Auto positive end-expiratory pressure (auto-PEEP) is defined as a positive alveolar pressure at the end of exhalation during mechanical ventilation. It is a form of occult PEEP that is determined as the equilibrium pressure after occluding the exhalation port at the end of exhalation while delaying the next breath. Auto-PEEP was present in 56% of the patients studied. Eighty-two percent of these patients had blunt trauma and 25% had pulmonary contusions. Chronic obstructive pulmonary disease was present in several of the difficult patients. Auto-PEEP was treated by minimizing minute ventilation, increasing flow rates, using bronchodilators selectively, and changing small nasotracheal tubes to larger orotracheal tubes. Several head injury patients could not be hyperventilated successfully due to hyperinflation and auto-PEEP. I:E ratios of 1:4 were used with increased inspiratory flow rates ranging from 40 to 120 L/min to allow longer exhalation times. Reversal of auto-PEEP levels greater than 5 cm H2O resulted in an increase in mean arterial blood pressure from 90 to 102 mm Hg and a decrease in central venous pressure from 13 to 7 mm Hg. J. Trauma 30:1316, 1990. Auto-PEEP may result in increased pleural pressures which depress cardiac output and artifactually increase pulmonary capillary wedge pressure. Failure to recognize auto-PEEP may lead to inappropriate fluid restriction and unnecessary use of vasopressors. Am. Rev. Respir. Dis. 126:166, 1982.
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