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We read with interest the findings of Evans et al concerning the changes in middle cerebral artery (MCA) haemodynamics over the first two days after birth in preterm infants.1
Evans et al demonstrated a significant change in several Doppler velocity measures in the MCA, including systolic, diastolic, and mean velocity, pulsatility index and velocity time integral, during days 1 and 2 after birth in a cohort of preterm infants. Blood flow (F), blood pressure (P), and vascular resistance (R) are closely related (F = P/R), and resistance changes are a function of changing vessel calibre. The authors attempted to measure MCA diameter, but because measurements were inaccurate the authors did not attempt to calculate values for MCA blood flow.
Our own studies of cerebral haemodynamics, using near infrared spectroscopy, supports the view of Evans et al that there are significant changes in cerebral blood flow over the first 36 hours after birth. The demographic details of our cohort were similar to the one studied by Evans et al, with 36 infants of mean (SD) gestation 26 (2) weeks and mean birth weight 929 (250) g. We found that a significant increase in cerebral blood flow between days 1 and 2 was accompanied by a significant decrease in cerebral fractional oxygen extraction (FOE). High cerebral FOE may protect the brain from hypoxicischaemic injury, a potential consequence of reduced cerebral blood flow.
The results presented by Evans et al give an insight into the complex relationships that exist within a dynamic fluid system. Systemic blood pressure was closely related to MCA mean velocity, but not estimated MCA diameter, thus implying that cerebral blood flow would vary independently of systemic blood pressure because of changes in cerebrovascular resistance. Our own observations have produced similar results. Cerebral FOE, which is expected to increase as cerebral blood flow decreases, is not related to mean arterial blood pressure.2 There is, however, a significant relationship between cerebral FOE and left ventricular output, which is a major determinant of central blood flow.2 This latter finding is in agreement with the observation of Evans et al that superior vena cava flow, also related to central blood flow, was significantly associated with estimated MCA diameter.
Evans et al remind us that velocity is not the same as flow. Their observations, and our own, stress the importance of vascular resistance in mediating the relationship between blood pressure and blood flow. In the sick preterm infant, the presence of an adequate mean arterial blood pressure is often achieved using pharmacological methods. Although this is reassuring, it does not guarantee the presence of good central blood flow, nor the adequate perfusion of the end organs, including the brain.
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