General anesthesia causes an increase in intrapulmonary shunt, which may impair oxygenation, and the magnitude of shunt is correlated with the formation of atelectasis. The atelectasis appears within minutes after anesthesia induction in nearly 90% of patients. The degree of atelectasis is larger with obese patients and when a higher fraction of inspired oxygen (FI02) is used. Using lower FIO2 (30%) during induction can effectively decrease the amount of atelectasis, however this is associated with a lower safety margin is patients who may be difficult to intubate. Another option is the use of positive end expiratory pressure (PEEP) after a recruitment breath during induction that prevents recurrence of atelectasis when high FIO2 is used. Complications of PEEP include decreasing cardiac output (reduced venous return, reduced ventricular compliance, increased RV outflow impedence, and ventricular external constraint by hyperinflated lungs) and well as lung injury/alveoli rupture when used in patients with localized lung disease.
During mechanical ventilation (positive pressure ventilation), atelectasis may occur when lungs are underinflated due to low tidal volumes, or when compression occurs (such as patient position or obesity). Use of volume control ventilation (A/C, IMV) will deliver a preset volume (normally 6-8mL/kg) and the pressure generated in the lung will then be dependent on the resistance and compliance of the respiratory system. In pressure controlled ventilation , a constant pressure is used to inflate the lungs, and the tidal volume delivered will be dependent on the resistance and compliance of the respiratory system. Either of these modes may contribute to atelectasis if the pressure or volume delivered to the lungs is insufficient to prevent the alveoli from collapsing.
Another method to decrease atelectasis is the use of inverse ratio ventilation (IRV) during pressure controlled ventilation. In this mode, PCV is combined with a prolonged inflation time, and the usual I:E ratio (1:2) is reversed (2:1). The prolonged inflation time can help prevent alveolar collapse. However, use of IRV is associated with inadequate emptying of the lungs which can lead to hyperinflation and auto-PEEP, that can decrease cardiac output.
Techniques for Prevention of Atelectasis Addition of PEEP (at least 10 cm H2O) Sigh maneuver (must be 30 cm H2O or more in order to work, although 40 cm is best) Avoidance of 100% oxygen Adding CPAP to 100% preoxygenation (will attenuate the subsequent atelectasis) Avoidance of paralysis / maintenance of spontaneous ventilation.