Near-Infrared Spectroscopy Hemoglobin Index Measurement During Fluid Challenge: A Prospective Study in Cardiac Surgery Patients

ObjectiveLittle is known about changes in near-infrared spectroscopy-derived tissue hemoglobin index (HbI). The authors tested the hypothesis that absolute values and changes in brain hemoglobin index (HbIb) and skeletal muscle hemoglobin index (HbIm) could differ from the reference arterial hemoglobin (Hb) during fluid challenge.DesignA prospective, monocenter observational study.SettingA 16-bed cardiac surgical intensive care unit in a teaching university hospital.ParticipantsFifty consecutive adult patients.InterventionsInvestigation before and after a fluid challenge.Measurements and Main ResultsSimultaneous comparative Hb, HbIb and HbIm data points were collected from a blood-gas analyzer and the EQUANOX device (Nonin Medical Inc., Plymouth, MN). Correlations were determined by linear regression. No significant relationship was found between absolute values of Hb and HbIb before (R2= 0.04, p = 0.627) and after (R2 = 0.00006, p = 0.956) fluid challenge. No significant relationship was found between absolute values of Hb and HbIm before (R2= 0.030, p = 0.226) and after (R2 = 0.05, p = 0.117) the fluid challenge. No significant relationship was found between changes in Hb and HbIb (R2= 0.26, p = 0.263) and between changes in Hb and HbIm (R2 = 0.001, p = 0.801) after the fluid challenge. Bland-Altman analysis showed a poor concordance between changes in Hb and HbIb, and changes in Hb and HbIm, with large limits of agreement.ConclusionsHbIb and HbIm cannot be used to provide continuous noninvasive estimation of Hb, and trends in HbIb and HbIm cannot be considered as noninvasive surrogates for the trend in Hb after cardiac surgery.