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

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Abstract
Neurological impairment may occur following cardiopulmonary bypass (CPB)and the effect of CPB on cerebrovascular control may be important in themechanism of cerebral injury. We have used near infrared spectroscopy(NIRS) to observe cerebral haemodynamics non-invasively before and duringCPB. We measured the change in cerebral blood volume (CBV) associated withchanging PaCO2 (CBVR). Patients (n = 19) were aged from 1 to 135 (median14) months. The cerebral blood volume response was determinedpre-operatively at normothermia under the influence of standardisedanaesthesia employing isoflurane (up to ET conc 0.5%) and duringsteady-state hypothermic bypass (22-32 degrees C) at an arterial pump flowrate 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), meanarterial pressure (MAP) and central venous pressure (CVP) was examinedusing analysis of covariance (P < or = 0.05) was accepted assignificant). The change in CBV associated with changing PaCO2 wascorrected for the effects of MAP and CVP. Preoperatively the median CBVRwas 0.130 (25th-75th percentile 0.079- 0.243) ml.100 g-1.kPa-1 and duringhypothermic bypass the median CBVR was 0.093 (25th-75th percentile0.026-0.255) ml.100 g-1.kPa-1. These values were compared with ourreference range derived for normal conscious children using theKruskal-Wallis test. There was not statistically significant differencebetween the three groups (P = 0.35). These results, indicating preservationof CBVR during the conditions of anaesthesia and bypass used, areconsistent with the observations of previous authors who measured cerebralblood flow response to carbon dioxide by a variety of other methods. Nearinfrared spectroscopy is proving to be a reliable, non-invasive techniquefor the investigation of cerebral haemodynamics during CPB.

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