Relationship of plasma volume to sympathetic tone in nulliparous women

We sought to determine whether resting supine plasma volume is related to sympathetic tone in healthy young nulligravid normotensive women.

Forty women were examined in the midfollicular phase. Alpha-adrenergic tone was estimated by an examination of the late phase II blood pressure response to the Valsalva maneuver. Resting heart rate was examined to evaluate the balance of sympathetic and parasympathetic input. Plasma catecholamines were measured during supine rest. Plasma volume was estimated by Evans blue dilution.

Plasma volume corrected for body surface area was correlated inversely to late phase II blood pressure response to the Valsalva maneuver (r = -0.31, P <.05) and was correlated directly to the cardiac R-R interval (r = 0.41, P <.01). There was no relationship of plasma volume corrected for body surface area to mean arterial pressure (r = -0.13, P not significant). We found no significant relationship of plasma epinephrine concentration (r = -0.05, P =.76) or plasma norepinephrine (r = -0.09, P =.60) with plasma volume corrected for body surface area.

We conclude that plasma volume is related inversely to both an estimate of alpha-adrenergic activation and heart rate. These findings are consistent with an adaptive physiologic response that is aimed at the maintenance of blood pressure in the face of reduced plasma volume.