How are some obstructive lung disease patients who develop intrinsic PEEP protected from the negative hemodynamic effects of low levels of applied PEEP?

A. because intrinsic PEEP improves the lung compliance of these patients
B. because intrinsic PEEP decreases the preload and left ventricular afterload
C. because pressure downstream from the site of flow limitation does not affect flow
D. because low levels of applied PEEP reduce airway resistance

?ANS: C
A. Incorrect response: Intrinsic PEEP does not improve lung compliance.
B. Incorrect response: PEEP does decrease the preload and left ventricular afterload. However, these hemodynamic influences do not account for the protection against applied PEEP afforded by intrinsic PEEP.
C. Correct response: Somewhat paradoxically, patients with intrinsic PEEP may be protected from the negative hemodynamic effects of applied PEEP. Patients whose obstructive lung disease is sufficiently severe to necessitate intubation are often flow-limited during their passive exhalation. A characteristic of flow-limited systems is that pressure downstream from the site of flow limitation does not affect flow. Application of low levels of PEEP has been shown neither to retard expiratory flow, to cause further hyperinflation, nor to decrease cardiac output in mechanically ventilated patients with intrinsic PEEP from obstructive lung disease. Higher levels of PEEP exceeding the closing pressures of the flow-limited airways will cause further hyperinflation and hypotension.
D. Incorrect response: Low levels of PEEP do not affect airway resistance.