Role of the environmental factors

The effects of the various pressures of oxygen and nitrogen have been studied by Hordnes & Tyssebotn60. Cardiac output and regional blood flow were studied using marked microspheres with conscious rats in three conditions: Group 1 : Normobaric Oxygen (NBO) (PO2 = 1 ata), Group 2 : Hyperbaric air with PO2 = 1 ata (5 ata, FiO2 = 0.20), Group 3 : Hyperbaric Nitrox with PO2 = 0.2 ata (5 ata, FiO2 = 0.04).

Global cardiac output decreased in the hyperoxic groups but did not change in the normoxic group (i.e., a PO2 rather than a pressure effect). Cerebral blood flow decreased only in the hyperoxic groups. Myocardial blood flow decreased in the hyperoxic groups whereas it increased in the hyperbaric normoxic group. Respiratory muscle blood flow increased in the hyperbaric groups.

To summarize, regional blood flows were reduced in proportion to cardiac output in the hyperoxic groups with the predominant effect being on cerebral blood flow. Myocardial blood flow was affected inversely by hyperoxia and hyperbaria, as was respiratory muscle blood flow. In a later study, Risberg & Tyssebotn61 showed that this was actually an effect of hydrostatic pressure (and gas density) and not of the actual pressure of nitrogen.

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