INTRODUCTION

Physiological limitations encountered during saturation diving are usually multifactorial. Respiratory problems, which represent only one facet of the difficulties encountered, have received wide attention in the last few years, since various ventilatory limitations have been observed during work at depth.

The respiratory system can be affected by saturation diving as a result of alteration of one or more of the various components of the environment which include: hydrostatic pressure, gas density, oxygen pressure and partial pressure of the inert gas species. Effects of abnormal oxygen pressures, hypoxia or 0, toxicity, can usually be controlled by manipulation of the 0, content of the gas mixture. The effects of gas density and pressure per se may act singularly or in combination as agents which may modify the functions of the respiratory system. Neural and chemical control of the system may be depressed or potentiated, depending on the absolute pressure.

The debilitating effects of high pressure nervous syndrome (HPNS) interfere with the ability of man to perform useful work at great depths. It is possible to attenuate HPNS by adding nitrogen to the helium-oxygen breathing mixture (heliox) to counterbalance the effect of rapid changes in pressure and/or of pressure per se1 However, the addition of nitrogen to heliox necessarily increases the density of the breathing gas. The net effect, therefore, might be an amelioration of HPNS symptoms accompanied by a gas density dependent reduction in ventilatory ability, a reduction in gas exchange reserve and of work capability.

Increased gas density has definitive effects on the mechanics of the system2 and may also affect performance of the respiratory muscles. Maximum expiratory flow decreases in a somewhat predictable fashion as a result of flow resistance changes caused by an increase in gas density, while fatigue of inspiratory muscles can influence ventilatory volumes. Efficiency of gas exchange may be improved by better distribution of the dense gas within the lung relative to distribution at normal density. On the other hand, poor intra-airway mixing or stratified lnhomogenelty may adversely influence gas exchange. Delivery of 0, may be compromised by an effect of pressure on hemoglobin affinity for 0,. Acidemia from retention of CO, and increased arterial lactic acidosis are other results of an increase in gas pressure and/or density. The interaction of all factors on work capacity is not predictable. Quantitative descriptions of these changes constitutes the major portion of this report.

METHODS

All data presented in this paper were collected during the past five years during deep dives in a dry chamber, involving three subjects per dive.

Studies were conducted while breathing a variety cf gas mixtures with densities ranging from 1.1 to 17.1 g/l (BTPS). The subjects breathed either heliox or a mixture of heliox and nitrogen (trimix) at pressures of 1509 feet sea water (fsw) or at 2132 fsw.

All gases contained 0, at a pressure of 0.5 ATA. Table 1 contains a more complete description of the depths and gas compositions used in this series which have been named the Atlantis Dives. The subjects varied in backgrounds ranging from professional divers (SP and LW) to a medical student (WB). Lung capacity and other gas exchange capabilities of the subjects are shown in Table 2.

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