In 15 patients with asthma attack, evidence of the uneven distribution of air flow during controlled ventilation was obtained by detection of ventilatory asynchronism expressed by the incurvated profile of tracheal pressure waves associated with the repetitive interruptions of air flow. It was observed that low values of PEEP (mean: 5 +/- 2.5 cm H 2O) induced an increase in transbronchial pressure able to overcome ventilatory asynchronism. In these conditions, an appropriate ventilation-perfusion ratio was restored and improved gas exchanges as indicated by the mean increase of arterial PO 2 from 66.3 mmHg (+/- 2.57) to 96.89 mmHg (+/- 4.41) (p = 0.0005) associated with a mean decrease in arterial PCO 2 from 53.66 mmHg (+/- 2.71) to 42.07 mmHg (+/- 1.64) (p = 0.0005). Simultaneously hemoglobin oxygen saturation rose from 82.31% (+/- 1.97%) to 95.74% (+/- 0.5%). In our patients, such values of PEEP were not high enough to influence the pulmonary arterial circulation. The means of the pulmonary arterial pressures obtained before (syst.: 32.3; diast.: 15.1; mean: 22.00 mmHg) were quite the same (p greater than 0.2) as with PEEP (syst.: 32.00; diast.: 14.00; mean: 21.1 mmHg). The mean of the wedge pressure was found to be 8.3 (+/- 74 mmHg) prior to and 8.4 (+/- 0.68 mmHg) after PEEP (p greater than 0.3). Mean cardiac output rose slightly from 5.27 l/min (+/- 0.24) to 5.77 l/min (+/- 0.38) during PEEP (p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)