Determination of respiratory point compensation in healthy adults: Can non-invasive near-infrared spectroscopy help?

Objectives: We tested the hypothesis that the respiratory compensation point can be accurately determined in healthy participants during incremental cycling exercise using non-invasive near-infrared spectroscopy-derived measures of deoxygenated hemoglobin (deoxyHb). Design: Validation study. Methods: 118 healthy men (average age 47 ± 19 yrs, range 20-79 yrs) performed an incremental cycling test to exhaustion. Breath-by-breath pulmonary oxygen uptake (over(V, ̇) O2) and other ventilatory and gas exchange variables were measured and used to determine respiratory compensation point. Vastus lateralis deoxyHb was monitored using a frequency domain multi-distance system near-infrared spectroscopy device and deoxyHb data were modeled with a piece-wise double-linear function from which the deoxyHb deflection point (deoxyHbDP) was determined. The absolute (L min-1) and relative (% maximal over(V, ̇) O2 [over(V, ̇) O2max]) over(V, ̇) O2 values associated with the respiratory compensation point and deoxyHbDP were determined for each individual. Results: DeoxyHb increased as a function of exercise intensity up to a point (deoxyHbDP) after which the signal displayed a "near-plateau". The deoxyHbDP corresponded to a over(V, ̇) O2 of 2.25 ± 0.69 L min-1 (74 ± 12% over(V, ̇) O2max) which was not significantly different from the over(V, ̇) O2 at respiratory compensation point (2.28 ± 0.70 L min-1 and 74 ± 10% over(V, ̇) O2max, p < 0.05). Both indices were highly correlated (r2 = 0.86) and Bland Altman analyses confirmed a non-significant bias for over(V, ̇) O2 (-0.024 L min-1) concomitant with a small imprecision of 0.26 L min-1. Conclusions: During incremental cycling exercise, the over(V, ̇) O2 associated with the onset of a plateau in near-infrared spectroscopy-derived deoxyHb occurs in coincidence with the over(V, ̇) O2 at respiratory compensation point suggesting that respiratory compensation point can be accurately estimated, non-invasively, using near-infrared spectroscopy-derived deoxyHb in alternative to the use of ventilatory-based techniques