Patients with Preoperative Hyponatremia A prospective validated cardiac surgery database of 4944 patients containing preoperative serum sodium levels

52 Hyponatremia is common in patients prior to cardiopulmonary bypass (CPB), usually secondary to diuretic therapy (1). However, rapid correction of hyponatremia, which potentially occurs on commencing of CPB, in patients with chronic hyponatremia may have disastrous consequences (2). Once the brain has adapted to hyponatremia, it is not well protected from the osmotic stress that accompanies correction (3,4). During rapid correction of chronic hyponatremia, which potentially occurs on CPB, the blood becomes hypertonic relative to the brain which will result in the movement of intracellular water causing cerebral dehydration, intramyelinic edema, a breakdown of the blood-brain barrier, and oligodendrocyte degeneration. Increasing osmotic pressure while correcting hyponatremia, as is the case when adding mannitol to CPB, may not decrease the risk of inducing myelinolysis (5). How rapid an increase in serum sodium concentration the brain can tolerate has been difficult to define. A figure of 12 mmol/L in 24 hours has been described (6,7). Correction while commencing CPB occurs instantly. To date, the potentially deleterious effects of acute hyponatremia and its rapid correction is well documented in urology and general medicine, where it is known as post transurethral resection of the prostate (TURP) syndrome and central pontine myelomatosis respectively, however it has not been described or investigated in cardiac surgery.