Effect of hyponatremia on response to volume depletion in uremic dogs

It has been suggested that sodium depletion can alter cardiovascular reflexes and vascular sensitivity to pressors. To define the effects of hyponatremia on the hemodynamic response to volume depletion in the setting of uremia, isolated ultrafiltration was performed in acutely uremic dogs previously rendered hyponatremic (127 + 2.5 [SD] mEq/L) for 72 hours by dialyzing against a low-sodium dialysate. Response was compared with that of the same animals in the normonatremic (150 +/- 3.6 mEq/L) condition. Animals in the hyponatremic and normonatremic states had similar initial values of blood volume (hyponatremic 2284 +/- 338 ml, normonatremic 2306 +/- 372 ml, P not significant), mean arterial pressure, cardiac output, and total peripheral resistance, but preultrafiltration body weight in the hyponatremic state was greater (hyponatremic 26.1 +/- 2.4 kg, normonatremic 24.6 +/- 1.7 kg, P less than 0.05). Isolated ultrafiltration was performed until a 30% fall in mean arterial pressure occurred. Body weight at the end of ultrafiltration in the hyponatremic state was still higher (24.7 +/- 2.4 kg) than in the normonatremic state (22.9 +/- 1.5 kg, P less than 0.05). At the end of ultrafiltration, similar values of blood volume (hyponatremic 1871 + 329 ml, normonatremic 1821 +/- 267, P not significant) cardiac output, and total peripheral resistance were again found. The results suggest that, in this model, hyponatremia is associated with an alteration in the relationship between body water and blood volume. The excess body water is difficult to remove by isolated ultrafiltration because blood volume depletion ensues at a higher body weight.(ABSTRACT TRUNCATED AT 250 WORDS)