Cardiac performance during graded exercise in acute hypoxia

Abstract

To evaluate the cardiocirculatory changes induced by acute hypoxic stress, rate corrected electro-mechanical systole (QS2)) left ventricular ejection time (LVET), and preejection period (PEP) were determined in 11 healthy males at sea level (SL) and after 2 h at a simulated altitude (HA) of 4,300 m. Recordings were made at rest and at 15 min of upright bicycle exercise at light and moderately heavy loads (40 and 75% of Vo2 max, respectively). Venous blood was withdrawn simultaneously for estimation of total plasma catecholamines (CATS). Exposure to HA produced a significant decrease in each interval at rest with differences narrowing progressively with exercise of increasing intensity. PEP/LVET ratio decreased progressively with graded exercise at SL indicating an enhancement of left ventricular performance that was directly related to the elevation in plasma CATS. Cardiac performance was augmented at rest during HA exposure, despite a normal level of CATS. However, during moderately heavy exercise at HA, when the plasma CATS were increased to twice the SL values, cardiac performance, as reflected by the PEP/LVET ratio, did not increase proportionately. It is postulated that the depressant effect of increased hypoxemia and acidosis during moderately heavy exercise in HA may override the cardiostimulatory influence of circulating CATS.