INFLUENCE OF ??-ADRENERGIC BLOCKADE ON THE HEART RATE PERFORMANCE CURVE DEFLECTION

Abstract

The deflection of the heart rate performance curve (HRPC) may be used for non-invasive determination of the anaerobic threshold using a heart rate tum point (HRTP) concept which has been shown to be related to the second lactate turn point (LTP2) and the maximal lactate steady state (MaxLaSS) in cases where deflection of the HRPC was present. However, in a certain number of subjects no or even an inverted deflection can be found questioning the physiological basis of the concept. Aim of the study was to investigate the influence of the highly ß1-selective adrenoreceptor antagonist bisoprolol on the degree and the direction of the HRPC. 10 healthy male subjects (age: 23 ± 3 yrs; height: 181 ± 6cm; weight: 76 ± 6 kg) were pretested before the study and randomly received oral placebo (P) or bisoprolol (B) (5mg/day) for two weeks, following a double blind cross over design. Before the study and in the second week of each period subjects performed an incremental cycle ergometer exercise test until exhaustion (increase of 20W/min). The first (LTP1) and the second (LTP2) lactate turn points as well as the degree and the direction of the HRPC (kHR) were calculated by means of computer aided analysis. HRPC was not significantly different between pretest and P however, heart rate (HR) was significantly reduced at rest and at all work load levels in B. kHR was not significantly different between pretest (0.20 ± 0.37) and P (0.24 ± 0.36), highly reproducible (R = 0.94; p < 0.001) but significantly lower in B (0.05 ± 0.34). HR at LTP1 (126 ± 7/105 ± 10 bpm), LTP2 (162 ± 11 / 138 ± 15 bpm) and HRmax (187 ± 8 / 168 ± 15 bpm) indicated a greater difference at lower workloads compared to workload above the LTP2 as indicated by a decrease in kHR. The change of kHR from P to B condition was not uniform and presented a trend towards a greater decrease in cases of regular HRPC response (kHR > 0.2). Conclusion: Highly ß1-selective adrenoreceptor blockade influenced the time course of the HRPC between LTP1 and maximal performance indicating differences in number and/or sensitivity of adrenoreceptors in the investigated subjects. Subjects with non regular HRPC presumably have lower responsiveness to catecholamines already under placebo conditions.