Regulation of plasma volume during obstructive sleep apnoea.

The evidence that plasma volume is altered in obstructive sleep apnoea is an indirect one, based on the observation of a paradoxical elimination of peripheral oedema along with a decrease in water and sodium excretion and of a decrease in haematocrit when apnoeas are eliminated with continuous positive airway pressure (CPAP) treatment. A suggested interpretation of these observations is that in the untreated condition, increased renal sodium excretion and increased vascular membrane permeability lead to increased urine and salt excretion and to a fluid shift from the plasma to the extracellular space, causing nocturnal polyuria, peripheral oedema and haemoconcentration. Treatment with continuous positive airway pressure reverses the increased membrane permeability and urine excretion, allowing the peripheral oedema to resolve and the haematocrit to decrease. Increased atrial natriuretic peptide release and decreased renin-angiotensin-aldosterone activity, along with an increased release of thromboxane and of endothelin (which have been reported in untreated obstructive sleep apnoea), could be the mechanisms of the observed alterations in fluid distribution in obstructive sleep apnoea.