Cardiopulmonary interactions in health and chronic heart failure

Our knowledge of cardiopulmonary interactions during exercise is based primarily upon the extrapolation of data derived from anesthetized animals. Our goal was to determine the effects of respiratory muscle pressure production on cardiovascular function during exercise. In the first two investigations, human subjects performed either a predominantly "ribcage" breathing pattern or a diaphragmatic breathing pattern while the intrathoracic pressure (PITP) excursion was held constant. Femoral venous blood flow (Q fv) was then measured using an ultrasound Doppler in the presence or absence of calf contraction exercise. Despite the fact that the two breathing patterns elicited directionally opposite within-breath changes in Qfv from the locomotor limb, net Qfv was not altered by the breathing pattern followed by the subjects nor the magnitude of the inspiratory PITP excursion in the steady-state. However, the addition of active expiration significantly reduced net Qfv in the steady state. In the remaining three investigations, we used a chronically instrumented dog model to measure changes in cardiac output and blood flow distribution in response to changes in intrathoracic pressure. We found that removing the normally produced inspiratory PITP excursion reduced stroke volume during submaximal exercise, though cardiac output and hindlimb blood flow were unchanged. In contrast, removing the normally produced inspiratory PITP excursion in dogs with pacing-induced chronic heart failure increased stroke volume and cardiac output, and preferentially redistributed blood flow towards the locomotor limbs. Increasing the expiratory PITP excursion significantly reduced cardiac output, stroke volume, and locomotor limb blood flow in both healthy dogs and dogs with pacing-induced CHF. We conclude that (1) the respiratory pump is not an independent contributor to increases in cardiac output and locomotor limb blood flow during exercise in healthy animals, but appears to be required for preload recruitment and a maximal stroke volume response to exercise in health, (2) the normally produced inspiratory PITP excursions are detrimental to the cardiac output and locomotor limb blood flow responses to exercise in CHF, and (3) augmented expiratory PITP excursions limit the increases in cardiac output and locomotor limb blood flow in response to exercise in both healthy dogs and dogs with pacing-induced CHF.