Mechanical ventilation is used to assist or replace spontaneous breathing. It is implemented with special devices that can support ventilatory function and improve oxygenation through the application of high-oxygen-content gas and positive pressure. The primary indication for initiation of mechanical ventilation is respiratory failure, of which there are two basic types: (1) hypoxemic, which is present when arterial O2 saturation (Sao2) <90% occurs despite an increased inspired O2 fraction and usually results from ventilation-perfusion mismatch or shunt; and (2) hypercarbic, which is characterized by elevated arterial carbon dioxide partial pressure (PCO2) values (usually >50 mmHg) resulting from conditions that decrease minute ventilation or increase physiologic dead space such that alveolar ventilation is inadequate to meet metabolic demands. When respiratory failure is chronic, neither of the two types is obligatorily treated with mechanical ventilation, but when it is acute, mechanical ventilation may be lifesaving.
The most common reasons for instituting mechanical ventilation are acute respiratory failure with hypoxemia (acute respiratory distress syndrome, heart failure with pulmonary edema, pneumonia, sepsis, complications of surgery and trauma), which accounts for ~65% of all ventilated cases, and hypercarbic ventilatory failure—e.g., due to coma (15%), exacerbations of chronic obstructive pulmonary disease (COPD; 13%), and neuromuscular diseases (5%). The primary objectives of mechanical ventilation are to decrease the work of breathing, thus avoiding respiratory muscle fatigue, and to reverse life-threatening hypoxemia and progressive respiratory acidosis.
In some cases, mechanical ventilation is used as an adjunct to other forms of therapy. For example, it is used to reduce cerebral blood flow in patients with increased intracranial pressure. Mechanical ventilation also is used frequently in conjunction with endotracheal intubation for airway protection to prevent aspiration of gastric contents in otherwise unstable patients during gastric lavage for suspected drug overdose or during gastrointestinal endoscopy. In critically ill patients, intubation and mechanical ventilation may be indicated before the performance of essential diagnostic or therapeutic studies if it appears that respiratory failure may occur during those maneuvers.
TYPES OF MECHANICAL VENTILATION
There are two basic methods of mechanical ventilation: noninvasive ventilation (NIV) and invasive (or conventional mechanical) ventilation (MV).
NIV has gained acceptance because it is effective in certain conditions, such as acute or chronic respiratory failure, and is associated with fewer complications—namely, pneumonia and tracheolaryngeal trauma. NIV usually is provided with a tight-fitting face mask or nasal mask similar to the masks traditionally used for treatment of sleep apnea. NIV has proved highly effective in patients with respiratory failure arising from acute exacerbations of chronic obstructive pulmonary disease. It is most frequently implemented as bilevel positive airway pressure ventilation or pressure-support ventilation. Both modes, which apply a preset positive pressure during inspiration and a lower pressure during expiration at the mask, are well tolerated by a conscious patient and optimize patient-ventilator synchrony. The major limitation to the widespread application of NIV has been patient intolerance: the tight-fitting mask required for NIV can cause both physical and psychological discomfort. In addition, NIV has had limited success in patients with acute hypoxemic respiratory failure, for whom endotracheal intubation and conventional MV remain the ventilatory method of choice.
The most important group of patients who benefit from a trial of NIV are those with exacerbations of COPD and respiratory acidosis (pH <7.35). Experience from several randomized trials has shown that, in patients with ventilatory failure characterized by blood pH levels between 7.25 and 7.35, NIV is associated with low failure rates (15–20%) and good outcomes (as judged by intubation rate, length of stay in intensive care, and—in some series—mortality rates). In more severely ill patients with a blood pH <7.25, the rate of NIV failure is inversely related to the severity of respiratory acidosis, with higher failure rates as the pH decreases. In patients with milder acidosis (pH >7.35), NIV is not better than conventional treatment that includes controlled oxygen delivery and pharmacotherapy for exacerbations of COPD (systemic glucocorticoids, bronchodilators, and, if needed, antibiotics).
Despite its benign outcomes, NIV is not useful in the majority of cases of respiratory failure and is contraindicated in patients with the conditions listed in Table 323-1. NIV can delay lifesaving ventilatory support in those cases and, in fact, can actually result in aspiration or hypoventilation. Once NIV is initiated, patients should be monitored; a reduction in respiratory frequency and a decrease in the use of accessory muscles (scalene, sternomastoid, and intercostals) are good clinical indicators of adequate therapeutic benefit. Arterial blood gases should be determined at least within hours of the initiation of therapy to ensure that NIV is having the desired effect. Lack of benefit within that time frame should alert the physician to the possible need for conventional MV.
TABLE 323-1Contraindications for Noninvasive Ventilation |Favorite Table|Download (.pdf) TABLE 323-1 Contraindications for Noninvasive Ventilation
|Cardiac or respiratory arrest |
|Severe encephalopathy |
|Severe gastrointestinal bleed |
|Hemodynamic instability |
|Unstable angina and myocardial infarction |
|Facial surgery or trauma |
|Upper airway obstruction |
|High-risk aspiration and/or inability to protect airways |
|Inability to clear secretions |
Conventional Mechanical Ventilation
Conventional MV is implemented once a cuffed tube is inserted into the trachea to allow conditioned gas (warmed, oxygenated, and humidified) to be delivered to the airways and lungs at pressures above atmospheric pressure. Care should be taken during intubation to avoid brain-damaging hypoxia. In most cases, the administration of mild sedation may facilitate the procedure. Opiates and benzodiazepines are good choices but can have a deleterious effect on hemodynamics in patients with depressed cardiac function or low systemic vascular resistance. Morphine can promote histamine release from tissue mast cells and may worsen bronchospasm in patients with asthma; fentanyl, sufentanil, and alfentanil are acceptable alternatives. Ketamine may increase systemic arterial pressure and has been associated with hallucinatory responses. The shorter-acting agents etomidate and propofol have been used for both induction and maintenance of anesthesia in ventilated patients because they have fewer adverse hemodynamic effects, but both are significantly more expensive than older agents. Great care must be taken to avoid the use of neuromuscular paralysis during intubation of patients with renal failure, tumor lysis syndrome, crush injuries, medical conditions associated with elevated serum potassium levels, and muscular dystrophy syndromes; in particular, the use of agents whose mechanism of action includes depolarization at the neuromuscular junction, such as succinylcholine chloride, must be avoided.
PRINCIPLES OF MECHANICAL VENTILATION
Once the patient has been intubated, the basic goals of MV are to optimize oxygenation while avoiding ventilator-induced lung injury due to overstretch and collapse/re-recruitment. This concept, known as the “protective ventilatory strategy” (see below and Fig. 323-1) is supported by evidence linking high airway pressures and volumes and overstretching of the lung as well as collapse/re-recruitment to poor clinical outcomes (barotrauma and volume trauma). Although normalization of pH through elimination of CO2 is desirable, the risk of lung damage associated with the large volume and high pressures needed to achieve this goal has led to the acceptance of permissive hypercapnia. This condition is well tolerated when care is taken to avoid excess acidosis by pH buffering.
Hypothetical pressure-volume curve of the lung in a patient undergoing mechanical ventilation. Alveoli tend to close if the distending pressure falls below the lower inflection point (A), whereas they overstretch if the pressure within them is higher than that of the upper inflection point (B). Collapse and opening of ventilated alveoli are associated with poor outcomes in patients with acute respiratory failure. Protective ventilation (purple shaded area), using a lower tidal volume (6 mL/kg of ideal body weight) and maintaining positive end-expiratory pressure to prevent overstretching and collapse/opening of alveoli, has resulted in improved survival rates among patients receiving mechanical ventilatory support.
Mode refers to the manner in which ventilator breaths are triggered, cycled, and limited. The trigger, either an inspiratory effort or a time-based signal, defines what the ventilator senses to initiate an assisted breath. Cycle refers to the factors that determine the end of inspiration. For example, in volume-cycled ventilation, inspiration ends when a specific tidal volume is delivered. Other types of cycling include pressure cycling and time cycling. The limiting factors are operator-specified values, such as airway pressure, that are monitored by transducers internal to the ventilator circuit throughout the respiratory cycle; if the specified values are exceeded, inspiratory flow is terminated, and the ventilator circuit is vented to atmospheric pressure or the specified pressure at the end of expiration (positive end-expiratory pressure, or PEEP). Most patients are ventilated with assist-control ventilation, intermittent mandatory ventilation, or pressure-support ventilation, with the latter two modes often used simultaneously (Table 323-2).
TABLE 323-2Characteristics of the Most Commonly Used Forms of Mechanical Ventilation |Favorite Table|Download (.pdf) TABLE 323-2 Characteristics of the Most Commonly Used Forms of Mechanical Ventilation
|Ventilatory Mode ||Variables Set by User (Independent) ||Variables Monitored by User (Dependent) ||Trigger Cycle Limit ||Advantages ||Disadvantages |
|ACMV (assist-control ventilation) || |
Peak, mean, and plateau airway pressures
Barotrauma and volume trauma
Every effective breath generates a ventilator volume
|PEEP level |
|Pressure limit |
|IMV (intermittent mandatory ventilation) || |
Mandatory ventilator rate
Peak, mean, and plateau airway pressures
Patient effort Timer
Comfort from spontaneous breaths
| ||Fio2 ||ABG || || |
| ||PEEP level ||I/E ratio || || || |
| ||Pressure limit || || || || |
| ||Spontaneous breaths between assisted breaths || || || || |
|PSV (pressure-support ventilation) || |
Inspiratory pressure level
No timer backup
|NIV (noninvasive ventilation) ||Inspiratory and expiratory pressure level || |
|Patient control ||Mask interface may cause discomfort and facial bruising |
| ||Fio2 || ||Leaks are common |
| || || || ||Hypoventilation |
Assist-Control Ventilation (ACMV)
ACMV is the most widely used mode of ventilation. In this mode, an inspiratory cycle is initiated either by the patient’s inspiratory effort or, if none is detected within a specified time window, by a timer signal within the ventilator. Every breath delivered, whether patient- or timer-triggered, consists of the operator-specified tidal volume. Ventilatory rate is determined either by the patient or by the operator-specified backup rate, whichever is of higher frequency. ACMV is commonly used for initiation of mechanical ventilation because it ensures a backup minute ventilation in the absence of an intact respiratory drive and allows for synchronization of the ventilator cycle with the patient’s inspiratory effort.
Problems can arise when ACMV is used in patients with tachypnea due to nonrespiratory or nonmetabolic factors, such as anxiety, pain, and airway irritation. Respiratory alkalemia may develop and trigger myoclonus or seizures. Dynamic hyperinflation leading to increased intrathoracic pressures (so-called auto-PEEP) may occur if the patient’s respiratory mechanics are such that inadequate time is available for complete exhalation between inspiratory cycles. Auto-PEEP can limit venous return, decrease cardiac output, and increase airway pressures, predisposing to barotrauma.
Intermittent Mandatory Ventilation (IMV)
With this mode, the operator sets the number of mandatory breaths of fixed volume to be delivered by the ventilator; between those breaths, the patient can breathe spontaneously. In the most frequently used synchronized mode (SIMV), mandatory breaths are delivered in synchrony with the patient’s inspiratory efforts at a frequency determined by the operator. If the patient fails to initiate a breath, the ventilator delivers a fixed-tidal-volume breath and resets the internal timer for the next inspiratory cycle. SIMV differs from ACMV in that only a preset number of breaths are ventilator-assisted.
SIMV allows patients with an intact respiratory drive to exercise inspiratory muscles between assisted breaths; thus it is useful for both supporting and weaning intubated patients. SIMV may be difficult to use in patients with tachypnea because they may attempt to exhale during the ventilator-programmed inspiratory cycle. Consequently, the airway pressure may exceed the inspiratory pressure limit, the ventilator-assisted breath will be aborted, and minute volume may drop below that programmed by the operator. In this setting, if the tachypnea represents a response to respiratory or metabolic acidosis, a change in ACMV will increase minute ventilation and help normalize the pH while the underlying process is further evaluated and treated.
Pressure-Support Ventilation (PSV)
This form of ventilation is patient-triggered, flow-cycled, and pressure-limited. It provides graded assistance and differs from the other two modes in that the operator sets the pressure level (rather than the volume) to augment every spontaneous respiratory effort. The level of pressure is adjusted by observing the patient’s respiratory frequency. During PSV, the inspiration is terminated when inspiratory airflow falls below a certain level; in most ventilators, this flow rate cannot be adjusted by the operator. With PSV, patients receive ventilator assistance only when the ventilator detects an inspiratory effort. PSV is often used in combination with SIMV to ensure volume-cycled backup for patients whose respiratory drive is depressed. PSV is well tolerated by most patients who are being weaned from MV; PSV parameters can be set to provide full ventilatory support and can be withdrawn to load the respiratory muscles gradually.
Other Modes of Ventilation
There are other modes of ventilation, each with its own acronym and each with specific modifications of the manner and duration in which pressure is applied to the airway and lungs and of the interaction between the mechanical assistance provided by the ventilator and the patient’s respiratory effort. Although their use in acute respiratory failure is limited, the following modes have been used with varying levels of enthusiasm and adoption.
PRESSURE-CONTROL VENTILATION (PCV)
This form of ventilation is time-triggered, time-cycled, and pressure-limited. A specified pressure is imposed at the airway opening throughout inspiration. Since the inspiratory pressure is specified by the operator, tidal volume and inspiratory flow rate are dependent, rather than independent, variables and are not operator-specified. PCV is the preferred mode of ventilation for patients in whom it is desirable to regulate peak airway pressures, such as those with preexisting barotrauma, and for post–thoracic surgery patients, in whom the shear forces across a fresh suture line should be limited. When PCV is used, minute ventilation is altered through changes in rate or in the pressure-control value, with consequent changes in tidal volume.
INVERSE-RATIO VENTILATION (IRV)
This mode is a variant of PCV that incorporates the use of a prolonged inspiratory time with the appropriate shortening of the expiratory time. IRV has been used in patients with severe hypoxemic respiratory failure. This approach increases mean distending pressures without increasing peak airway pressures. It is thought to work in conjunction with PEEP to open collapsed alveoli and improve oxygenation. However, no clinical-trial data have shown that IRV improves outcomes.
CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP)
CPAP is not a true support mode of ventilation because all ventilation occurs through the patient’s spontaneous efforts. The ventilator provides fresh gas to the breathing circuit with each inspiration and sets the circuit to a constant, operator-specified pressure. CPAP is used to assess extubation potential in patients who have been effectively weaned and who require little ventilatory support and in patients with intact respiratory system function who require an endotracheal tube for airway protection.
Nonconventional Ventilatory Strategies
Several nonconventional strategies have been evaluated for their ability to improve oxygenation and reduce mortality rates in patients with advanced hypoxemic respiratory failure. These strategies include high-frequency oscillatory ventilation (HFOV), airway pressure release ventilation (APRV), extracorporeal membrane oxygenation (ECMO), and partial liquid ventilation (PLV) using perfluorocarbons. Although case reports and small uncontrolled cohort studies have shown benefit, randomized controlled trials have failed to demonstrate consistent improvements in outcome with most of these strategies. A recent randomized trial of ECMO documented positive outcomes, but the technique remains controversial because older studies failed to document positive results. Currently, these approaches should be thought of as “salvage” techniques and considered for patients with hypoxemia refractory to conventional therapy. Prone positioning of patients with refractory hypoxemia has also been explored because, in theory, lying prone should improve ventilation-perfusion matching. Several randomized trials in patients with acute lung injury did not demonstrate a survival advantage with prone positioning despite demonstration of a transient physiologic benefit. The administration of nitric oxide gas, which has bronchodilator and pulmonary vasodilator effects when delivered through the airways and improves arterial oxygenation in many patients with advanced hypoxemic respiratory failure, also failed to improve outcomes in these patients with acute lung injury.
The design of new ventilator modes reflect attempts to improve patient-ventilator synchrony—a major practical issue during MV—by allowing patients to trigger the ventilator with their own effort while also incorporating flow algorithms that terminate the cycles once certain preset criteria are reached; this approach has greatly improved patient comfort. New modes of ventilation that synchronize not only the timing but also the levels of assistance to match the patient’s effort have been developed. Proportional assist ventilation (PAV) and neurally adjusted ventilatory-assist ventilation (NAV) are two modes that are designed to deliver assisted breaths through algorithms incorporating not only pressure, volume, and time but also overall respiratory resistance as well as compliance (in the case of PAV) and neural activation of the diaphragm (in the case of NAV). Although these modes enhance patient-ventilator synchrony, their practical use in the everyday management of patients undergoing MV needs further study.
PROTECTIVE VENTILATORY STRATEGY
Whichever mode of MV is used in acute respiratory failure, the evidence from several important controlled trials indicates that a protective ventilation approach guided by the following principles (and summarized in Fig. 323-1) is safe and offers the best chance of a good outcome: (1) Set a target tidal volume close to 6 mL/kg of ideal body weight. (2) Prevent plateau pressure (static pressure in the airway at the end of inspiration) exceeding 30 cm H2O. (3) Use the lowest possible fraction of inspired oxygen (Fio2) to keep the Sao2 at ≥90%. (4) Adjust the PEEP to maintain alveolar patency while preventing overdistention and closure/reopening. With the application of these techniques, the mortality rate among patients with acute hypoxemic respiratory failure has decreased to ~30% from close to 50% a decade ago.
Once the patient has been stabilized with respect to gas exchange, definitive therapy for the underlying process responsible for respiratory failure is initiated. Subsequent modifications in ventilator therapy must be provided in parallel with changes in the patient’s clinical status. As improvement in respiratory function is noted, the first priority is to reduce the level of mechanical ventilatory support. Patients on full ventilatory support should be monitored frequently, with the goal of switching to a mode that allows for weaning as soon as possible. Protocols and guidelines that can be applied by paramedical personnel when physicians are not readily available have proved to be of value in shortening ventilator and intensive care unit (ICU) time, with very good outcomes. Patients whose condition continues to deteriorate after ventilatory support is initiated may require increased O2, PEEP, or one of the alternative modes of ventilation.