Myocardial blood flow and lactate metabolism at rest and during exercise with reduced arterial oxygen content

Abstract

The effect of a reduction in arterial oxygen content, equivalent to acute exposure to an altitude of 2300 metres above sea level, on myocardial blood flow and oxygen and lactate exchange was studied by coronary sinus catheterization in 12 healthy men. Measurements were made at rest, during atrial pacing and during submaximal and maximal exercise both breathing air and breathing 15% oxygen (hypoxia). Coronary sinus blood flow was measured by thermodilution and the possibility of a simultaneous uptake and release of lactate by the heart was calculated using intravenous infusion of 14C lactate. At all levels of cardiac power output myocardial oxygen consumption was the same during hypoxia as during air breathing. At rest this was achieved entirely by a more complete extraction of oxygen from the coronary blood, during maximal exercise entirely by a greater coronary sinus blood flow, while at intermediate levels of cardiac power output a combination of these mechanisms prevailed. At rest and during submaximal work myocardial lactate extraction was lower with hypoxia than air breathing suggesting a change in myocardial redox state, while the 14C lactate data suggested no significant lactate release or possibly limited areas with some lactate production. During maximal exercise, however, there was no difference in myocardial lactate net extraction between hypoxia and air breathing, which together with the greater blood flow suggests that the heart has a 'coronary flow reserve' permitting maximal exercise at moderate altitude without anaerobic myocardial metabolism.

Myocardial perfusion during exercise in endurance-trained and untrained humans

“…In the present study the myocardial perfusion values were found to be slightly higher than in some other studies (10,11). One possible explanation could be methodological differences between studies.…”

“…In humans, Grubbstrom et al (10) found with coronary sinus catheterization and thermodilution measurement that maximal coronary blood flow was 3.5-fold higher than at rest in sedentary subjects, although further increase was observed when maximal exercise was performed during hypoxia (10). However, by extrapolating from the results of the present study peak myocardial perfusion in humans would be ϳ5 ml⅐g Ϫ1 ⅐min Ϫ1 (ϳ7-fold higher than resting myocardial perfusion), assuming that the relationship between myocardial perfusion and workload remains linear (Fig.…”

“…One possible explanation could be methodological differences between studies. In human studies (10,11) and in several animal experiments (e.g., Refs. 26, 33, 39) blood flow in the coronary arteries was measured, and only in some cases were the estimates of flow per gram of tissue given.…”

Evidence that reduced skeletal muscle recruitment explains the lactate paradox during exercise at high altitude

“…Indeed coronary flow reserve exists during maximal exercise in hypoxia, confirming the absence of myocardial hypoxia (7,8). These data prove 1) that cardiac function during exercise in hypoxia is not regulated to maximize blood flow to the exercising limbs and 2) that the progressively lower cardiac outputs during maximal exercise with increasing hypoxia cannot be due to myocardial failure.…”

Cardiorespiratory responses to exercise in acute hypoxia, hyperoxia and normoxia

“…During maximal exercise the compensatory mechanism is an increase in coronary , heart rate (f c ) and stroke volume (SV) at a given oxygen uptake ( _ V O 2 ). See text for the results of the regression analysis blood¯ow, indicating that a normal heart has à`c oronary¯ow reserve'' that can be used in hypoxia (Grubbstrom et al 1991;Kaijser et al 1990). As the increase in¯ow is accompanied by an increase in coronary arterial pressure and stiness (Templeton et al 1972), the stiening of the ventricle could shift the enddiastolic pressure (EDP)/end-diastolic volume (EDV) curve to the left (Janicki et al 1996).…”