A Novel Cardiac Force Index Measured by Digital Devices and Applied to the G Tolerance Test among Military Aircrew: An Observational Study (Preprint)

BACKGROUND Military aircrew are occupationally exposed to a high-G environment. G force causes blood to flow to the lower body region and challenges their cardiac function and anti-G straining maneuver effectiveness. A tolerance test is necessary for every military aircrew member before undergoing flight training. A novel cardiac force index (CFI) has been developed and used to assess long-distance running by mobile health (mHealth) technology. There is still no study to monitor the CFI by wearable devices during the G tolerance test. OBJECTIVE We added the cardiac function parameter CFI to the G tolerance test and elucidated the relationship between cardiac function and G tolerance among military aircrew. METHODS A noninvasive device, BioHarness 3.0, was used to measure heart rate (HR) and activity while resting and walking on the ground. The mathematical formula for cardiac function calculation is CFI = weight × activity/HR. The cardiac force ratio (CFR) is calculated by walking CFI (WCFI)/resting CFI (RCFI). G tolerance includes relaxed G tolerance (RGT) and straining G tolerance (SGT) tested by a human centrifuge under the gradual-onset-rate profile. RESULTS In total, 92 male participants voluntarily completed this study. The average values of RCFI, WCFI, and CFR were 0.02 [SD 0.04], 0.15 [SD 0.04], and 10.77 [SD 4.11], respectively. The mean RGT and SGT were 5.1G [SD 0.9] and 7.8G [SD 1.1], respectively, in the centrifuge. The percentages of participants with RGT greater than 5G or SGT greater than 8G were equally noted as 54.3%. Each 100-unit increase in WCFI increased RGT by 0.14G [SE 0.02, 95% CI 0.09 to 0.19] and by 0.17G [SE 0.03, 95% CI 0.11 to 0.22], corresponding to SGT. In addition, there was an increased chance of RGT values higher than 5G and SGT values higher than 8G according to the increase in WCFI. CONCLUSIONS Our results suggested that WCFI is positively correlated with G tolerance in the centrifuge and has the potential to be used for military aircrew selection.