Autonomic Nervous System Responses to Aviation Live-Fire Combat Training: An Effective Tool for Operator State Monitoring of United States Warfighter Aircrew.

BACKGROUND: Warfighters perform highly complex and dangerous combat missions are both physically and cognitively challenging. Meeting these physiological demands are regulated by the autonomic nervous system (ANS). Thus, continuously monitoring ANS activity during combat and training may critically inform military leadership of the current physiological state and ability of their warfighters to effectively execute mission tasks. The purpose of this study was to quantify ANS response throughout 540 minutes (9 hours) of aviation-related combat training drills performed in the UH-60 helicopter.

MATERIALS AND METHODS: Aircrew members recruited from the United States Army Aeromedical Research Laboratory participated in the current study. ANS function was measured using the Warfighter Monitor (WFM, Tiger Tech Solutions, Inc., Miami, FL, United States), an armband monitor equipped with electrocardiographic capabilities. Subjects wore the WFM on the upper arm throughout their 540-minute combat training which consisted of aviation-related tasks. The WFM continuously monitored heart rate (HR), HR variability (HRV), and respiration rate (RR). Two HRV time-domain metrics were calculated: SD of the NN interval (SDNN) and the root mean square of successive differences of NN intervals (rMSSD). Studentized t-tests were performed to evaluate within- and between-group differences in ANS activity.

RESULTS: Thirty-four healthy, aircrew members aged between 18 and 45 years and mostly male, participated in the current study. For pre-flight events, average HRs ranged between 76.1 and 91.2 bpm. On average, HRs increased between +10.2 and +12.9 bpm. HRV metrics were between 72.4 and 91.3 milliseconds for rMSSD and from 101.8-139.9 ms for SDNN. RRs for aircrew members ranged between 12.4 and 13.5 breaths/min and increased by +1.4 to +1.7 breaths/min. For in-flight operations, significantly higher HRs (88.2-100.2 bpm) were observed. Similarly, significantly different increases were reported (+14.7 to +26.4 bpm). HRV metrics were significantly lower (rMSSD: 62.8-83.7 milliseconds; SDNN: 93.8-116.7 milliseconds). The average and increases in RRs were significantly higher, 12.8-17.3 breaths/min and +1.5 to +4.2 breaths/min.

CONCLUSIONS: Significant increases in sympathetic drive were observed in U.S. warfighter aircrew during combat mission training, reflected by higher HRs, RRs, and lower HRV. The magnitudes in the ANS response, however, differed between pre- and in-flight operations with the latter inducing a larger response, possibly attributed to greater physical and cognitive workloads. These observations may aid warfighters and military leadership in designing effective preparedness training and implementing immediate intervention for managing the heightened sympathetic drive while preventing negative outcomes.