Effects of thermoregulatory vasoconstriction on pulse hemoglobin measurements using a co-oximeter in patients undergoing surgery

Study Objective: To validate intraoperative pulse hemoglobin (SpHb) measurements in anesthetized patients with large forearm temperature - fingertip temperature gradients. Design: prospective and observational study. Setting: Operating room of a university hospital. Patients: 28 patients undergoing surgery during general anesthesia, requiring arterial blood withdrawal. Interventions: Radial arterial blood pressure, forearm and fingertip skin surface temperatures, and SpHb were monitored. Measurements: Paired SpHb and arterial hemoglobin (Hb) measurements at different skin-surface temperature gradients. Main Results: A total of 175 paired SpHb and arterial Hb measurements were analyzed. The mean SpHb to arterial Hb differences in each group were 0.33 ± 1.41 g/dL in the < 1°C group of the forearm temperature - fingertip temperature gradient, -0.31 ± 1.24 g/dL in the 1 - 2°C group, - 0.59 ± 1.11 g/dL in the 2 - 3°C group, and - 0.53 ± 0.87 g/dL in the > 3°C group (P < 0.05). The percentage of nonmeasurable SpHb due to low perfusion state was 0% (0 of 115 paired measurements) in the < 1°C group, 6.7% (2 of 30 pairs) in the 1 - 2°C group, 16.7% (3 of 18 pairs) in the 2 - 3°C group, and 66.7% (8 of 12 pairs) in the > 3°C group. Conclusion: SpHb measured at fingertip was significantly affected by the perfusion state, with lower perfusion associated with lower SpHb. Thermoregulatory vasoconstriction affects measurement of SpHb.