Non-invasive ‘point of care’ haemoglobin monitoring coming closer

’Point of care’ monitoring of haemoglobin levels is nearing reality in operating rooms as a spectrophotometric haemoglobin sensor is providing a new approach to non-invasive monitoring of blood haemoglobin levels during surgery.

The spectrophotometric haemoglobin (SpHb) technology could reduce the need for invasive monitoring or the need for blood transfusion during surgery. But further development will be needed to make SpHb sufficiently accurate for clinical use, according to the study by Dr Ronald D Miller of University of California, San Francisco.

The researchers evaluated SpHb for continuous monitoring of blood haemoglobin levels in 20 patients undergoing spine surgery with general anaesthesia. Similar to the familiar ‘finger clip’ pulse oximeter, SpHb works by shining different light wavelengths through blood-perfused tissues. However, the SpHb monitor provides more detailed information on a number of different blood variables, including haemoglobin. During major surgery, regular measurements of haemoglobin - the oxygen-carrying compound in the blood - are made to assess the need for blood transfusion.

Haemoglobin was also measured in blood samples, using both a standard invasive laboratory test and a newer point of care device called HemoCue, which provides rapid results in the operating room. The accuracy of the SpHb was compared with these standard approaches, based on different cut-off points for transfusion decisions. (Transfusions may be needed when haemoglobin is less than 10 g/dL and are almost always needed at less than 7 g/dL.) The study, reported in the April issue of Anesthesia & Analgesia, official journal of the International Anesthesia Research Society (IARS), included a total of 78 comparisons.

The SpHb monitor was fairly accurate, compared to the standard laboratory test. The difference in haemoglobin levels was less than 1.5 g/dL for 61% of comparisons, but more than 2.0 g/dL in 22%. The SpHb technique tended to be more accurate in longer surgical procedures and at higher levels of tissue blood perfusion. In contrast, the HemoCue measurements were almost always within 1.0 g/dL of the standard laboratory test.

The ability to measure haemoglobin without the need for a blood sample would help to simplify patient monitoring during surgery. The new results show that SpHb is a fairly good indicator of haemoglobin levels, compared to standard tests. “Yet our study indicates that SpHb may not be as accurate as clinically necessary in some patients,” Dr Miller and co-authors write.

“Improved refinement of continuous, non-invasive technology, such as SpHb, could address important clinical requirements,” they add. In the meantime, the researchers suggest that SpHb might be used to guide the timing of direct measurement of haemoglobin in blood samples.

The International Anesthesia Research Society is a non-political, not-for-profit medical society founded in 1922 to advance and support scientific research and education related to anaesthesia, and to improve patient care through basic research. Additional information about the society and the journal may be found at www.iars.org and www.anesthesia-analgesia.org.