Active Exhalation

A rise in baseline pressure during exhalation accompanied by an increase in expiratory flow indicates expiratory muscle contraction, which raises intrathoracic pressure. Under normal circumstances, expiration is a passive process driven by the elastic recoil of the respiratory system and produces a smooth exponential decay in the expiratory flow waveform.

However, when the patient actively contracts the expiratory muscles, this normal decay pattern is lost and the expiratory flow rises abruptly. Recognizing expiratory muscle contraction on waveforms is important because the waveform pattern immediately before this contraction may resemble an ineffective (failed) trigger. Differentiating between the two prevents misinterpretation and ensures appropriate clinical management.