Effect of β-adrenergic blockade on$\left( {\dot VO_2 } \right)$ kinetics during pseudorandom binary sequence exercise

Summary

The effect of β-adrenergic blockade on the dynamic response characteristics of oxygen uptake$\left( {\dot VO_2 } \right)$ was examined in seven healthy men. Cycle ergometer exercise consisted of a pseudorandom binary sequence (PRBS) with 15 units per sequence, each unit lasting 15 s, for a total period of 225 s. Work rate was either 25 W or 125 W. Six identical sequences were performed continuously in a single exercise test. Each subject was studied on three occasions after 48 h treatment of placebo (CON); a nonselective β-blocker, propranolol (PROP); and a β₁-selective blocker, metoprolol (MET).$\dot V_{O_2 }$ was measured breath-by-breath. The data were analyzed in the frequency domain by Fourier analysis to yield amplitude and phase shift coefficients for the relationship between the input work rate and output$\dot V_{O_2 }$. Time domain analysis was performed on the cross-correlational analysis of$\dot V_{O_2 }$ (output) · work rate (input) to determine the total lag time. The amplitude of the$\dot V_{O_2 }$ · W⁻¹ relationship was lower (P<0.0022) and the phase shift was greater (P<0.0039) with β-blockade compared to CON; no differences were found between PROP and MET. The total lag time was longer (P<0.0053) in PROP (34.3±2.2s) and MET (34.4±1.6s) compared to CON (29.8±1.3s). These data indicate that β-adrenergic blockade causes a significant slowing of kinetics of the O₂ transport system as determined by both frequency and time domain analysis. In addition, the PRBS exercise test is a sensitive indicator of change in function of the O₂ transport system.