AIM: Early low systemic blood flow is common in preterm infants. This study examined the relationship between low systemic blood flow and early changes in serum lactate and potassium(K)levels.

METHODS: 126 infants born before 30 weeks(mean gest 27 weeks, mean BW 991g) were studied with Doppler echocardiography at 5,12,24 and 48 hours of age. Superior vena cava flow(SVC flow) was assessed by Doppler echocardiography as a measure of upper body systemic blood flow uncorrupted by systemic to pulmonary shunts. Other measures included the color Doppler diameters of ductal and atrial shunts and the right and left ventricular outputs. Whole blood K levels on each blood gas sample were measured by a Ciba-Corning analyser in 119 infants. In 49 infants the serum lactate levels were measured at both 5 and 24 hours by a standard assay method.

RESULTS:Mean(range) serum lactate levels fell between 5 hours(2.30mmol/L,0.53-6.58)and 24 hours (1.92mmol/L,0.61-4.51). At 5 hours higher lactate levels correlated with low Apgar score at 1 minute, maternal antihypertensive use and death. At 24 hours higher lactate levels were associated with low SVC flow(p=0.01),maternal antihypertensive use(p=0.004), caesarean section(p=0.01) and death(p=0.009). Most infants had a variable degree of rise in K in the immediate postnatal period, K decreasing over the study period in just 6 infants. The mean rate of rise in K was 0.17mmol/hr(0-1.33). The mean peak K was 5.54mmol/L(3.75-8.93) and the mean time of peak K was 20 hours(2-62). The peak K occurred after the lowest measured SVC flow in 84%(p<0.001) of the infants. There was a significant relationship between the mean(p=0.001) and peak(p=0.001) K in the first 24 hours and the lowest measured SVC flow. Low SVC flow at 5 hours best predicted the rate of K rise(r=0.28,p=0.002) and at 12 hours best predicted the peak K(r=0.47,p<0.001). Mean minimum SVC flow in the 17 babies that became hyperkalaemic(peak K≤6.5mmol/L) was 29.5ml/kg/min(10.3-47.3) vs 46.2ml/kg/min(16.6-95.9) in the 102 infants with peak K<6.5mmol/L(p<0.001). The normal range for SVC flow is more than 40ml/kg/min. There was no relationship between K levels and blood pressure or left ventricular output.

CONCLUSIONS:While both raised serum lactate and hyperkalaemia reflect preceeding systemic hypoperfusion, hyperkalaemia is the more specific marker of the two. Low systemic blood flow is an important factor in the aetiology of hyperkalaemia in preterm infants.