Patients require mechanical ventilation because of apnea, acute or impending acute respiratory failure, or severe refractory hypoxemia.
The 2 basic forms of mechanical ventilation are pressure ventilation (peak airway pressure constant, tidal volume variable) and volume ventilation (tidal volume constant, peak airway pressure variable).
Although numerous modes of ventilation exist, few data are available to differentiate the benefits of one mode over another, and no mode has been shown to improve patient outcome.
A major concern during assisted ventilation is patient–ventilator synchrony—the ventilator should be set to match the patient's ventilatory demands.
Auto positive end-expiratory pressure (PEEP) is a common cause of patient–ventilator dyssynchrony; in patients with obstructive lung disease, properly set applied PEEP can improve synchrony and decrease patient efforts to ventilate.
It is unnecessary to achieve normal PaO2 and PaCO2. In most critically ill patients, a PaO2 of 60 mm Hg or higher is acceptable, and permissive hypercapnia may be useful in treating some patients.
Ventilator-induced lung injury is primarily caused by localized overdistension and the opening and closing of unstable lung units.
In most patients who are ventilated, the tidal volume should be 4 to 8 mL/kg predicted body weight (PBW) and the plateau pressure should be less than 30 cm H2O, and PEEP should be set to avoid the collapse of unstable lung units.
High-frequency ventilation and airway pressure release ventilation, although useful in managing patients with acute respiratory distress syndrome (ARDS), show no outcome benefits over conventional pressure or volume ventilation.
Noninvasive positive-pressure ventilation is useful to transition patients who are at high risk of extubation failure from invasive ventilation to spontaneous breathing.
Most surgical patients do not require support of the respiratory system beyond the immediate postoperative period. However, others with preexistent chronic respiratory diseases, trauma patients, and patients who require extensive surgical procedures may require more lengthy periods of respiratory support. This chapter focuses on mechanical ventilation of the postoperative patient. It primarily discusses invasive ventilation but also reviews selected data for noninvasive respiratory support and the application of noninvasive positive-pressure ventilation (NPPV) and continuous positive airway pressure (CPAP) by mask.
The main objectives of mechanical ventilation in the postoperative patient are to decrease the work of breathing and the load on the cardiovascular system, and to reverse life-threatening hypoxemia or respiratory acidosis. Table 81-1 summarizes some of the specific indications for mechanical ventilation.1,2 In an international survey of 1638 patients requiring mechanical ventilation reported by Esteban et al, acute respiratory failure was the indication for mechanical ventilation in most patients (66%), followed by coma (15%), an acute exacerbation of chronic obstructive pulmonary disease (13%), and neuromuscular disorders (5%).3 Included under the heading of respiratory failure were postoperative respiratory failure, acute respiratory distress syndrome (ARDS), heart failure, pneumonia, sepsis, and complications of trauma.3