Cardiac autonomic function during hypothermia and its measurement repeatability

Abstract

This study examined the effect of mild hypothermia (a 0.5°C decrease in rectal temperature) on heart rate variability (HRV), with the identical hypothermia protocol performed twice and compared using intraclass correlation coefficient (r) analysis to study the repeatability. Twelve healthy males each completed a Neutral (23°C) and two Cold (0°C) trials. In the Neutral trial participants sat quietly for 30 min. In the Cold trials, baseline data were obtained from a 5-min sample following 30 min of quiet sitting at 23°C, followed by passive exposure to 0°C; hypothermic measures were taken from a 5-min period immediately prior to rectal temperature decreasing by 0.5°C. HRV was obtained from a three-lead ECG. There were no differences (all p>0.05) in baseline measures between the Neutral and the two Cold trials, suggesting no pre-cooling anxiety related to the Cold trials. Heart rate along with HRV measures RMSSD, TINN, LF, and HF increased (all p<0.05) with mild hypothermia and showed excellent reliability between the two Cold trials (all r≥0.81). In contrast, HF/LF ratio decreased (p<0.05) and had only fair reliability between the two Cold trials (r=0.551). In general, hypothermia led to increases in heart rate along with most measures of HRV. While counter-intuitive that both sympathetic and vagal influence would increase simultaneously, these changes likely reflect increased stress from whole body cooling along with marked cardiovascular strain and sympathetic nervous system activity from shivering to defend core body temperature. An important methodological consideration for future studies is the consistent and repeatable HRV responses to hypothermia.

Diving in the Arctic: Cold Water Immersion’s Effects on Heart Rate Variability in Navy Divers

“…It is a combination of both surface and deep body temperature, and it takes into account changes in blood redistribution. Cold is a known promoter of PNS activity (Vesoulis et al, 2017;Hodges et al, 2019). Since MBT decreased during the dives we would assume that temperature is an important factor in inducing the increase in PNS activity over time.…”

Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress

“…This autoregulatory mechanism in the core (cerebral) vasculature is widely different from systemic autoregulation and has been quantitatively investigated in various ways. The methods commonly used to study SNA include plasma kinetic analysis of the sympathetic neurotransmitters—norepinephrine and epinephrine (Reed et al, 1989; Esler and Kaye, 2000; Guild et al, 2010; Seravalle et al, 2013), direct recordings of neural activity from intrafascicular electrodes implanted in muscles (microneurography) (Anderson et al, 1989; Ramchandra et al, 2008; Van Vliet et al, 2008; Hart et al, 2017), heart rate variability analysis of electrocardiograms (ECG) (Appelhans and Luecken, 2006; Thayer et al, 2012; Deng et al, 2018; Hodges et al, 2018), low frequency analysis of impedance cardiography (Burgess et al, 2004), and analysis of the low-frequency oscillation in Photoplethysmograms (Nitzan et al, 1994, 2009; Penzel et al, 2002). However, these studies have been inconclusive, and there is a lack of PPG related studies investigating SNA.…”

Commentary: Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress

“…Regarding the first point, Budidha and Kyriacou (2018, p. 2) base their goal to assess SNA using HRV on four articles. "The methods commonly used to study SNA include [...] HRV analysis of ECG (Appelhans and Luecken, 2006;Thayer et al, 2012;Deng et al, 2018;Hodges et al, 2019) [...]." However, none of these papers support the author's interpretation of the LF/HF "to quantify the changes in the sympathetic activity" (Budidha and Kyriacou, 2018, p. 5), but refer to it as reflecting the HRV assessment of the "sympathovagal balance."…”

“…Regarding the first point, Budidha and Kyriacou ( 2018 , p. 2) base their goal to assess SNA using HRV on four articles. “The methods commonly used to study SNA include [...] HRV analysis of ECG (Appelhans and Luecken, 2006 ; Thayer et al, 2012 ; Deng et al, 2018 ; Hodges et al, 2019 ) [...].” However, none of these papers support the author's interpretation of the LF/HF “to quantify the changes in the sympathetic activity” (Budidha and Kyriacou, 2018 , p. 5), but refer to it as reflecting the HRV assessment of the “sympathovagal balance.” Previous research indicates that the LF/HF ratio does not bear a significant correlation with SNA (Saul et al, 1990 ; Kingwell et al, 1994 ; Van de Borne et al, 1997b ). The LF band (0.04–0.15 Hz) is produced by both the sympathetic and the parasympathetic nervous system (Akselrod et al, 1981 ; Berntson et al, 1997 ).…”

“…Finally, regarding the last point, evidence suggests that cold water immersion causes hyperventilation (Tipton, 1989 ; Eglin and Tipton, 2005 ; Mantoni et al, 2008 ). As mentioned by Hodges et al ( 2019 ), the frequency domains of HRV are especially vulnerable to bias in cold pressor tests because of their dependency on the participants' breathing pattern. HF, calculated in the range 0.15–0.4Hz, is expected to reflect cardiac vagal tone when breathing frequency is comprised between 9 and 24 cycles per minute (Malik et al, 1996 ; Berntson et al, 1997 ; Laborde et al, 2017 ).…”