Feasibility of Using Near Infrared Spectroscopy in Determining VO₂ for Preoperative Risk Assessment

Abstract

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.