The influence of furosemide on heart rate and oxygen uptake in exercising man.

We recently observed that heart rate (HR) related methods for assessing physical fitness lead to an overestimation of endurance capacity in subjects treated with furosemide. To gain a more detailed description of this effect, the relationships between work load (WL), oxygen uptake (VO2), and HR were determined in the present study. To this end, nine healthy male subjects performed two incremental exercise tests (10 W increase per 30 s) on a bicycle ergometer. In one test 40 mg furosemide (Lasix) was applied orally 90 min before exercise started. Compared with control conditions, furosemide led to a change in mean blood volume of -4.5% (range: +7.8% to -11.5%). Neither the maximal VO2 (VO2max) nor the maximal work load (WLmax) were significantly altered after furosemide application. Though the WL-VO2 relationship was not significantly affected, the HR-VO2 relationship showed significant alterations which depended on both the loss of blood volume (BV) and work intensity: When the reduction in BV was less than approximately 5%, HR was found to be lowered at all workloads. When the BV reduction was greater than about 5% HR was significantly reduced only in the lower ranges of work load but significantly increased at the higher work intensities. Since BV reductions are known to increase HR during exercise, our findings suggest that, in addition to the blood volume induced changes in HR, furosemide exerts further direct or indirect effects on heart rate adjustment.