Slower adaptation of $\dot V$O₂ to steady state of submaximal exercise with Β-blockade

Summary

The kinetics of oxygen uptake (VO₂) were assessed in 17 normal subjects with Β-blockade and placebo. Β-blockade was achieved with either 50 mg oral metoprolol or 40 mg oral propranolol, each twice per day. Tests were conducted on the cycle ergometer at work rates approximating 80% of the work rate at ventilatory anaerobic threshold. Work rate was initiated as a square wave starting from prior rest. Data obtained 48 h, 1 week, and 4 weeks after starting drug or placebo were pooled to increase the number of points for regression analysis of kinetic parameters. While there were no differences in the plateau values for $\dot V$O₂ with and without Β-blockade, the rate of adaptation to steady state was significantly slower with Β-blockade than with placebo (p<0.05). This resulted in an increase of oxygen deficit by approximately 200 ml O₂. Cardiac output measured by CO₂ rebreathing was significantly reduced by Β-blockade (metoprolol by 4.1%, propranolol by 12.2%, both P<0.05). Blood lactate concentration was unaffected by Β-blockade. It was concluded that the influence of Β-blockade on the oxygen transport system was responsible for the significantly slower increase of $\dot V$O₂ to steady state in submaximal exercise.