Plasma volume and endocrine responses to water immersion with intermittent positive-pressure breathing in men

The effect of intermittent positive-pressure breathing (PB), induced by expiring against a resistance of 12.5 mm Hg, on plasma volume and endocrine responses to standing water immersion, was studied in seven male subjects, 28-49 years of age. The men were immersed to the neck (35 +/- 0.5 degrees C) for 90 min with PB from 30 to 60 min. Compared to control values, the hematocrit and hemoglobin concentration decreased (p less than 0.001) during immersion while plasma osmolality was unchanged, indicating an isotonic increase in plasma volume (hemodilution) which peaked after 75 min at +15.5% of the preimmersion plasma volume. This hemodilution was not significantly affected by PB. Plasma renin activity and vasopressin and aldosterone concentrations decreased progressively throughout immersion (p less than 0.001) and were unaffected by PB. The magnitude of these hormonal decreases was accentuated by preexisting, presyncopal symptoms in four subjects. It is concluded that intermittent PB as 12.5 mm Hg failed to compensate for the negative-pressure breathing of standing subjects immersed in water to the neck.