The influence of abnormal blood gases on cerebral blood flow velocity in the preterm newborn.

The influence of hypercarbia, combined hypercarbia and hypoxemia, and hypocarbia on cerebral blood flow of preterm infants with a gestational age of less than 34 weeks was investigated by measuring peak systolic flow velocity (PSFV), end diastolic flow velocity (EDFV), pulsatility index (PI) and area under the velocity curve (AUVC) of the anterior cerebral artery (ACA) using transcutaneous Doppler technique. Mild and moderate hypocarbia did not change cerebral blood flow velocity. During severe hypercarbia (PaCO2 greater than or equal to 6.7 kPa) significantly lower PI-values were detected, mainly caused by an increase of EDFV, indicating a decreased cerebrovascular resistance and increased cerebral blood flow. A highly significant decrease in PI-values during combined severe hypercarbia and hypoxemia (PaO2 less than 6.0 kPa) was found suggesting that hypoxemia superimposed on hypercarbia strengthens the increase of cerebral blood flow. While the increase of EDFV is thought to be due to carbon dioxide-induced vasodilation of cerebral arterioles, we assume that the accompanied increase of PSFV during combined hypercarbia and hypoxemia is caused by an increase in cardiac output due to hypoxemia which can alter the blood flow velocity wave form of the ACA.