Tissue Oxygen Saturation by NIRS During 60 and 100 RPM Ramp Tests

Abstract

0176 Faster cycling cadence is associated with greater oxygen uptake (VO2) at all external work rates up to around peak VO2. It is less clear in humans how circulatory adjustments and O2 delivery are integrated with these increased metabolic demands at higher muscle contraction rates. This balance between blood flow and metabolic rate can be followed noninvasively by near infrared spectroscopy (NIRS) measurement of tissue oxygen saturation. PURPOSE: To test the hypothesis that changes in tissue saturation (%HbO2) during ramp cycle exercise would be similar at 100 vs. 60 rpm. METHODS: To date, four subjects have completed the protocol, which consisted of incremental ramp exercise on a cycle ergometer at either 60 or 100 rpm on different days. Pulmonary gas exchange and heart rate were monitored breath-by-breath, while tissue saturation (%HbO2) of the vastus lateralis was followed by surface NIRS (OxyPlexTS, ISS). RESULTS: Peak VO2 was similar for each rpm test (60 rpm: 3.31 ± 1.08; 100 rpm: 3.40 ± 1.10 L/min), as was the estimated lactate threshold (60 rpm: 1.71 ± 0.70 L/min; 100 rpm: 1.80 ± 0.72 L/min). Likewise, %HbO2 was similar at peak VO2 (60 rpm: 59.6 ± 4.7%; 100 rpm: 62.0 ± 5.0%). Interestingly, baseline (unloaded cycling) VO2 was higher at 100 rpm (1.23 ± 0.29 L/min) than at 60 rpm (0.64 ± 0.17 L/min) while %HbO2 was similar (100 rpm: 71.0 ± 22.0%; 60 rpm: 74.1 ± 0.8%). From the Fick equation, this suggests that baseline muscle blood flow was substantially greater at 100 than 60 rpm, and matched to the elevated metabolic rate. Further, assuming that the scattering coefficient changed slowly over several seconds, pronounced contraction-induced oscillations in all of the NIRS- derived responses (except deoxyhemoglobin concentration) were noted. CONCLUSION: Within each subject, fatigue during incremental exercise was associated with a similar minimum level of tissue saturation, irrespective of contraction frequency. The similar peak VO2 and minimum tissue saturation also predict similar peak muscle blood flow responses at both 60 and 100 rpm, suggesting that muscle blood flow is not compromised at the higher contraction rate during cycle exercise. Finally, muscle contraction may have appreciable effects on capillary gas exchange during the contraction-relaxation cycle. supported by AHA Grant-in-Aid 0151183Z