Positive end-expiratory pressure (PEEP) in treatment of asthma may be beneficial by dilating airways or detrimental by increasing hyperinflation. Several studies have reported beneficial results but with conflicting effects on lung volume. We studied the effects of PEEP on pulmonary hyperinflation, gas exchange, and circulation in six patients (59 +/- 19 yr, four men, two women) with severe airflow obstruction requiring mechanical ventilation (four with asthma, two with an exacerbation of chronic airflow obstruction). Three levels of PEEP (5, 10, and 15 cm H2O) were studied. All patients were paralyzed and ventilated with a tidal volume of 1.0 L, and respiratory rates (R) of 10, 16, and 22 breaths per min. End-inspiratory lung volume (VEI) or the degree of pulmonary hyperinflation above functional residual capacity (FRC) was quantified by measuring total exhaled gas volume during a period of apnea following steady-state tidal inspiration (1). Two patients were not studied at 15 cm H2O PEEP because of hypotension. Without PEEP, all patients showed gas trapping above FRC that increased progressively as R was increased (i.e., expiratory time decreased). At each R, increases in PEEP progressively increased FRC up to 1.42 +/- 0.43 L (mean +/- SD) at 15 cm H2O PEEP (n = 4) and progressively reduced the degree of gas trapping above the PEEP FRC.(ABSTRACT TRUNCATED AT 250 WORDS)