Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android. Learn more here!


Several million people worldwide participate in diving activities, including recreational, commercial, military, and competitive breath-hold diving. This chapter provides an introduction to the physiology of diving and clinical presentations of dysbarism (i.e., medical conditions arising from changes in ambient pressure), as well as an outline of fitness-to-dive principles for medical screening.


Breath-hold diving was undoubtedly first performed thousands of years ago, and it continues today during international competitions and commercial pearl diving, mostly in the Asia-Pacific region. The maximum depth of breath-hold diving is limited predominantly by the depletion of oxygen and the mechanical effects of pressure on the thoracic contents, as is discussed in more detail below.

Breathing at shallow depth can be performed using a snorkel; however, the maximum depth is constrained by the degree to which the inspiratory muscles can breathe against the transthoracic pressure imposed by the hydrostatic pressure of the water column. Maximum inspiratory pressure varies with age and sex; for a healthy young adult it is impossible to inhale through a snorkel at depths greater than one meter1 (Fig. 92-1). Prolonged excursions underwater can, therefore, only be performed by delivering compressed gas to the diver at the ambient pressure of the environment.

Figure 92-1

Limits of snorkel. A. The average adult is limited to a functional depth of 1 m due to limited strength of the inspiratory muscles to overcome the hydrostatic pressure applied to the chest. B. Maximum inspiratory pressure in males and females as a function of age. (Data from Sclauser Pessoa IM, Franco Parreira V, Fregonezi GA, et al. Reference values for maximal inspiratory pressure: a systematic review. Can Respir J. 2014;21(1):43–50.)

In 1942 Jacques Cousteau and Emile Gagnan designed the Aqua-Lung, an open-circuit, self-contained underwater breathing apparatus (SCUBA). With this system, the diver breathes from a regulator that delivers gas at ambient pressure. Breathing gas is stored in pressurized steel or aluminum tanks and is delivered by a series of first- and second-stage regulators, which reduce the gas pressure to the ambient pressure of the diver; exhaled gas is released into the environment.

An alternative technology, developed by naval forces at about the same time, is the closed-circuit rebreather. This device allows the diver to rebreathe expired gas, thus eliminating bubbles, increasing efficiency, and maintaining secrecy. Carbon dioxide is removed by a chemical absorbent, while oxygen is replenished from a pressurized oxygen tank. In the 1940s, rebreathers used 100% oxygen, which was discovered to produce fatal central nervous system oxygen toxicity due to seizures at depths beyond 8 to 9 meters (25–30 ft). Today’s rebreathers rely on complex sensors to deliver gas at a desired partial pressure, thus allowing deeper dives. Surface-supplied diving, or hookah diving, allows for an individual diver with an enclosed ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.