Changes in electrocardiogram parameters during acute nonshivering cold exposure and associations with brown adipose tissue activity, plasma catecholamine levels, and brachial blood pressure in healthy adults

Physiological ReportsVolume 9, Issue 3 e14718 ORIGINAL ARTICLEOpen Access Changes in electrocardiogram parameters during acute nonshivering cold exposure and associations with brown adipose tissue activity, plasma catecholamine levels, and brachial blood pressure in healthy adults Juho R. H. Raiko, Corresponding Author Juho R. H. Raiko juho.raiko@utu.fi orcid.org/0000-0001-8879-4738 Turku PET Centre, Turku University Hospital, Turku, Finland Correspondence Juho R. H. Raiko, Turku PET Centre, University of Turku, c/o Turku University Hospital, P.O. Box 52, 20521 Turku, Finland. Email: juho.raiko@utu.fiSearch for more papers by this authorTeemu Saari, Teemu Saari Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this authorJanne Orava, Janne Orava Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this authorNina Savisto, Nina Savisto Turku PET Centre, University of Turku, Turku, FinlandSearch for more papers by this authorRiitta Parkkola, Riitta Parkkola Turku PET Centre, Turku University Hospital, Turku, Finland Department of Radiology, Turku University Hospital and University of Turku, Turku, FinlandSearch for more papers by this authorMerja Haaparanta-Solin, Merja Haaparanta-Solin Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this authorPirjo Nuutila, Pirjo Nuutila Turku PET Centre, Turku University Hospital, Turku, Finland Department of Endocrinology, Turku University Hospital, Turku, FinlandSearch for more papers by this authorKirsi A. Virtanen, Kirsi A. Virtanen Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this author Juho R. H. Raiko, Corresponding Author Juho R. H. Raiko juho.raiko@utu.fi orcid.org/0000-0001-8879-4738 Turku PET Centre, Turku University Hospital, Turku, Finland Correspondence Juho R. H. Raiko, Turku PET Centre, University of Turku, c/o Turku University Hospital, P.O. Box 52, 20521 Turku, Finland. Email: juho.raiko@utu.fiSearch for more papers by this authorTeemu Saari, Teemu Saari Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this authorJanne Orava, Janne Orava Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this authorNina Savisto, Nina Savisto Turku PET Centre, University of Turku, Turku, FinlandSearch for more papers by this authorRiitta Parkkola, Riitta Parkkola Turku PET Centre, Turku University Hospital, Turku, Finland Department of Radiology, Turku University Hospital and University of Turku, Turku, FinlandSearch for more papers by this authorMerja Haaparanta-Solin, Merja Haaparanta-Solin Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this authorPirjo Nuutila, Pirjo Nuutila Turku PET Centre, Turku University Hospital, Turku, Finland Department of Endocrinology, Turku University Hospital, Turku, FinlandSearch for more papers by this authorKirsi A. Virtanen, Kirsi A. Virtanen Turku PET Centre, Turku University Hospital, Turku, FinlandSearch for more papers by this author First published: 13 February 2021 https://doi.org/10.14814/phy2.14718 Funding information: This study was financially supported by the Academy of Finland Center of Excellence in Cardiometabolic Research, EU project DIABAT, Southwestern Finland Medical District funding, Turku University Hospital Foundation, Outpatient Care Research Foundation, Päivikki and Sakari Sohlberg Foundation, Jalmari and Rauha Ahokas Foundation, Maire Taponen Foundation, and Aarne Koskelo Foundation. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Background Sympathetic activity causes changes in electrocardiogram (ECG) during cold exposure and the changes have been studied mostly during hypothermia and less during mild acute nonshivering cold exposure. Cold-induced sympathetic activity also activates brown adipose tissue (BAT) and increases arterial blood pressure (BP) and plasma catecholamine levels. We examined changes in ECG parameters during acute nonshivering cold exposure and their associations with markers of sympathetic activity during cold exposure: brachial blood pressure (BP), plasma catecholamine levels, and BAT activity measured by positron emission tomography (PET). Methods and results Healthy subjects (M/F = 13/24, aged 20–55 years) were imaged with [15O]H2O (perfusion, N = 37) and [18F]FTHA to measure plasma nonesterified fatty acid uptake (NEFA uptake, N = 37) during 2-h nonshivering cold exposure. 12-lead ECG (N = 37), plasma catecholamine levels (N = 17), and brachial BP (N = 31) were measured at rest in room temperature (RT) and re-measured after a 2-h nonshivering cold exposure. There were significant differences between RT and cold exposure in P axis (35.6 ± 26.4 vs. 50.8 ± 22.7 degrees, p = 0.005), PR interval (177.7 ± 24.6 ms vs.163.0 ± 28.7 ms, p = 0.001), QRS axis (42.1 ± 31.3 vs. 56.9 ± 24.1, p = 0.003), and QT (411.7 ± 25.5 ms vs. 434.5 ± 39.3 ms, p = 0.001). There was no significant change in HR, QRS duration, QTc, JTc, and T axis during cold exposure. Systolic BP (127.2 ± 15.7 vs. 131.8 ± 17.9 mmHg, p = 0.008), diastolic BP (81.7 ± 12.0 vs. 85.4 ± 13.0 mmHg, p = 0.02), and plasma noradrenaline level increased during cold exposure (1.97 ± 0.61 vs. 5.07 ± 1.32 µmol/L, p = 0.001). Cold-induced changes in ECG parameters did not correlate with changes in BAT activity, brachial BP, plasma catecholamines, or skin temperature. Conclusions During short-term nonshivering cold exposure, there were increases in P axis, PR interval, QRS axis, and QT compared to RT in healthy adults. Cold-induced changes in ECG parameters did not correlate with BAT activity, brachial BP, or plasma catecholamine levels which were used as markers of cold-induced sympathetic activity. 1 INTRODUCTION Cold exposure is known to induce changes in the electrocardiogram (ECG). Short-term cold exposure results in higher T-wave amplitudes and shortening of QTc (Hintsala et al., 2014), while cold pressor test results in heart rate increase and shortening of QT interval correlating with simultaneous cold-induced sympathetic activity (Doytchinova et al., 2017). Acute cold exposure also activates nonshivering thermogenesis in brown adipose tissue (BAT) due to sympathetic activation mediated by catecholamines (Zhu et al., 2014). Catecholamines adrenaline and noradrenaline participate in sympathetic cardiac regulation during cold exposure. During hypothermia (core temperature <35°C), common ECG features are tremor artifact from shivering, initial increase in heart rate (HR) in mild hypothermia and decreased HR in more severe hypothermia, J waves, bradycardias, and prolongation of PR, QRS, and QT intervals (Drake & Flowers, 1980; Gould, 1985; Mareedu et al., 2008). Rapid immersion in cold water can trigger the autonomically mediated cold shock response which includes tachycardia, peripheral vasoconstriction, and hypertension (Tipton, 1989) and the coactivation of the sympathetic and parasympathetic autonomic nervous system can produce especially supraventricular and nodal cardiac arrhythmias (Datta & Tipton, 2006; Tipton et al., 1994). However, few studies have examined the effects of acute mild nonshivering cold exposure most studies focusing on hypothermia. Sympathetic activity increases during cold exposure (Leppäluoto et al., 2005) resulting in higher HR and blood pressure (BP) (Hanna, 1999) which may contribute to the observed peak in cardiovascular morbidity and mortality during colder months (Pell & Cobbe, 1999) although the link between sympathetic nervous system activity and mortality during cold periods remains speculative. Higher incidence of both arrhythmias (Anand et al., 2007; Frost et al., 2002) and acute myocardial infarctions have been observed during colder periods (Jia et al., 2012). As BAT activity and arterial blood pressure are regulated by the sympathetic nervous system, we find it interesting if cold-induced BAT activity, plasma catecholamine levels, and brachial blood pressure would correlate with ECG parameters measured in cold due to vascular resistance and the chronotropic, inotropic, dromotropic, and lusitropic effects of catecholamines on the heart. Thus, subjects with high BAT activity might share a similar ECG phenotype. Cold-induced increase in BAT activity and BP could be considered as surrogate markers of sympathetic activity during cold exposure, while plasma catecholamine levels would act as a more direct marker of sympathetic activity. BAT is activated by elevated plasma catecholamine levels in humans (Wang et al., 2011) and catecholamines also increase the number of brown adipocytes and upregulates uncoupling protein 1 expression in BAT (Cannon & Nedergaard, 2004), which would support the hypothesis of potential correlations between BAT activity and ECG parameters associated with sympathetic activity. Our study hypothesis is that ECG parameters associated with sympathetic activity (i.e., HR, conduction and repolarization times) would alter during acute nonshivering cold exposure in healthy adults and associate with surrogate markers of cold-induced sympathetic activity such as supraclavicular BAT activity and brachial BP and also with cold-induced changes in plasma catecholamine levels. In the current study, we examined the effects of acute 2-h mild nonshivering cold exposure on conventional resting 12-lead ECG parameters in healthy adults compared to ECG measurements performed in room temperature. Additionally, we studied associations between cold-induced changes in ECG parameters and supraclavicular BAT activity measured with positron emission tomography (PET) in room temperature (RT) and during nonshivering cold exposure. The effect of mild superficial cold exposure on cardiac electrical function in healthy adults is not well established as most previous studies have examined ECG changes in severe whole-body cold exposure and hypothermia. Thus, our study also provides insight into changes in cardiac electrophysiology in healthy adults during acute nonshivering cold exposure which can, from a clinical standpoint, alter ECG parameters in clinical patients exposed to mild superficial cooling prior to acquiring ECGs. Additionally, since BAT-activating interventions are researched as potential treatments for obesity and diabetes, associations between BAT activity and ECG parameters may possess further clinical significance. 2 MATERIALS AND METHODS 2.1 Ethical approval Prior to acceptance in the study, the subjects gave written informed consent and the studies conformed to the standards set by the latest revision of the Declaration of Helsinki. The study was approved by the properly constituted local medical ethics committee of the Southwestern Finland Medical District (the ethical review approval number: T231/2013) and the project has been registered in the Clinical Trials Database (NCT01985503). The cohort consisted of study subjects who participated in research projects into BAT activity in healthy adults (U Din, 2018). The measured ECG parameters and their associations with BAT activity, plasma catecholamines, and brachial BP have not been previously reported elsewhere. The study subjects were recruited with newspaper adverts. Subjects were required to be nondiabetic, non-hypertensive and without a history of cardiovascular disease. Exclusion criteria were diabetes, hypertension, dyslipidemia, history of cardiovascular disease and thyroid dysfunction, and pregnancy or breastfeeding prior to acceptance in the study. Subjects with abnormal ECG changes (ischemia, bundle branch blocks, AV blocks) were excluded from the study. The study cohort consisted of 22–55-year-old healthy subjects (N = 37, M/F 13/24). Study structure is shown in Figure 1. FIGURE 1Open in figure viewerPowerPoint Description of the study structure. All subjects underwent ECG measurement in room temperature, while ECG in cold stimulus was performed only in subjects imaged with [18F]FTHA. ECG measurements used in our analyses were performed during Study Visit 1 as shown in Figure 1. The study subjects came to the research center at 8 AM and had been instructed to avoid strenuous physical exercise on the day prior and during the morning of Study Visits 1 and 2. The study subjects lay in supine position on a bed for a period of around 10 min before the acquisition of the resting the 12-lead conventional ECG measurement at RT. This was performed prior to the setting of intravenous connection to avoid the potential effect from the pain of the procedure on ECG parameters. Skin was wiped clean with alcohol and body hair was shaved on the site of the leads when necessary. Additionally, 12-lead ECG measurements were performed after 2 h of cold exposure in supine position and the cooling continued during the ECG recording. During Study Visit 2, only the PET scan in RT was performed. Plasma catecholamine levels were drawn from a venous catheter in room temperature prior to the cold exposure and after the 2-h cold exposure in 17 study subjects as described previously (U Din, 2018). Additionally, blood pressure was measured with an Omron blood pressure monitor (Omron Healthcare Inc., US) in 31 subjects in RT and then after 2 h of cold exposure. Brachial BP was measured during Study Visit 1 first in RT after a 10 min period in the supine position, and then, after the 2 h cold exposure. Single measurements from the right arm (blood sampling venous cannula was in the left arm) were used to minimize the effect of consecutive BP measurements on BP and sympathetic activity. BP measurements were performed after the acquisition of ECG. Cooling started once all measurements in RT were performed and cooling was stopped once all measurements at 120 min of cooling were performed. The study subjects lay in the supine position throughout the period between the ECG measured in RT and the ECG measured after the cooling. The ECG recordings were 10 s in duration. The recordings were checked for technical quality and segments without baseline variation from breathing, muscle movement or tremor or other technical artifacts were selected for analysis. P axis, PR interval, QRS axis, QRS duration, QT, QTc, JTc, and T axis were measured manually by a single researcher. The values were averaged from three heartbeats. The end of the T-wave was determined with a tangent drawn to the steepest last limb of the T-wave and the end of the T-wave was the intersection of this tangent and the baseline. Healthy adult subjects underwent positron emission tomography (PET) with [18F]FTHA and [15O]H2O (N = 37) to measure nonesterified fatty acid (NEFA) uptake and perfusion in the supraclavicular BAT depot during cold exposure (U Din, 2018). Intravenous injection of [18F]FTHA was given and a dynamic emission scan was performed (frames: 1 × 60 s, 6 × 30 s, 1 × 60 s, 3 × 300 s, 2 × 600 s). Nonshivering cold exposure with adjustable cooling blankets had started 2 h prior to and continued during the PET imaging and 20 subjects were reimaged at RT with [15O]H2O and [18F]FTHA to measure nonstimulated BAT metabolism. All PET scans were performed after an overnight fast. The study subject lay on a bed with one cooling blanket below them and the other placed on top of them. The cooling blanket under the study subject reached form the feet to the thoracic region and the blanket placed on the subject reached from the feet to the abdomen. The temperature of the cooling blanket was adjusted to prevent muscle tremor and the temperature of the cooling blanket was recorded at 5 min intervals during the cooling to calculate the mean cooling temperature. The cooling blanket included plastic tubing where cooled water was circulated and the water temperature was increased if muscle tremor started to appear as described previously (U Din, 2018). The study subjects wore underpants and light pyjama-like patient clothing with long sleeves and pants during the cold exposure. Skin temperature during cooling was measured with a contact thermometer attached laterally on the abdominal region midway between the iliac crest and the lowest ribs on the sight side of the study subject. Skin temperature was recorded at 5 min intervals and the skin temperature at the start of the cooling was used as skin temperature in RT and skin temperature at 120 min of cooling was used as the skin temperature after cold exposure. The study subjects were in supine position during the measurements in RT and during cold exposure. A constant room temperature was maintained during the RT studies (mean ± SD 22.5 ± 0.4°C). QTc was calculated as follows: QTc = QT/√RR interval. JTc was calculated as follows: JTc = QTc − QRS duration. PET images were analyzed with Carimas 2.8. imaging analysis software (Turku PET Centre, University of Turku). 2.2 Statistical analysis The results are expressed as the mean group values with standard deviations (SD). Spearman's correlation was used to measure bivariate correlations between variables. Paired two-way t-test was used to examine differences in mean levels between RT and cold exposure. We calculated the changes in parameters from baseline in RT to measurements performed during cold exposure and then analyzed correlations between changes in ECG parameters and BAT activity, plasma catecholamine levels, and brachial BP to assess if there were simultaneous cold-induced changes in these variables. Due to the large number of variables in our correlation analyses, Bonferroni correction was used to assess a critical P-value corrected for multiple comparisons. SPSS 23 (IBM) was used as the statistical software. 3 RESULTS 3.1 Description of cohort characteristics The study cohort characteristics are displayed in Table 1. Mean age was 38.4 ± 9.9 years and mean BMI was 27.4 ± 4.6 kg/m2. Mean ECG parameters in RT were within normal range. Mean cold-activated BAT NEFA uptake in cold in the cohort (N = 37) was 0.77 ± 0.49 μmol/100 g/min, and BAT perfusion was 19.1 ± 17.5 ml/100 g/min (Table 1 displays the mean values of subjects who were PET imaged both at RT and during cold exposure, N = 20). TABLE 1. Description of study cohort Variable Room temperature Mean±SD Cold exposure Mean±SD P for comparison Age 38.4 ± 9.9 NA NA Males/Females (N) 13/24 NA NA HR at rest (1/min) 60.5 ± 6.6 59.7 ± 7.3 0.42 P axis (degrees) 35.6 ± 26.4 50.8 ± 22.7 0.005 PR interval (ms) 177.7 ± 24.6 163.0 ± 28.7 0.001 QRS duration (ms) 91.±10.2 89.4 ± 11.8 0.31 QRS axis (degrees) 42.1 ± 31.3 56.9 ± 24.1 0.003 QT (ms) 411.7 ± 25.5 434.5 ± 39.3 0.001 QTc (ms) 423.9 ± 26.4 426.8 ± 31.0 0.52 JTc (ms) 333.8 ± 28.8 337.4 ± 36.3 0.35 T axis (degrees) 27.6 ± 18.9 43.3 ± 20.7 0.07 Waist (cm) 93.4 ± 16.0 NA NA BMI (kg/m2) 27.4 ± 4.6 NA NA Systolic BP (mmHg) 127.2 ± 15.7 131.8 ± 17.9 0.008 Diastolic BP (mmHg) 81.7 ± 12.0 85.4 ± 13.0 0.02 Plasma adrenaline (µmol/L) 0.169 ± 0.044 0.176 ± 0.081 0.74 Plasma noradrenaline (µmol/L) 1.97 ± 0.61 5.07 ± 1.32 0.001 BAT NEFA uptake (umol/100 g/min) 0.67 ± 0.70 1.15 ± 1.65a a Includes only subjects with PET scans performed both at RT and during cold exposure. 0.19 BAT perfusion (ml/100 g/min) 11.0 ± 11.1 16.0 ± 14.5a a Includes only subjects with PET scans performed both at RT and during cold exposure. 0.08 Mean cooling temperature (°C) NA 7.2 ± 2.6 NA Skin temperature (°C) 32.2 ± 1.6 32.0 ± 2.0 0.48 a Includes only subjects with PET scans performed both at RT and during cold exposure. 3.2 Cold-induced changes in ECG parameters, BAT activity, brachial BP, and plasma catecholamines Table 1 displays the comparison of ECG parameters between baseline measurements in RT and measurements performed during cold exposure to examine the potential effects of cold-induced changes. Differences between RT and cold exposure were significant in P axis (35.6 ± 26.4 vs. 50.8 ± 22.7°, p = 0.005), (PR interval (177.7 ± 24.6 ms vs.163.0 ± 28.7 ms, p = 0.001), QRS axis (42.1 ± 31.3 vs. 56.9 ± 24.1, p = 0.003), and QT (411.7 ± 25.5 ms vs. 434.5 ± 39.3 ms, p = 0.001). There was no significant difference in HR, QRS duration, QTc, JTc, and T axis. Systolic (127.2 ± 15.7 vs. 131.8 ± 17.9 mmHg, p = 0.008) and diastolic BP (81.7 ± 12.0 vs. 85.4 ± 13.0 mmHg, p = 0.02) both increased during cold exposure. BAT NEFA uptake did not increase significantly in cold (0.67 ± 0.70 vs. 1.15 ± 1.65 µmol/100 g/min, p = 0.19) and increase in BAT perfusion was also nonsignificant (11.0 ± 11.1 vs. 16.0 ± 14.5 ml/100 g/min, p = 0.08). Figure 2 shows the cold-induced increase in supraclavicular BAT compared to RT in a PET/CT image. FIGURE 2Open in figure viewerPowerPoint Baseline supraclavicular BAT perfusion in room temperature during fasting (a) and elevated BAT perfusion during cold exposure in the same subject (b). Highest tracer activity is marked with green in the PET/CT fusion images. Cold exposure induced a significant increase in plasma noradrenaline level (1.97 ± 0.61 vs. 5.07 ± 1.32 µmol/L, p = 0.0001), while plasma adrenaline level did not change (p = 0.74). Although brachial BP and noradrenaline level increased suggesting a significant sympathetic response to our cooling method, skin surface temperature did not change significantly during the cold exposure (32.2 ± 1.6 vs. 32.0 ± 2.0°C, p = 0.48). 3.3 Correlations between cold-induced changes in ECG parameters and BAT activity, brachial BP, and plasma catecholamines Table 2 shows the correlations between cold-induced changes in ECG parameters and BAT activity, BP, and plasma catecholamines. We observed no significant correlations between the examined variables. TABLE 2. Correlation between cold-induced changes in ECG parameters and BAT activity, plasma catecholamines and brachial BP. Only ECG parameters with significant changes during cold exposure were included Variables ΔPR interval ΔP axis ΔQRS axis ΔQT ΔBAT NEFA uptake −0.233 (p = 0.37) −0.132 (p = 0.76) −0.014 (p = 0.96) −0.394 (p = 0.12) ΔBAT perfusion −0.100 (p = 0.68) −0.347 (p = 0.40) −0.203 (p = 0.40) −0.068 (p = 0.78) ΔNoradrenaline −0.189 (p = 0.48) −0.230 (p = 0.42) −0.435 (p = 0.09) −0.097 (p = 0.72) ΔAdrenaline −0.189 (p = 0.48) 0.301 (p = 0.30) −0.435 (p = 0.09) 0.013 (p = 0.96) ΔSystolic blood pressure 0.155 (p = 0.57) 0.140 (p = 0.61) −0.066 (p = 0.81) −0.241 (p = 0.37) ΔDiastolic blood pressure 0.190 (p = 0.50) 0.059 (p = 0.78) −0.052 (p = 0.79) −0.242 (p = 0.20) All variables are the absolute changes from RT measurements to measurements performed during cold exposure. Bonferroni's adjusted critical p < 0.002. 3.4 Correlations between ECG parameters and BAT activity, brachial BP, and plasma catecholamines in room temperature Correlations between ECG parameters and BAT activity, BP, plasma catecholamines, and skin temperature were examined in Table 3. BAT NEFA uptake in RT correlated significantly with QRS axis (r = 0.456, p = 0.04) and JTc (r = 0.500, p = 0.03). Additionally, plasma noradrenaline level correlated with HR (r = 0.687, p = 0.002). However, all correlations exceeded the Bonferroni's corrected critical p < 0.001. TABLE 3. Correlation between ECG parameters and BAT activity, plasma catecholamines and brachial BP in room temperature (RT) Variables HR PR interval P axis QRS duration QRS axis QT QTc JTc T axis BAT NEFA uptake in RT 0.202 (p = 0.39) −0.016 (p = 0.95) −0.151 (p = 0.62) −0.368 (p = 0.11) 0.456 (p = 0.04) 0.158 (p = 0.51) 0.409 (p = 0.07) 0.500 (p = 0.025) 0.309 (p = 0.31) BAT perfusion in RT 0.048 (p = 0.84) −0.089 (p = 0.70) −0.209 (p = 0.49) −0.313 (p = 0.17) 0.271 (p = 0.23) 0.111 (p = 0.63) 0.125 (p = 0.59) 0.281 (p = 0.22) 0.231 (p = 0.30) Noradrenaline in RT 0.687 (p = 0.002) −0.467 (p = 0.06) 0.371 (p = 0.47) 0.056 (p = 0.83) −0.023 (p = 0.93) −0.204 (p = 0.43) 0.440 (p = 0.08) 0.304 (p = 0.24) 0.087 (p = 0.87) Adrenaline in RT 0.251 (p = 0.33) −0.123 (p = 0.64) −0.143 (p = 0.79) 0.139 (p = 0.60) −0.046 (p = 0.86) −0.072 (p = 0.78) 0.183 (p = 0.48) 0.018 (p = 0.95) −0.377 (p = 0.46) Systolic blood pressure in RT 0.044 (p = 0.80) 0.232 (p = 0.18) −0.471 (p = 0.09) 0.067 (p = 0.70) 0.068 (p = 0.69) 0.133 (p = 0.45) 0.123 (p = 0.48) 0.086 (p = 0.62) 0.126 (p = 0.67) Diastolic blood pressure in RT −0.054 (p = 0.76) 0.219 (p = 0.21) 0.070 (p = 0.81) −0.078 (p = 0.66) 0.095 (p = 0.59) −0.117 (p = 0.50) −0.234 (p = 0.18) −0.202 (p = 0.24) 0.199 (p = 0.49) Skin temperature in RT −0.097 (p = 0.59) 0.139 (p = 0.43) −0.506 (p = 0.07) 0.033 (p = 0.85) 0.009 (p = 0.96) −0.079 (p = 0.65) −0.210 (p = 0.23) −0.162 (p = 0.35) −0.161 (p = 0.36) Bonferroni's adjusted critical p < 0.001. 3.5 Correlations between ECG parameters and BAT activity, brachial BP, and plasma catecholamines during cold exposure In Table 4, we examined correlations between ECG measurements and BAT activity, brachial BP, plasma catecholamines, skin temperature, and mean cooling temperature during cold exposure. HR correlated with systolic BP (r = 0.382, p = 0.03) and skin temperature (r = −0.423, p = 0.04). These correlations nonetheless exceeded the Bonferroni's corrected critical p < 0.001. TABLE 4. Correlation between ECG parameters and BAT activity, plasma catecholamines, brachial BP, skin temperature, and mean cooling temperature during cold exposure Variables HR PR interval P axis QRS duration QRS axis QT QTc JTc T axis BAT NEFA uptake during cold exposure 0.210 (p = 0.23) −0.205 (p = 0.25) −0.095 (p = 0.63) −0.205 (p = 0.25) −0.259 (p = 0.14) −0.073 (p = 0.68) 0.216 (p = 0.22) 0.272 (p = 0.12) 0.341 (p = 0.08) BAT perfusion during cold exposure 0.176 (p = 0.33) −0.233 (p = 0.19) −0.240 (p = 0.19) −0.105 (p = 0.56) −0.198 (p = 0.27) −0.121 (p = 0.50) 0.041 (p = 0.83) 0.238 (p = 0.18) −0.075 (p = 0.71) Noradrenaline during cold exposure 0.111 (p = 0.67) −0.459 (p = 0.07) −0.240 (p = 0.41) −0.124 (p = 0.64) 0.034 (p = 0.90) −0.209 (p = 0.42) 0.025 (p = 0.93) 0.115 (p = 0.67) −0.024 (p = 0.94) Adrenaline during cold exposure 0.183 (p = 0.48) −0.375 (p = 0.14) −0.183 (p = 0.53) −0.055 (p = 0.83) 0.079 (p = 0.76) 0.158 (p = 0.55) 0.322 (p = 0.21) 0.313 (p = 0.22) −0.257 (p = 0.37) Systolic blood pressure during cold exposure 0.382 (p = 0.03) 0.127 (p = 0.50) −0.025 (p = 0.90) −0.213 (p = 0.25) −0.007 (p = 0.97) −0.294 (p = 0.11) 0.135 (p = 0.47) 0.138 (p = 0.46) 0.106 (p = 0.60) Diastolic blood pressure during cold exposure 0.117 (p = 0.54) 0.181 (p = 0.34) 0.059 (p = 0.78) −0.267 (p = 0.16) −0.052 (p = 0.79) −0.242 (p = 0.20) −0.077 (p = 0.69) 0.007 (p = 0.97) 0.130 (p = 0.53) Skin temperature after 2 h cold exposure −0.423 (p = 0.04) −0.102 (p = 0.59) −0.011 (p = 0.96) 0.179 (p = 0.34) 0.332 (p = 0.07) 0.286 (p = 0.13) −0.024 (p = 0.90) −0.082 (p = 0.67) 0.044 (p = 0.84) Mean cooling temperature between 60–120 min 0.034 (p = 0.84) 0.107 (p = 0.53) 0.203 (p = 0.17) −0.080 (p = 0.64) 0.111 (p = 0.51) 0.052 (p = 0.76) 0.089 (p = 0.60) 0.077 (p = 0.65) 0.013 (p = 0.94) Bonferroni's adjusted critical p < 0.001. 4 DISCUSSION According to our hypothesis, acute nonshivering cold exposure would increase HR and prolong conduction and repolarization times in conventional resting ECG. In our results, P axis increased, PR interval decreased and QRS axis and QT increased significantly during cold exposure compared to RT. Our cold exposure method did not alter surface skin temperature but brachial BP and plasma noradrenaline level increased during cold exposure suggesting nonshivering cooling was sufficient to induce a measurable sympathetic response. Cold-induced changes in BAT activation, brachial BP, and plasma catecholamine levels was expected to associate with changes in ECG parameters displaying chronotropy, cardiac conductance, and repolarization such as HR, QTc, and JTc. However, changes in these surrogate markers of cold-stimulated sympathetic activity did not correlate with cold-induced-changes in ECG parameter levels. Neither was there any correlation between BAT activity level and ECG parameters in cold. This might suggest that the autonomic regulation of cardiac electrophysiology may be somewhat independent of cold-stimulated BAT activity, brachial BP, and plasma catecholamine levels during mild acute cold exposure. However, nonstimulated BAT fatty acid metabolism in RT correlated directly with QRS axis and JTc. Additionally, plasma noradrenaline level correlated with HR in RT. HR in cold correlated with systolic BP and skin temperature during cold exposure. Additionally, the lack or presence of significant correlations in our study setting does not prove or disprove causal relationships between the examined variables since we only examined correlations. Previous studies with severe cooling have shown prolonged conduction and repolarization times during hypothermia (Drake & Flowers, 1980; Gould, 1985; Mareedu et al., 20