Applied physiology of diving

Recreational diving is a popular sport, although human ability to stay in and under water is severely limited physiologically. An understanding of these limitations enhances safety and enjoyment of sports diving. Breath-hold diving involves head-out water immersion, apnoea and submersion, exercise, cold stress, and pressure exposure. Each of these components, by itself, elicits prominent and specific physiological effects. Combination of these factors produces a unique and interesting physiological response generally known as diving reflex. Humans display weak diving responses, but exhibit no oxygen conservation function. Nevertheless, application of diving-induced physiological changes is now finding its way into clinical practice. Apnoea, face immersion, and head-out water immersion all show promise of clinical application. There are several spin-offs from diving research worth noting. Diuresis, enhancement of cardiac performance, and redistribution of blood flow, all produced by head-out water immersion, have been shown to be clinically useful, besides providing physiological data useful to space travel. Results from investigations on apnoea have been shown to be relevant to the following: treating some forms of cardiac arrhythmias; understanding drowning, sudden infant death syndrome and sleep apnoea; and confirming hyperventilation as the major cause of drowning. In comparison to marine mammals, humans are poor divers because of severe physiological constraints which limit their breath-hold time, diving depth, and ability to conserve body heat. Although under special circumstances humans can achieve unusually long breath-hold time and reach exceptional depth with a single breath, the sustainable working time and depth are only about 1 minute and 5 metres, respectively. Hypothermia inevitably results in divers working in the ocean. Without thermal protection, the intolerable limit of 35 degrees C is reached within 30 minutes in winter (10 degrees C) water and within 60 to 90 minutes in summer. Nevertheless, effective harvest work can be performed by humans in the ocean, and recreational benefits enhanced when these physiological limitations are respected. An unusual circulatory state exists during head-out water immersion in that there is a sustained increase of stroke volume. This results in 30% increase in cardiac output when the subject is resting in thermal neutral water, indicating a substantial overperfusion for the oxygen requirement. Furthermore, animal experiments showed that the elevated blood flow is preferentially channeled to the liver, fat, and the organs in the splanchnic region.(ABSTRACT TRUNCATED AT 400 WORDS)