Data on pulmonary gas exchange were collected in breathhold dives to 90 feet in a tank and in open-sea breathhold dives to depths of 217.5 and 225 feet. Thoracic blood volume displacements were measured at depths of 25, 50, 90, and 130 feet, by use of the impedance plethysmograph. The open-sea dives were carried out with an average speed of descent of 3.95 feet per second and an average rate of ascent of 3.50 feet per second. End-dive alveolar oxygen tensions did not fall below 36 millimeters of mercury, while alveolar carbon dioxide tension did not rise above 40 millimeters of mercury except in one case. These findings indicate that for diver Croft, who has unusual lung capacity, neither hypoxia nor hypercapnia determined the depth limits under those conditions. At depths of 90 and 130 feet blood was forced into the thorax, amounting to 1047 and 850 milliliters respectively.

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To construct and test a system for the measurement of respiratory tidal volume and respiratory rate that avoids the limitations and disadvantages of the usual mouthpiece, face masks, and neck seals in a plethysmograph. FINDINGS A respirometer based on the use of two pairs of magnetic coils placed on the chest and abdomen, as described by Mead and co-workers, will reproducibly measure tidal volume as well as respiratory rate. APPLICATIONS This system, at its present stage of development, can be used in the laboratory for measurement of tidal volume and respiratory rate when the use of mouthpieces, face masks, and neck seals is undesirable. By the use of a miniature tape recorder and a pressureproof underwater housing, it should be satisfactory for measurement of respiratory rate and tidal volume on SCUBA, Hoka, and hard-hat divers. ADMINISTRATIVE INFORMATION This investigation was conducted as part of In-House Work Unit Number MR011.01-5000-Physiological Limits in Saturation-Excursion Diving. The present report is No. 1 on that Work Unit.