Hyponatremia as a complication of cardiac catheterization: a prospective study

Background: A decrease in plasma sodium (P(Na)) concentration is common after surgery and attributed to the secretion of antidiuretic hormone in response to such nonosmotic stimuli as pain or nausea. In this setting, acute hyponatremia may lead to seizures, coma, and permanent neurological damage. Sporadic case reports have described severe neurological symptoms caused by hyponatremia occurring within hours after cardiac catheterization. We evaluated the prevalence, contributing clinical circumstances, and course of hyponatremia in patients undergoing cardiac catheterization.

Methods: We prospectively studied 309 consecutive patients scheduled for an elective cardiac catheterization. Plasma and urine electrolytes and urine osmolarity were measured at baseline and again 1 to 4 hours and 24 hours after the procedure.

Results: P(Na) level was 139.4 +/- 2.3 mEq/L at baseline. At 1 to 4 hours, P(Na) level decreased to 134.2 +/- 3.6 mEq/L (P < 0.0001). Mild (> or =5 to 10 mEq/L), moderate (11 to 14 mEq/L), and severe (> or =15 mEq/L) reductions in P(Na) levels occurred in 50%, 5%, and 0.3% of patients, respectively. At the 24-hour point, P(Na) level increased to 137.3 +/- 2.4 mEq/L, but was significantly lower compared with baseline (P < 0.0001). In hyponatremic patients at the 1- to 4- and 24-hour points, mean urine osmolarity values were 428 +/- 139 and 420 +/- 204 mOsm/kg, respectively; almost every urine sample was inappropriately concentrated. Multivariate logistic regression identified the amount of electrolyte-free water administered to be a predictor for the development of hyponatremia (3.7-fold incremental risk for every 1 L administered to a 70-kg patient).

Conclusion: An acute reduction in P(Na) level commonly occurs shortly after cardiac catheterization. The cause of hyponatremia appears to be related to the administration of hypotonic fluids, together with impaired urinary dilutional capacity. Although symptomatic hyponatremia is rare, the diagnosis should be entertained when neurological symptoms develop in this setting.