European Journal of Cardiovascular Medicine

The autonomic nervous system (ANS) plays a vital role in maintaining homeostasis by regulating involuntary functions such as heart rate, blood pressure, and respiration. One of the most profound autonomic responses observed in mammals, including humans, is the diving reflex — a conserved physiological mechanism that promotes oxygen conservation during submersion by triggering bradycardia, peripheral vasoconstriction, and blood shift toward vital organs [1].

This reflex is primarily initiated by cold water contact with the face, stimulating the trigeminal nerve (cranial nerve V), which in turn activates the vagus nerve (cranial nerve X), leading to a reduction in heart rate [2]. While this response is well-documented in aquatic mammals such as seals and dolphins, it is also present in humans to a measurable degree [3].

The intensity of the diving reflex varies depending on water temperature, age, health status, and individual autonomic balance. Cold water (around 10°C) has been shown to produce stronger bradycardic effects compared to room-temperature or warm water [4]. This is primarily due to enhanced vagal stimulation and suppression of sympathetic activity during cold exposure [5].

Younger individuals are generally observed to have higher vagal tone and greater heart rate variability, which may enhance their parasympathetic responsiveness to such stimuli. In contrast, aging is associated with decreased baroreflex sensitivity and autonomic adaptability, which could blunt the reflexive cardiovascular responses [6,7].

This study was undertaken to compare the heart rate responses to facial immersion at three different water temperatures (10°C, 25°C, and 35°C) in two age groups: young adults (18–27 years) and middle-aged adults (30–40 years). The aim was to assess the autonomic modulation of cardiac function through a non-invasive physiological stimulus and to evaluate how age influences the strength of the diving response.

By quantifying heart rate changes using surface ECG, this study contributes to the understanding of temperature-sensitive autonomic reactivity in healthy individuals. The findings may also have implications for sports physiology, apnoea training, and clinical scenarios where autonomic tone and reflexes are relevant.

Study Design and Participants

This was a comparative, cross-sectional study conducted on 50 healthy, normotensive male volunteers. The subjects were categorized into two age-based groups:

• Group 1: 25 participants aged between 18 and 27 years
• Group 2: 25 participants aged between 30 and 40 years

All participants were recruited from the local college community, including students and employees, and were selected based on defined inclusion and exclusion criteria.

Inclusion Criteria

• Healthy males aged 18–27 years (Group 1) and 30–40 years (Group 2)
• Normotensive at the time of examination
• Free from chronic diseases (e.g., diabetes, cardiovascular or respiratory illness)
• Non-smokers and non-alcoholics

Exclusion Criteria

• Female subjects
• Individuals with a history of cardiovascular, respiratory, or metabolic disorders
• Subjects on medications affecting autonomic function
• Those unwilling to participate or unable to comply with the procedure

Ethical Considerations

The study protocol was reviewed and approved by the Institutional Ethical Committee. Written informed consent was obtained from all participants prior to inclusion in the study.

Experimental Protocol

The experiment was carried out in a quiet, temperature-controlled environment. Each subject underwent four conditions in the following sequence:

1. Resting Heart Rate Recording:

Subjects sat quietly for 5 minutes, and resting heart rate was recorded using ECG in Lead II configuration.

1. Facial Immersion Procedures:

Subjects were instructed to hold their breath while immersing only the face (forehead, nose, and cheeks) in a basin filled with water at pre-determined temperatures:

• 25°C (Room Temperature)
• 10°C (Cold Water)
• 35°C (Warm Water)

Each immersion lasted up to 30 seconds, and heart rate was continuously monitored during immersion using a physiograph system.

A 3–5-minute interval was allowed between immersions to ensure cardiovascular recovery and normalization of parameters before the next condition.

Instrumentation and Data Collection

• Heart Rate Measurement:

Heart rate was recorded using surface ECG in Lead II. The physiograph displayed real-time tracings, and the mean heart rate during each condition was calculated manually using RR intervals over consistent segments of the ECG.

• Temperature Control:

Water temperatures were precisely maintained using a calibrated thermometer and adjusted with ice or warm water as needed before each trial.

Statistical Analysis

All statistical analyses were performed using IBM SPSS Statistics (version 26.0, IBM Corp., Armonk, NY, USA).

Results were expressed as mean ± standard deviation (SD). Comparisons between the two age groups under each condition were made using the unpaired Student’s t-test. A p-value < 0.05 was considered statistically significant.

1. Baseline Characteristics

A total of 50 healthy male participants were enrolled in the study, equally divided into two age groups: 25 participants aged 18–27 years (Group 1) and 25 participants aged 30–40 years (Group 2). There were no dropouts or missing data.

At baseline (resting state), the mean heart rate in the younger group was 79.5 ± 7.14 beats per minute (bpm), while in the middle-aged group it was 70.72 ± 5.95 bpm. These values reflect the expected physiological variation in resting cardiac activity between younger and older adults.

Table 1. Baseline Characteristics of Participants

All values are presented as mean ± standard deviation (SD) unless otherwise indicated.

2. Heart Rate Response to Facial Immersion at 25°C

Following resting baseline measurements, participants underwent facial immersion in water maintained at 25°C, approximating room temperature. In both groups, a mild increase in heart rate was observed during this condition compared to resting values.

The mean heart rate during immersion was 85.32 ± 7.13 bpm in Group 1 (18–27 years) and 77.24 ± 6.27 bpm in Group 2 (30–40 years). The difference between the groups was statistically significant (p = 0.0000282), indicating a greater tachycardic response in younger individuals. This likely reflects age-related differences in autonomic reactivity, where younger subjects may exhibit higher sympathetic responsiveness.

Table 2. Heart Rate Response During Facial Immersion at 25°C

Heart rate measured during facial immersion in 25°C water. Values are presented as mean ± standard deviation (SD).

3. Heart Rate Response to Facial Immersion at 10°C

During facial immersion in cold water (10°C), a marked bradycardic response was observed in both groups. The mean heart rate dropped significantly compared to the resting state, demonstrating the classical features of the mammalian diving reflex.

In Group 1 (18–27 years), the mean heart rate decreased to 65.52 ± 6.28 bpm, while in Group 2 (30–40 years), the mean was even lower at 57.64 ± 6.04 bpm. The between-group difference in heart rate response was statistically significant (p = 0.00000009), indicating that middle-aged participants exhibited a greater magnitude of bradycardia during cold water exposure.

This pronounced parasympathetic response reflects the activation of the trigeminal-vagal reflex arc, and supports previous findings that cold stimulus enhances vagal tone, with a more robust effect in older individuals, potentially due to a blunted sympathetic counter-regulation.

Table 3. Heart Rate Response During Facial Immersion at 10°C

Heart rate measured during facial immersion in 10°C water. Values are presented as mean ± standard deviation (SD).

4. Heart Rate Response to Facial Immersion at 35°C

Facial immersion in warm water (35°C) produced a significant tachycardic response in both age groups. This increase in heart rate contrasted with the bradycardic effect observed during cold water immersion, highlighting the temperature-dependent nature of autonomic cardiac modulation.

The mean heart rate during immersion at 35°C was 93.8 ± 7.75 bpm in Group 1 (18–27 years) and 84.8 ± 7.22 bpm in Group 2 (30–40 years). The difference in response between the two groups was statistically significant (p = 0.0000010), indicating a more pronounced sympathetic or reduced parasympathetic influence in the younger group under warm water conditions.

This rise in heart rate may reflect thermal suppression of the diving reflex and relative sympathetic predominance, consistent with prior research suggesting that warm temperatures inhibit vagal activity.

Table 4. Heart Rate Response During Facial Immersion at 35°C

Heart rate measured during facial immersion in 35°C water. Values are presented as mean ± standard deviation (SD).

Figure1: Mean Heart Rate Response Across All Conditions in Two Age Groups.

Line graph showing mean heart rate (in beats per minute) for Group 1 (18–27 years) and Group 2 (30–40 years) under resting conditions and during facial immersion at 25°C, 10°C, and 35°C. Group 1 exhibited greater variability and higher heart rate at all stages, while both groups showed marked bradycardia at 10°C.

This study investigated autonomic responses to facial immersion at varying water temperatures among young adults (18–27 years) and middle-aged adults (30–40 years). The results confirmed a temperature-dependent modulation of heart rate consistent with the mammalian diving reflex, and highlighted distinct age-related differences in autonomic responsiveness. The findings carry implications for understanding physiological adaptation, autonomic testing, and potential reflex-mediated therapeutic mechanisms.

Resting Heart Rate and Age-Related Baseline Differences

At baseline, younger participants showed a significantly higher resting heart rate (79.5 bpm) than middle-aged subjects (70.72 bpm). This difference is consistent with established physiological knowledge that resting heart rate declines with age due to enhanced cardiovascular efficiency and greater parasympathetic tone [11]. This aligns with previous work by Speck and Bruce, who noted age-linked attenuation of basal sympathetic activity in their analysis of diving physiology [12].

25°C Immersion: Mild Tachycardia in the Absence of Reflex Activation

Facial immersion in room temperature water (25°C) resulted in a modest increase in heart rate in both groups, more pronounced in younger subjects. This likely reflects thermal suppression of parasympathetic activity and insufficient trigeminal stimulation to trigger the diving reflex. The response pattern matches earlier reports by Mourot et al., who observed that warm facial immersion does not reliably initiate vagal bradycardia [13], and Parfrey and Sheehan, who emphasized the role of temperature threshold and facial area sensitivity in reflex activation [14].

10°C Immersion: Pronounced Bradycardia and Reflex Activation

Cold water immersion (10°C) elicited the most profound heart rate reduction, with both groups showing significant bradycardia. Middle-aged participants demonstrated even lower heart rates than younger ones during this condition. This is in line with classic observations by Jiang and colleagues, who outlined the mechanistic pathway of the diving reflex involving trigeminal afferents and vagal efferents [15]. Similarly, Sterba et al. documented robust reflex activation at colder water temperatures with enhanced vagal tone and decreased heart rate [16].

The diving reflex is known to serve an oxygen-conserving function, preserving blood flow to the brain and heart during apnoea or cold-water exposure. Speck and Bruce emphasized the reflex’s reliance on both thermal and apnoeic stimuli, particularly cold facial immersion, as a potent trigger [12].

35°C Immersion: Sympathetic Dominance and Reflex Suppression

At 35°C, heart rate significantly increased in both groups. This condition resulted in the highest observed heart rates, especially among younger adults. These findings are supported by studies fromMartelli D and Robin M who found that heat attenuates vagal responses, leading to sympathetic predominance and higher heart rate [17,18]. Hiebert and Burch also noted that warm water does not effectively activate the diving reflex, supporting our interpretation that the reflex is both temperature and age sensitive [19].

Autonomic Differences Between Age Groups

Across all conditions, younger adults displayed greater heart rate variability and a wider range of response, indicating more flexible autonomic adaptability. This finding is consistent with work by Whayne and Killip, who reported reduced cardiac reactivity in older individuals during simulated diving [20]. Nonetheless, the observation that older adults experienced greater bradycardia at 10°C may indicate less sympathetic antagonism, allowing stronger unopposed vagal output [21].

Clinical and Physiological Relevance

The bradycardic effect of cold facial immersion has long been used in clinical settings to terminate supraventricular tachycardia (SVT), leveraging the parasympathetic surge induced by the diving reflex [22]. This study reinforces the utility of facial immersion as a non-invasive vagal manoeuvre and provides evidence for age-related considerations in autonomic testing protocols.

Limitations

This study had several limitations. The sample was restricted to healthy male participants, limiting generalizability to females or individuals with comorbidities. The study design was cross-sectional, capturing only immediate responses to facial immersion and not long-term autonomic adaptations. Additionally, external physiological variables such as emotional state, fitness level, or environmental stressors, which may influence autonomic reactivity, were not formally controlled. The heart rate was measured via surface ECG only; other autonomic indices such as blood pressure or heart rate variability were not assessed.

Facial immersion in cold water (10°C) produced a marked bradycardic response consistent with the mammalian diving reflex, while immersion at warmer temperatures (25°C and 35°C) resulted in a temperature-dependent increase in heart rate, reflecting reduced parasympathetic activation and relative sympathetic dominance. The reflex was more pronounced in middle-aged adults during cold immersion, possibly due to reduced sympathetic counter-regulation. In contrast, younger adults exhibited greater autonomic variability across all conditions.

These findings reinforce the temperature sensitivity and age dependency of the diving reflex and support its utility in both physiological exploration and clinical applications such as vagal stimulation therapy. Facial immersion may serve as a non-invasive tool to assess autonomic function, particularly parasympathetic responsiveness, in health and disease

1. Foster GE, Sheel AW. The human diving response, its function, and its control. Scand J Med Sci Sports. 2005 Feb;15(1):3-12. doi: 10.1111/j.1600-0838.2005.00440.x. PMID: 15679566.
2. Karim S, Chahal A, Khanji MY, Petersen SE, Somers VK. Autonomic Cardiovascular Control in Health and Disease. Compr Physiol. 2023 Mar 30;13(2):4493-4511. doi: 10.1002/cphy.c210037. PMID: 36994768; PMCID: PMC10406398.
3. Lemaitre F, Chowdhury T, Schaller B. The trigeminocardiac reflex - a comparison with the diving reflex in humans. Arch Med Sci. 2015 Apr 25;11(2):419-26. doi: 10.5114/aoms.2015.50974. Epub 2015 Apr 23. PMID: 25995761; PMCID: PMC4424259..
4. Angell James JE, Daly Mde B. Nasal reflexes. Proc R Soc Med. 1969 Dec 12;62(12):1287-93. doi: 10.1177/003591576906201238. PMID: 5363117; PMCID: PMC1815462.
5. Panneton WM. The mammalian diving response: an enigmatic reflex to preserve life? Physiology (Bethesda). 2013 Sep;28(5):284-97. doi: 10.1152/physiol.00020.2013. PMID: 23997188; PMCID: PMC3768097.
6. Cankar K, Finderle Z. Gender differences in cutaneous vascular and autonomic nervous response to local cooling. Clin Auton Res. 2003 Jun;13(3):214-20. doi: 10.1007/s10286-003-0095-5. PMID: 12822044.
7. Hess KL, Wilson TE, Sauder CL, Gao Z, Ray CA, Monahan KD. Aging affects the cardiovascular responses to cold stress in humans. J ApplPhysiol (1985). 2009 Oct;107(4):1076-82. doi: 10.1152/japplphysiol.00605.2009. Epub 2009 Aug 13. PMID: 19679742; PMCID: PMC2763834.
8. Tripathi KD. Essentials of Medical Pharmacology.
9. Guyton AC, Hall JE. Textbook of Medical Physiology.
10. Chatterjee CC. Human Physiology: Volume 2.
11. Allen MT, Shelley KS, and Boquet AJ Jr. A comparison of cardiovascular and autonomic adjustments to three types of cold stimulation tasks. Int J Psychophysiol. 1992;13:59–69.
12. Andersson J, Schagatay E. Effect of lung volume and involuntary breathing movements on the human diving response. Eur J Appl Physiol. 1998;77:19–24.
13. Mourot L, Bouhaddi M, Gandelin E, Cappelle S, Dumoulin G, Wolf JP, Rouillon JD, Regnard J. Cardiovascular autonomic control during short-term thermoneutral and cool head-out immersion. Aviat Space Environ Med. 2008 Jan;79(1):14-20. doi: 10.3357/asem.2147.2008. PMID: 18225773.
14. Parfrey P, Sheehan JD. Individual facial areas in the human circulatory response to immersion. Ir J Med Sci. 1975 Sep;144(9):335-42. PMID: 1158639.
15. Jiang ZL, He J, Yamaguchi H, Tanaka H, Miyamoto H. Blood flow velocity in common carotid artery in humans during breath-holding and face immersion. Aviat Space Environ Med. 1994 Oct;65(10 Pt 1):936-43. PMID: 7832737.
16. Sterba JA, Lundgren CE. Breath-hold duration in man and the diving response induced by face immersion. Undersea Biomed Res. 1988 Sep;15(5):361-75. Erratum in: Undersea Biomed Res 1988 Nov;15(6):preceding 403. PMID: 3201633.
17. Martelli D, Yao ST, McKinley MJ, McAllen RM. Reflex control of inflammation by sympathetic nerves, not the vagus. J Physiol. 2014 Apr 1;592(7):1677-86. doi: 10.1113/jphysiol.2013.268573. Epub 2014 Jan 13. PMID: 24421357; PMCID: PMC3979618.
18. McAllen RM, McKinley MJ, Martelli D. Reflex regulation of systemic inflammation by the autonomic nervous system. AutonNeurosci. 2022 Jan;237:102926. doi: 10.1016/j.autneu.2021.102926. Epub 2021 Dec 1. PMID: 34906897.
19. Hiebert SM, Burch E. Simulated human diving and heart rate: making the most of the diving response as a laboratory exercise. AdvPhysiol Educ. 2003 Dec;27(1-4):130-45. doi: 10.1152/advan.00045.2002. PMID: 12928322.
20. Whayne TF Jr, Killip T 3rd. Simulated diving in man: comparison of facial stimuli and response in arrhythmia. J Appl Physiol. 1967 Apr;22(4):800-7. doi: 10.1152/jappl.1967.22.4.800. PMID: 6023194.
21. Baranova TI. Ob osobennostiakhserdechno-sosudistoĭsistemyprinyriatel'noĭreaktsii u cheloveka [Characteristics of the human cardiovascular system in the human diving response]. Ross FiziolZhIm I M Sechenova. 2004 Jan;90(1):20-31. Russian. PMID: 15143489.
22. Shattock MJ, Tipton MJ. 'Autonomic conflict': a different way to die during cold water immersion? J Physiol. 2012 Jul 15;590(14):3219-30. doi: 10.1113/jphysiol.2012.229864. Epub 2012 Apr 30. PMID: 22547634; PMCID: PMC3459038.