Man in cold water : heart rate and electrocardiographic responses
Date
1977
Authors
McKay, William Ross
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Abstract
The heart rate and electrocardiographic (ECG) response to immersion in cold ocean water (4.6 - 18.2°C) was studied in conscious normal humans in relation to several variables. Individual responses were studied but most results were based on means of 12 (usually 6 male and 6 female) subjects. Continuous monitoring of heart rate, ECG, rectal temperature and oxygen consumption was carried out on subjects before and during cold immersion (until rectal temperature fell to 35.0°C), and subsequently during rewarming in a heated whirlpool bath.
In immersions in water below 10.56C where subjects held still, heart rate remained elevated (95 bpm at 4.6°C) or increased (82 bpm to 90 bpm at 10.5°C) as rectal temperature decreased while in the cold water. A positive correlation between heart rate and metabolic rate (oxygen consumption) was found, accounting for this increase. In the early rewarm period heart rate declined as rectal temperature continued to drop, then increased as subjects began to rewarm. In water at 18.2°C heart rate showed an overall increase of 5 bpm during the immersion. Although heart rate level was inversely related to ambient temperature, no significant differences were found in this relatively narrow range of temperatures. Heart rate level was directly related to the amount of thermal protection of clothing, but no significant differences were found. Sex and body size differences did not significantly affect heart rate response but due to confounding factors these relationships could not be accurately evaluated. Significant regressions of heart rate against rectal temperature showed a direct relationship in the first 22 minutes of cold immersion and the first 22 minutes of the rewarming phase but an inverse relationship between these periods. These relationships were complicated by the relationship of heart rate and metabolic rate, anxiety and muscular movement.
In immersions while subjects swam moderately, heart ·rate was significantly higher than while holding still (122 bpm active; 87 bpm still (p < 0.01)) but no significant difference was observed during the rewarming phase.
The intermittent ECG recordings of 33 subjects participating in 102 total immersions and cooling to an approximate rectal temperature of 34°C were analysed for significant changes. Quantitative analysis of ECG changes showed significant increases in the QTc interval and in the amplitude of the T wave with progressive hypothermia. PR and QRS time components were not increased significantly. Qualitatively, occasional atrial and ventricular extrasystoles and sinus arrhythmias were observed on immersion and with hypothermia. J point deflection, ST segment elevation and ST depression were only rarely seen.
No life-threatening cardiovascular changes were observed in this young, fit, healthy sample subjected to mild hypothermia. However, definite non-hypothermic and hypothermic stresses act upon subjects exposed to cold water. Their effect depends upon the age and health of those exposed and the severity of the exposure. These considerations are discussed.