Expiratory reserve volume and vital capacity of the lungs during immersion in water.

The effects of immersion by 5-cm increments on the expiratory reserve volume of the lungs (ERV) and on the vital capacity were studied in the sitting and supine positions. These effects were compared to those produced by continuous negative-pressure breathing when the subjects were in air and were counteracted by positive pressure breathing during immersion. The depth of immersion was also related to definable anatomic landmarks. In the sitting position about one-fourth of the decrease in the ERV was accounted for by the hydrostatic pressure of the water on the abdomen and the remainder by the pressure on the thorax. Immersion to the level of the spinous process of the seventh cervical vertebra was equivalent to 28 cmH2o continuous negative pressure breathing in air. In the supine position, a comparable value was 8 cmH2o. These observations agree well with those of others if differences in the levels of immersion are accounted for.