The cerebrovascular effects of carbon dioxide in ventilator-dependent preterm infants.

Changes in arterial carbon dioxide tension (PaCO2) have a major influence on cerebral blood flow (CBF). The following work firstly reviews the role of PaCO2 and other factors which regulate CBF. Secondly a study was undertaken to assess whether changes in CBF following a 1 kPa rise in PaCO2 predicted subsequent periventricular leukomalacia (PVL - identified on cranial ultrasound) in sick preterm infants, since PVL is an important cause of neurological deficit in such infants and has been hypothesised to occur from inadequate regulation of CBF. Changes in CBF (estimated by Doppler cerebral blood flow velocity, [CBFV]) following a rise in PaCO2 in the first day of life in infants 30 weeks gestation appeared greatly dependent on the concomitant changes in mean arterial blood pressure (MABP). Similar dependence also occurred following administration of pancuronium to infants whose CBFV response was previously independent of changes In MABP. This dependency lasted for the duration of paralysis. Changes in CBFV following a rise in PaCO2 did not however predict subsequent ultrasonographic changes indicative of PVL. Since serial studies on individuals were often performed at different ventilator settings, the circulatory effects of ventilatory rate in 20 therapeutically paralysed preterm infants were studied to determine whether fast rates might compromise MABP and hence CBF. At the fastest rates used (l00.min-1), changes in CBFV appeared influenced by changes in MABP. This again may have resulted from the use of pancuronium. A Doppler technique was also used to study changes in cardiac output following a similar carbon dioxide (CO2) "challenge" in 21 of the infants. The rise in MABP observed was not accompanied by a rise in cardiac output, suggesting that components of peripheral resistance influenced blood pressure in such infants. These results firstly confirm the major cardio- and cerebrovascular influence of PaCO2 in sick preterm infants. Secondly they underline the importance of continuous monitoring of and the avoidance of large changes in blood pressure in such infants. The circulatory effects of pancuronium warrant further study.