Low systemic blood flow and hyperkalemia in preterm infants

Objective: Early low systemic blood flow is common in preterm infants. This study examines the relationship among low flow, renal function, and early changes in blood potassium (K+). Methods: Preterm infants (n = 119) born before 30 weeks' gestational age underwent serial Doppler echocardiographic studies. Superior vena cava flow (SVC flow) was assessed as a measure of upper body systemic blood flow uncorrupted by systemic to pulmonary shunts. Serial whole blood K+ concentrations on each arterial blood gas sample and urinary output in the first 48 hours were recorded. Results: Most infants had a variable degree of rise in K+ during the first 24 hours of life. The mean rate of rise was 0.17 mmol/L/h, the mean peak K+ was 5.54 mmol/L, and the mean time of peak K+ was 20 hours. The peak K+ occurred after the lowest measured SVC flow in 84'% of infants. A significant positive relationship was found between the lowest measured SVC flow and the mean (r = 0.31, P =.001) and peak (r = 0.31, P =.001) K+ in the first 24 hours. Low SVC flow at 5 hours best predicted the rate of K+ rise (r = 0.28, P =.002) and at 12 hours best predicted the peak K+ concentration (r = 0.47, P <.001). The mean minimum SVC flow in the 17 babies who became hyperkalemic was 29.5 mL/kg/min versus 46.2 mL/kg/min in the 102 infants with normokalemia. Urine output in the first 24 hours was significantly lower in the hyperkalemic infants. A K+ rate rise exceeding 0.12 mmol/L/h in the first 12 hours predicted low SVC flow with 93% accuracy. Conclusions: The data are consistent with a role for low systemic blood flow leading to reduced urinary output and subsequent hyperkalemia in preterm infants.