Perioperative Calibration of Noninvasive Hemoglobin Monitoring

To the Editor The articles by Park et al.1 and Sjöstrand et al.2 deal with the accuracy of the noninvasive hemoglobin (SpHb) measurement of the Masimo Radical 7 monitor. While we agree that SpHb may be of value as a trend monitor to decrease the number of invasive measurements, appropriate timing of the determination of the baseline value is essential. Although SpHb readings are stable after mere seconds in steady-state conditions, induction of general anesthesia or variations in fractions of inspired oxygen (FIO2) in our experience almost systematically induce an increase in SpHb (Fig. 1), often by as much as 40%, even in ASA physical status I and II patients scheduled for elective surgery.Figure 1: Change of SpHb during induction of general anesthesia in a typical patient. TCI = target-controlled infusion; FIO2 = fractions of inspired oxygen.Likewise, the methodology described by Sjöstrand et al.2 of using SpHb may certainly be of interest for pharmacokinetic studies, but if it were to be translated to clinical situations, the sources of drift in SpHb should be addressed first. Massive fluid resuscitation in emergency settings is often accompanied by induction of general anesthesia and changes in FIO2. Colloid administration may even independently decrease SpHb accuracy,3 casting additional bias. While changes in perfusion index affect signal reliability, our observed systematic increase in SpHb in patients with only minor comorbidity and adequate perfusion index suggests there may be a more fundamental causality. This may be either related to concomitant changes in FIO2 and thus peripheral oxyhemoglobin saturation or to vasodilation following induction of anesthesia. Determination of the baseline value (and in newer models manual recalibration of the device) using an invasive gold standard assessment would likely be performed early during anesthesia, often even before induction of anesthesia. We suggest, however, that this (virtual) calibration should be performed only after both steady-state anesthesia and FIO2 are reached and SpHb is stable over several minutes. Moreover, since the sources of drift in SpHb during induction of anesthesia are not yet fully understood, we recommend reassessment of the reference value of SpHb if major changes in hemodynamic and/or ventilatory conditions occur. Patient Consent Statement: Use of observational data approved by the Institutional Review Board METc 2011.052, University Medical Center Groningen, The Netherlands. Alain F. Kalmar, MD, PhD Marieke Poterman, MD Thomas WL Scheeren, MD, PhD Department of Anesthesiology University of Groningen University Medical Centre Groningen Groningen, The Netherlands [email protected]