Circulatory functions during immersion and breath-hold dives in humans.

An unusual circulatory state exists in humans immersed in water at or near 35 degrees C. This circulatory state is characterized by a persistent and elevated stroke index (SI) but heart rate (HR) changes little; hence an increased cardiac index (CI) results. In comparison of measurements in air and immersion in water up to the neck level that are based on the weighted averages of 45 subjects from 8 studies, SI increased by 29%, CI increased by 24%, and HR decreased by 6%. Evidence is presented to support the finding that the increase in stroke volume is a result of enhanced preload of the heart, and that alterations in afterload and contractility play an unimportant role in affecting cardiac performance during immersion in thermoneutral water up to the level of the neck. The circulatory state represents hyperperfusion, since there is no concurrent rise in metabolic demand or heat stress. To what extent this preexisting circulatory state affects the cardiovascular responses to breath-hold (BH) diving is in most part unknown. However, the BH bradycardiac responses are more potent in a natural setting than in the laboratory where the breath hold is performed with only face immersion in water. In contrast, in the natural setting, the divers perform BH dives while they are immersed up to the neck in cold water (much lower than 35 degrees C) and are exercising. It is desirable in future studies to compare systematically other aspects of circulatory responses to BH dives while immersed in water and while exercising.