Effects of expiratory threshold loading on thoracoabdominal motion in cats.

Expiratory muscle activity may alter rib cage and abdominal configuration at end-expiration and thereby affect the pattern of thoracoabdominal motion during subsequent inspiration. In this study, expiratory muscles were stimulated by the application of a series of expiratory threshold loads (ETL) (range: 0 to + 10 cm H2O) in 10 cats lightly anaesthetized with pentobarbitone. Thoracoabdominal motion was monitored by inductance plethysmography. Peak internal intercostal and abdominal muscle electrical activity increased proportionately with the magnitude of the ETL, suggesting comparable activation of both expiratory muscle groups. Increases in end-expiratory abdominal volume, however, were greater than increases in end-expiratory rib cage volume, during ETL. This resulted in a shift of the end-expiratory thoracoabdominal position to the right of the relaxation characteristic. During ETL, there was inward movement of the abdomen in early inspiration and this decrease in abdominal volume correlated closely with the change in end-expiratory abdominal volume and the magnitude of the ETL (mean r = 0.95, P less than 0.01). In three animals, sectioning the abdominal muscles resulted in a further increase in end-expiratory abdominal volume for a given ETL. Deepening anesthesia progressively reduced and eventually abolished expiratory muscle activation and under these conditions, ETL increased end-expiratory rib cage and abdominal volume along the thoracoabdominal relaxation characteristic. The results of this study indicate that while abdominal muscle activation acts to decrease abdominal volume at end-expiration, the action of the expiratory intercostal muscles predominates during ETL to produce a smaller rib cage volume at the expense of an enlargement of abdominal volume.