Simultaneous calf and forearm blood flow during immersion in man

Core cooling and thermal responses during whole-head, facial, and dorsal immersion in 17 °C water

This study isolated the effects of dorsal, facial, and whole-head immersion in 17 °C water on peripheral vasoconstriction and the rate of body core cooling. Seven male subjects were studied in thermoneutral air (∼28 °C). On 3 separate days, they lay prone or supine on a bed with their heads inserted through the side of an adjustable immersion tank. Following 10 min of baseline measurements, the water level was raised such that the water immersed the dorsum, face, or whole head, with the immersion period lasting 60 min. During the first 30 min, the core (esophageal) cooling rate increased from dorsum (0.29 ± 0.2 °C·h–1) to face (0.47 ± 0.1 °C·h–1) to whole head (0.69 ± 0.2 °C·h–1) (p < 0.001); cooling rates were similar during the final 30 min (mean, 0.16 ± 0.1 °C·h–1). During the first 30 min, fingertip blood flow (laser Doppler flux as percent of baseline) decreased faster in whole-head immersion (114 ± 52%·h–1) than in either facial (51 ± 47%·h–1) or dorsal (41 ± 55%·h–1) immersion (p < 0.03); rates of flow decrease were similar during minutes 30 to 60 (mean, 22.5 ± 19%·h–1). Total head heat loss over 60 min was significantly different between whole-head (120.5 ± 13 kJ), facial (86.8 ± 17 kJ), and dorsal (46.0 ± 11 kJ) immersion (p < 0.001). The rate of core cooling, relative to head heat loss, was similar in all conditions (mean, 0.0037 ± 0.001 °C·kJ–1). Although the whole head elicited a higher rate of vasoconstriction, the face did not elicit more vasoconstriction than the dorsum. Rather, the progressive increase in core cooling from dorsal to facial to whole-head immersion simply correlates with increased heat loss.

Thermal effects of whole head submersion in cold water on nonshivering humans

This study isolated the effect of whole head submersion in cold water, on surface heat loss and body core cooling, when the confounding effect of shivering heat production was pharmacologically eliminated. Eight healthy male subjects were studied in 17°C water under four conditions: the body was either insulated or uninsulated, with the head either above the water or completely submersed in each body-insulation subcondition. Shivering was abolished with buspirone (30 mg) and meperidine (2.5 mg/kg), and subjects breathed compressed air throughout all trials. Over the first 30 min of immersion, exposure of the head increased core cooling both in the body-insulated conditions (head out: 0.47 ± 0.2°C, head in: 0.77 ± 0.2°C; P < 0.05) and the body-exposed conditions (head out: 0.84 ± 0.2°C and head in: 1.17 ± 0.5°C; P < 0.02). Submersion of the head (7% of the body surface area) in the body-exposed conditions increased total heat loss by only 10%. In both body-exposed and body-insulated conditions, head submersion increased core cooling rate much more (average of 42%) than it increased total heat loss. This may be explained by a redistribution of blood flow in response to stimulation of thermosensitive and/or trigeminal receptors in the scalp, neck and face, where a given amount of heat loss would have a greater cooling effect on a smaller perfused body mass. In 17°C water, the head does not contribute relatively more than the rest of the body to surface heat loss; however, a cold-induced reduction of perfused body mass may allow this small increase in heat loss to cause a relatively larger cooling of the body core.

Mechanism of the human diving response

The diving response in human beings is characterized by breath-holding, slowing of the heart rate (diving bradycardia), reduction of limb blood flow and a gradual rise in the mean arterial blood pressure. The bradycardia results from increased parasympathetic stimulus to the cardiac pacemaker. The reduction in limb blood flow is due to vasoconstriction resulting from increased activity of the sympathetic nerves supplying arteries in the arms and legs. Essentially the response is produced by the combination of water touching the face and either voluntary or involuntary (reflex) arrest of breathing. The nervous inputs and outputs for the response are coordinated in the brain stem by the respiratory, vasomotor and cardioinhibitory "centers." The diving response in human beings can be modified by many factors but the most important are water temperature, oxygen tension in the arterial blood and emotional factors.

Cardiovascular responses elicited by simulated diving and their habituation in man

The cardiovascular responses of 24 subjects were investigated under various simulated diving conditions. Muscle blood flow in forearm and calf, arterial pressure, heart rate and intrathoracic pressure were monitored. Breath holding with face immersion in water at 18 degrees C gave a typical diving response at intrathoracic pressure of 0 and 20 mmHg, (23% bradycardia, greater than 60% muscle vasoconstriction). Breath holding alone at 20 mmHg intrathoracic pressure resulted in vasoconstriction (50%) and bradycardia (4%). Breath holding at 0 mmHg intrathoracic pressure induced a muscle vasoconstriction (5%). These results indicate that both increased intrathoracic pressure and facial immersion can produce a typical diving response individually but that the full 'diving response' requires the presence of both conditions. Diving often activated two responses, the typical 'diving response' and a superimposed defence reaction. Cardiovascular components of the defence reaction (muscle vasodilatation and tachycardia) which was elicited in some divers masked the diving response. In those subjects in whom the diving response was initially absent during repetition of diving manoeuvres the cardiovascular components of the defence reaction were habituated and the characteristic diving response gradually emerged: the initial tachycardia diminished and was replaced by bradycardia, while vasodilatation in the forearm and calf was replaced by vasoconstriction.