Introduction: Cardiopulmonary exercise testing (CPX) has been used to identify elderly patients at high risk for major surgery; Older demonstrated that postoperative cardiovascular-related deaths were predicted by an anaerobic threshold (AT) < 11 ml/min/kg¹. This methodology is limited by the uncomfortable and claustrophobic facemask used for standard CPX. During cycling, pulmonary oxygen consumption (VO₂) is equivalent to twice muscle VO₂². Our research group has developed novel methods of using near infrared spectroscopy (NIRS) to determine muscle oxygen saturation (SmO₂), muscle pH and hematocrit.

Hypothesis: NIRS, in combination with heart rate (HR) monitoring, may be used to determine VO₂.

Methods: Ten healthy subjects (5M/5F) performed CPX. Pulmonary VO₂ was determined with a metabolic cart simultaneously with NIR spectra from the thigh. Muscle VO₂ was calculated using the Fick equation (VO₂ = SV x HR x C(a-v)O₂) where stroke volume was estimated from HR. Oxygen content difference was calculated from hematocrit and SmO₂ obtained with NIRS. Pulmonary and NIRS VO₂ were compared by Bland-Altman analysis. AT was identified from spectrally determined pH³.

Results: SV was gender specific and a mathematical equation was developed to calculate SV from HR during exercise. Our data showed that NIRS VO₂ closely approximated pulmonary VO₂ up to the AT. The bias between pulmonary and NIRS-measured VO₂ was -0.05 L/min and the limits of agreement were -0.6 and +0.5 L/min; R² = 0.89. Larger errors were observed for VO₂> AT.

Conclusion: Our results demonstrate the feasibility of using NIRS to determine VO₂for preoperative risk assessment.

References:
1. Older P, et al. Chest. 1999, 116:355-362.
2. Grassi et al. J Appl Physiol. 1996, 80:988-998.
3. Soller BR, et al. J Appl Physiol.2008, 104:837-844.

Funded by FAER and NSBRI through NASA NCC9-58.