Cold Therapy - Protocols & safety
Cold shock, blood pressure and who should be careful
Last updated 30 September 2026. How we write this .
Cold shock is the gasp, fast breathing and jump in heart rate and blood pressure in your first minute in water below 15°C. It causes most cold-water immersion deaths, and heart problems raise the risk.
Start here Cold shock is the gasp, fast breathing and jump in heart rate and blood pressure in your first minute in water below 15°C. It causes most cold-water immersion deaths, and heart problems raise the risk.
By the end, you will understand What happens in the first minute? Why does cold water raise blood pressure? Who should be careful with cold water? Key words used in this lesson Meta-analysis A statistical method for combining the results of multiple studies into a single estimate. More reliable than any one study, but bounded by the quality of what went into it. Full plain-English glossary
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Cold shock is your body's automatic reaction to sudden cold on the skin. Within seconds of entering water below about 15°C you gasp, start breathing fast and hard, and your heart rate and blood pressure jump. It peaks around 30 seconds in and settles over roughly two minutes, and that short window, rather than hypothermia, is where most of the danger sits.
That matters for anyone trying ice baths, sea dips or open-water swimming. The RNLI points out that average sea temperatures around the UK and Ireland are just 12°C, so for most of the year British water is cold enough to set off the full response.
What happens in the first minute?
Cold receptors in the skin fire all at once. The first thing most people notice is an involuntary gasp, followed by breathing they cannot control. The RNLI says breathing rate can rise as much as tenfold. If your face is underwater during that gasp, you breathe in water, and the RNLI notes that about half a pint of sea water in the lungs is enough to start an adult man drowning.
Lab studies put numbers on this. In a 2000 University of Portsmouth study led by Mike Tipton, healthy male volunteers lowered into 10°C water for the first time breathed on average 47 times a minute in the first 30 seconds, with heart rates around 128 beats a minute. Breath-holding collapses too. In a 2006 Portsmouth study of 32 volunteers in 11°C water, the average maximum breath-hold on entry was about 24 seconds.
This is how a strong swimmer can get into trouble a few metres from the edge. Beat Knechtle and colleagues, reviewing the evidence in 2020, concluded that this initial shock accounts for the majority of cold-water immersion deaths.
Why does cold water raise blood pressure?
To protect your core, the blood vessels in your skin clamp shut. That raises the resistance your heart has to pump against, while the heart is also beating faster. Pressure rises and the heart's workload rises at the same moment. The RNLI warns that cold water shock can cause heart attacks, even in people who are relatively young and healthy.
There is a second, less obvious problem. Cold water on the face, especially with a held breath, sets off the diving response, which slows the heart. Cold shock pushes it to speed up. In 2012 Michael Shattock of King's College London and Mike Tipton called this autonomic conflict: the two branches of the nervous system that control heart rate pulling in opposite directions at once. When people submerge in cold water while holding their breath, 62 to 82% of young, fit, healthy participants develop heart rhythm disturbances. Shattock and Tipton argue that in vulnerable people the same clash may explain some deaths that were put down to drowning or hypothermia.
Be clear about the limits of this evidence. Much of it comes from small lab studies, often of young men, and none of it can predict how one particular person will respond. Controlled cold exposure also behaves differently from a sudden plunge. A 2024 meta-analysis of 24 studies in healthy people, covering cold water immersion and cryotherapy chambers, found on average only a slight rise in blood pressure and a small fall in heart rate. The sharpest danger belongs to sudden, unplanned immersion, especially when the head goes under.
Who should be careful with cold water?
People with heart disease or a heart rhythm problem. The British Heart Foundation advises discussing cold water swimming with your doctor or nurse specialist first, and cites cardiac rehabilitation guidance that water between 26 and 33°C is safest for people with heart conditions. People with high blood pressure, particularly if it is not yet under control. The cold shock surge lands on top of whatever pressure you already have. People with long QT syndrome, a heart rhythm condition that can be inherited or caused by some medicines. Shattock and Tipton point to a strong link between swimming and sudden cardiac arrest in children with the inherited form. People with narrowed heart arteries (ischaemic heart disease), where a sudden rise in the heart's demand for oxygen meets a reduced supply. Anyone who has been drinking. Alcohol speeds the onset and progression of hypothermia, and Knechtle's review advises avoiding it before winter swimming. Beginners. The same review flags people who are unfamiliar with cold water, or not adapted to it, as being at particular risk of dying from cold shock. Does getting used to cold water make it safe?
It makes it safer, not safe. In Tipton's 2000 study, eight men who completed a short series of immersions in 15°C water cut their breathing rate in the first 30 seconds from 47 to 24 breaths a minute, and their heart rate from 128 to 109. Seven months later the effect was still clear. After 14 months, heart rate stayed lower but breathing had largely drifted back.
Getting used to it blunts the shock but does nothing for the later stages. After about three minutes, cooling nerves and muscles in the arms and legs start to make swimming harder. After about 30 minutes, the body is heading towards hypothermia, a core temperature below 35°C, which the NHS treats as a medical emergency. Knechtle's review is plain: even the most experienced ice swimmers still face a risk of death, and cold water swimming should only be done with appropriate supervision.
What should you do if you fall in, or someone gets into trouble?
If you end up in cold water unexpectedly, the RNLI's advice is to take a minute. The worst of the shock passes in under a minute, so resist the urge to swim hard straight away. Relax, float, get your breathing under control, then call for help or swim to safety if you can.
If you plan to go in, the RNLI suggests checking conditions and water temperature first, wearing a wetsuit suited to how long you will be in, and using a flotation device, which greatly improves your chance of getting through the initial shock.
Afterwards, watch for signs of hypothermia: shivering, pale and cold skin, slurred speech, slow breathing, tiredness or confusion. The NHS says to call 999 or go to A&E if you think someone has hypothermia.
This lesson is general education about cold water risks, not medical advice. If you have a heart condition, high blood pressure or another long-term illness, talk to your GP before trying cold water immersion.
Related in this classroom Related in other classrooms The studies behind this page Cold water shock - RNLICold water swimming: benefits and risks. A narrative review - International Journal of Environmental Research and Public Health, 2020Permanence of the habituation of the initial responses to cold-water immersion in humans - European Journal of Applied Physiology, 2000Breath-hold performance during cold water immersion: effects of psychological skills training - Aviation, Space, and Environmental Medicine, 2006'Autonomic conflict': a different way to die during cold water immersion? - Journal of Physiology, 2012The effects of cold exposure (cold water immersion, whole- and partial-body cryostimulation) on cardiovascular and cardiac autonomic control responses in healthy individuals: a systematic review, meta-analysis and meta-regression - Journal of Thermal Biology, 2024Cold water swimming: is it bad for your heart? - British Heart Foundation, 2023Hypothermia - NHSSources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.