Peak Total‐[Hemoglobin + Myoglobin] During Incremental Dynamic Handgrip Exercise and Post Occlusive Hyperemia

Abstract

IntroductionNear infrared spectroscopy (NIRS) has been used previously to measure changes in the total hemoglobin + myoglobin (Total‐[Hb+Mb]) signal, thought to reflect changes in microvascular hematocrit (HCT). During rest the hematocrit of the microvasculature is lower compared to the systemic hematocrit. However, during exercise microvascular hematocrit increases towards that seen in the systemic circulation. This increase in hematocrit provides an increase in the surface area available for O2 exchange. Post‐occlusive reactive hyperemia (PORH) tests have also utilized changes in NIRS signals, including Total‐[Hb+Mb], to evaluate function/dysfunction of the circulatory system. However, it is currently unclear how the peak Total‐[Hb+Mb] observed transiently following cuff release during PORH might relate to the peak Total‐[Hb+Mb] seen at task failure during incremental exercise (active hyperemia). These results could provide insight into the underlying mechanisms which control microvascular HCT.PurposeThe purpose of this investigation was to determine how the peak Total‐[Hb+Mb] achieved during incremental dynamic handgrip exercise compared to that seen during PORH.MethodsEleven men completed two testing protocols; 1) Incremental dynamic handgrip exercise to failure and 2) PORH. Exercise and PORH were performed in a supine position, with the right arm abducted to ~80° at heart level. During incremental exercise the work rate progressively increased 0.5 Watts every 30 sec until the subject could not maintain the correct contraction rate for at least 3 consecutive contractions despite verbal encouragement. The PORH test included: 1 min of baseline, 5 min occlusion of the brachial artery, and 3 min recovery after cuff release. Near‐infrared spectroscopy (NIRS) measurements were made continuously during both tests, from which the Total‐[Hb+Mb] signal was derived.ResultsThere was no difference in the mean values for peak Total‐[Hb+Mb] during the incremental test (93.0 ± 20.3 μM) and that observed during the PORH test (93.1 ± 17.7 μM) (p = 0.992). Further, peak Total‐[Hb+Mb] during PORH was significantly correlated to peak Total‐[Hb+Mb] during incremental handgrip exercise (R2 = 0.657, p = 0.002). The slope of the regression line for Peak PORH vs. Incremental was 0.930.ConclusionThese data suggest that in a given subject, similar peak values of microvascular Total‐[Hb+Mb], and thus microvascular HCT, are achieved during active and reactive hyperemia.