Circulatory effects of fast ventilator rates in preterm infants

Cerebral blood flow velocity and pulsation in neonatal respiratory distress syndrome and periventricular hemorrhage

The present study addressed the hypotheses that cerebral ischemia and/or excessive cerebral blood pulsation contribute to periventricular hemorrhage in preterm newborns with respiratory distress and that the pulse width is a valuable tool to estimate the contribution of cerebral blood pulsation. These hypotheses were tested by following preterm newborns at risk for respiratory distress and periventricular hemorrhage. We monitored for cerebral blood flow velocity (CBFV), cerebral pulse width, and cerebral pulsatility index; for patent ductus arteriosus, capillary Pco2, heart rate (HR) and behavior; and for the occurrence of respiratory distress and periventricular hemorrhage (PVH). The data obtained were analyzed with linear regression with the mode of respiration (spontaneous or supported) and postnatal age as additional covariates. We observed that (a) respiratory distress, either uncomplicated or complicated by PVH, correlates with a low CBFV and a high cerebral pulsatility index; (b) PVH also correlates with a high cerebral pulse width; (c) the increased pulse width precedes the onset of the hemorrhage; and (d) these CBF alterations can be partly attributed to ductal shunting and are ameliorated by mechanical ventilation.

Circulatory effects of ventilator rate and end-expiratory pressure in unparalysed preterm infants

Impairment of cerebrovascular autoregulation may be important in the pathogenesis of ischaemic brain injury in preterm infants. A previous study in ventilated preterm infants paralysed with pancuronium showed that changes in cerebral blood flow velocity (CBFV) were related to concomitant changes in arterial blood pressure. In a similar study in unparalysed infants, changes in CBFV in response to changes in ventilator rate or end-expiratory pressure were independent of associated changes in the arterial blood pressure. These results emphasize the importance of avoiding large swings in blood pressure in paralysed infants. Whether alternative paralysing agents have similar effects warrants further study.

Cerebrovascular carbon dioxide reactivity and failure of autoregulation in preterm infants

Changes in cerebral blood flow velocity (CBFV) in response to a rise in arterial carbon dioxide tension (PaCO2) in 94 ventilated preterm infants were determined using Doppler ultrasound to assess whether the nature of this change might predict subsequent neurological injury. Concomitant changes in mean arterial pressure (MAP) were recorded. Both CBFV and MAP rose significantly in response to the rise in PaCO2, the response being significantly less in the first 24 hours. Analysis indicated that the change in CBFV in the first day of life in infants less than or equal to 30 weeks' gestation was dependent to a great extent on the concomitant change in MAP. Similar dependence also occurred after administration of pancuronium to infants whose CBFV response was previously independent of changes in MAP. This dependency lasted for the duration of paralysis. Changes in CBFV after a rise in PaCO2 did not predict subsequent neurological injury. The influence of pancuronium on cerebrovascular regulation warrants further study.

Cardiovascular effects of carbon dioxide in ventilated preterm infants

Sick preterm infants may, under certain conditions, demonstrate blood pressure passive cerebral blood flow in response to changes in arterial carbon dioxide tension. Blood pressure in turn depends on cardiac output and peripheral resistance. A Doppler technique for assessing cardiac output compared favourably in terms of reproducibility to a thermodilution technique in a group of infants undergoing cardiac catheterization for congenital heart disease. Doppler was subsequently used to monitor changes in cardiac output following an increase in arterial carbon dioxide tension of 1 kPa in 25 ventilated preterm infants. Blood pressure increased significantly (p = 0.006). However, heart rate did not change significantly (p = 0.16) and, in addition, both stroke and minute volume decreased (p = 0.023, p = 0.02, respectively). This suggests that accompanying changes in components of peripheral resistance exert important effects on blood pressure in the preterm neonate in response to changes in arterial carbon dioxide tension.