Changes in potassium concentration and haematocrit associated with cardiopulmonary bypass in paediatric cardiac surgery

Introduction: A blood prime is frequently required for paediatric bypass surgery to maintain adequate haematocrit (Hct). However, stored blood can have high extracellular potassium levels and this study aims to investigate the effect of stored blood on the potassium concentration, both in the prime and subsequently in the patient after cardiopulmonary bypass (CPB) has been established. In neonatal surgery, the stored blood may be irradiated if there is a question of impaired immunity. Irradiation may cause a further increase in potassium levels.

Methods: Blood-primed circuits prepared for 320 consecutive paediatric bypass cases were analysed for electrolyte levels, Hct and acid-base status before and immediately after establishment of CPB. Patients were divided into three groups according to body weight (<5kg, 5—10 kg and > 10 kg) and both stored blood and irradiated blood primes were compared.

Results: The potassium concentration was above the physiological range in all bypass primes pre-CPB and was significantly higher when using irradiated blood (8.12 ± 2.54 mmol/L versus 4.94 ± 3.35 mmol/L, p < 0.0001).

Despite this, on commencing CPB, the potassium level remained within the physiological range in the majority of patients (4.16 ± 2.72 mmol/L for stored blood prime and 4.55 ± 1.01 mmol/L for irradiated blood, p = 0.02). However, in smaller patients (< 5 kg) who had irradiated blood prime potassium level > 7.0 mmol/L, there was resultant hyperkalaemia (5.60 ± 0.90 mmol/L) on commencing CPB, that returned to normal later. No adverse clinical events were associated with the hyperkalaemia. Hct was well maintained on CPB (22—25%) in all groups and was not related to patient weight.

Conclusion: Blood primes result in high potassium concentrations in the prime fluid that is more severe if irradiated blood is used. The concentration is not sufficient to cause hyperkalaemia in the patients on commencing CPB except when irradiated blood prime is used in infants < 5 kg. Hct is well maintained in all patient groups with the use of blood prime. Perfusion (2007) 22, 87—92.