Is the use of non-invasive haemoglobin measurements accurate and realiable during the transfer of sick and injured patients utilising air ambulance aircraft?

Introduction: Traditional methods for measuring haemoglobin require the drawing of blood and lab analysis, which is not an optimally timely or a risk-free process in the aeromedical transport environment. Technology that allows non-invasive and continuous monitoring of haemoglobin levels became commercially available in 2008. If this technology is not affected by altitude or the stressors of flight, its use would be advantageous for both patients and medical staff during aeromedical transfers. Aim: This study set out to determine the precision and accuracy of a specific method of non-invasive haemoglobin (Hb) measurement and monitoring during aeromedical retrievals with a focus on the effects of altitude and flight cabin changes. Method: This was a simple interventional before and after study design with volunteer subjects exposed to atmospheric pressure changes first while reference haemoglobin concentrations were measured. Subjects were taken rapidly to altitude in a hypobaric chamber to simulate aircraft cabin pressure (6,000 feet and 12,000 feet). Non-invasive Hb measurements were recorded every 10 seconds at both 6,000 feet and then at 12,000 feet. Ground (sea level) measurements of non-invasive and invasive Hb were also recorded for each participant. Statistical analysis compared the invasive laboratory Hb taken at sea level and the non-invasive measurement recorded at ground level and at altitude, using paired measurement methods of comparison to assess limits of agreement. Results: A total of 64 subjects took part in the study. The mean difference (SD) between lab Hb and non-invasive Hb at ground level was -3.36 (12.87) g/L. A Bland-Altman bias plot showed that at relatively low Hb values the non-invasive measure tends to overestimate Hb, and at higher levels there is a tendency to underestimate Hb. In general, there was a small negative bias for the non-invasive (test) measure. For the measurements taken at 6,000 feet the mean (SD) difference between lab and non-invasive Hb was -8.64 (12.93) g/L, while at 12000 feet it was -13.15 (17.52) g/L, though at this higher ‘altitude’ level, there was a smaller sample. A strong relationship was noted between degree of perfusion as assessed by the spectrophotometric test device at the sample site and low or high non-invasive Hb measurements. Discussion: This is the first study we are aware of which examines effects of altitude on the accuracy of the test device, and we have found that with increasing altitude, there were small but systematic increases in the test measurement of Hb concentration. At altitude the non-invasive measurement tends to overestimate Hb in cases where Hb levels are lower, and conversely when levels of Hb are higher there is a tendency to underestimate Hb. These results are consistent with other studies analysing non-invasive Hb in different areas of medicine. Conclusion: I conclude from these findings that the use of this new non-invasive Hb measuring technology during aeromedical retrievals shows reliability even with changes in cabin pressure which allows assessment of clinical Hb levels, and particularly to confirm approximate normal Hb levels, though not with precision and accuracy which is needed for scientific (reference) measurements. Non-invasive continuous haemoglobin monitoring, while lacking precision, is largely unaffected by the aviation environment, and has clinical application during the unusual conditions experienced during aeromedical retrieval. At least, it is no worse than traditional invasive methods, and provides clinical information that is timelier than periodic sampling. Moreover, the elimination of the need to take frequent blood samples reduces the risks to both the patient and aeromedical attendant. To have real-time continuous Hb measurements available during aeromedical retrievals can improve medical safety for the patient and medical crews