Factors Determining the Circulatory Adjustments to Diving: II. Asphyxia

A ndersen , H. T. Factors determining the circulatory adjustments to diving II. Asphyxia. Acta physiol. scand. 1963. 58. 186–200. — The significance of asphyxia and its two components hypoxia and hypercapnia for the development of the circulatory adjustments to diving has been studied in the domestic duck by ventilating the lungs and air‐sacs with various gas mixtures during submergence. The heart rate has been used as an index of the cardio‐vascular changes. Asphyxia produces a marked bradycardia in the duck, but only after a delay of up to 45 sec. The very conspicuous bradycardia seen during protracted dives is, however, neither due to submergence per se nor to asphyxia alone, for it has been found that the effects of the two summate. Hypercapnia appears to influence the heart rate much more than hypoxia does, although the latter is not without any effect, since it has been shown that the respiratory and circulatory adjustments are not abolished unless the asphyxia is relieved; or the hypercapnia is relieved at a point when the hypoxia is not too severe. Passive mechanical stretching of the lungs and air‐sacs have very little effect on the heart rate during diving.