Effect of Lower Body Negative Pressure on Phase I Cardiovascular Responses atExercise Onset

摘要

We hypothesised that vagal withdrawal and increased venous return interact in determining therapid cardiac output (CO) response (phase I) at exercise onset. We used lower body negativepressure (LBNP) to increase blood distribution to the heart by muscle pump action and reduceresting vagal activity. We expected a larger increase in stroke volume (SV) and smaller forheart rate (HR) at progressively stronger LBNP levels, therefore CO response would remainunchanged. To this aim ten young, healthy males performed a 50 W exercise in supineposition at 0 (Control), −15, −30 and −45 mmHg LBNP exposure.On single beat basis, we measured HR, SV, and CO. Oxygen uptake was measured breath-by-breath.Phase I response amplitudes were obtained applying an exponential model. LBNP increased SVresponse amplitude threefold from Control to −45 mmHg. HR response amplitudetended to decrease and prevented changes in CO response. The rapid response of CO explainedthat of oxygen uptake. The rapid SV kinetics at exercise onset is compatible with an increasedvenous return, whereas the vagal withdrawal conjecture cannot be dismissed for HR. The rapid COresponse may indeed be the result of two independent yet parallel mechanisms, one acting on SV,the other on HR.