Cardiovascular and pulmonary responses to breath-hold diving in humans.

Cardiovascular and pulmonary responses to breath-hold diving (breath holding, submersion, and compression) were investigated. In addition, the effects of transitions between dry conditions and head-out immersion during eupnea were studied. Surface breath holds at a large lung volume with relaxed respiratory muscles resulted in a positive esophageal (transthoracic) pressure and a reduced cardiac output. In contrast, the esophageal pressure (relative to ambient pressure) was decreased, and cardiac output was at least partially restored, when lung gas volume was reduced by compression during breath-hold diving. The increased cardiac output that accompanied eupneic transition from dry to immersed conditions was associated with a short-lasting increase of alveolar gas exchange, whereas the decreased cardiac output during immersion-to-dry transition was associated with a long-lasting decrease of alveolar gas exchange, both reflecting changes in the tissue gas stores of the body. Surface breath holds were associated with a decreased O2 uptake from the lung to the blood, and breath-hold dives were associated with a large transient increase of O2 uptake at depth which resulted in a restoration of the time-averaged O2 uptake to the eupneic control level: these changes reflected changes in tissue O2 stores. Compared to surface breath holds, breath-hold dives were associated with larger tissue retention of CO2 during breath holds, and prolonged recovery for CO2 elimination after breath holds. The distribution of pulmonary perfusion, as indicated by expirograms obtained immediately after breath holds, was made more homogeneous by submersion and the distribution was further improved by compression during breath-hold dives. All of these different effects on the gas exchange in breath-hold diving and in eupneic headout immersion can to a large extent be explained by associated changes in cardiac output in combination with redistributions of peripheral blood flow and venous blood volume. Thus, the different components of breath-hold diving have profound cardiovascular and pulmonary effects. Changes in the intrathoracic pressure and in the distribution of venous blood volume induce changes in cardiac output. All of these changes affect the temporal and spatial distributions of pulmonary perfusion and peripheral blood flow. Also, the circulatory changes affect the temporal and spatial distributions of alveolar gas exchange and of tissue gas stores of the body.