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> Then, after five miles, I stopped and started walking. Another guy >came up and asked if I was still running, so I ran with him. He was >pretty tough, and picked the pace up. My rate went up to 170, and after >a mile I felt like I was getting too hot, so I broke off and ran back to >the house, where I got into the pool. At this point my rate was 95, but >when I stuck my head in the water, it dropped to 60 immediately, and >then back to 95 a few seconds after I took my head out. > > This seems wierd to me, since when we swim the heart rate goes way up >and stays up. I wonder what would happen to the rate from a resting >rate. You probably experienced a "diving response". We have cold receptors in the forehead. Activation of these receptors by head immersion can produce a rapid bradycardia. It has been shown that the higher the temperature difference between the air and the wet milieu, the higher the response. This diving response is some kind of protective mechanism. When the body gets in a position where air could becomes unavailable (i.e. head in water), several things happen in order to reduce to a maximum the oxygen consumption. In humans, this diving response is not very impressive. It usually involves bradycardia, changes in cardiac output and redistribution of peripheral blood flow. In diving mammals, this diving response can be much more extensive. Waddel seals, for example, can hold their breath for an hour. They acheive this by an extensibe diving response that include sustained bradycardia, reduced cardiac output, redistribution of peripheral blood flow, spleenic contraction (thus liberating many red blood cells and causing an increased hematocrit which in turn increases the oxygen carrying capacity of the blood). If you want to know more and if you have access to scientific journals, the following papers might be of interest: Marsh N, Askew D, Beer K, Gerke M, Muller D, Reichman C: Relative contributions of voluntary apnoea, exposure to cold and face immersion in water to diving bradycardia in humans. Clinical & Experimental Pharmacology & Physiology. 22(11):886-7, 1995 Nov 1996;73(1-2):1-6 Hong SK, Moore TO, Lally DA, Morlock JF: Heart rate response to apneic face immersion in hyperbaric heliox environment. Journal of Applied Physiology 1973 Jun;34(6):770-4 Lin YC, Moore TO, McNamara JJ, Hong SK: Oxygen consumption and conservation during apnea in the anesthetized dog. Respiration Physiology 1975 Sep;24(3):313-24 Sterba JA, Lundgren CE: Diving bradycardia and breath-holding time in man. Undersea Biomedical Research 1985 Jun;12(2):139-50 Sterba JA, Lundgren CE: Breath-hold duration in man and the diving response induced by face immersion [published erratum appears in Undersea Biomed Res 1988 Nov;15(6):preceding 403]. Undersea Biomedical Research 1988 Sep;15(5):361-75 Etienne -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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