Prevalence Of Tachycardia During Sudden Immersion In Cold Water In An Outdoor Setting

Sudden immersion in cold water (CWI) induces a rapid onset of the cold shock response (CSR), which is suggested to account for more than half of CWI deaths in outdoor settings. CSR may include an initial inspiratory gasp, tachycardia, vasoconstriction, hypertension, and hyperventilation. CSR has primarily been measured in laboratory environments with relatively small sample sizes, yet CSR prevalence has not been adequately described in large populations during outdoor CWI. The current work focuses on determining the prevalence of one CSR element, tachycardia, during outdoor CWI. PURPOSE: Determine the prevalence of tachycardia during outdoor CWI and examine the relationship between pre-immersion heart rate (HR) and in-water tachycardia. METHODS: Seventy-seven (N = 77) military personnel (mean ± SD age: 26 ± 6 yrs; ht: 176 ± 8 cm; wt: 81 ± 12 kg) completed outdoor CWI (air: - 4.2 °C). Prior to immersion (PRE) participants stood next to a pond (water: 1.3 °C) for 10 min until they were instructed to enter the water (clothed in non-cold weather military uniform). Participants entered the water (IMM) and remained immersed to the level of the neck for 10 min, after which they egressed from the water and began rewarming. HR was measured each minute before and during CWI. Frequency of tachycardia (HR ≥ 100 bpm) during CWI was analyzed and participants were grouped based on presence (TAC; n = 61) or absence (NSR, normal sinus rate; n = 16) of tachycardia. Mean HR responses for HRPRE, HRIMM, and HRPEAK were compared among groups using non-paired T-tests with significance at p < .05. RESULTS: Tachycardia during CWI was present in 79% of participants. HRPRE (mean ± SD) was 19 beats higher in TAC (98 ± 16 bpm) vs NSR (79 ± 9 bpm), p < .001; HRIMM was 21 beats higher in TAC (105 ± 12 bpm) vs NSR (84 ± 10 bpm), p < .001; and HRpeak (mean ± SD) was 32 beats higher in TAC (123 ± 14 bpm) vs NSR (92 ± 8 bpm), p < .001. Additionally, the HRPEAK - HRPRE difference was two times higher in TAC (25 ± 14 bpm) vs NSR (13 ± 7 bpm), p < .001. CONCLUSIONS: Findings suggest that most individuals participating in outdoor CWI experienced tachycardia. Those with elevated HR prior to immersion sustain higher HR responses throughout CWI, possibly due to acute anxiety of anticipated CWI. Efforts to reduce anxiety prior to CWI may alleviate cardiovascular stress during CWI and reduce the magnitude of CSR.