CARDIOVASCULAR AND PULMONARY RESPONSES TO BREATH-HOLD DIVING IN HUMANS

Cardiovascular and pulmonary responses to breath-hold diving (breath h olding, submersion, and compression) were investigated. In addition, t he effects of transitions between dry conditions and head-out immersio n during eupnea were studied. Surface breath holds at a large lung vol ume with relaxed respiratory muscles resulted in a positive esophageal (transthoracic) pressure and a reduced cardiac output. In contrast, t he esophageal pressure (relative to ambient pressure) was decreased, a nd cardiac output was at least partially restored, when lung gas volum e was reduced by compression during breath-hold diving. The increased cardiac output that accompanied eupneic transition from dry to immerse d conditions was associated with a shortlasting increase of alveolar g as 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 O-2 uptak e from the lung to the blood, and breath-hold dives were associated wi th a large transient increase of O-2 uptake at depth which resulted in a restoration of the time-averaged O-2 uptake to the eupneic control level: these changes reflected changes in tissue O-2 stores. Compared to surface breath holds, breath-hold dives were associated with larger tissue retention of CO2 during breath holds, and prolonged recovery f or CO2 elimination after breath holds. The distribution of pulmonary p erfusion, as indicated by expirograms obtained immediately after breat h holds, was made more homogeneous by submersion and the distribution was further improved by compression during breath-hold dives. All of t hese different effects on the gas exchange in breath-hold diving and i n eupneic head-out immersion can to a large extent be explained by ass ociated 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 pulm onary effects. Changes in the intrathoracic pressure and in the distri bution of venous blood volume induce changes in cardiac output. All of these changes affect the temporal and spatial distributions of pulmon ary perfusion and peripheral blood flow. Also, the circulatory changes affect the temporal and spatial distributions of alveolar gas exchang e and of tissue gas stores of the body.