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 ...