[Cardiovascular function in man during experimental dives at 26 ATA (helium-nitrogen-oxygen mixture)]

During two human experimental dives at 26 ATA (helium-nitrogen-oxygen gaz mixture; PIO2 = 400 mbar), the cardiac frequency (Fc) and radial arterial pulse were continuously recorded, at rest and during periods of maximal expiratory (Valsalva) or inspiratory (Müller) manoeuvres, used to increase or decrease the intrathoracic pressure, respectively. Cardiovascular variables were measured at 1 and 26 ATA in resting individuals and during the maximal respiratory manoeuvres. Discontinuous measurement of arterial blood pressure using a sphygomanometer allowed to calculate the mean arterial pressure. The value of mean arterial pressure was maintained against the membrane of a radial pulse sensor. This procedure, proposed by Posey et al. (1969), gives a continuous approximation and recording of arterial blood pressure and its components. The present results did not show significant variation in the values of Fc nor systolic or diastolic blood pressures measured at rest or during Müller manoeuvres performed at 1ATA at the maximal depth. On the other hand, Valsalva manoeuvres performed at depth induced significant variations in circulatory variables compared to the normobaric response. The most important effect was an enlargement of differential pressure due to a marked decrease in the diastolic blood pressure. These observations are discussed in terms of enlarged sensitivity of the baroreflex arch under hyperbaric condition.