Effect of posture on expiratory muscle use during breathing in the dog.

The electromyographic (EMG) activity of the triangularis sterni and abdominal external oblique muscles was studied in 10 lightly anesthetized, spontaneously breathing dogs during postural changes from supine to head up and head down. The triangularis sterni was active during expiration in all animals when breathing in supine posture, whereas the external oblique was active in only 6 animals. A change from the supine to head up posture elicited a marked increase in triangularis sterni and external oblique expiratory EMG activity in all animals. A change from the supine to head down posture conversely caused the triangularis sterni expiratory activity to decrease and the external oblique expiratory activity, when present in supine posture, to be markedly reduced or abolished. The external oblique was generally silent after bilateral cervical vagotomy and no longer responded to posture. In contrast, bilateral vagotomy had no clear-cut effect on either the active use of the triangularis sterni in supine posture or on the muscle response to posture. These results indicate that: the active nature of spontaneous quiet expiration in the dog is not specific to the supine posture; the adoption of the head up posture actually is associated with considerable phasic expiratory recruitment of the triangularis sterni and abdominal muscles, so that moving the chest wall during breathing is proportionately more an expiratory act and less an inspiratory act than in supine posture; and in contrast to the abdominal muscles, the postural changes in triangularis sterni use during breathing are largely independent of vagal inputs.