# Ice-water self-rescue raised soldiers’ heart rates while ear thermometers falsely suggested hypothermia

> A fall through lake ice can trigger gasping, rapid breathing, panic and a sharp rise in the work demanded from the heart. Military personnel train for such emergencies because the first moments in freezing water can decide whether someone can control their...

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Published: 2026-08-23T13:25:02+00:00
Categories: Explainer, Health

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A fall through lake ice can trigger gasping, rapid breathing, panic and a sharp rise in the work demanded from the heart. Military personnel train for such emergencies because the first moments in freezing water can decide whether someone can control their breathing and climb back onto the ice.

A study involving 80 German Mountain Infantry soldiers found that supervised **ice-water immersion** produced strong signs of physical stress without causing life-threatening complications in the healthy participants. Their heart rates rose, while the timing between beats became much more variable. Lung tests remained broadly stable.

The most troubling measurement came from the thermometers. Ear readings suggested that many soldiers had become hypothermic, although swallowed temperature capsules showed that their internal temperatures remained near normal. The research was published in [**Frontiers in Physiology**](https://doi.org/10.3389/fphys.2025.1679550) on November 21, 2025.

## Inside the frozen-lake rescue drill

The exercise took place in Norway during a German Armed Forces training program called "Self-Rescue in an Ice Breakthrough Emergency." Air temperature stood at minus 10 degrees Celsius, while the water beneath the ice measured only 0.5 degrees Celsius.

Trainers cut a five-meter-wide square opening in a frozen lake. The soldiers entered the water fully clothed in standard field uniforms, without waterproof or insulated outer layers. Their skis and backpacks were removed and they carried ski poles to help pull themselves back onto the ice.

Most participants briefly went beneath the surface for less than five seconds. They then spent about 30 seconds controlling their breathing, avoiding another submersion and planning their escape. After climbing out with the poles, they rolled in snow to remove water from their clothing. Medical measurements continued in a nearby tent and during later outdoor survival training. The trial had been registered in the [German Clinical Trials Register](https://www.drks.de/search/de/trial/DRKS00032345/details) before the results were published.

## How sudden immersion stressed the heart

Cold water striking the skin produces an immediate **cold shock response**. Blood vessels near the skin narrow to reduce heat loss, which sends more blood toward the center of the body. The heart then has to pump against greater resistance. Sudden gasping and rapid breathing add to the strain.

Cold water on the face can also activate the diving reflex. The brain responds as though the body is preparing to remain underwater, which can slow the heart through a branch of the nervous system that supports rest and energy conservation. At the same time, fear, cold and physical effort can speed the heart through the body's emergency response.

Researchers call the overlap **autonomic conflict**. Two powerful nerve responses influence the heart at once, one pushing its rate upward and the other applying a slowing force. In some people, especially those with heart disease, the mixed signals may contribute to an abnormal rhythm.

## No dangerous rhythm changes appeared

Usable electrocardiogram recordings were collected from 34 soldiers. Their median heart rate rose from 114 beats per minute before immersion to 131 afterward, an increase of 17 beats per minute. The already high starting rate may have reflected anticipation, cold exposure and activity before the jump.

The spacing between normal heartbeats also changed sharply. A measure of **heart rate variability** rose from a median of 46.5 milliseconds to 136.5 milliseconds. Researchers interpreted the increase as evidence that the nerve pathway linked to the diving reflex was active even while heart rate climbed.

Two participants had five isolated extra beats from the heart's lower chambers. The beats did not develop into an unstable rhythm. The study abstract reported that the exercise ended "without malignant arrhythmias," referring to dangerous conditions such as ventricular fibrillation. All participants remained stable and reported no clinical symptoms during the observation period.

The lack of **malignant arrhythmias** provides useful evidence for supervised training, although it does not erase the known dangers of an uncontrolled fall into cold water. The soldiers had passed fitness and cardiac health checks, medical specialists were nearby and the exercise followed a planned rescue procedure.

## Lung function remained stable

Sudden immersion often causes an involuntary gasp followed by fast breathing. A person who cannot regain control may inhale water, develop numbness, or lose the strength needed for self-rescue. Heavy breathing can also lower carbon dioxide in the blood, which may cause dizziness and reduced blood flow to the brain.

The researchers used **spirometry**, a test that measures how much air a person can force from the lungs and how quickly it moves. Complete results were available for 26 soldiers, with 23 included in the reported table of lung measurements.

Major measures of lung capacity and airflow showed no clinically important decline after immersion. No participant reported breathing trouble during the monitored period. Physical effort during the escape may have helped keep the airways open because the body's stress hormones can relax airway muscles for a short time.

The result describes brief exposure in healthy, trained adults. Longer immersion allows internal temperature to fall and can gradually weaken breathing. People with asthma or sensitive airways may also respond differently to severe cold.

## Ear thermometers reported misleading temperatures

Medical teams need an accurate **core body temperature** when deciding how to treat a cold-water casualty. A reading below 35 degrees Celsius can support a diagnosis of hypothermia, which affects handling, transport, rewarming and hospital choice.

The study compared infrared **tympanic thermometry**, which measures temperature inside the ear canal, with a swallowed electronic capsule. Twenty-three usable capsule records and 34 pairs of ear readings were available.

Before immersion, the capsules recorded a median temperature of 37.6 degrees Celsius. The median fell by only 0.2 degrees, reaching 37.4 degrees after the rescue. Such a small change fits the short time spent in the water, since the body's deeper tissues cool more slowly than exposed skin.

The unprotected ear gave a median reading of 34.5 degrees Celsius after immersion, down from 36.1 degrees. The protected ear read 34.7 degrees, down from the same starting value. After the exercise, ear readings were a median 2.8 degrees below capsule measurements.

Those readings could have placed many participants below the usual hypothermia threshold even though the capsule data showed normal internal temperatures. Clinical guidance on [accidental hypothermia](https://journals.sagepub.com/doi/10.1016/j.wem.2019.10.002) stresses the value of core measurements and careful clinical assessment when reliable equipment is unavailable.

## Why an earplug failed to improve accuracy

One ear canal was sealed with a standard military foam earplug before immersion, while the other remained open. Researchers wanted to learn whether blocking direct water contact would preserve a more accurate ear temperature.

The plug produced no significant improvement. Its foam resisted water to some degree, although it did not create a waterproof seal. Water could still reach the canal and cold surrounding air could cool the outer ear after the soldier left the lake.

Infrared ear devices also estimate temperature from a small area whose conditions can change quickly. Water, a poorly positioned probe, cold skin, or an exposed ear canal can alter the result. The swallowed **ingestible temperature capsule** remained deep inside the body, where brief surface cooling had much less immediate influence.

The paper concluded that "Tympanic thermometry proved unreliable following immersion, even after ear channel occlusion." Emergency workers could therefore misclassify a cold-water casualty if they depend on a standard ear reading alone.

## What the findings mean for cold-water training

Structured practice may help fit personnel learn a sequence that supports survival. A person can focus first on controlling the initial gasp, keeping the face above water and planning a route back onto firm ice. The soldiers in the study used tools and followed a known procedure under close supervision.

The results support the safety of carefully planned exercises for screened participants. Medical professionals monitored the session, a heated tent stood nearby and the trainees moved into heat-preservation tasks after leaving the water. Such controls differ greatly from an accidental fall involving injury, isolation, heavy equipment, or an uncertain escape route.

The [CDC overview of cold illness](https://www.cdc.gov/yellow-book/hcp/environmental-hazards-risks/heat-and-cold-illness-in-travelers.html) notes that water removes body heat rapidly. A short exposure can produce cold shock before the deeper body has cooled enough to meet the definition of hypothermia. Longer exposure adds progressive cooling, weaker muscles and reduced mental ability.

For rescue teams, the thermometer findings may have the broadest practical value. A falsely low reading could lead to an incorrect estimate of severity. Better field devices are needed for alert cold-water casualties when invasive temperature probes are unsuitable.

## Limits of a study involving fit soldiers

The participants formed a narrow group. Their average age was 25.7 years and 78 of the 80 soldiers were male. All had been cleared for military training, while 12 had previous cold-water immersion experience. The results may differ for older adults, children, people with heart conditions and anyone with limited swimming or rescue ability.

Operational demands also meant that every soldier could not complete every measurement. Motion during self-rescue interfered with ECG recordings, so the researchers studied short segments before, during and immediately after immersion. Blood pressure was not measured and the study had no separate control group.

The capsules also followed an unusual schedule. Temperature pills are commonly swallowed several hours before measurements, giving them time to travel farther through the digestive system. Training conditions required the soldiers to swallow prewarmed capsules shortly before immersion. Researchers waited for stable, physically plausible readings, although the altered timing remains a source of uncertainty.

The study was a field feasibility project involving a **healthy military cohort**, which gave the researchers realistic conditions at the cost of laboratory control. Larger studies could include a wider range of participants, longer monitoring and temperature methods designed specifically for cold-water emergencies. The paper and author details are also indexed in [PubMed](https://pubmed.ncbi.nlm.nih.gov/41357169/).

For now, the findings show how strongly the nervous system can react during a brief ice-water escape while deep body temperature changes very little. They also reveal how quickly a convenient thermometer can give rescuers the wrong picture after a person's head meets freezing water.