Running Or Bleeding? What Should We Measure to Assist Remote Triage Decisions in a Military Environment?

Abstract

Traditional physiological measurements available to a combat medic do not differentiate between a wounded soldier and an active soldier. Although not a major concern for the medic with physical access to their patients, the inability to triage from a remote location carries the risk that a medic may be placed in harm's way attempting to assist a soldier who may not be injured or may be unsalvageable. PURPOSE: To compare changes in the ratio of high-frequency to low-frequency (HF/LF) R-R interval spectral power during simulated hemorrhage with those of exercise in order to establish the potential utility of remotely differentiating active from bleeding soldiers. The HF/LF was used because of its association with trauma patient outcome. METHODS: Nine healthy volunteer subjects were exposed to progressive (5 stages) lower body negative pressure (LBNP) to −70 mmHg. Beat-to-beat heart rate (HR) blood pressure (BP) and stroke volume (SV) were measured continuously. On a separate day, subjects completed a supine cycle ergometer protocol. The workload for exercise was determined by matching HR responses to each of the LBNP levels. Data were analyzed in both time and frequency domains. RESULTS: HR increased from 65 ± 4 bpm to 113 ± 4 bpm by −70 mmHg LBNP (p ≤0.001). HR was successfully matched within 5% of LBNP measurements during exercise. As HR increased with the progression of LBNP and exercise, pulse pressure (PP) and SV responses diverged. By the final stage of LBNP, PP and SV decreased by 36 ± 4% and 50 ± 4% (p ≤0.001) compared with increases of 35 ± 7% and 18 ± 10% during exercise (p ≤0.001). The HF/LF ratio decreased from baseline during both LBNP (134 ± 42 ms2 to 41 ± 23 ms2) and exercise (234 ± 99 ms2 to 59 ± 17 ms2). CONCLUSION: The HF/LF ratio cannot by itself distinguish conditions of exercise and reduced central blood volume. However, monitoring of PP and SV would clearly distinguish a bleeding from an active soldier. Future technologies that allow telemetry of PP and SV or their surrogates would provide a combat medic with remote decision support to assess soldier status on the battlefield.