Cardiac Interbeat Intervals To Assess Combat Readiness.

Abstract

PURPOSE: Chronic fatigue/physical exhaustion (FPE) impacts combat readiness but is difficult to identify. We tested the hypothesis that resting cardiac interbeat intervals, including both time- and frequency-domain assessments of heart rate variability (HRV), would correlate with hydration status and aerobic capacity in military recruit-age men and women with varying fitness levels. METHODS: Cardiac interbeat intervals were recorded using a Polar R-R monitor during 20 minutes of quiet, supine rest with paced breathing (0.25 Hz). HRV metrics included: R-R intervals (RRI); R-R interval standard deviation (RRISD); the percentage of adjacent R-R intervals varying by ≥ 50 ms (pNN50); and integrated areas of R-R interval spectral power at the high (0.15 - 0.4 Hz; RRIHF) and low (0.04 - 0.15 Hz; RRILF) frequencies. Treadmill VO2 peak; segmental bioimpedance estimates of total body water (TBW); and urine specific gravity (USG) were also assessed. RESULTS: All dependent variables of interest were within expected ranges, although absolute ranges of individual values were considerable. RRI correlated with VO2 peak (r = 0.49; p < 0.001), TBW (r = 0.38; p < 0.001) and inversely with USG (r = -0.23; p = 0.02). RRISD correlated with VO2 peak (r = 0.21; p = 0.03), but not with TBW or USG. pNN50 correlated inversely with urine specific gravity (r = -0.21; p = 0.03) but not VO2 peak or TBW. R-R interval spectral power at the high and low frequencies correlated poorly with VO2 peak, TBW, and USG. CONCLUSIONS: These results provide direction as to how fitness level and hydration status may affect cardiac function via changes in autonomic tone, highlighting the potential utility of applying field-based assessment of various interbeat interval metrics to identify FPE and other aspects of combat readiness.