AIM: Low blood flow to the preterm upper body and brain is common in the first 24 hours and is associated with a high risk of subsequent intraventricular hemorrhage. We hypothesised low flow would be associated with early large ductal shunts and higher mean airway pressure (MAP). This study aimed to analyse how these and other clinical and echocardiographic factors relate to these low flow states.

METHODS: 126 babies born before 30 weeks (mean gest 27 weeks, mean BW 991g) were studied with Doppler echocardiography and cerebral ultrasound at 5, 12, 24 and 48 hours of age. Superior vena cava (SVC) flow was assessed by Doppler echocardiography as a measure of systemic blood flow returning from the upper body and brain. This is necessary because atrial and ductal shunts can cause either ventricular output to significantly overestimate systemic blood flow. Other measures included colour Doppler diameters of ductal and atrial shunts, Doppler assessment of shunt direction and velocity and right and left ventricular outputs. Upper body vascular resistance (UBVR) was calculated from mean blood pressure (MBP) and SVC flow.

RESULTS: Babies with low SVC flow (below the 10th centile for uncomplicated preterm babies); at 5 hrs, had larger diameter patent ductus arteriosus (PDA)(2 vs 1.5mm, p<0.001) and higher MAP (11 vs 9 cmsH2O, p=0.02); at 12 hours,were of lower gestation (26 vs 28 weeks, p=0.002), were less likely to have mothers on antihypertensives (12% vs 34%, p=0.05) and higher MAP (9 vs 7cmsH2O, p=0.002); at 24 and 48 hours, were of lower gestation (26 vs 28 weeks, p<0.001).At all scans, babies with low SVC flow had higher UBVR (0.93 vs 0.46 mmHg/ml.kg.min, p<0.001). Babies who maintained normal SVC flow in the presence of a large PDA or high MAP had lower UBVR and were more mature.

The relationship between MBP and SVC flow was weak (r < 0.26) signifying a close inverse relationship between SVC flow and UBVR (r > 0.94). That high vascular resistance might be causative of low flows was supported by the finding that the 28 babies whose mothers had received vasodilating antihypertensives were significantly protected from low SVC flow at 12 hours, p=0.02.

CONCLUSIONS: Low SVC flow (and by inference total systemic blood flow) may result from an immature myocardium struggling to adapt to extrauterine vascular resistances. Critically low flow can occur when this is compounded in the first 12 hours of life by high MAP and large shunts out of the systemic circulation through the ductus arteriosus.