323: Mechanical Ventilatory Support

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.

INDICATIONS

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).

Noninvasive Ventilation

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.

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.

MODES OF VENTILATION

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).

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.

PATIENT MANAGEMENT

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.

Patients for whom mechanical ventilation has been initiated usually require sedation and analgesia to maintain an acceptable level of comfort. Often, this treatment consists of a combination of a benzodiazepine and an opiate administered intravenously. Medications commonly used for this purpose include lorazepam, midazolam, diazepam, morphine, and fentanyl. Oversedation must be avoided in the ICU because most (but not all) studies show that daily interruption of sedation in patients with improved ventilatory status results in a shorter time on the ventilator and a shorter ICU stay.

Immobilized patients receiving mechanical ventilatory support are at risk for deep venous thrombosis and decubitus ulcers. Venous thrombosis should be prevented with the use of subcutaneous heparin and/or pneumatic compression boots. Fractionated low-molecular-weight heparin appears to be equally effective for this purpose. To help prevent decubitus ulcers, frequent changes in body position and the use of soft mattress overlays and air mattresses are employed. Prophylaxis against diffuse gastrointestinal mucosal injury is indicated for patients undergoing MV. Histamine-receptor (H₂-receptor) antagonists, antacids, and cytoprotective agents such as sucralfate have all been used for this purpose and appear to be effective. Nutritional support by enteral feeding through either a nasogastric or an orogastric tube should be initiated and maintained whenever possible. Delayed gastric emptying is common in critically ill patients taking sedative medications but often responds to promotility agents such as metoclopramide. Parenteral nutrition is an alternative to enteral nutrition in patients with severe gastrointestinal pathology who need prolonged MV.

COMPLICATIONS OF MECHANICAL VENTILATION

Endotracheal intubation and mechanical ventilation have direct and indirect effects on the lung and upper airways, the cardiovascular system, and the gastrointestinal system. Pulmonary complications include barotrauma, nosocomial pneumonia, oxygen toxicity, tracheal stenosis, and deconditioning of respiratory muscles. Barotrauma and volutrauma overdistend and disrupt lung tissue; may be clinically manifest by interstitial emphysema, pneumomediastinum, subcutaneous emphysema, or pneumothorax; and can result in the liberation of cytokines from overdistended tissues, further promoting tissue injury. Clinically significant pneumothorax requires tube thoracostomy. Intubated patients are at high risk for ventilator-associated pneumonia as a result of aspiration from the upper airways through small leaks around the endotracheal tube cuff; the most common organisms responsible for this condition are Pseudomonas aeruginosa, enteric gram-negative rods, and Staphylococcus aureus. Given the high associated mortality rates, early initiation of empirical antibiotics directed against likely pathogens is recommended. Hypotension resulting from elevated intrathoracic pressures with decreased venous return is almost always responsive to intravascular volume repletion. In patients who are judged to have respiratory failure on the basis of alveolar edema but in whom the cardiac or pulmonary origin of the edema is unclear, hemodynamic monitoring with a pulmonary arterial catheter may be of value in helping to clarify the cause of the edema. Gastrointestinal effects of positive-pressure ventilation include stress ulceration and mild to moderate cholestasis.

WEANING FROM MECHANICAL VENTILATION

The Decision to Wean

It is important to consider discontinuation of mechanical ventilation once the underlying respiratory disease begins to reverse. Although the predictive capacities of multiple clinical and physiologic variables have been explored, the consensus from a ventilatory weaning task force cites the following conditions as indicating amenability to weaning: (1) Lung injury is stable or resolving. (2) Gas exchange is adequate, with low PEEP/Fio₂ (<8 cmH₂O) and Fio₂(<0.5). (3) Hemodynamic variables are stable, and the patient is no longer receiving vasopressors). (4) The patient is capable of initiating spontaneous breaths. A “wean screen” based on these variables should be done at least daily. If the patient is deemed capable of beginning to wean, the recommendation is to perform a spontaneous breathing trial (SBT), whose value is supported by several randomized trials (Fig. 323-2). The SBT involves an integrated patient assessment during spontaneous breathing with little or no ventilatory support. The SBT is usually implemented with a T-piece using 1–5 cmH₂O CPAP with 5–7 cmH₂O or PSV from the ventilator to offset resistance from the endotracheal tube. Once it is determined that the patient can breathe spontaneously, a decision must be made about the removal of the artificial airway, which should be undertaken only when it is concluded that the patient has the ability to protect the airway, is able to cough and clear secretions, and is alert enough to follow commands. In addition, other factors must be taken into account, such as the possible difficulty of replacing the tube if that maneuver is required. If upper airway difficulty is suspected, an evaluation using a “cuff-leak” test (assessing the presence of air movement around a deflated endotracheal tube cuff) is supported by some internists. Despite all precautions, ~10–15% of extubated patients require reintubation. Several studies suggest that NIV can be used to obviate reintubation, particularly in patients with ventilatory failure secondary to COPD exacerbation; in this setting, earlier extubation with the use of prophylactic NIV has yielded good results. The use of NIV to facilitate weaning in respiratory failure of other etiologies is not currently indicated.

Prolonged Mechanical Ventilation and Tracheostomy

From 5% to 13% of patients undergoing MV will go on to require prolonged MV (>21 days). In these instances, critical care personnel must decide whether and when to perform a tracheostomy. This decision is individualized and is based on the risk and benefits of tracheostomy and prolonged intubation as well as the patient’s preferences and expected outcomes. A tracheostomy is thought to be more comfortable, to require less sedation, and to provide a more secure airway and may also reduce weaning time. However, tracheostomy carries the risk of complications, which occur in 5–40% of these procedures and include bleeding, cardiopulmonary arrest, hypoxia, structural damage, pneumothorax, pneumomediastinum, and wound infection. In patients with long-term tracheostomy, complex complications include tracheal stenosis, granulation, and erosion of the innominate artery. In general, if a patient needs MV for more than 10–14 days, a tracheostomy, planned under optimal conditions, is indicated. Whether it is completed at the bedside or as an operative procedure depends on local resources and experience. Some 5–10% of patients are deemed unable to wean in the ICU. These patients may benefit from transfer to special units where a multidisciplinary approach, including nutrition optimization, physical therapy with rehabilitation, and slower weaning methods (including SIMV with PSV), results in successful weaning rates of up to 30%. Unfortunately, close to 2% of ventilated patients may ultimately become dependent on ventilatory support to maintain life. Most of these patients remain in chronic care institutions, although some with strong social, economic, and family support may live a relatively fulfilling life with at-home ventilation.