A comparison of non-invasive versus invasive methods of haemoglobin estimation in patients undergoing intracranial surgery

Background: Until recently, invasive monitoring techniques were the only ones available for the estimation of haemoglobin (Hb) in the blood. However, following the introduction of a new non-invasive monitor, Hb concentration could be non-invasively and continuously monitored. It allows immediate and ongoing Hb changes to be displayed during surgery, which may aid in the rapid detection of clinically significant blood loss. To test the accuracy of this non-invasive monitor, we compared Hb levels obtained using standard invasive techniques (laboratory and arterial blood gas machine analysis) with those derived from a non-invasive monitor. Method: Thirty patients undergoing various intracranial surgeries were enrolled in the study. Blood samples were withdrawn for Hb estimation from the laboratory [total haemoglobin mass (tHb)] and arterial blood gas (ABG) machine (aHb), using automated Hb analyser techniques randomly at any point during the surgery. At the same time, the Hb values displayed on the non-invasive monitor [Radical-7 (R) Pulse Co-oximeter (SpHb (R))], together with the perfusion index (PI), were also noted. Results: The perfusion index (PI) was less than 1.4 in eight patients. The obtained Hb values were unreliable, as per the manufacturer's recommendations. Statistical analysis showed poor correlation between the invasive and non-invasive techniques of Hb estimation in the remaining 22 patients. Conclusion: Non-invasive Hb monitoring (SpHb (R)) may not have sufficient accuracy to minimise the need for invasive Hb monitoring. However, it may allow continuous monitoring of Hb and could guide clinicians as to the need for invasive monitoring.