The increased expiratory muscle use in upright dogs: role of cardiovascular receptors

Abstract

A change from the supine to upright posture in anesthetized dogs promotes increased expiratory muscle use during breathing. To examine the role of cardiovascular receptors in eliciting this expiratory muscle recruitment, the electromyographic (EMG) activity of the triangularis sterni (TS) and abdominal external oblique (EO) muscles was recorded in seven spontaneously breathing animals during head up tilting and during occlusion of the inferior vena cava. Head up tilting was associated with a reduction in cardiac output, a transient fall in systemic blood pressure, and considerable increases in TS and EO expiratory EMG activity. On an average (mean ± SE), the amount of TS and EO expiratory activity in the supine posture was 44.7 ± 12.9 and 10.3 ± 7.3%, respectively, of the activity recorded in the 80° head up posture. When occlusion of the inferior vena cava in the supine animals induced a reduction in cardiac output and a fall in systemic blood pressure that were comparable to those measured during head up tilting, the TS and EO expiratory EMG activity also increased. This activity, however, always remained smaller than that recorded during breathing in the upright posture; for the seven animals, the amount of TS and EO expiratory activity during vena cava occlusion was only 58.4 ± 5.7 and 17.9 ± 10.4% of the activity in upright posture, respectively ( P < 0.001 for both muscles). We conclude, therefore, that the reduced venous return and systemic hypotension of the upright posture are not the critical sensory events for promoting the increased expiratory muscle use in this posture. It must, therefore, be elicited by respiratory receptors.