Mechanical ventilation is a therapeutic method that is used to assist or replace spontaneous breathing. The primary indication for initiation of mechanical ventilation is respiratory failure, of which there are two basic types: hypoxemic respiratory failure, which is present when arterial O2 saturation (Sao2) <90% occurs despite an increased inspired O2 fraction, and hypercarbic respiratory failure, which is characterized by arterial PCO2 values >50 mmHg. When it is chronic, neither of the two types is obligatorily treated with mechanical ventilation, but when 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, followed by causes of hypercarbic ventilatory failure such as coma (15%), exacerbations of chronic obstructive pulmonary disease (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, such as its use in reducing cerebral blood flow in patients with increased intracranial pressure. Mechanical ventilation also is used frequently in conjunction with endotracheal intubation 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 essential diagnostic or therapeutic studies if it appears that respiratory failure may occur during those maneuvers.
Types of Mechanical Ventilation
In its broadest sense, there are two distinct methods for ventilating patients: noninvasive ventilation (NIV) and invasive ventilation or conventional mechanical ventilation (MV).
Noninvasive ventilation has been gaining more 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. Noninvasive ventilation usually is provided by using a tight-fitting face mask or nasal mask similar to the masks traditionally used for treatment of sleep apnea. Noninvasive ventilation has proved highly effective in patients with respiratory failure from acute exacerbations of chronic obstructive pulmonary disease and is most frequently implemented by using bilevel positive airway pressure ventilation or pressure support ventilation. In both of these modes, a preset positive pressure is applied during inspiration and a lower pressure is applied during expiration at the mask. Both modes are well tolerated by a conscious patient and optimize patient-ventilator synchrony. The major limitation to its widespread application has been patient intolerance because the tight-fitting mask required for NIV can cause both physical and emotional 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 acute exacerbations of chronic obstructive pulmonary disease (COPD) leading to respiratory acidosis (pH <7.35). Experience from several well-conducted 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 (intubation rate, length of stay in intensive care, and in some series mortality rates). In more severely ill patients with pH <7.25, the rate of NIV failure is inversely related to the severity of respiratory acidosis, with greater failure as the pH decreases. In patients with milder acidosis (pH >7.35), NIV is not better than conventional therapy that includes controlled oxygen delivery and pharmacotherapy for exacerbations of COPD (systemic corticosteroids, 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 269-1. Experience shows that NIV can delay lifesaving ventilatory support in those cases and actually results 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 obtained at least within hours of the initiation of therapy to ensure that NIV is having the desired effect and that it is safe to continue its application. Lack of benefit within that time frame should alert one to the possible need for conventional MV.
Table 269–1. Contraindications for Noninvasive Ventilation
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Table 269–1. Contraindications for Noninvasive Ventilation
|Cardiac or respiratory arrest|
|Severe gastrointestinal bleed|
|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 mechanical ventilation 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. Great care has to be taken during the act of intubation to avoid brain-damaging hypoxia. In some patients, intubation can be achieved without added sedation. In most patients, the administration of mild sedation may help 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; it should be used with caution in patients with hypertensive crisis or a history of psychiatric disorders. Newer agents such as etomidate and propofol have been used for both induction and maintenance of anesthesia in ventilated patients. They are shorter-acting, and etomidate has fewer adverse hemodynamic effects, but both agents are significantly more expensive than older agents. Great care must be taken to avoid the use of neuromuscular paralysis during intubation; in particular, the use of agents whose mechanism of action includes depolarization at the neuromuscular junction, such ...