Haemodilution in nine neonates resulted in significant mean (SEM) decrease of packed celi volume (0.67 (0.01) to 0-55 (0.01)) and increases in cardiac output (250 (16) to 308 (25) ml/min/kg) and blood flow velocities of the internal carotid artery and the coeliac artery (+20%). However, red cell flows in the aorta, carotid and coeliac arteries did not change during haemodilution, thereby indicating that haemodilution did not improve oxygen transport. (Arch Dis Child 1994; 71: F53-F54) Neonatal polycythaemia increases the risk of pulmonary hypertension, renal failure, necrotising enterocolitis, cerebral ischaemia, intracranial haemorrhage, and developmental retardation.' The clinical manifestations of polycythaemia result from the rise in blood viscosity.2 Previous studies have shown that cardiac output and cerebral blood flow velocity in polycythaemic neonates increased more than 30% during isovolaemic haemodilution (partial exchange transfusion). 3 Blood flow in gastrointestinal arteries of polycythaemic infants has not been studied. However, experiments in puppies have shown that polycythaemia decreases gastrointestinal blood flow by more than 40%.4 The present study was designed to evaluate the effects of polycythaemia and haemodilution on cardiac output and blood flow velocities of cerebral and coeliac arteries in newborn infants.were clamped within 20 seconds of birth.In the polycythaemic infants, isovolaemic haemodilution was performed with serum (Biseko, Biotest) via an umbilical vein catheter. The haemodilution procedure lasted about two hours and was continued until the packed cell volume was about 055.Cardiovascular measurements in the polycythaemic infants were done before and one to two hours after haemodilution. During the examinations, infants were either sleeping or quiet and in supine position. Blood flow velocities and cardiac output were measured using an Interspec XL pulsed Doppler ultrasound system (Interspec Inc). Details of the cardiac output method have been reported elsewhere.5 Systolic blood flow velocities were measured using a 5-0 MHz pulsed Doppler transducer. The arteries were identified by duplex scan mode. The right and left internal carotid artery were localised via the anterior fontanelle. As there were no significant differences between the two internal carotid arteries, the mean velocities of both arteries were calculated for each infant. The coeliac artery was localised by ultrasound from a longitudinal abdominal section and blood flow velocity was determined close to the origin of the artery from the abdominal aorta.Packed cell volume was determined by the microhaematocrit method. Mean arterial blood pressure was measured in the right and left upper arm using an oscillometric technique (Dinamap 847, Critikon). Systemic flow resistance was calculated as mean pressure to cardiac output ratio.

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Circulatory adaptation was studied serially in 11 healthy term neonates on days 1, 3, and 5 by cross sectional and pulsed Doppler echocardiography. Changes in the determinants of blood viscosity (packed cell volume, plasma viscosity, red cell aggregation, and red cell deformability) were studied on day 1 and day 5. There was a 27% increase in the cardiac output as a result of increasing stroke volume, whereas heart rate did not change significantly. Mean blood pressure increased by nearly the same extent as cardiac output (21%), so that the overall resistance remained unchanged. Packed cell volume, red cell aggregation, and red cell deformability did not change significantly during the first five postnatal days. Plasma viscosity rose significantly (by 12%) so that whole blood viscosity increased during that period. As there was no change in overall systemic vascular resistance the vascular hindrance-calculated as the ratio of resistance: blood viscositydecreased, thereby indicating vasodilation.Blood flow in circular vessels is determined by Poiseuille's law with the Hagenbach extension. This means that blood flow (Q) increases with increasing pressure gradient (that is, pressure drop divided by vessel length, oP/L) and increasing vessel radius (r), and decreases with increasing blood viscosity (n): r oP r4 Q=-x-x-8 L r

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