Analysis of cerebroarterial Doppler flow velocity waveforms in newborn infants: towards an index of cerebrovascular resistance

Doppler ultrasound has been used extensively to study cerebral hemodynamics in the human newborn. This report presents a continuous recording in an infant with epileptic seizures. Heart rate, mean aortic blood pressure and mean flow velocity, obtained from the anterior cerebral artery by Doppler ultrasound, all increased markedly during the seizures. The Poucelot index was calculated from the cerebral blood flow velocity waveform and is supposed to reflect cerebrovascular resistance. This index decreased. Three mutually contradictive interpretations were possible in terms of cerebral metabolic rate, cerebral blood flow, metabolism-flow coupling and pressure-flow autoregulation. Sympathetic activation and its effects on cerebral hemodynamics is discussed. It is pointed out that the competence of the pressure-flow autoregulation may not be studied reliably during changes in blood pressure associated with altered sympathetic activity. An electrical model of the systemic arterial system is presented and used to demonstrate that the resistance index is likely to be severely affected by changes in cerebroarterial compliance, peripheral resistance, duration of systole as a fraction of heart cycle and patency of the arterial ductus. A pulsatility index ratio, which is corrected for arterial pulse pressure, is suggested to avoid some of these errors.