The effect of age on mechanisms of exercise tolerance: Reduced arteriovenous oxygen difference causes lower oxygen consumption in older people.

Abstract

To assess the effect of age on mechanisms of exercise tolerance. Prospective observational study recruited 71 healthy individuals divided into two groups according to their age i.e. younger (≤40years of age, N=43); and older (≥55years of age, N=28). All participants underwent maximal graded cardiopulmonary exercise stress testing using cycle ergometer with simultaneous non-invasive gas-exchange and central haemodynamic measurements. Using the Fick equation, arteriovenous O2 difference was calculated as the ratio between measured O2 consumption and cardiac output. The mean age of younger and older participants was 26.0±5.7years, and 65.1±6.6years respectively. Peak O2 consumption was significantly lower in older compared to the younger age group (18.8±5.2 vs 34.4±9.8mL/kg/min, p<0.01). Peak exercise cardiac output and cardiac index were not significantly different between the younger and older age groups (22.7±5.0 vs 22.1±3.9L/min, p=0.59; and 12.4±2.9 vs 11.8±1.9L/min/m2, p=0.29). Despite demonstrating significantly lower peak heart rate by 33 beats/min (129±18.3 vs 162±19.9, p<0.01), older participants demonstrated significantly higher stroke volume and stroke volume index compared to the younger age group (173±41.5 vs 142±34.9mL/min, p<0.01; and 92.1±18.1 vs 78.3±19.5mL/m2, p<0.01). Arteriovenous O2 difference was significantly lower in older compared to younger age group participants (9.01±3.0 vs 15.8±4.3 mlO2/100mL blood, p<0.01). Ability of skeletal muscles to extract delivered oxygen represented by reduced arteriovenous O2 difference at peak exercise appears to be the key determinant of exercise tolerance in healthy older individuals.