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When anesthetized, paralyzed patients were mechanically ventilated at rapid frequencies such that the time available for exhalation was insufficient, gas was retained in the thorax. Gas trapping became more pronounced as respiratory frequency increased. For any patient, the minimum length of time needed for virtual completion of exhalation could be predicted from measurements of total respiratory compliance and resistance. At moderately rapid respiratory frequencies, incomplete exhalation due to insufficient available time was partially compensated by increases in transthoracic pressure that permitted exhalation of larger volumes in the time available and were accompanied by increases in end-expiratory lung volume.