Vascular distension in muscles contributes to respiratory control in sheep

It has recently been proposed that afferent fibers from skeletal muscle could sense the state of the microvascular circulation, linking ventilation to the degree of peripheral perfusion or vascular distension (Huszczuk et al., Respir. Physiol., 91:207-226, 1993). Ventilatory and circulatory responses to manipulation of peripheral vascular pressures in the hind limbs of anaesthetized (sodium thiopental) sheep were examined. Inflatable balloons were placed at the caudal ends of the abdominal aorta and the vena cava (Vc). Aortic (Ao) occlusion induced a consistent normocapnic decrease in minute ventilation (VE). In contrast, VE increased significantly during vena cava obstruction, leading to hypocapnia. Small changes in systemic blood pressure were observed (+7 mmHg for Ao occlusion and -12 mmHg during Vc obstruction). Moreover, inflation of the caval balloon superimposed on a previously established Ao occlusion, preventing venous drainage of anastomotic inflow, resulted in a significant rise in distal vascular pressures with trivial changes in systolic blood pressure. This led to a gradual rise of VE, despite further reduction of the CO2 flux to the lungs. The subsequent deflation of the aortic balloon, exposing the hindlimb vasculature to aortic pressure, resulted in an even more profound hypocapnic hyperpnea. The concurrent arterial blood pressure changes were too small to possibly involve the ventilatory component of the arterial baroreflex. We therefore hypothesize, that perfusion-related afferent signals within the muscles could contribute to respiratory homeostasis by maintaining ventilation of the lungs commensurate with the circulatory state of the muscular apparatus.