Relationship Between Heart Rate Threshold, Lactate Turn Point and Myocardial Function

Abstract

We examined the relationship between heart rate threshold (HRT), lactate turn point (LTP) and myocardial function expressed as left ventricular ejection fraction (LVEF) determined by radionuclide ventriculography. Two groups of subjects (G I: N = 8; G II: N = 7) with and without a deflection of heart rate performance curve (HRPC) underwent sitting cycle ergometry. HRT (G I), aerobic threshold (AeT; G I, G II), and LTP (G I, GII) were determined by means of linear regression break point analysis. Also, a break point in LVEF performance curve (LVEFBP) was obtained.

Power output at HRT and at LTP was not significantly different between G I and G II (272.5 ±38.7 W; 294.3 ±20.6 W). Power output at LVEFBP (G I: 182.6±31.7 W; GII: 211.8 ±21.5 W) was not significantly different to power output at LTP (G I: 194.2 ± 32.7 W; GII: 215.2 ±24.4 W) and HRT (G I: 193.0±38.2 W). Significant differences (p<0.05) could only be found between GI and GII for heart rate (HR) at LTP (G I: 163.5±5.8b·min^-1; G II: 154.4±6.7b·min^-1) and LVEF at the end of the load (LVEFend) (G I: 77.9 ± 2.9%; GII: 71.3 ± 7.0%). The drop of LVEF at LVEFBP was significantly related to LTP in all cases.

The present data suggest that the noninvasive determination of anaerobic threshold by means of heart rate curve analysis is not always possible due to different response of myocardial function and heart rate to graded cycle ergometer exercise. The cause for the differing behaviour of HR and LVEF requires further investigation.

Key words

Bicycle ergometer - heart rate threshold - lactate turn point - left ventricular ejection fraction - anaerobic threshold