Non-invasive Perioperative Hb Monitoring in Spinal Surgery

The decision to transfuse or not to transfuse blood products is one of the main issues in healthcare systems. Timely transfusion of blood to a patient bleeding, either due to the surgery or because of the trauma, can prevent tissue hypoxia and ischemia in vital organs. However, transfusion of blood products should be done carefully and cautiously due to the potential complications of transfusions, and also because of the cost of providing blood products 1, 2, 3. Hence, during surgeries, hemoglobin level has a vital role in the diagnosis and management of patients with anemia.

The standard laboratory method of hemoglobin assessment is time-consuming, gives intermittent data, and requires venipuncture which is invasive and painful. In the past decade, the use of non-invasive and faster methods, which allow physicians to measure hemoglobin levels at the patient's bedside, have become widespread. One of the tools that make this possible is Spectrophotometric Hemoglobin (SpHb Masimo Rainbow Co-oximeter). This procedure is applied by a digital probe and allows physicians to monitor hemoglobin levels continuously. 4, 5, 6 However, one of the main concerns regarding this method is its accuracy. Actually, the difference between the measured hemoglobin level and the actual hemoglobin level of the patient, might make it impossible for physicians to decide with sufficient confidence3.

From the time that SpHb has been available in operating rooms, various studies have evaluated its accuracy. While its accuracy has been reported relatively acceptable (mean difference is about 1-1.5 g/dl comparing to the lab data),4 there are still concerns regarding this issue. Furthermore, it is not clear whether this difference would remain the same during the surgery or not. Because blood transfusion is rarely needed when the hemoglobin level of patient is higher than 10g/dl during the surgery. Also, blood transfusion becomes obviously necessary if hemoglobin levels drop to 6g/dl or lower during the surgery. Therefore, the accuracy of the tool in assessing hemoglobin levels at this critical interval (6-10g/dl) is of greater importance.6,7 Experimental studies have confirmed that accuracy could change in different hemoglobin levels5,8, and with the passage of time.9 In addition, it has been reported that after significant blood loss or even following injection of blood, the average difference between SpHb and laboratory might change, as well. 5,8 Since the accuracy of this assessment depends on the extent of perfusion of the organ on which the probe is placed, methods that enhance blood circulation in the organ (including local heating of the organ and use of digital nerve blocks (DNB), proposed to increase its accuracy.10, 11 In a study conducted in 2012 by Miller et al. on 20 patients undergoing spinal surgery, DNB was performed using lidocaine. The differences between SpHb and lab hemoglobin were recorded first after injection, and then at almost hourly intervals. These differences were compared to data from another study without DNB. Thirty seven percent of data in the intervention group and 12% of data in the control group was considered as "very accurate". Results of this study showed that DNB increased perfusion indices and SpHb accuracy.10 Following the previous study, Miller et al. carried out a study in 2014 to compare the effects of DNB with lidocaine and bupivacaine on 12 healthy volunteers. DNB was performed on two identical fingers on both hands of the volunteers. Lidocaine was used on one hand and bupivacaine on the other one. Results of this study showed that both lidocaine and bupivacaine raised the perfusion indices and temperatures of the fingers. The duration and extent of the rise in temperature were more for bupivacaine, and there was a direct correlation between changes in perfusion index and those in SpHb.11 Since, the previous study data had been gathered from healthy individuals, this study aims at investigate the effects of DNB on the accuracy SpHb in patients undergoing surgery.