Sodium reduction during cardiopulmonary bypass: Plasma-Lyte 148 versus trial fluid as pump primes

We compared effects on plasma sodium concentrations plus calculated plasma tonicity of two "balanced" crystalloid solutions used as 2 L pump primes during cardiopulmonary bypass (CPB): Plasma-Lyte 148 (sodium concentration, 140 mmol/L; potassium concentration, 5 mmol/L) versus a bicarbonate-balanced fluid (sodium concentration, 140 mmol/L; potassium concentration, 0 mmol/L).

We analysed pooled data from two prospective interventional studies performed in university-affiliated hospitals, from 50 patients undergoing elective cardiac surgery.

Participants were allocated equally to Plasma-Lyte 148 or bicarbonate-balanced fluid, with plasma electrolytes measured by direct ion selective electrodes immediately before bypass (pre-CPB), within 3 minutes of commencement (T2), and before bypass cessation (end-CPB).

Plasma sodium fell at T2 in 46 patients (92%) (P<0.0005). With Plasma-Lyte 148, the mean sodium decreased by 3.0 mmol/L (SD, 1.7 mmol/L), and with bicarbonate-balanced fluid it decreased by 2.2 mmol/L (SD, 1.1 mmol/L) (P=0.002). The mean tonicity fell by >5 mOsm/kg for both groups (P<0.0005). At end-CPB, the mean sodium for both groups remained reduced by >2 mmol/L (P<0.0005). In the group receiving Plasma-Lyte 148, 52% of patients were hyponatraemic (sodium<135 mmol/L) at T2 and end-CPB.

Sodium reductions were common with both priming solutions, but more severe with Plasma-Lyte 148. Crystalloid priming solutions require sodium concentrations>140mmol/L to ensure normonatraemia throughout CPB.