Cerebral blood volume response to changes in carbon dioxide tension before and during cardiopulmonary bypass in children, investigated by near infrared spectroscopy.

Neurological impairment may occur following cardiopulmonary bypass (CPB) and the effect of CPB on cerebrovascular control may be important in the mechanism of cerebral injury. We have used near infrared spectroscopy (NIRS) to observe cerebral haemodynamics non-invasively before and during CPB. We measured the change in cerebral blood volume (CBV) associated with changing PaCO2 (CBVR). Patients (n = 19) were aged from 1 to 135 (median 14) months. The cerebral blood volume response was determined pre-operatively at normothermia under the influence of standardised anaesthesia employing isoflurane (up to ET conc 0.5%) and during steady-state hypothermic bypass (22-32 degrees C) at an arterial pump flow rate of 1.9-2.4 lm-2.min-1. Complete data was available for 10 patients. The relation between CBV, arterial carbon dioxide tension (PaCO2), mean arterial pressure (MAP) and central venous pressure (CVP) was examined using analysis of covariance (P < or = 0.05) was accepted as significant). The change in CBV associated with changing PaCO2 was corrected for the effects of MAP and CVP. Preoperatively the median CBVR was 0.130 (25th-75th percentile 0.079-0.243) ml.100 g-1.kPa-1 and during hypothermic bypass the median CBVR was 0.093 (25th-75th percentile 0.026-0.255) ml.100 g-1.kPa-1. These values were compared with our reference range derived for normal conscious children using the Kruskal-Wallis test. There was not statistically significant difference between the three groups (P = 0.35). These results, indicating preservation of CBVR during the conditions of anaesthesia and bypass used, are consistent with the observations of previous authors who measured cerebral blood flow response to carbon dioxide by a variety of other methods. Near infrared spectroscopy is proving to be a reliable, non-invasive technique for the investigation of cerebral haemodynamics during CPB.