Many patients with severe chronic obstructive pulmonary disease (COPD) contract the transversus abdominis (TA) muscle during expiration. The mechanical correlates of this contraction, however, are uncertain. In the present study, we have measured airflow, esophageal (Pes), and gastric (Pga) pressures, the anteroposterior (AP) diameter of the abdomen, and the electromyogram of the TA during resting breathing in 25 seated patients with severe COPD (FEV1 = 28 +/- 8% of predicted). Nine patients (Group I: FEV1 = 29 +/- 8% of predicted) in general had no TA activity during breathing, and 16 patients (Group II: FEV1 = 27 +/- 8% of predicted) had invariable phasic expiratory TA activity. In all patients of Group II, the decrease in abdomen AP diameter during expiration was associated with a gradual fall in Pga. In contrast, in 13 patients of Group II, the expiratory decrease in abdomen AP diameter was associated with a rise in Pga; this rise in Pga averaged 2.8 +/- 2.2 cm H2O. Furthermore, most patients had positive alveolar pressure at end-expiration, as shown by the time lag between the fall in Pes at the beginning of the inspiratory effort and the onset of inspiratory flow. However, whereas end-expiratory alveolar pressure averaged 2.4 +/- 2.2 cm H2O in Group II, it was only 0.8 +/- 0.6 cm H2O in Group I (p < 0.05). For the patient group as a whole, there was a close relationship between the rise in Pga during expiration and end-expiratory alveolar pressure (r = 0.87; p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)