Background Combat-related traumatic injury (CRTI) has been linked to an increased cardiovascular disease (CVD) risk in servicemen returning from military operations. While Heart Rate Variability (HRV) has been established as an objective and non-invasive marker of CVD risk, the long-term impact of unselected CRTI on HRV has not been explored to date. This study aimed to investigate the impact of CRTI, injury mechanism and injury severity on HRV. We hypothesised that HRV would be lower in injured servicemen than in uninjured servicemen.

Methods This was a baseline analysis from the ArmeD SerVices TrAuma and RehabilitatioN OutComE (ADVANCE) prospective cohort study. The sample consisted of British servicemen with CRTI sustained during deployment to Afghanistan (2003–2014) and an uninjured comparison group who were frequency-matched to the injured group based on age, rank, deployment period and role-in-theatre. Root mean square of successive differences between normal heartbeats (RMSSD) was reported as a measure of HRV. RMSSD was measured over an ultra-short time period (HRV_UST) of up to 16s continuous recording of the femoral arterial pulse waveform signal via the Vicorder device. Other measures included the New Injury Severity Scores (NISS) as a measure of injury severity and injury mechanism (blast and others).

Results Overall 862 participants aged 33.9±5.4 years (range 23–59 years) were included, of who 428 (49.6%) were injured and 434 (50.3%) were uninjured. The median (interquartile range) NISS for those injured was 12 (6–27; range 1–66) with blast being the predominant injury mechanism (76.8%). The median (IQR) RMSSD was significantly lower in the injured versus the uninjured [39.47 ms (27.77–59.77) vs 46.22 ms (31.14–67.84), p<0.001]. Using multiple linear regression (adjusting for age, rank, ethnicity and time from injury), CRTI was associated with a 13% lower geometric mean ratio (GMR) RMSSD versus the uninjured group (GMR 0.87, 95%CI 0.80–0.94, p<0.001). A higher injury severity (NISS>25) (GMR 0.78, 95%CI 0.69–0.89, p<0.001) and blast mechanism of injury (GMR: 0.86, 95%CI 0.79–0.93, p<0.001) were also independently associated with lower RMSSD.

Conclusion These results suggest an inverse association between CRTI, its worsening severity and blast mechanism of injury with HRV. These findings may help understand the cardiovascular profile of the population with traumatic injuries. Further research from longitudinal studies and examination of the potential mediating factors in this CRTI-HRV relationship are needed to better understand the cardiovascular profile of the military personnel.