A systematic review of HRV during diving in very cold water

Figures & data

Figure 1. PRISMA flow chart with the article search process. After the removal of duplicates and studies that did not fulfil the pre-defined criteria for the review, a final number of 26 articles were included in the review. Of those included, only three articles met the criteria for very cold water studies (< 5°C).

Table 1. Studies included in this review. The table shows the characteristics of subjects, information on the type of exposure and temperature during exposure as well as the main findings in the studies. The reported temperatures are water temperatures for in-water studies and air/gas temperatures for studies from hyperbaric chambers and facilities. Studies from very cold water are highlighted with grey background colour.

	Subjects	Exposure and breathing gas					Temperature	Autonomic nervous system
	n	Subjects (as described in article)					Water/gas (°C)	Main finding(s)
Systematic review and meta-analysis[30]	311	n/a	n/a	n/a	n/a	n/a	n/a	PNS activity increased during exposure but not after exposure. Total body immersion had a greater effect than forehead cooling.
Original articles
SCUBA diving
[31]	25	sport divers	n/a	n/a	dynamic	air	n/a	PNS activity increased during diving. HRV measure length shortening to 3 min (time domain) and to 5 min (frequency domain) without loss of information possible.
Schipke and Pelzer, 2001[32]	25	trained SCUBA divers	0–4	10	static	air	27	The trigeminocardiac reflex did not show a significant role in warm water. Main promotor of PNS activity was head out of water immersion.
[19]	10	SCUBA divers	19.7	39.5	static	air	16	PNS activity increased and SNS activity decreased during dives.
Flouris et al. 2009[33]	10	healthy adults	5	20	static	air	27	PNS activity decreased during dives due to psychological stress.
[34]	10	navy divers	4.6	360	static	air	32–33	Consecutive day diving showed a successively decreasing vagal PNS responce to diving.
[35]	24	experienced SCUBA & rebreather divers	10–61	40	n/a	air/nitrox/trimix	10–15	PNS activity increased during diving, and it increased proportionally more with greater depth and higher oxygen partial pressure.
[36]	13	experienced divers	n/a	n/a	dynamic	air/nitrox	n/a	PNS activity increased during diving, and higher oxygen partial pressure increased PNS activity more.
[37]	4	navy divers	6	80–91	static	air	0	PNS activity increased during diving. PNS activity increase due to the trigeminocardiac reflex was short lived.
[38]	7	experienced rebreather divers	90–120	180	dynamic	trimix	27	PNS activity was increased after diving, compared to pre-dive measures.
[39]	18	experienced divers	30	20–65	dynamic	air/nitrox	17–33	PNS activity increased during diving, and higher oxygen partial pressure increased PNS activity more.
[18]	26	experienced rebreather divers	45	66	static/dynamic	trimix	2–4	PNS activity increased during diving. PNS activity increase due to the trigeminocardiac reflex is short lived. Cold seemed to be the strongest promoter of PNS activity.
[40]	18	divers from commers./priv. dive clubs	5.7–40.1	35	static	air/nitrox	5–23	Non-selected divers in real-world conditions showed an overall increase in ANS (SNS and PNS) activity without PNS dominace.
Hyperbaric chambers and facilities
[41]	4	experienced saturation divers	450	n/a	static	heliox	n/a	For unknown reasons, breath holding in increased pressure caused a progressive shortening in cardiac R-R intervals.
[42]	10	professional divers	15	75	static	air/oxygen	n/a	PNS activity increased more in a chamber with 2.5 bar when breathing 100% oxygen than when breathing air.
[43]	16	normal volunteers	330	n/a	static	heliox	n/a	PNS activity increased during the exposure and had a negative correlation to urinary cathecolamines.
[44]	6	volunteers	300–400	n/a	static	heliox	n/a	PNS and SNS activity increased concurrently after the exposure.
[9]	10	divers	15	60	static	air/oxygen	n/a	Non-linear and frequency domain measures showed a strong correlation in this study. Therefore, it is justified to report either parameter in hyperbaric experiments.
[45]	10	divers	0–30	40	static	air	n/a	PNS activity increased as pressure was elevated.
[46]	10	healthy individuals	45	n/a	static	trimix/nitrox/oxygen	n/a	Post-exposure PNS activity increased only for a normobaric control group, not for those exposed to elevated pressure.
[47]	17	healthy subjects	0–40	n/a	static	air	n/a	Self-organizing maps (artificial intelligence) could be trained to use HRV to predict different body responces during exposure to elevated pressure.
Static apnea and breath hold diving
[48]	9	elite breath-hold divers	n/a	n/a	static/dynamic	air	26.9	PNS activity increased in both static and in dynamic apnea. In dynamic apnea, the PNS response blunts the effects of exercise induced SNS activity.
[49]	11	trained breath-hold divers	n/a	n/a	static	air	26–27	Breathholding under water and in air caused a triphasic heart rate kinetics: first a strong decrease, then a short linear increase and finally a decrease.
Immersed subjects
[50]	10	healthy subjects	0	360	static	air	32–33	Estimated with HRV measures, a differential activation-depression path was seen in the cold pressure and handgrip test compared to the head-up-tilt test.
Elite synchr. swimmers (incl. immersion)
Solana-Tramunt 2019[51]	12	elite synchonized swimmers	n/a	n/a	dynamic	air	n/a	Pronounced bradycardia was observed during training sessions corresponding to apneic exercise, but no clinically meaningful changes in HRV measures occured.
Case reports
[52]	3	experienced divers	25	39	dynamic	nitrox	5–10	Both PNS and SNS activity increased shortly after a stressufull incident in diving.
[53]	4	military divers	n/a	10	n/a	air	−1.7	PNS activity decreased from baseline during diving, when measured 10 min starting from 2 min after the beginning of the dives.

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