Experimental facts

First observed by Dautrebande & Haldane, its importance was made evident by Fagraeus3. These authors studied the heart rate and other physiological variables in healthy subjects performing a standardized exercise in various atmospheric conditions: air at 1 ata (A), oxygen at 1 ata (B) and air at 4.5 ata (C). Since pressures of oxygen were the same in conditions B and C, the effect of the increase in pressure of oxygen could be distinguished from the effect of the other environmental factors. The authors showed that for the four levels of exercise studied, there was a decrease of 12 to 16 beats per minute in condition C, whereas this decease, when it occurred, was only 4 to 7 beats per minute in condition B. They concluded the bradycardia observed in compressed air was mainly caused by high pressures of oxygen, but this only explained in part the bradycardic effect, the part of bradycardia which is not oxygen-dependent being related to other environmental factors.

The relative importance of the oxygen dependent and non-dependent parts was questioned by Shida & Lin4. Studying the heart rate of non anaesthetized rats in various atmospheric conditions (O2-N2 and O2-He mixtures) and pressures, these authors showed that, for pressures between 1 and 10 ata this increase in oxygen pressure was the primary cause for the bradycardia. Variations in density, pressure and inert gases produced only marginal effects.

However there are several explanations for the seeming contradiction: beyond the differences in species (man and rats) both the conditions (exercise and rest) and pressures of oxygen differed in range. Fagraeus's study was carried out at a pressure of oxygen of 1 ata whereas Shilda & Lin mostly worked with pressures of 0.6 ata. Kenmure5 showed the bradycardic effect was close to its maximum at 1 ata oxygen and increasing PO2 over 1 ata only produced a small effect.

Thus the following conclusion can be made : in the range of pressures used in HBO (1-3 ata, FiO2 = 1), the decrease in heart rate is mostly due to the increase in oxygen pressure, this decrease is nearly completely achieved as soon as normobaric oxygen (NBO) is provided and a higher increase in pressure causes no further decrease in heart rate.

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