Mechanical Ventilation

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Chapter 48 Mechanical Ventilation

I Introduction

Mechanical ventilation is frequently used to provide respiratory support in times of critical illness or in patients undergoing general anesthesia. The main goals of mechanical ventilation are oxygenation and carbon dioxide elimination, which are ensured by maintaining adequate tidal volumes and respiratory rates. Since the foot-pump ventilation apparatus was designed by Fell O’Dwyer in 1888, significant advances have been made in positive-pressure ventilation. Because the mechanical ventilator can injure the lung, safe application to limit ventilator-induced lung injury (VILI) and negative interactions with other organ systems is fundamental in managing patients. This chapter reviews different modes of mechanical ventilation and describes their characteristics, attributes, and shortcomings.

When describing mechanical ventilation, we refer only to positive-pressure ventilation, which is used in contemporary settings. Negative-pressure ventilation was employed with the advent of mechanical ventilation to treat patients affected by poliomyelitis, but it is no longer used and not discussed in this chapter.

Esteban and colleagues reviewed the use of mechanical ventilation in intensive care units (ICUs) in North America, South America, Spain, and Portugal. Among the indications for mechanical ventilation, acute respiratory failure was the most common (66% of patients), followed by coma (15%), exacerbation of chronic obstructive pulmonary disease (COPD, 13%), and neuromuscular weakness (5%). The principal causes of acute respiratory failure across all centers were pneumonia (16%), sepsis (16%), postoperative infection (15%), heart failure (12%), acute respiratory distress syndrome (ARDS, 12%), trauma (12%), unspecified causes (13%), and aspiration (3%). Endotracheal tubes were used three times more often than tracheostomies to provide artificial airways. There was some variability in the modes of ventilation used in the different countries participating in the study. Assist-control ventilation (ACV) was the most common worldwide, followed by synchronized intermittent mandatory ventilation (SIMV) with pressure support and by pressure-support ventilation (PSV) alone. However, in North American ICUs, ACV and SIMV were used equally.1

To initiate mechanical ventilation, the patient must have in place an artificial airway with which to interface with the ventilator. Various types of airway devices are discussed in other chapters of this textbook. Patients are connected to a mechanical ventilator with an orotracheal tube (i.e., endotracheal tube), nasotracheal tube, or tracheostomy.

Common reasons for insertion of an artificial airway are to maintain airway patency, to prevent aspiration, to facilitate clearance of secretions, and to allow mechanical ventilatory support.2 There are several indications for mechanical ventilation:

II Initiation of Mechanical Ventilation

Mechanical ventilation can be delivered to the patient by invasive or noninvasive methods. Noninvasive positive-pressure ventilation (NIPPV) is delivered by an external nasal or a naso-oral interface such as a face mask. The decision to use invasive or noninvasive mechanical ventilation depends on the severity and rapid anticipated reversibility of the underlying condition and the mental status of the patient. NIPPV is useful in cases of hypercapnic respiratory failure, especially associated with COPD; obstructive sleep apnea; cardiogenic pulmonary edema; and hypercapnic respiratory insufficiency in persons with adequate mental status to remain communicative.35 Application of NIPPV requires frequent assessments to ensure that the desired goal of oxygenation or ventilation is being achieved.

After the deciding to initiate invasive positive-pressure mechanical ventilation, several variables must be considered for effective implementation. They include tidal volume, respiratory rate, positive end-expiratory pressure (PEEP), fraction of inspired oxygen (FIO2), peak flow, plateau pressure, trigger sensitivity, flow rate, and flow pattern.

A Tidal Volume

Tidal volume is the volume of air delivered to the lungs with each breath by the mechanical ventilator. Historically, initial tidal volumes were set at 10 to 15 mL/kg of actual body weight for patients with neuromuscular diseases. Over the past 2 decades, VILI has been associated with excessive tidal volume leading to alveolar distention.6,7 The mechanism of lung injury includes regional overinflation,8 stress of repeated opening and closing of lung units,9,10 and sheer stress between adjacent structures with differing mechanical properties.11

The low-tidal-volume strategy, which uses 6 mL/kg of predicted body weight, has become the standard of care for patients with ARDS, following the Acute Respiratory Distress Syndrome Network (ARDS Network) publication in 2000.12 The ARDS Network prospectively studied intubated patients with acute lung injury (ALI) or ARDS to determine whether a low-tidal-volume strategy, compared with a traditional-tidal-volume strategy, could improve mortality and decrease the total number of ventilator days. The final analysis showed a 23% reduction in all-cause mortality and a 9% absolute decrease in mortality with the use of a tidal volume of 6 mL/kg of predicted body weight and plateau pressures of 30 cm H2O or less, compared with the usual practice of 12 mL/kg of predicted body weight and plateau pressures of 50 cm H2O or less. Low tidal volume or so-called lung protective ventilation is recommended for all patients with ARDS. In patients without ARDS, a retrospective review demonstrated the relationship between ALI and the use of tidal volumes greater than 10 mL/kg of predicted body weight.13 Considering the current evidence, tidal volumes greater than 10 mL/kg of predicted body weight should not be routinely used in the care of the mechanically ventilated patient.12,13

B Respiratory Rate

The respiratory rate setting depends on the desired minute ventilation. Minute ventilation is a product of the respiratory rate and the tidal volume, and it is expressed in liters per minute. After a patient is intubated and placed on the mechanical ventilator, it is important to ensure adequate minute ventilation since the underlying pathophysiology or pharmacologic interventions can suppress the patient’s ability to compensate for metabolic demands. In most scenarios, the rate is determined by observing the patient’s native respiratory rate before intubation. The normal rate of minute ventilation is 5 to 7 L/min. In patients with sepsis or diabetic ketoacidosis, the native minute ventilation may be as high as 12 to 15 L/min, requiring a high respiratory rate. To adequately compensate for the acid-base derangements and ensure adequate minute ventilation, it is necessary to titrate the set respiratory rate until the desired pH and PaCO2 goals are met. Permissive hypercapnia (i.e., allowing the PaCO2 to increase intentionally to achieve other goals) may be appropriate in certain clinical conditions. Auto-PEEP (i.e. intrinsic PEEP) must be evaluated to ensure it remains at less than 5 cm H2O. After the goal is achieved, it is a safe practice to set the rate at 4 breaths/min below the spontaneous breathing rate in the event that intrinsic or extrinsic factors suppress respiration. In patients on SIMV, the rate is initially set to meet up to 80% of the minute ventilation demands. Initial respiratory rates are usually 12 to 16 breaths/min, but rates of breaths per minute in the high 20s to low 30s may be required in patients with ARDS. In those with obstructive lung disease (e.g., asthma), a lower respiratory rate is desired, with significant risk of developing auto-PEEP. Assessment and management of auto-PEEP are discussed separately.

C Positive End-Expiratory Pressure

PEEP is the alveolar pressure above the atmospheric pressure at end-expiration. Applied PEEP (i.e., extrinsic PEEP) through mechanical ventilation allows delivery of positive pressure at the end of expiration to keep the unstable lung units from collapse.14 PEEP increases the peak inspiratory pressure, which directly overcomes the opening pressure of the unstable lung units. Low levels of PEEP (3 to 5 cm H2O) are routinely used in patients on mechanical ventilation. It can decrease alveolar collapse at end-expiration and may reduce the incidence of ventilator-associated pneumonia.15 Higher levels of PEEP are employed to improve oxygenation in patients with hypoxic respiratory failure. Goals in managing ARDS are to optimize alveolar recruitment and decrease cycles of recruitment and derecruitment of alveolar lung units. Several strategies are used to determine optimal PEEP, but there are limited data to support their routine use. Determining the lower inflection point of the pressure-volume curve (Pflex), which reflects the transition from low to higher compliance, and applying PEEP of 2 cm H2O greater than this point may be used to estimate the appropriate level of applied PEEP (Fig. 48-1).16

Because it is often impractical to routinely obtain pressure-volume curves, algorithms have been developed (e.g., in ARDS Network trials), with PaO2/FIO2 ratios to set the recommended PEEP (Table 48-1).12 Measuring esophageal pressures to estimate transpulmonary pressures has been studied as a method to determine the appropriate applied PEEP in patients with ARDS, and this approach has demonstrated improvement in oxygenation and compliance.17 Trials of increasing or decreasing PEEP can also be used.18,19 Higher levels of PEEP in postoperative patients have had no benefit.20

Lung injury in patients with hypoxic respiratory failure is heterogeneous. Since the collapse and repeated opening and closing of unstable lung units leads to further injury, its prevention would be the optimal ventilator strategy. High PEEP has been used mitigate alveolar collapse and cyclic alveolar stress. Several trials demonstrated that high PEEP increased oxygenation but did not improve mortality rates.21,22 However, a meta-analysis of high PEEP trials indicated a mortality benefit for patients with a PaO2/FIO2 ratio of less than 200.23 The optimal method of applying adequate PEEP has not been established.14 Trials in ARDS patients demonstrate PEEP requirements are usually between 12 and 20 cm H2O.

D Fraction of Inspired Oxygen

On initiation of mechanical ventilation, the FIO2 usually is set at 1.0. The goal is to rapidly reduce the FIO2 to the target PaO2 and SpO2 to limit the consequences of supplemental oxygen. In most patients, a target PaO2 of 60 mm Hg and SpO2 of 90% meets oxygenation requirements. However, some patients may have higher PaO2 targets based on their underlying cardiopulmonary status (e.g., myocardial ischemia, pulmonary hypertension). In patients with ARDS, targeting a PaO2 as low as 50 mm Hg may be appropriate to limit alveolar injury.24 A prolonged high level of FIO2 has been associated with airway and parenchymal injury, atelectasis from nitrogen washout, and increased risk of diffuse alveolar damage, which is even higher in patients receiving bleomycin therapy.25,26 If the need for supplemental FIO2 remains greater than 0.6, FIO2 should be reduced with strategies such as applied PEEP and alternative ventilator modes.

H Flow Rate

Flow rate, or peak inspiratory flow rate, is the maximum flow at which a set tidal volume breath is delivered by the ventilator. Most modern ventilators can deliver flow rates between 60 and 120 L/min. Flow rates should be titrated to meet the patient’s inspiratory demands.31 If the peak flow rate is too low for the patient, dyspnea, patient-ventilator asynchrony, and increased work of breathing may result. High peak flow rates increase peak airway pressures and lower mean airway pressures, which may decrease oxygenation.27

In most patients, peak flow rates of 60 L/min are adequate. Higher flow rates are required in patients with higher ventilator demands.31 Higher peak flow rates may also be necessary in patients with obstructive lung disease to decrease inspiratory time, thereby increasing the expiratory time and reducing the risk of developing auto-PEEP.32,33

III Common Modes of Mechanical Ventilation

The three most commonly used modes of mechanical ventilation are ACV, SIMV, and PSV. Each mode describes whether breaths are volume constant or pressure constant; which are mandatory or spontaneous, or both; and which variables determine a change in function. All three modes have uses throughout the spectrum of stabilization of ventilation, maintenance of ventilation, and weaning from mechanical support.

Choice of the type of mechanical ventilation is most often determined by whether resting of respiratory muscles is indicated. Patients who are hemodynamically compromised, patients with severe oxygenation or ventilation derangements, and those undergoing general endotracheal anesthesia qualify for a rest of respiratory muscles. In these cases, it is prudent to choose a mode of ventilation that accomplishes ventilation without the need for spontaneous respirations; ACV is most often used. However, if use of muscles of respiration is desired, SIMV or PSV should be considered. Patients in whom the use of respiratory muscles is desired are usually those being weaned from mechanical ventilation or undergoing assessment of muscle strength and adequacy of spontaneous work of breathing. PSV is the only mode of the three that entirely relies on the patient spontaneously breathing. Table 48-2 shows the set and variable parameters in each common mode of ventilation. Control mode of ventilation (CMV) is the original mode of ventilation. In CMV, the patient receives a positive-pressure breath at a set rate without the ability to influence how it is delivered.

A Assist-Control Ventilation

Volume assist-control ventilation (VACV) is the most frequently used initial mode of ventilation, and it has several advantages in stabilization and maintenance of adequate ventilation. Using VACV allows adequate oxygenation and ventilation, and it decreases the work of breathing while treating a pathologic process. It is commonly used in patients expected to be passive, as in routine use in the operating room for general anesthesia and in comatose patients.

VACV is a combination mode of ventilation in which the preset tidal volume is delivered in response to the inspiratory effort or if no patient effort occurs within a set period of time. The period is determined by the backup respiratory rate set on the ventilator. A patient-triggered breath is sensed by a change in airway flow or pressure. When the change reaches the trigger threshold, the ventilator delivers the predetermined tidal volume. In ACV, the limit variable that increases to the set threshold before inspiration ends is volume or flow, or both. The cycle variable that ends inspiration is volume or time. Peak inspiratory airway pressure and plateau pressure are variable in this setting. In patients with deep sedation or neuromuscular blockade, the ACV mode functions like CMV. The advantage of ACV is that it substantially decreases the work of breath and decreases myocardial oxygen demand. The disadvantages of ACV in the active patient are that it is less comfortable than spontaneous breathing and that it can induce respiratory alkalosis and breath-stacking (Table 48-3).

TABLE 48-3 Advantages and Disadvantages of Conventional Modes

Ventilation Mode Advantages Disadvantages
ACV

SIMV PSV

ACV, Assist-control ventilation; PEEP, positive end-expiratory pressure; PSV, pressure-support ventilation; SIMV, synchronized intermittent mandatory ventilation.

When ACV is used in a volume-targeted mode, airway pressures vary. When patients with severe hypoxemia (e.g., ARDS) require high PEEP and FIO2 settings to maintain adequate oxygenation, the airway pressures that are generated to deliver the desired tidal volume increase. This increasing pressure can be measured as the peak inspiratory pressure, the mean airway pressure, or the plateau pressure, all of which attempt to describe the pressures that are transmitted through the airways at different levels and at different points in the respiratory cycle. As the plateau pressures increase, reflecting increasing alveolar pressure, it may be prudent to use a pressure-control variant of the ACV mode.

Similar to volume-targeted ACV, the pressure-targeted ACV mode requires the user to input the frequency (i.e., desired respiratory rate), PEEP, and FIO2, but instead of a desired tidal volume, the user sets the upper limit of the inspiratory pressure that is allowable. As the ventilator delivers a breath, the inspiratory flow continues until the maximum pressure or allotted time is reached, and the flow then ceases. In pressure-targeted ACV, the tidal volume varies, and consistency is sacrificed to prevent barotrauma by high pressures (Table 48-4).35 Figure 48-3 depicts the differences in VACV and pressure assist-control ventilation (PACV) in graphs of pressure versus time and airflow versus time.

TABLE 48-4 Comparison of Volume-Targeted and Pressure-Targeted Assist-Control Ventilation

Parameter Volume-Targeted Ventilation Pressure-Targeted Ventilation
Frequency (rate) Set Set
Tidal volume Set Variable
Inspiratory flow Set Set
Peak inspiratory pressure Variable Set
PEEP Set Set
FIO2 Set Set

PEEP, Positive end-expiratory pressure; FIO2, fraction of inspired oxygen.

image

Figure 48-3 Graphs of airway pressure versus time and airflow versus time compare volume assist-control ventilation (VACV) and pressure assist-control ventilation (PACV).

(From Marini JJ: Point: Is pressure assist-control preferred over volume assist-control mode for lung protective ventilation in patients with ARDS? Yes. Chest 140:286–290, 2011.)

ARDS is commonly seen in medical and surgical patients and presents dilemmas in treatment.36 According to the 1994 American-European Consensus Conference definition, ARDS is recognized as a spectrum, which includes ALI, as defined by a ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2/FIO2) of 300 or less, and ARDS, which is defined as a PaO2/FIO2 ratio of 200 or less. Other characteristics of ARDS are the acute onset of bilateral pulmonary infiltrates and a pulmonary capillary wedge pressure of less than 18 mm Hg (or no evidence of elevated left atrial pressure). ARDS is synonymous with noncardiogenic pulmonary edema.37 ALI has many direct and indirect causes. Examples of direct injury are pneumonia, orogastric fluid aspiration, and inhalation injury; indirect causes of injury include severe sepsis, shock, pancreatitis, blood product transfusion, and narcotic overdose.38

Ventilatory strategies for the management of ARDS rest on the results of the ARDS Network studies, which demonstrated that patients given tidal volumes of 6 mL/kg of predicted body weight had improved mortality rates compared with patients with tidal volumes of 12 mL/kg of predicted body weight. Another finding was that plateau pressures less than 30 cm H2O protect the lung (Fig. 48-4 and Table 48-5).12 In 1998, Amato and colleagues demonstrated the mortality benefit of lower tidal volumes and a lower rate of barotraumas (Fig. 48-5).16 Two meta-analyses demonstrated decreased mortality rates with the use of low tidal volume ventilation (i.e., lung-protective ventilation).39,40 In ARDS management, plateau pressures should be less than or equal to 30 cm H2O or the lowest possible level. A high-PEEP strategy decreased the mortality rate in a meta-analysis of 2299 ARDS patients.23 Randomized trials of ventilation in ARDS patients are summarized in Table 48-6.

image

Figure 48-4 The Kaplan-Meier curve from the ARDS Network study compares survival to 180 days and discharge to home without breathing assistance in the lower tidal volume group and the traditional tidal volume group.

(From Acute Respiratory Distress Syndrome Network: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 342:1301–1308, 2000.)

image

Figure 48-5 Comparison of the 28-day survival of patients with acute respiratory distress syndrome (ARDS) assigned to protective or conventional mechanical ventilation.

(From Al-Saady N, Bennett ED: Decelerating inspiratory flow waveform improves lung mechanics and gas exchange in patients on intermittent positive-pressure ventilation. Intensive Care Med 11:68-75, 1985.

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