FUNDAMENTALS OF MECHANICAL VENTILATION

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CHAPTER 86 FUNDAMENTALS OF MECHANICAL VENTILATION

Soumitra R. Eachempati, Marc J. Shapiro, Philip S. Barie

Mechanical ventilation is often required to manage trauma or critical illness, whether for airway protection, administration of general anesthesia, or management of acute respiratory failure (ARF) (Table 1). New technology now provides several modes by which a patient may be ventilated, with the goals of improved gas exchange, better patient comfort, and rapid liberation from the ventilator. Moreover, noninvasive positive-pressure ventilation permits some cases of ARF to be managed without insertion of an artificial endotracheal airway, and some patients who are extubated with marginal reserves to avoid reintubation. Nearly all ventilators can be set to allow full support of the patient on the one hand, and periods of exercise on the other. Thus, the choice of ventilator settings is a matter of physician preference for the majority of patients (Table 2). Controlled ventilation with suppression of spontaneous breathing leads rapidly to respiratory muscle atrophy; therefore modes of assisted ventilation are preferred wherein machine-delivered breaths are triggered by the patient’s own inspiratory efforts. Basic modes of assisted ventilation include assist-control ventilation (ACV), synchronized intermittent mandatory ventilation (SIMV), and pressure support ventilation (PSV).

Table 1 General Indications for Mechanical Ventilation

Airway maintenance or protection

Airway obstruction

General inhalational anesthesia

Hemodynamic instability

Hypoxemia

Metabolic acidosis

Pulmonary toilet (excessive secretions)

Table 2 Glossary of Basic Terminology of Mechanical Ventilation

Control: regulation of gas flow

Volume-controlled—volume limited, volume targeted; airway pressure is variable

Pressure-controlled—pressure limited, pressure targeted; volume delivered is variable

Dual-controlled—volume targeted (guaranteed), pressure limited; seldom used in practice

Cycling: ventilator switching from inhalation to exhalation after volume or pressure target (or limit) has been reached

Time-cycled—example: pressure control ventilation

Flow-cycled—example: pressure support ventilation

Volume-cycled—example: volume-controlled ventilation

Triggering: causes the ventilator to cycle to inhalation

Time-triggered—ventilator cycles at a set frequency as determined by the controlled rate

Pressure-triggered—ventilator senses the patient’s inspiratory effort from a decrease of airway pressure

Flow-triggered—constant flow of gas is delivered throughout the respiratory cycle (flow-by). Altered flow caused by patient inhalation is detected by the ventilator, which delivers a breath. Less work is done by the patient than with pressure-triggering

Breaths: cause the ventilator to cycle from inhalation to exhalation

Mandatory—controlled; determined by the respiratory rate

Assisted—examples: assist control, synchronized intermittent mandatory ventilation, pressure support

Spontaneous—no additional assistance, as in continuous positive airway pressure (CPAP)

Flow pattern: constant, decelerating, or sinusoidal

Sinusoidal—examples: spontaneous breathing and CPAP

Constant— flow continues at a constant rate until the set tidal volume is delivered, seldom used in practice

Decelerating—inhalation slows from a high initial flow rate as alveolar pressure increases; example: pressure-targeted ventilation, often used in volume-targeted ventilation, as it causes lower peak airway pressures than constant flow

Mode (breath pattern)

Controlled mechanical ventilation—controlled ventilation, without allowances for spontaneous breathing, typical of anesthesia ventilators

Assist-control—assisted breaths simulate controlled breaths

Intermittent mandatory ventilation—admixes controlled and spontaneous breaths, which may also be synchronized to prevent “stacking”

Pressure support—patient controls all aspects of breath except pressure limit

Most patients are started on mechanical ventilation for management of ARF, during which the work necessary to initiate a breath increases by a factor of four to six. The most common reason to initiate mechanical ventilation is to decrease the work of breathing by the patient. Additional potential benefits of mechanical ventilation include improved gas exchange, enhanced coordination between support and the patient’s own efforts, resting of respiratory muscles, prevention of deconditioning, and prevention of iatrogenic lung injury while promoting healing. However, unless settings are chosen carefully to synchronize with the patient’s own central respiratory drive, mechanical ventilation can cause an increase in work. Regardless of the mode chosen, all mechanical ventilation is a modification of the manner in which positive pressure is applied to the airway, and the interplay of the mechanical support and the patient’s own efforts.

NONINVASIVE VENTILATION

Ventilatory support delivered without establishing an endotracheal airway is “noninvasive ventilation” (NIV). Noninvasive ventilation was administered previously with intermittent negative pressure, but the current technique utilizes positive-pressure ventilation delivered through a nasal or face mask, and usage is expanding in the management of acute and chronic respiratory failure and possibly for some patients with heart failure.

Putative benefits of NIV are numerous, owing to avoidance of the complications of endotracheal intubation. Noninvasive ventilation preserves swallowing, feeding, speech, cough, and physiologic air warming and humidification by the nasooropharynx. Nonintubated patients communicate more effectively, require less sedation, and are more comfortable. In addition, patients are often able to continue with standard oral nutrition. Noninvasive ventilation eliminates complications such as trauma with tube insertion, mucosal ulceration, aspiration, infection (e.g., pneumonia, sinusitis), and dysphagia after extubation.

In a randomized, prospective trial following pulmonary resection of 48 patients with acute hypoxemic respiratory insufficiency, Auriant et al. compared standard invasive mechanical ventilation with nasal mask NIV. The need for postoperative reintubation and mortality were clearly reduced in patients receiving NIV as a part of respiratory support. Similarly, Squadrone et al. randomized 209 patients with respiratory failure in the postanesthesia care unit after major abdominal surgery to oxygen alone, or with continuous positive airway pressure (CPAP) via a mask. Patients who received oxygen plus CPAP had a significantly lower intubation rate, and also lower rates of pneumonia, infection, and sepsis.

Contraindications to Noninvasive Ventilation

Crucial to successful NIV is an awake, cooperative, spontaneously breathing patient. Airway, electrocardiographic, or hemodynamic instability argues against the use of NIV. An additional requirement is an intact cough reflex and ability to clear secretions, the absence of which is a common reason for failure of NIV. Relative contraindications include the inability to fit and seal the mask adequately, inability to cough with prompting, or inability to remove the mask in the event of emesis. A hypothetical contraindication is recent gastrointestinal surgery with aerophagia and gut distention. If pressures used to ventilate the patient are kept below 30 mm Hg, the closing pressure of the lower esophageal sphincter should not be exceeded, and aerophagia should be avoided. Morbid obesity is also a relative contraindication secondary to increased ventilatory pressure requirements arising from body habitus and the weight of the chest wall and abdominal viscera while the patient is supine.

Complications of Noninvasive Ventilation

The most common complication of NIV is focal skin necrosis, which is most common over the bridge of the nose but may also occur over the zygoma. The incidence is 7%–10% among patients receiving full-face-mask NIV. Other complications (incidence, 1%–2% each) include gastric distention, aspiration, and pneumothorax. Conceptual concerns with gastric distention are subsequent vomiting, aspiration, and pneumonia. Conjunctivitis may develop secondary to air leaks near the eyes in around 2% of patients.

The most serious complication is failure to recognize when noninvasive ventilation is not providing a patient with adequate ventilation, oxygenation, or airway patency. Delayed placement of an artificial airway, or failure of placement thereof, may cause continued deterioration or the death of a patient.

PRESSURE SUPPORT VENTILATION

Pressure support is a method of assisting spontaneous breathing in a ventilated patient, either partially or fully. The patient controls all parts of the breath except the pressure limit. The patient triggers the ventilator, which delivers a flow of gas in response up to a preset pressure limit (for example, 10 cm H₂O) depending on the desired minute ventilation (V_E). Gas flow cycles off when a certain percentage of peak inspiratory flow (usually 25%) has been reached. Tidal volumes (V_T) may vary, just as they do spontaneously.

Positive end-expiratory pressure (PEEP, also called continuous positive airway pressure [CPAP]) is added to restore functional residual capacity (FRC) to normal for the patient. When lung volumes are low, the work of breathing during early inhalation is reduced. Noncompliant lungs require higher intrapleural pressures to inflate to a normal V_T, even with CPAP. The addition of pressure support (PS) assists the patient to move up the pressure–volume curve (larger changes in volume for a given applied pressure, i.e., increased lung compliance). The term “pressure support ventilation” describes the combination of pressure support and PEEP (or CPAP). Although useful in the patient breathing spontaneously, PS may be used to assist spontaneous breaths in SIMV. Weaning may be facilitated using this combination, as the backup (SIMV) rate is weaned initially, and then the PS.

HELIOX

Helium has significantly lower density than air or nitrogen. Substituting helium for nitrogen reduces the density of the gas in direct proportion to the amount of helium admixed. Breathing heliox (the concentration in clinical use ranges from 80:20 to 60:40) results in more laminar flow, reduced airway resistance, and reduced work of breathing. Heliox may be useful in clinical situations where resistance to airflow is high, including asthma, acute exacerbations of chronic bronchitis or chronic obstructive pulmonary disease, other causes of bronchospasm, and upper airway obstruction with stridor. Breathing heliox is well tolerated, without significant adverse effects. Disadvantages include high cost and limited utility when a high F_IO₂ is required. Ventilators must also be recalibrated when heliox is delivered by ventilator rather than nebulizer (the usual route), to ensure that the flow of gas is measured correctly.

MODES OF MECHANICAL VENTILATION

Assist Control Ventilation

The ACV mode is the most commonly used mode in medical/surgical critical care units. Set parameters in ACV mode are inspiratory flow rate, frequency (f), and V_T. The ventilator delivers a set number of equal breaths per minute, each of a given V_T. Tidal volume and flow determine inspiratory (I) and expiratory (E) time and the I:E ratio. Plateau or alveolar pressure is related to V_T and respiratory system compliance. The patient has the ability to trigger extra breaths by exerting an inspiratory effort exceeding a preset trigger level. Typically, each patient will display a preferred rate for a given V_T and will trigger all breaths when f is set a few breaths per minute below the patient’s rate. In this mode, the control rate serves as adequate support should the patient stop initiating breaths. When high inspiratory effort continues during a ventilator-delivered breath, the patient may trigger a second superimposed breath. Patient effort can be increased, if desired, by increasing the triggering threshold or lowering V_T.

Synchronized Intermittent Mandatory Ventilation

In a passive patient, SIMV cannot be distinguished from ACV. Ventilation is determined by f and V_T. However, if the patient is not truly passive, respiratory work may be performed during mandatory breaths. In addition, the patient may trigger additional breaths by spontaneous effort. If the triggering effort comes in a brief, defined interval before the next mandatory breath, the ventilator will deliver the mandatory breath ahead of schedule to synchronize with patient inspiratory effort. If a breath is initiated outside the synchronization window, V_T, flow, and I:E are determined by patient effort and respiratory system mechanics, not by ventilator settings. These spontaneous breaths tend to be of low V_T and are variable from breath to breath. The SIMV mode is often used to augment patient work of breathing gradually by lowering the mandatory breath frequency or V_T, compelling the patient to breathe more rapidly in order to maintain adequate V_E. Some ventilators allow combinations of modes. A useful combination is SIMV plus PSV as a means to add “sigh” breaths and decrease atelectasis. Because SIMV plus PSV guarantees some backup V_E that PSV alone does not, this combination may be particularly useful for patients at high risk for deteriorating central respiratory drive, and it is also popular as an adjunct to weaning the ventilator.

Positive End-Expiratory Pressure

Although it is a ubiquitous form of ventilatory support, positive end-expiratory pressure (PEEP) can be confusing because the positive pressure is actually applied throughout the respiratory cycle and is more correctly termed CPAP. Using PEEP accomplishes three goals: prevention of alveolar derecruitment by restoring FRC, which is decreased in acute lung injury (ALI) and atelectasis, to the physiologic range; protection against injury during phasic opening and closing of atelectatic units; and assisting cardiac performance during heart failure, by increasing mean intrathoracic pressure.

The FRC is the lung’s physiologic reserve; loss of chest wall or lung compliance (the rate of change of volume in response to pressure) causes reduced FRC. The FRC is the volume of gas that remains in the lungs at the end of a normal tidal breath (∼2.5 liters); gas exchange does occur. At FRC, the tendency for the lungs to collapse is balanced by the tendency for the chest wall to move outward. A small vacuum in the pleural space assists in maintaining equilibrium, which is lost when pneumothorax is present.

The FRC is determined by the compliance of the lung and chest wall. Anything that constrains chest wall expansion reduces its compliance; likewise, anything that reduces lung volume reduces lung compliance. The FRC is composed of two volumes, the expiratory reserve volume (ERV) and the residual volume (RV). Below FRC, exhalation is active; lung tissue must be compressed to express gas. The RV (∼1 liter in adults) is the point where no more gas can be expressed from the lungs regardless of the pressure applied, because alveolar pressure exceeds atmospheric pressure and the gradient along the airway is reversed. Being filled with gas and coated with surfactant, alveoli are difficult to compress, but airways are compressible. When intrathoracic pressure exceeds pressure in the small airways, “dynamic airway collapse” occurs and gas is trapped in alveoli. Airway collapse increases the work of breathing and leads to ventilation-perfusion (V/Q) mismatch. Collapsed airways are difficult to reinflate, leading to a huge increase in the work of breathing and oxygen consumption.

The concept behind PEEP is to increase FRC; in essence, to allow alveoli to deflate only to the point just above where inflation remains easy (called the lower inflection point of the pressure–volume curve). The patient requires sufficient PEEP to prevent alveolar derecruitment, but not so much PEEP that alveolar overdistension, dead space ventilation from collapse of the alveolar microcirculation, or hypotension due to reduced right ventricular preload, right ventricular output, and ultimately cardiac output occur.

Auto-PEEP is caused by gas trapped in alveoli at end-expiration. This gas is not in equilibrium with the atmosphere and is at positive pressure, increasing the work of breathing. In patients with obstructive airways disease, increased bronchial tone leads to resistance to both inhalation and exhalation. Shortening of E (e.g., small airways disease, mucus plugging, pressure-controlled ventilation with inverted I:E) results in gas trapping at end-expiration, hyperinflation, and increased intrathoracic pressure, which abolishes the alveolar pressure gradient. Auto-PEEP can be ameliorated by lengthening E, shortening I, or decreasing the respiratory rate.

The ideal level of PEEP is controversial (Table 3). It may be that which prevents derecruitment of the majority of alveoli, while causing minimal overdistension; alternatively, application of PEEP is a recruitment maneuver, arguing for higher pressures to be applied to overcome alveolar collapse. Applying PEEP to put the majority of lung units on the favorable part of the pressure–volume curve will maximize gas exchange and minimize overdistention, but is easier said than done because the lower inflection point is sometimes indistinct. Undoubtedly, the combination of PEEP and low V_T prevents volutrauma, but the exact amount of PEEP to apply is controversial. The reason for this is hysteresis—the tendency of the lungs, due to surfactant, to exist at higher volumes in exhalation than in inhalation.

Table 3 Protocol Summary for Institution of Mechanical Ventilation for Acute Lung Injury/Acute Respiratory Distress Syndrome

Initial ventilator settings

Use a volume-controlled mode initially to ensure that VT is delivered.

Initial VT 8 ml/kg; reduce by 1 ml/kg/2 hours until 6 ml/kg is reached. Minimum VT is 4 ml/kg.

Set ventilator rate at 12–20 breaths/minute. Maximum ventilator rate 35 breaths/minute.

Adjust ventilator subsequently based on goals of arterial pH (7.25–7.45; ventilator rate) and end-inspiratory plateau pressure (Pplat) (<30 cm H₂O; VT).

Measure arterial pH upon admission to the ICU, every morning, and 15 minutes after each change in respiratory rate or VT.

Alkalemia is managed by decreasing ventilator rate by at least two breaths/minute.

Mild acidemia (pH 7.15–7.25) is managed by increasing ventilator rate until pH > 7.25 or PaCO₂ < 25 mm Hg up to 35 breaths/minute. If ventilator rate = 35 or PaCO₂ < 25 mm Hg, give sodium bicarbonate.

Severe acidemia (pH < 7.15) is managed by increasing the ventilator rate up to 35 breaths/minute. If ventilator rate = 35 and pH < 7.15, and sodium bicarbonate has been given, increase VT in increments of 1 ml/kg until pH > 7.15. It may be necessary to exceed the target Pplat under these conditions.

Keep Pplat < 30 cm H₂O. Measure Pplat at least every 8 hours, and 5 minutes after each change in PEEP or VT, and more frequently when changes in lung compliance are likely. Accurate measurement of Pplat requires a patient who is not moving or coughing.

If Pplat cannot be measured because of an air leak, peak inspiratory pressure may be substituted.

Target ranges for PaO₂ are 55–80 mm Hg, or SaO₂ > 88%. The combination of PEEP and F_IO₂ is discretionary, but F_IO₂:PEEP should generally be < 5 if F_IO₂ > 0.45.

When increasing PEEP above 10 cm H₂O, increase by 2–5 cm increments up to a maximum of 35 cm H₂O, until target ranges for PaO₂ are reached. Reduce PEEP to the previous level of PEEP if the change does not increase PaO₂ > 5 mm Hg or if decreased oxygen delivery results from a decrease of cardiac output.

Assess arterial oxygenation by blood gas determination or oximetry at least every 4 hours.

If arterial oxygenation is below the target range, increase F_IO₂ incrementally (up to 1.0), then PEEP (up to 35 cm H₂O within 30 min). Reassess every 15 min after each adjustment until target ranges for PaO₂ are regained. Brief periods of SaO₂ < 88% (<5 min) may be tolerated. F_IO₂ = 1.0 may be used transiently (<10 min) for arterial desaturation or during suctioning or bronchoscopy.

PEEP, Positive end-expiratory pressure.

Adapted from Nathens AB, Johnson JL, Mine JP, et al. Inflammation and the Host Response to Injury Investigators: Inflammation and the host response to injury, a large-scale collaborative project: patient-oriented research core—standard operating procedures for clinical care. I. Guidelines for mechanical ventilation of the trauma patient. J Trauma 59:764–769, 2005.