Efficiency, maximal blood flow, and aerobic work capacity of canine diaphragm

Abstract

During heavy ventilatory work loads the maximum aerobic capacity of the diaphragm may be approached or exceeded. An attempt was made to define the aerobic capacity of the canine diaphragm. Using anesthetized mongrel dogs, blood flow was measured with radioactive microspheres and mechanical work on the diaphragm with a lower-body plethysmography. By samplng blood from the aorta and left inferior phrenic vein, the difference between arterial and venous O2 content was measured across the diaphragm; the latter multiplied by blood flow yielded diaphragmatic O2 consumption. Mechanical efficiency was estimated from the inverse of the slope of the relationship between O2 consumption and mechanical work of the diaphragm as work was progressively increased by inspiratory resistive loads. Mechanical efficiency of the diaphragm averaged 23% and was constant as work load was increased. When active vascular tone in the diaphragm was abolished by vasodilator infusions or hypoxia, vascular conductance of the diaphragm (in ml .cntdot. min .cntdot. Torr-1 .cntdot. g muscle-1) was directly related to mean aortic perfusion pressure (.hivin.Pa, in Torr); thus maximal diaphragmatic blood flow (.ovrhdot.Qdimax) was related to the square of .hivin.Pa according to the equation .ovrhdot.Qdimax = (1.32 .hivin.Pa2 + 29.6 .hivin.Pa) .times. 10-4. Aerobic work capacity of the diaphragm could be maintained until phrenic venous O2 tension fell to < 10-12 Torr; below this ventilatory failure and CO2 retention occurred. Phrenic venous O2 content could fall to as low as 1 ml/dl at heavy work. Aerobic work capacity of the diaphragm (.ovrhdot.Wdimax) was estimated for different perfusion pressures. .ovrhdot.Wdimax = [(1.32 .hivin.Pa2 + 29.6 .hivin.Pa) (CaO2 = 1.0) .times. 10-6 - 1.54]1.16, where CaO2 is arterial O2 content.