE 44-6 Effect of ventilator flow waveform on peak and mean airway pressure with changing lung mechanics. Generally, flow waveforms that tend to increase mean airway pressure also decrease peak pressure (PIP) and vice versa. Consequently, if increasing mean airway pressure is the goal, decelerating (down ramp) flow waveforms may be helpful. However, in the care of patients with cardiovascular compromise, in whom reducing mean airway pressure may be helpful, a square wave or accelerating flow (up ramp) may be valuable. Accelerating flow waveforms are no longer available on newer critical care ventilators. (Modified from Rau JL, Shelledy DC: The effect of varying inspiratory flow waveforms on peak and mean airway pressures with a time-cycled volume ventilator: a bench study. Respir Care 36:347, 1991.)

During PCV or PSV, a decreasing flow waveform is delivered. The initial peak flow typically is reached rapidly. Flow decreases throughout inspiration until the breath is terminated. With PSV, inspiration ends when the flow decreases to a preset value, typically adjustable from 5% to 85% of the peak flow in some newer ventilators. With PCV, flow continues to decrease until the inspiratory time has elapsed. In the PCV mode, increasing inspiratory time tends to increase V_T until zero flow is reached at end inspiration. Further increases in inspiratory time do not increase V_T, although distribution of inspired air may improve, and mean airway pressure does increase.

Inspiratory Pause

In addition to inspiratory time or flow, most ventilators have an option for setting an inspiratory pause or hold in the volume-control mode. A brief inspiratory pause (up to 10%) has been recommended in the past for improving the distribution of the inspired air and PaO₂.31,32 Use of an inspiratory pause has been suggested for administration of bronchodilators to improve medication delivery. However, in the treatment of patients with COPD, a 5-second inspiratory pause did not result in significant improvement in measures related to the effectiveness of the bronchodilator.³¹ If a brief inspiratory pause is used, I : E ratio and mean airway pressure increase. An inspiratory pause of 0.5 to 2 seconds applied for a single breath is used for measurement of P_plat and in estimation of airway resistance (Raw):

Raw=PIP−Pplat/Inspiratory flow(L/sec)

where PIP is peak inspiratory pressure. An inspiratory pause should never be set in a spontaneously breathing patient except for a single breath in attempts to measure P_plat because it increases the level of asynchrony causing the patient to fight the pause and to try to exhale during the pause.

An inspiratory pause can also be used to ensure a full inspiration before a chest radiograph is obtained, and this step may improve the quality of the resulting radiograph.³³ Use of an extended inspiratory pause should be limited because of the resultant increase in mean airway pressure and risk of impeding venous return and cardiac output, especially in patients who are hypovolemic or hypotensive or whose condition is hemodynamically unstable.

Rule of Thumb

An end inspiratory pause should be used only to estimate the end inspiratory P_plat. It should never be applied continuously to a patient actively triggering the ventilator.

Oxygen Percentage (Fractional Inspired Oxygen)

FiO₂ selected on initiation of mechanical ventilation varies with the patient’s condition. If little is known about the patient or if the patient’s condition appears to be grave, 100% O₂ is the preferred starting point. Examples of disease states or conditions that typically warrant initial FiO₂ of 1 include acute pulmonary edema, ARDS, near drowning, cardiac arrest, severe trauma, suspected aspiration, severe pneumonia, carbon monoxide poisoning, and any disease state or condition resulting in a large right-to-left shunt. After initiation of mechanical ventilation with FiO₂ of 1, the FiO₂ should be reduced as soon as is practical to avoid O₂ toxicity and absorption atelectasis. In most patients, FiO₂ should always be adjusted to ensure PaO₂ of 60 mm Hg or greater or oxygen saturation by pulse oximeter (SpO₂) of 90% or greater.

Patients who have undergone previous blood gas measurement or oximetry who are doing well clinically and patients with disease states or conditions that normally respond to low to moderate concentrations of O₂ may begin ventilation with a lower O₂ concentration (50% to 70% O₂). These typically are patients with normal ventilation/perfusion () or a imbalance without shunt ( but >0). Patients who often do well with low to moderate concentrations of O₂ include patients with acute exacerbation of COPD, emphysema, chronic bronchitis, drug overdose without aspiration, or neuromuscular disease and postoperative patients with normal lungs. For example, a patient with an acute exacerbation of COPD who needs mechanical ventilatory support may have had PaO₂ of 50 mm Hg with a nasal cannula at 4 L/min before intubation and mechanical ventilation. This patient would probably do well with FiO₂ of about 0.50 when adequate ventilation is restored. The patient can begin with 50% O₂ and be immediately assessed for assurance of adequate SpO₂. FiO₂ can be adjusted according to the patient’s response.

Positive End Expiratory Pressure and Continuous Positive Airway Pressure

PEEP and CPAP are effective techniques for improving and maintaining lung volume and improving oxygenation for patients with acute restrictive disease such as ALI, pneumonia, pulmonary edema, and ARDS.8,29 PEEP and CPAP should be cautiously applied in the treatment of patients with an already elevated FRC, such as patients with COPD or acute asthma, except at levels that are applied to offset auto-PEEP and air trapping.⁸ Generally, the indications for PEEP or CPAP in the critical care setting are inadequate arterial O₂ level with moderate to high concentrations of O₂ caused by unstable lung units that are collapsed. PaO₂ less than 50 to 60 mm Hg with FiO₂ greater than 0.40 to 0.50 is a good general starting place for considering use of PEEP or CPAP.

In terms of ventilator initiation, initial PEEP or CPAP levels usually are 5 cm H₂O even in the absence of unstable lung units or auto-PEEP. Most experts advocate for the use of 5 cm H₂O “physiologic” PEEP for all patients who have an artificial airway in place. Intubation results in small reductions in FRC,8,29 which can be balanced with the application of PEEP or CPAP. Whether the application of physiologic PEEP has important benefits in terms of patient outcome is unknown.

PEEP has been advocated in the presence of auto-PEEP, in particular, in the care of patients with obstructive lung disease.³⁴Applied PEEP in the presence of auto-PEEP is indicated only if the patient has difficulty triggering the ventilator. During controlled ventilation, increasing PEEP in the presence of auto-PEEP is usually not indicated. Because the patient is spontaneously breathing, the measurement of auto-PEEP (end expiratory pause) is very difficult; the patient frequently forces exhalation negating the measurement. The presence of auto-PEEP is generally indicated by the expiratory flow waveform (Figure 44-7) or the fact that there are missed triggered breaths. That is, the patient inspires but is unable to trigger the ventilator. In this case, PEEP is slowly applied in increments of 1 to 2 cm H₂O, rechecking the patient and ventilator response rate with each adjustment. When the patient is able to trigger the ventilator with each inspiratory effort, the PEEP level is set properly.²⁹ The exact auto-PEEP level may never be known in these patients; however, this is unimportant. What is important is that each patient’s effort triggers the ventilator. The literature indicates that if the amount of PEEP applied does not exceed approximately 80% of the measured auto-PEEP, the patients’ lung mechanics are not affected by the application of PEEP.³⁵ Auto-PEEP ideally should be rechecked to ensure intrinsic PEEP does not increase as PEEP is applied. Box 44-11 summarizes methods for minimizing the effects of auto-PEEP. An absolute contraindication to PEEP is an uncontrolled tension pneumothorax. However, PEEP should be cautiously applied in any patient with severe intrinsic lung disease, hypotension, and elevated intracranial pressure.

FIGURE 44-7 Airway flow, pressure, and volume in a patient with severe airflow obstruction and auto-PEEP. There is a rapid decrease in expiratory flow at the onset of exhalation because of the obstruction *(arrow A)*, and there is a lack of return of flow to baseline at the end of the breath *(arrow B)*. This type of expiratory flow pattern, regardless of the expiratory time, indicates auto-PEEP. The amount of auto-PEEP cannot be determined from this example, but anytime end expiratory flow is greater than zero, auto-PEEP is present.

Open Lung Strategy, Recruitment Maneuvers, and Positive End Expiratory Pressure

In the care of patients with ALI/ARDS, it is usually necessary to initiate PEEP at 10 to 15 cm H₂O.21,22,35,36 However, many clinicians use the ARDS Clinical Network PEEP/FiO₂ tables to set PEEP initially during the establishment of ventilatory support (see Box 44-7). When patients are stabilized, the use of an open lung ventilation strategy in early-stage ARDS has been recommended.^5,29 Such a strategy incorporates V_T of 4 to 8 ml/kg IBW with either pressure-targeted or volume-targeted ventilation and a PEEP level set after a lung recruitment maneuver using a decremental PEEP trial.^5,29,37 The lung recruitment maneuver is intended to open collapsed lung units, and the setting of PEEP using a decremental PEEP trial is intended to apply PEEP based on the patient’s lung mechanics to keep the lung units recruited open.

Although all patients with ARDS/ALI require PEEP, not all patients with ALI respond to PEEP, and patients with pulmonary (vs. nonpulmonary) causes of ALI/ARDS, such as pneumonia, may be less likely to respond to low to moderate levels of PEEP.¹¹ Nonpulmonary causes of ALI/ARDS (e.g., extrathoracic trauma, intraabdominal sepsis) seem to respond well to PEEP.¹¹ In practice, some authors have suggested that higher levels of PEEP (>15 cm H₂O) be reserved for patients with a high percentage of recruitable lung.³⁸ High levels of PEEP have been shown to improve outcomes in ARDS in patients with the most severe forms of ARDS (PaO₂/FiO₂ < 150 mm Hg).^39,40 (See later section on the performance of recruitment maneuvers and the setting of PEEP by decremental trial.)

Pressure Rise Time or Slope

Most newer critical care ventilators include an inspiratory pressure rise time or pressure slope. This control functions only with pressure-limited breaths (PSV, PCV, PRVC, volume support, airway pressure release ventilation, pressure SIMV). The purpose of this control is to adjust the rate at which flow increases from baseline to peak.41–43 Generally, rise time should be set at a value that ensures adequate inspiratory gas flow (meeting or exceeding patient demand) without an excessive “overshoot” of the pressure at the beginning of inspiration. A slow or low rise time may increase the patient’s WOB.^41–43 The effect of changing inspiratory rise time is described in Figure 44-8 and discussed more in the section on patient-ventilator synchrony.

FIGURE 44-8 Effect of changing rise time during pressure-targeted breaths for a patient who prefers a moderate flow. A, Flow exceeds patient demand, and a pressure spike and short inspiratory time result. B, As flow is decreased, inspiratory time lengthens, and the pressure spike disappears. Machine output matches patient demand. C, When flow is reduced further, patient demand exceeds machine flow; the result is deformation of the pressure waveform and a decrease in inspiratory time. (Modified from Branson RD, Campbell RS, Davis K, et al: Altering flow rate during maximum pressure support ventilation (PSV_max): effect on cardiorespiratory function. Respir Care 35:1056–1069, 1990.)

Limits and Alarms

Ventilator alarms and limits warn of ventilator malfunction and changes in patient status. Ventilator malfunction alarms include power or gas supply loss and electronic or pneumatic malfunction. These alarms usually are preset by the manufacturer.

Patient status alarms usually are set by the respiratory therapist (RT). These include maximum inspiratory pressure, low-pressure and low-PEEP alarms, high-volume and low-volume and rate alarms, O₂ and humidification alarms, and apnea alarms. After initiation of ventilation, alarms and limits are readjusted as needed. Alarms usually are set so that they warn the clinician of important changes or problems. Without proper setting, these alarms can become a nuisance by falsely signaling problems that are not real.⁸

In volume ventilation, a pressure limit should be set. Generally, before the patient is connected to the ventilator, the limit should be set at 40 cm H₂O to avoid overpressuring the system when the patient is connected. After the patient is connected to the ventilator, the peak and plateau pressures should be assessed. If P_plat is greater than 30 cm H₂O, consideration should be given to decreasing the set V_T. If P_plat is less than 30 cm H₂O, the high pressure limit can be adjusted to 10 to 15 cm H₂O above PIP. One can decrease peak pressure by decreasing the peak flow rate, increasing the inspiratory time, changing the inspiratory flow waveform from a square to a down ramp, or decreasing the delivered V_T. For spontaneously breathing patients, inspiratory flow and time must meet or exceed the patient’s inspiratory demand to ensure one does not increase the patient’s WOB further (see the section on Patient-Ventilator Interaction). Preset or adjustable alarms common to most ventilators include pressure (high-low), volume (high-low V_T, minute ventilation), apnea, O₂ percentage, and temperature. Suggested initial settings for these alarms and backup ventilator settings are presented in Table 44-5.

TABLE 44-5

Alarm and Backup Ventilation Setting of Initial Ventilatory Setup (Adults)

Low pressure	8 cm H₂O or 5-10 cm H₂O below PIP
Low PEEP/CPAP	3-5 cm H₂O below PEEP
High pressure limit	50 cm H₂O, which is adjusted to 10-20 cm H₂O above PIP
Low exhaled V_T	100 ml or 10%-15% below set V_T
Low exhaled minute ventilation	2-5 L/min or 10%-15% below minimum SIMV or assist-control backup minute ventilation
High minute ventilation	5 L/min or 10%-15% above baseline minute ventilation
O₂ percentage (FiO₂)	5% above and below set O₂ percentage
Temperature	2° C above and below set temperature, high temperature not to exceed 37° C
Apnea delay	20 sec
Apnea values	V_T and rate set to achieve full ventilatory support (V_T 8-10 ml/kg; rate 10-12 breaths/min) with 100% O₂

Humidification

Humidification is required during both invasive and noninvasive mechanical ventilation. A heated humidifier or a heat and moisture exchanger (HME) should provide a minimum of 30 mg/L of water with a temperature of 30° C or greater.⁴⁴ Use of HMEs should be avoided in the care of patients with secretion problems and patients with low body temperature (<32° C), high spontaneous minute ventilation (>10 L/min), or air leaks in which exhaled V_T is less than 70% of delivered V_T.⁴⁴ Heated humidifiers may be used to deliver 100% body humidity at 37° C. Current clinical practice guidelines suggest an inspired gas temperature of 33° C ± 2° C, although inspiratory gas temperatures of 33° C to 37° C are acceptable in most patients.⁴⁴ We prefer an optimal humidity approach and use of a heated humidifier to deliver gas in the range of 33° C to 37° C at the airway in most intubated patients and at a temperature consistent with patient comfort during noninvasive ventilation. However, in patients without primary pulmonary dysfunction and short-term ventilation, HMEs are very useful; generally, these are postoperative patients after elective surgery, patients in the emergency department, and patients recovering from an overdose.

Periodic Sighs

Constant, monotonous tidal ventilation at a small volume (<7 ml/kg) may result in progressive atelectasis.8,45 Periodic deep breaths or sighs taken every 6 to 10 minutes reverse this trend.^8,45 During the 1960s and 1970s, it was common to ventilate patients with a smaller V_T (5 to 7 ml/kg) and no PEEP. As a result, an intermittent sigh function was incorporated into most volume ventilators. Sighs were programmed at to 2 times the set V_T at an interval of every 6 to 10 minutes. Sometimes multiple sighs were included at a preset interval of up to 10 times per hour. Because of the use of PEEP, sighs are no longer routinely included. PEEP prevents the formation of atelectasis in a patient on ventilation with constant small V_T. This is the primary reason why 5 cm H₂O of PEEP is routinely used on patients, including patients with healthy lungs. General guidelines for the initial ventilator settings for most adult patients are described in Box 44-12.