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. Author manuscript; available in PMC: 2024 Dec 1.
Published in final edited form as: J Therm Biol. 2023 Oct 14;118:103727. doi: 10.1016/j.jtherbio.2023.103727

Cardiovascular and mood responses to an acute bout of cold water immersion

Emma L Reed 1, Christopher L Chapman 1, Emma K Whittman 1, Talia E Park 1, Emily A Larson 1, Brendan W Kaiser 1, Lindan N Comrada 1, Karen Wiedenfeld Needham 1, John R Halliwill 1, Christopher T Minson 1
PMCID: PMC10842018  NIHMSID: NIHMS1941439  PMID: 37866096

Abstract

Cold water immersion (CWI) may provide benefits for physical and mental health. Our purpose was to investigate the effects of an acute bout of CWI on vascular shear stress and affect (positive and negative). Sixteen healthy adults (age: 23±4 y; (9 self-reported men and 7 self-reported women) completed one 15-min bout of CWI (10°C). Self-reported affect (positive and negative) was assessed at pre-CWI (Pre), 30-min post-immersion, and 180-min post-immersion in all participants. Brachial artery diameter and blood velocity were measured (Doppler ultrasound) at Pre, after 1-min and 15-min of CWI, and 30-min post-immersion (n=8). Total, antegrade, and retrograde shear stress, oscillatory shear index (OSI), and forearm vascular conductance (FVC) were calculated. Venous blood samples were collected at Pre, after 1-min and 15-min of CWI, 30-min post-immersion, and 180-min post-immersion (n=8) to quantify serum β-endorphins and cortisol. Data were analyzed using a one-way ANOVA with Fisher’s least significance difference and compared to Pre. Positive affect did not change (ANOVA p = 0.450) but negative affect was lower at 180-min post-immersion (p < 0.001). FVC was reduced at 15-min of CWI and 30-min post-immersion (p <0.020). Total and antegrade shear and OSI were reduced at 30-min post-immersion (p <0.040) but there were no differences in retrograde shear (ANOVA p =0.134). β-endorphins did not change throughout the trial (ANOVA p = 0.321). Cortisol was lower at 180-min post-immersion (p =0.014). An acute bout of CWI minimally affects shear stress patterns but may benefit mental health by reducing negative feelings and cortisol levels.

Keywords: blood flow, shear stress, cold-shock response, stress, mental health, affect

1. Introduction

In uncontrolled settings, cold water immersion is associated with negative outcomes such as cardiac arrythmias, hypothermia, and drowning (Bierens et al., 2016). However, cold water immersion has been used in an intentional and controlled manner for centuries across multiple disciplines (Allan et al., 2022; Wang et al., 2006). For example, it is used during the post-exercise recovery period (Crowther et al., 2017) to attenuate inflammation and edema by reducing muscle temperature and blood flow (Leeder et al., 2012; Smith, 1991; Vromans et al., 2019). Pre-cooling with cold water immersion prior to exercise in the heat is associated with improved performance (Ranalli et al., 2010). It is also the most effective cooling modality in the treatment of heat-related illnesses (Casa et al., 2007). Cold water immersion has been suggested as a tool to provide benefits beyond performance or rehabilitation settings (Leeder et al., 2012; Versey et al., 2013). It is used between sauna bathing intervals, a practice known as contrast bathing, utilizing opposing environmental stressors for physiological adaptations associated with improved health (Heinonen & Laukkanen, 2018). Lastly, repeated cold exposure has been suggested to improve mood, anxiety, psychological well-being (Holland, 2020; van Tulleken et al., 2018; Williams, 2022), provide cardioprotective benefits (Kralova Lesna et al., 2015; Tibenska et al., 2020), and metabolic adaptations (i.e., brown adipose tissue activation) (van der Lans et al., 2013). While it is evident the use of cold water immersion continues to grow in popularity, the mechanisms which contribute to these purported cardiovascular and psychological outcomes remain largely unexplored.

It is well established that cold water immersion is a robust physiological stressor, particularly of the cardiovascular system, as indicated by increased heart rate, total peripheral resistance, and arterial blood pressure (Castellani & Young, 2016; Tipton, 1989; Tipton et al., 2017). Other physiological stressors of the cardiovascular system, such as exercise or passive heating, result in responses, such as shear stress, that can benefit cardiovascular health (Carter et al., 2014; Tinken et al., 2010). Shear stress is the frictional force along the innermost endothelial lining in arteries and is an important modulator of endothelial function (Paszkowiak & Dardik, 2003; Tinken et al., 2009).

During cold stress, peripheral vasoconstriction reduces peripheral blood flow, mainly in cutaneous and likely skeletal muscle vasculatures (Castellani & Young, 2016; Mugele et al., 2023; Wilson et al., 2007). Peripheral hemodynamic responses can be quantified by ultrasound measurement of blood flow in conduit arteries (i.e., femoral and brachial arteries) that supply both the skin and muscle vasculatures. For example, lower-limb immersion in cold water (10 min at 8 and 22°C) has been shown to reduce femoral artery blood flow at the end of immersion and 30 min after immersion (Gregson et al., 2011). Vasoconstriction in resistance vessels distal to the conduit arteries during sympathetic activation, such as cold pressor test and post-exercise ischemia, can alter the proximal shear stress patterns (i.e., increase retrograde shear) in the conduit arteries (Halliwill & Minson, 2010; Padilla et al., 2010). In the context of cold water immersion, it is unclear how the cold-induced peripheral vasoconstriction effects vascular shear stress in a conduit artery during and following immersion. While repeated exposures to cold may show potential cardiovascular benefits (Kralova Lesna et al., 2015; Tibenska et al., 2020), it is valuable to first understand the acute shear stress responses to determine if this is an avenue for targeting cardiovascular outcomes.

Guided by claims of improvement in mood, anxiety, and psychological well-being, many individuals incorporate frequent, short bouts of cold stress including cold baths, cold showers, or winter swimming into their lifestyle routine (Holland, 2020; Kelly & Bird, 2022; Shevchuk, 2008; van Tulleken et al., 2018; Williams, 2022). However, some of these reports are either small case studies, anecdotal, or include exercise (Leppaluoto et al., 2008; Massey et al., 2020; Shevchuk, 2008; van Tulleken et al., 2018). Additionally, previous investigations only include subjective ratings of mood but do not include objective biomarkers (e.g., blood analytes) associated with mood (Kelly & Bird, 2022). Improvements in mood or alleviation of symptoms of depression or anxiety with cold water immersion could result from acute stimulation of the acute stress response (i.e., autonomic nervous system activation, catecholamines, cortisol), increases in circulating β-endorphins, or reductions in inflammatory cytokines (Shevchuk, 2008; Tipton et al., 2017). It is necessary to quantify objective responses to cold water immersion in addition to subjective responses, as selection and social biases may confound the subjective ratings of mood.

Therefore, the purpose of our study was to quantify shear stress patterns in the brachial artery, subjective ratings of positive and negative affect, and objective blood biomarkers associated with mood (cortisol and β-endorphins) during and after an acute bout of cold water immersion in healthy, young adults. We tested the hypotheses that an acute bout of cold water immersion would 1) decrease shear stress in the brachial artery, 2) increase positive affect and decrease negative affect, 3) acutely increase cortisol during immersion followed by a decrease in cortisol after immersion, and 4) increase in β-endorphins. Together, we aimed to address current gaps in knowledge surrounding both vascular and affect outcomes due to the application and continued popularity in the use of cold water immersion.

2. Materials & Methods

2.1. Participants

Sixteen healthy adults (age: 23±4 y; body mass index: 22±2 kg/m2; 9 self-reported men and 7 self-reported women) participated in the study. Participants were non-smokers who were free of cardiovascular disease and medical conditions that reduce cold tolerance (e.g., anemia, anorexia, hypothyroidism, Raynaud’s disease, disorders of the hypothalamus, fibromyalgia, or previous frostbite injury), and naïve to cold therapy or not currently undergoing cold therapy. Participants of child-bearing potential were not pregnant (confirmed via a commercially available urine pregnancy test) or undergoing treatment to increase sperm count. Participants were not taking prescription medications other than hormonal contraceptives (n=5). For the women participants, we did not control for different phases of the menstrual when scheduling the single study visit for this protocol as this was not a main purpose of the protocol. All participants provided written consent once fully informed of the experimental procedures and risks. This study was approved by the Institutional Review Board at the University of Oregon and conformed to the principles of the Declaration of Helsinki. Participants were instructed to abstain from heavy exercise for 24 h; alcohol, supplements, and medications (except for hormonal contraceptives) for 12 h; caffeine for 6 h; and food for 2 h prior to arriving to the laboratory for the single study visit.

This study was conducted during the COVID-19 Pandemic between January-June 2021 in adherence with the safety limitations dictated by the Incident Management Team within Safety and Risk Services at the University of Oregon. These restrictions specified that human research protocols were limited to one experimental session per week with no more than 2 h participant engagement and 15 min of close contact (within 2 m of research personnel). Due to these restrictions, participants were assigned to one of two groups (ultrasonography or serial blood sampling; described below) but the overall study timeline was identical for all participants. Therefore, measurements obtained in both groups were combined for analysis when applicable. From self-report, no participants had tested positive for COVID-19 prior to or during participation in the study protocol.

2.2. Experimental Protocol

Upon arrival to the climate-controlled laboratory (23.0°C [SD: 1.5] and 31% [SD: 3] relative humidity), participants entered a private room to self-insert a rectal thermistor probe (YSI Series 400, Yellow Spring Instruments, Yellow Springs, OH) ~10 cm past the anal sphincter and changed into a swimsuit. Participants were then seated on a hydraulic lift chair (S.R. Smith, Canby, OR) and instrumented with a 3-lead electrocardiogram for measurement of heart rate (Cardiocap 5, Datex-Ohmeda, St. Louis, MO) and a pressure cuff on the left arm for automated auscultatory measurement of arterial blood pressure (Tango M2, Suntech Medical, Raleigh, NC). For the ultrasound group (n=8), the right arm was supported at the level of the heart for imaging of the right brachial artery with a 10.0 MHz linear array ultrasound transducer probe (Terason t3000cv, Teratech, Burlington, MA). The right brachial artery was located and marked with indelible ink by the same experienced sonographer (BWK or EAL) within a single visit. For the blood sampling group (n=8), an intravenous catheter was placed in the right arm to obtain venous blood samples. Positive and negative affect was assessed using the Positive and Negative Affect Schedule (PANAS) Survey (Watson et al., 1988). The PANAS was utilized due to its inclusion of both positive and negative mood ranking versus only positive or negative mood and strong internal reliability (between 0.86–0.90 for positive affect and 0.84–0.87 for negative affect)(Watson et al., 1988).

A schematic of the experimental visit timeline is shown in Figure 1. After instrumentation, participants then remained seated for 20 min of quiet rest followed by a 5-min period of pre-immersion data collection (Pre). The hydraulic lift chair was used to transfer the participants into a tub filled with cold, circulating water (10.5°C [SD 0.2]; iCoolSport, iCool USA Inc., Hartselle, AL) until the water level reached the xiphoid process of the sternum. Participants remained seated in the water for 15 min with both arms supported above the level of the water to accommodate equipment. At the end of the 15 min cold water immersion, the hydraulic lift chair was used to transfer the participant out of the water and return them to their original position next to the tub. Participants remained seated for 30 min following cold water immersion and were allowed a blanket for their lower limbs upon request. The post-immersion period was utilized to 1) quantify short-term responses in the primary outcomes (shear stress, mood, and blood biomarkers) following immersion and 2) to monitor participants due to the likelihood of “afterdrop” following cold water immersion (Romet, 1988). After the post-immersion period, participants were de-instrumented and exited the laboratory.

Figure 1.

Figure 1.

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Schematic of the experimental visit timeline. Participants were assigned to either the ultrasound or blood sampling group. All participants completed the physical affect survey (black square), heart rate, blood pressure, and rectal temperature measurements (black triangle). The ultrasound group had the right brachial artery scanned for one minute (ultrasound probe) whereas a venous blood sample (black circle) was obtained from an intravenous catheter in the blood sampling group.

The experimental conditions of the protocol were selected to safely elicit a significant cold stress (i.e., cold shock) while aiming to increase external validity to cold water immersion protocols. The 1-min timepoint is transferable to the use of cold water with contrast bathing which is several sauna bathing sessions lasting 5 to 15 min separated by short bouts of ice-bath immersion (typically 5 seconds to a few min) (Heinonen & Laukkanen, 2018). The 15-min timepoint may provide further insight to cold water immersion employed during post-exercise recovery as it is recommended to use cold water between 10–15°C for 10–15 min due to reductions in muscle temperature (Allan et al., 2022; Machado et al., 2016; Vromans et al., 2019).

Rectal temperature, heart rate, and arterial blood pressure were recorded at Pre, 1-min and 15-min of immersion, and 30-min post-immersion. Affect was recorded at Pre, 30-min and 180-min post-immersion. Ultrasound scans and blood draws were obtained at identical timepoints in the two groups of participants (Pre, 1-min of immersion, 15-min of immersion, and 30-min post-immersion). Participants in the blood sampling group returned to the laboratory 180 min after immersion for a blood sample (180-min post-immersion) and affect. During the window between 30-min post-immersion and 180-min post immersion, participants departed from the laboratory and were instructed to avoid consuming food, caffeine, and alcohol or performing exercise which exceeded completing activities of daily living. The ultrasound group reported the 180-min post-immersion affect over the phone. These participants were sent home with a printed copy of the PANAS survey to refer to while providing their response to each question over the phone. A research team member provided the same prompt prior to completion of the survey in person and over the phone.

2.3. Details of measurements

Mean arterial pressure was calculated as: (diastolicbloodpressure+(systolicbloodpressure-diastolicbloodpressure3) and used for the calculation of forearm vascular conductance (FVC) as: (bloodflowmeanarterialpressure).

The right brachial artery was imaged at an insonation angle of 60° with a 10.0 MHz linear array ultrasound probe. At each time point, ultrasound images were captured for 1 min at 20 Hz with video recording software (Camtasia, TechSmith, Okemo, MI). The ultrasound recording was analyzed offline with automated determination of vessel diameter and the peak blood velocity envelope (antegrade and retrograde) for each cardiac cycle (Brachial Analyzer for Research, Medical Imaging Applications, LLC, Coralville, IA). The time averaged peak blood velocity was calculated as: (peakantegradebloodvelocity-peakretrogradebloodvelocity). The time averaged mean blood velocity was calculated as: (timeaveragedbloodvelocity2). Blood flow (total, antegrade, and retrograde) was calculated as: (crosssectionalarea×timeaveragedmeanbloodvelocity). Shear rate (total, antegrade, and retrograde) was calculated as: (4×(timeaveragedbloodvelocity/2)diameter). Oscillatory shear index, the axial direction wall shear stress where a value of 0 implies unidirectional flow and a value of 0.5 implies multidirectional flow (Ascuitto et al., 2017), was calculated as: (retrogradeshearrate(retrogradeshearrate+antegradeshearrate)) (Moore et al., 1994).

Venous blood was collected in serum separator tubes, allowed to clot at room temperature for 30 min, centrifuged at 4°C for 10 min at 1,500 g and stored in a −80°C freezer for later analysis. β-endorphins were quantified with the QuickDetect® ELISA kit (BioVision, Milpitas, CA) and total cortisol was quantified with the Invitrogen™ ELISA kit (Fisher Scientific Co. L.L.C, Pittsburgh, PA). The pre-immersion blood samples were obtained between 9:00 AM-12:30 PM and the 180-min post-immersion blood draws were obtained between 1:30–5:30 PM.

2.4. Statistical Analysis

A one-way repeated-measures analysis of variance (ANOVA) was used to examine the changes in outcome variables over time (Prism, Version 9.1, GraphPad, La Jolla, CA). In the cases of missing values, a mixed-effects model was used. When the P-value for the overall ANOVA F statistic was <0.05, a post-hoc Fisher’s Least Significant Difference (LSD) test was used to compare each time point to pre-immersion values and between the last minute of immersion and end of post-immersion. The data were assessed for normality with the Shapiro-Wilk test. If the alpha was < 0.05, the data were log transformed, analyzed, and the statistics from the log transformation are reported whereas the tables and/or figures show the non-transformed, physiological data. The following variables were log transformed upon review of the Shapiro-Wilk test: retrograde blood velocity, mean blood flow, antegrade blood flow, retrograde blood flow, retrograde shear stress, oscillatory shear index, forearm vascular conductance, positive affect and negative affect. We excluded the following values: n=3 from rectal temperature due to a significant decrease (> 0.5°C) upon immersion in the water, n=1 from blood pressure (systolic, diastolic, and mean) at 1- and 15-min of immersion due to excess shivering interfering with the measurement, n=1 from the ultrasound analysis 1-min of immersion due to excessive shivering interfering with the quality of the recording, n=1 from β-endorphins due to inadequate sample, and n=2 from affect at 180-min post due to researcher error. Values are reported as [mean with (95% confidence intervals)] unless otherwise noted. Statistical significance was set to p < 0.05. This study was considered exploratory, therefore an a priori power analysis was not calculated.

3. Results

3.1. Rectal Temperature

Rectal temperature (Figure 2A) was reduced compared to pre-immersion baseline [37.5°C (37.3, 37.7)] during immersion at 1-min [37.4°C (37.2, 37.6)] and 15-min [37.1°C (36.7, 37.4)] (both p < 0.001). Rectal temperature was further reduced at 30-min of post-immersion [36.5°C (36.2, 36.9)] compared to during immersion (p < 0.001).

Figure 2.

Figure 2.

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Responses for rectal temperature (A; n=13; 7 women), heart rate (B; n=16; 7 women), systolic blood pressure (C; n=16, 7 women), diastolic blood pressure (D; n=16; 7 women), and mean arterial pressure (E; n=16; 7 women) before (Pre), during (Immersion; 1-min and 15-min), and after (Post; 30-min) cold water immersion. A n=1 from blood pressure (systolic, diastolic, and mean) at 1- and 15-min of immersion was excluded due to excess shivering interfering with the measurement. Data are presented as mean with 95% CI and individual responses. Data were analyzed with a one-way ANOVA, or mixed effects model if missing values, and post-hoc Fisher’s LSD when the overall ANOVA P-value was <0.05. * denotes p<0.05 versus Pre.

3.2. Cardiovascular & Hemodynamics

As shown in Figure 2B, heart rate was elevated during 1-min of immersion [85 bpm (76, 94)] but reduced at 15-min of immersion [65 bpm (59, 71)] and 30-min of post-immersion [63 bpm (58, 68)] compared to pre-immersion baseline [76 bpm (70, 82)] (all p < 0.007). There was no change in heart rate from 15-min of immersion to 30-min of post-immersion [63 bpm (58, 68)] (p = 0.313). Systolic blood pressure (Figure 2C, p < 0.007), diastolic blood pressure (Figure 2D, p < 0.012), and mean arterial pressure (Figure 2E, p < 0.001) were elevated during immersion (1-min SBP: [135 mmHg (127, 143)], DBP: [81 mmHg (78, 83)], MAP: [98 mmHg (95, 103)]; 15-min SBP: [119 mmHg (112, 126)], DBP: [71 mmHg (65, 76)], MAP: [87 mmHg (81, 92)]) and post-immersion (SBP: [125 mmHg (116, 133)], DBP: [73 mmHg (69, 77)], MAP: [90 mmHg (85, 95)]) compared to pre-immersion baseline (SBP: [111 mmHg (104, 118)], DBP: [66 mmHg (62, 70)], MAP: [81 mmHg (77, 86)]).

3.3. Brachial Artery Ultrasound

The brachial artery hemodynamics are presented in Table 1. Brachial artery diameter did not change from pre-immersion baseline at 1-min of immersion (p = 0.064) but was reduced at 15-min of immersion and 30-min of post-immersion (p < 0.011). Mean blood velocity did not change during immersion (p > 0.178) but was reduced at 30-min of post-immersion (p = 0.016) compared to pre-immersion baseline. Antegrade blood velocity did not change during immersion (p > 0.275) but was reduced at 30-min of post-immersion (p = 0.023) compared to pre-immersion baseline. Antegrade blood velocity was further reduced at 30-min of post-immersion (p = 0.022) compared to 15-min of immersion. Retrograde blood velocity did not differ from pre-immersion baseline during or after immersion (p > 0.065) or from 15-min of immersion to 30-min of post-immersion (p = 0.547). Mean blood flow did not change from pre-immersion baseline at 1-min of immersion (p = 0.121) but was reduced at 15-min of immersion and 30-min of post-immersion (p < 0.025). Mean blood flow did not change from 15-min of immersion to 30-min of post-immersion (p = 0.180). Antegrade blood flow did not change from pre-immersion baseline at 1-min of immersion (p = 0.312) but was reduced at 15-min of immersion and 30-min of post-immersion (p < 0.028) compared to pre-immersion. Antegrade blood flow did not change from 15-min of immersion to 30-min of post-immersion (p = 0.172). Retrograde blood flow did not differ from pre-immersion baseline during or after immersion (p > 0.210) or from 15-min of immersion to 30-min of post-immersion (p = 0.984).

Table 1.

Measured and calculated responses of the brachial artery before (Pre; n=8, 3 women), during (Immersion; 1-min, n=7 and 15-min, n=8, both 3 women), and after (Post; 30-min; n=8, 3 women) cold water immersion. Data are presented as mean with 95% CI and individual responses. Data were analyzed with a one-way ANOVA, or mixed effects model if missing values, and post-hoc Fisher’s LSD when the overall ANOVA P-value was <0.05.

Variable ANOVA Pre Min 1 Immersion Min 15 Immersion Post
Diameter (mm) 0.019 3.88 (3.37, 4.39) 3.63 (3.08, 4.17) 3.36 (2.79, 3.94)* 3.52 (2.95, 4.10)*
Blood Velocity (cm/s)
 Mean 0.020 7.10 (5.07, 9.13) 6.08 (4.72, 7.43) 5.66 (4.15, 7.16) 4.11 (3.34, 4.87)*†
 Antegrade 0.023 8.44 (5.99, 10.90) 7.91 (5.84, 9.97) 7.48 (5.75, 9.22) 5.69 (4.62, 6.77) *†
 Retrograde 0.105 1.34 (0.57, 2.10) 1.83 (0.88, 2.78) 1.83 (0.63, 3.02) 1.59 (0.88, 2.29)
Blood Flow (mL/min)
 Mean 0.007 52.70 (28.41, 76.98) 39.85 (20.12, 59.57) 30.43 (19.17, 41.69)* 25.22 (15.92, 34.51)*
 Antegrade 0.014 63.50 (32.29, 94.71) 52.48 (22.41, 82.55) 42.30 (22.28, 62.33)* 35.49 (21.604, 49.35)*
 Retrograde 0.479 10.80 (2.50, 19.10) 12.64 (1.61, 23.66) 11.88 (−0.55, 24.30) 10.29 (3.94, 16.64)
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*

denotes p<0.05 versus Pre

†

denotes p<0.05 versus 15-min of immersion.

The shear stress responses to cold water immersion and post-immersion are shown in Figure 3. Total shear stress (Figure 3A) did not change during immersion (1-min: [68.32 s−1 (53.37, 83.27)], 15-min: [71.08 s−1 (48.11, 94.05)] (all p > 0.461) but was reduced at 30-min of post-immersion [47.98 s−1 (37.50, 58.45)] (p = 0.021) compared to pre-immersion [73.91 s−1 (52.33, 95.48)]. Total shear stress was further reduced at 30-min of post-immersion compared to 15-min of immersion (p = 0.028). Antegrade shear rate (Figure 3B) did not change during immersion at 1-min [88.50 s−1 (69.07, 107.92) or 15-min [91.99 s−1 (69.20, 114.77) (all p > 0.670) but was reduced at 30-min of post-immersion [65.74 s−1 (53.18, 78.29)] (p = 0.039) compared to pre-immersion [87.29 s−1 (62.73, 110.85)]. Antegrade shear rate was reduced at 30-min of post-immersion compared to 15-min of immersion (p = 0.017). Retrograde shear rate (Figure 3B) did not differ during (1-min: [20.18 s−1 (11.72, 28.63)], 15-min: [20.90 s−1 (10.99, 30.82)]) or after immersion (min-30: [17.78 s−1 (10.71, 24.84)]) compared to pre-immersion baseline [13.39 s−1 (6.92, 19.85)] (ANOVA p = 0.134). Oscillatory shear index (Figure 3C) did not change during immersion (1-min: [0.18 a.u. (0.13, 0.23)], 15-min: [0.19 a.u. (0.12, 0.25)] from pre-immersion [0.14 a.u. (0.09, 0.18)) (both p < 0.120) but was greater at 30-min post-immersion: [0.21 a.u. (0.16, 0.26)]) (p = 0.015). Forearm vascular conductance (Figure 3D) trended to be reduced at 1-min of immersion [0.43 ml/min/mmHg (0.19, 0.66)] (p = 0.066) and was reduced at 15-min of immersion [0.33 ml/min/mmHg (0.22, 0.44)] and 30-min of post-immersion [0.28 ml/min/mmHg (0.17, 0.38)] (p < 0.010) compared to pre-immersion baseline [0.64 ml/min/mmHg (0.34, 0.95)]. Forearm vascular conductance did not change from 15-min of immersion to 30-min of post-immersion (p = 0.252).

Figure 3.

Figure 3.

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Responses for total shear (A), antegrade and retrograde shear (B), oscillatory shear index (C), and forearm vascular conductance (D) before (Pre; n=8, 3 women) during (Immersion; 1-min, n=7 and 15-min, n=8, both 3 women), and after (Post; 30-min; n=8, 3 women) cold water immersion. Data are presented as mean with 95% CI and individual responses. Data were analyzed with a one-way ANOVA, or mixed effects model if missing values, and post-hoc Fisher’s LSD when the overall ANOVA P-value was <0.05. * denotes p<0.05 versus Pre; † denotes p<0.05 versus 15-min of immersion.

3.4. Affect & Blood Biomarkers

The affect and blood biomarker responses to cold water immersion and post-immersion are shown in Figures 4 and 5. Positive affect (Figure 4A) did not change at 30-min [29 a.u. (35, 32)] or 180-min post-immersion [29 a.u. (35, 33)] compared to pre-immersion baseline [30 a.u. (37, 34)] (ANOVA p = 0.450). Negative affect (Figure 4B) did not change at 30-min after immersion [11 a.u. (10, 12)] (p = 0.741) but was reduced 180-min of post-immersion [9 a.u. (9, 10)] (p < 0.001) compared to pre-immersion [11 a.u. (10, 12)]. Serum β-endorphins (Figure 5A) did not change during (1-min: [23 pg/ml (19, 26)], 15-min: [23 pg/ml (19, 27)]) or after immersion (30-min: [23 pg/ml (19, 27)], 180-min: [24 pg/ml (19, 28)]) compared to pre-immersion [23 pg/ml (19, 27)] (ANOVA p = 0.321). Serum cortisol (Figure 5B) did not change during immersion (1-min: [1475 pg/ml (933, 2017)], 15-min: [1638 pg/ml (1013, 2262)]) (p > 0.168) or 30-min after immersion [1270 pg/ml (735, 1804)] (p = 0.525) but was reduced 180-min after immersion [750 pg/ml (415, 1084)] (p = 0.014) compared to pre-immersion baseline [1430 pg/ml (782, 2077)].

Figure 4.

Figure 4.

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Responses for positive affect (A) and negative affect (B) measured with the Positive and Negative Affect Schedule Survey before (Pre; n=16, 7 women) and after (Post; 30-min; n=16, 7 women and 180-min, n=14, 3 women) cold water immersion. Data are presented as mean with 95% CI and individual responses in arbitrary units (a.u.). Data were analyzed with a one-way ANOVA, or mixed effects model if missing values, and post-hoc Fisher’s LSD when the overall ANOVA P-value was <0.05. * denotes p<0.05 versus Pre.

Figure 5.

Figure 5.

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Responses for serum β-endorphins (A, n=7, 3 women) and serum cortisol (B, n=8, 4 women) before (Pre), during (Immersion; 1-min and 15-min), and after (Post; 30-min and 180-min) cold water immersion. Data are presented as mean with 95% CI and individual responses. Data were analyzed with a one-way ANOVA, or mixed effects model if missing values, and post-hoc Fisher’s LSD when the overall ANOVA P-value was <0.05. * denotes p<0.05 versus Pre.

4. Discussion

This study explored the effect of acute cold water immersion on vascular shear patterns, affect, and blood biomarkers associated with stress and mood. The main findings were that 15 min of acute cold water immersion: 1) minimally altered shear stress patterns during immersion but reduced total and antegrade shear rate 30 min post-immersion, 2) modestly lowered negative affect ratings 3 h post-immersion, and 3) reduced serum cortisol 3 h post-immersion. Together, these findings suggest a single bout of cold water immersion may not serve as a modality to target vascular function, but could be a means to reduce cortisol following immersion and contribute to less negative feelings. As cold water immersion continues to be used in a controlled, intentional way, these findings contribute to further the understanding of physiological responses with the use of cold water immersion across a variety of settings. It is important to note that the recreational use of cold exposure varies with modalities (immersion, showers, etc.), temperature, and timing. Our protocol reflects the specific conditions within this study (single bout of 15 min in ~10°C water) and these may not be representative of all cold therapy practices or the effects of repeated immersions.

4.1. Shear Stress Responses

Cold water immersion is a robust cardiovascular stressor marked by the increases in heart rate, total peripheral resistance, and arterial blood pressure. Other stressors to the cardiovascular system, such as exercise or heat stress, can alter shear stress patterns that are linked with improvements in cardiovascular outcomes (Carter et al., 2014; Tinken et al., 2010). However, to our knowledge, the shear stress patterns during and after cold water immersion have not been previously investigated. Due to peripheral vasoconstriction with cold temperatures, the shear stress patterns in the brachial artery were calculated to investigate the implications of the vascular responses during and after an acute bout of cold water immersion. Understanding the shear stress patterns could determine if cold water immersion is another physiological stressor to target cardiovascular outcomes.

We hypothesized that there would be a reduction in peripheral shear stress due to peripheral vasoconstriction. During cold water immersion, there were no changes to shear stress patterns in the brachial artery amidst reductions in rectal temperature (~0.4°C), antegrade and mean blood flow, and forearm vascular conductance. A caveat to this interpretation is the measured limb was not immersed in the cold water, so that localized cooling effects of the upper limb were not present during immersion and we did not have a measure of skin temperature on the upper extremities. The 42% and 49% reduction in brachial artery blood flow and forearm vascular conductance were similar to reductions in femoral artery blood flow after 10 min of lower limb immersion in 8 and 22°C water (Gregson et al., 2011). Shear stress was not calculated by this previous research group to draw direct comparisons. Nonetheless, this distinction could be important when considering whole body cold exposure, which generates systemic responses, versus local limb cold exposure targeting specific muscle groups (i.e., inflammation, edema) during recovery from exercise.

We did not have a control group with thermoneutral water to quantify if the vascular responses were due to water immersion, independent of water temperature. This is a limitation of the current investigation and would have strengthened the interpretations of the results. Carter et. al investigated the shear stress patterns in the brachial artery during 10 min of water immersion in thermoneutral water (Carter et al., 2017). Our results are similar in that there was also a reduction in antegrade blood flow, antegrade shear, and vascular conductance in the brachial artery during immersion (Carter et al., 2017). Our results differ in that Carter et. al reported a greater retrograde shear areas under the curve and no change to brachial artery diameter (Carter et al., 2017). We report no change to retrograde shear despite a previous work showing increased retrograde shear with sympathetic activation (Padilla et al., 2010) and we report a reduction in brachial artery diameter. From this, it appears the vascular shear responses may be due to water immersion alone when comparing thermoneutral and cold water.

We observed substantial reductions in mean (35%) and antegrade (25%) shear stress at 30-min post-immersion. Oscillatory shear stress index did not change during immersion but was increased after cold exposure. These shear stress patterns coincided with the greatest reduction in rectal temperature (~1°C) from pre-immersion. This continued decline in rectal temperature after cold stress is defined as “afterdrop” and is the result of conductive cooling (Romet, 1988; Savard et al., 1985; Taylor et al., 2014). With this continued decrease in rectal temperature, peripheral vasoconstriction contributed to both the elevated blood pressure via peripheral resistance and alterations to shear stress patterns at the end of the post-immersion period. As stated above, sympathetic-mediated vasoconstriction in the small, downstream resistance vessels can shift the shear stress balance (greater retrograde/lessor anterograde) in part by increasing the critical closing pressure above central arterial pressure (Baccelli et al., 1985; Halliwill & Minson, 2010; Padilla et al., 2010).

Overall, the shear stress patterns suggest a greater relative contribution of retrograde shear stress and less anterograde shear to total shear stress. Increased retrograde/decreased anterograde shear balance tends to reduce flow-mediated dilation (FMD), indicative of endothelial dysfunction (Brunt & Minson, 2021). On the contrary, increasing antegrade shear, such as with an acute bout of exercise or heat stress, enhances the FMD responses therefore implying improvement of endothelial function (Brunt et al., 2016; Naylor et al., 2011; Tinken et al., 2009). While we did not measure endothelial function or FMD, based on previous work, the shear pattern following acute cold water immersion would have a transient negative impact on endothelial function.

The question remains if the shear stress patterns might differ during a more aggressive return to normal body temperature such as with rewarming (e.g., hot tub, sauna, warm shower) (Romet, 1988) or over a longer passive recovery period. This is a relevant question, as cyclic vascular patterns, such as with oscillatory lower body negative pressure, have been associated with beneficial vascular adaptations (Holder et al., 2019). It is unknown whether the combination of heat and cold stress, such as with contrast bathing (several sauna bathing sessions lasting 5 to 15 min separated by short 5 s to a few min of ice-bath immersion) could generate shear patterns beneficial to vascular health (Heinonen & Laukkanen, 2018). Contrast bathing has been associated with improvements in cardiovascular disease risk and mortality (Heinonen & Laukkanen, 2018). Based on our data, it is unlikely that the magnitude of changes in rectal temperature that resulted in altered shear stress would occur within the short cold immersion times (< 5 min) typically used in contrast bathing.

4.2. Mood & Blood Biomarkers Responses

Cold stress (water immersion, cold showers, winter swimming) is an example of a physiological stressor, such as exercise, that has been hypothesized to improve psychological health (Shevchuk, 2008). For example, regular winter swimmers reported a reduction in tension and an improvement in mood after four months of winter swimming compared to non-swimming controls. However, there could be an interaction effect due to the independent effects of swimming exercise on mood (Massey et al., 2020). Two recent reports of a single, acute bout of cold water immersion (one in open seawater and the other in a research laboratory at 20°C for 5 min) both reported reductions in negative mood and increase in positive mood in healthy, young adults, assessed with the Profile of Mood States and the PANAS (Kelly & Bird, 2022; Yankouskaya et al., 2023). Yankouskaya et al. further reported the increase in positive emotions were associated with the coupling of multiple regions of the brain whereas there was no association with the changes in negative emotions. This provides additional support for the use of objective measures to understand the potential mechanisms associated with changes in mood. A recent meta-analysis reported an increase in positive affect coincided with a decrease in negative affect and there was a stronger negative response than positive response, or “negativity bias”, to negative and positive stimuli (Joseph et al., 2020). We hypothesized that there would be an increase in positive affect and a reduction in negative affect. Our results differ from these previous reports as we only reported a reduction in negative affect with no change to positive affect. This raises the question of whether reducing negative affect or increasing positive affect, independently, is more relevant for overall psychological well-being. Frederickson and Losada’s Theory of Positivity Ratio states that for positive mental well-being, individuals must report three times more positive affect compared to negative affect (Fredrickson & Losada, 2005). Therefore, by reducing negative affect with no change to positive affect, these changes presumably contribute to a greater positivity ratio in healthy, young adults.

The proposed mechanisms of cold stress for improving mood and alleviating depressive and/or anxiety symptoms include acute stimulation of the stress response (i.e., sympathetic activation), parasympathetic stimulation, increases in β-endorphins, reductions in inflammatory cytokines, and the activation of feelings of wakefulness or energy (Shevchuk, 2008; Tipton et al., 2017). We hypothesized there would be an increase in β-endorphins and positive affect following immersion. Our results did not reveal changes in β-endorphins nor positive affect under our experimental conditions in a controlled-laboratory setting. Previous work with acute cold stress (water immersion or cold pressor tests) reported increases in both β-endorphins and dopamine (Sramek et al., 2000; Suzuki et al., 2007). Elevated levels of β-endorphins and/or dopamine could support an enhancement in mood, feelings of euphoria, or a reduction in depressive symptoms (Dunlop & Nemeroff, 2007; Froehlich, 1997). Our immersion protocol was completed indoors and was head-out water immersion compared to outdoors and full submersion including the face, both of which may occur with immersion in natural bodies of water. Exposure to both green (e.g., forests) and blue spaces (e.g., lakes, rivers, ocean) are associated with reductions in stress, anxiety, and depression and improvements in overall well-being (Beyer et al., 2014; de Vries et al., 2016; Maund et al., 2019). Submersion of the face in cool water stimulates the diving reflex (Friedman & Thayer, 1998; Lemaitre et al., 2015; Paulev, 1968) and increases parasympathetic activation (Hayashi et al., 1997). Greater negative affect, including feelings of anxiety, depression, and anger, is associated with reduced parasympathetic activity (Kemp et al., 2012; McCraty et al., 1995). Stimulation of the parasympathetic nervous system via facial cooling during cold water immersion may reduce this negative affect (McCraty et al., 1995) and contribute to a reduced stressor response for subsequent stressful situations (Pulopulos et al., 2018). As we only measured affect after one acute bout of cold water immersion, it could be that methodological differences such as repeated bouts of cold water immersion, water temperature, face submersion, or nature exposure are needed to promote changes in positive mood in healthy individuals. Lastly, it is possible that this exploratory investigation was underpowered to detect changes in positive affect.

In prior work, acute cold water immersion (14°C for up to 120 min) increased cortisol in men and catecholamines (norepinephrine and epinephrine) in both men and women, highlighting the acute stress response (Solianik et al., 2014). We hypothesized an acute increase during immersion followed by a decrease after immersion in cortisol and a reduction in negative affect. In contrast, we did not observe a rise in cortisol during immersion, but it is possible the peak of cortisol occurred between blood collection timepoints in the current study. We reported a 47% reduction in cortisol 180-min after immersion, which coincided with participants reporting less negative affect at the same timepoint. Higher levels of cortisol have been associated with more negative affect (Smyth et al., 1998) but it is important to note this interaction is complex (Wirth et al., 2011).

When interpretating our results, it is important to note the potential confounding factor of the diurnal rhythmicity of cortisol. Cortisol peaks within one hour of waking and gradually declines thereafter throughout the day, with transient increases following meal consumption (Debono et al., 2009). We did not control for the start time of the study visits within our protocol such that participants were at different portions of the cortisol circadian curve. The participants within the blood sampling group began the study visit between 9:00AM-12:30PM and therefore the 180-min blood draw was obtained between 1:30PM-5:00PM. Additionally, participants were instructed to avoid consumption of food at least 2 hours prior to the start of the study visit. Together, these may have confounded our results. The use of a control group, controlling for start times, a greater window of fasting, and/or controlling for meal composition would have strengthened our interpretations.

Debono et al. quantified serum cortisol across 24 hours in healthy adults (Debono et al., 2009) and these values were utilized as reference for the calculations of percent change across our participants. Debono et. al reveal about a 50% reduction in cortisol from 9:00 AM-5:00 PM, which is similar to the reduction shown in our cohort of individuals when grouped together for pre-immersion to 180-min post-immersion. Due to the diurnal rhythmicity of cortisol, we categorized participants into a morning (start time before 11:30AM) and afternoon (start time after 12:00PM). For our morning participants, there were similar reductions in cortisol (~36%) from pre to 180-min post compared to Debono et al. (~41%) from 9:00 AM-1:30 PM (Debono et al., 2009). For our afternoon participants, there was about a ~60% reduction in cortisol from pre-immersion to 180-min post-immersion which is much larger than the ~22% reduction reported by Debono et. al between the times 12:00–4:30 PM (Debono et al., 2009). Together, while the grouped data implies similar changes to the normal rhythmicity of cortisol, it appears that cold water immersion could have further reduced cortisol beyond the typical reductions in the afternoon group.

4.2. Considerations & Limitations

As stated in our Methods, we were limited in the amount of time we could spend with the participants and they could not stay within the laboratory during the 180-min post-immersion period. Participants were instructed to remain fasted, with the exception of drinking water, and avoid physical exercise. We do not have record of what occurred during these windows of time (i.e., fluid intake, physical activity) to confirm adherence. There is also the potential that participants were exposed to alternative stimuli that could influence variables including but not limited to affect. It is important to note the potential confounding influence of meal timing and composition on cortisol secretion (Slag et al., 1981). Our participants were instructed to refrain from the consumption of food for at least 2 hours prior to the start of the study visit but a longer period without food may be required. Also, we did not control for the quantity or quality of food prior to the study visit. While we included women participants, we did not schedule the study visit during a specific phase of the menstrual cycle. This was not a main outcome of the current investigation but could be considered for future investigations. As stated above, the use of a thermoneutral water immersion control condition and/or time-control without water immersion and consideration for confounding factors including changes in body posture and/or blood volume after immersion (Norsk et al., 1985) would have strengthened the interpretation of our results. In regard to the affect responses, we recruited individuals who were naïve to cold therapy to reduce the likelihood of social bias on responses. We used the PANAS survey as it includes rankings of both positive and negative feelings to reduce the bias towards only positive or negative feelings.

5. Conclusion

In summary, this exploratory study demonstrates that during an acute bout of cold water immersion, there were minimal changes to peripheral vascular shear stress in the brachial artery and the greatest changes occurred at the end of the 30-min post-immersion period. Further, 3 h after immersion participants reported fewer negative affect and cortisol levels were reduced. Cold water immersion is utilized across multiple disciplines and recreationally to target performance, recovery, cardiovascular health, and psychological well-being. Based on our results, it appears that vascular function may not be directly affected by the use of acute cold water immersion as performed in this study. Additionally, the coinciding subjective ratings (e.g., reduced negative feelings) with objective levels of circulating stress biomarkers (e.g., cortisol) highlight the potential use of cold water immersion to target psychological health.

Highlights.

  • Cold water immersion had a minimal effect on peripheral shear stress patterns.

  • β-endorphins and positive mood did not change after cold water immersion.

  • Cortisol and negative mood were lower 180 minutes after cold water immersion.

Acknowledgments

We thank the volunteers who participated in the protocol.

Funding Information

This work was supported in part by the Kenneth and Kenda Singer Endowed Professorship in Human Physiology (to C.T. Minson), the National Institutes of Health [R01HL144128], and the American Heart Association [19TPA34890033].

Abbreviations:

CWI

Cold Water Immersion

FVC

Forearm Vascular Conductance

OSI

Oscillatory Shear Index

Footnotes

Declaration of Interest: None

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