Effect of the adjunct of carbon dioxide during cardiopulmonary bypass on cerebral oxygenation

Over the last few years, near-infrared spectroscopy (NIRS) has been introduced to study cerebral haemodynamics and oxygenation. This paper points out how the use of an external source of CO2 effects on the absolute value of cerebral NIRS during cardiac surgery on cardiopulmonary bypass.

Between January 2010 and September 2011, 368 patients underwent congenital heart disease correction on cardiopulmonary bypass (CPB). Nineteen patients, with a mean age of 26 days (range 6-120 days), required an external source of CO2 to correct hypocarbia during cardiopulmonary bypass. Different parameters were monitored: NIRS value, oxygen saturation, oxygen partial pressure, CO2 partial pressure, haematocrit, mean arterial pressure and pH. They were analyzed during different phases of the surgical procedure: before, during and after CO2 infusion.

There were no deaths during the hospital stay. The NIRS value increased significantly (p<0.05) after the addition of CO2, from a starting value of 52.9 to a final value of 63.4. PaCO2 was found to increase too: from 31.3 mmHg to 40.6 mmHg. On the other hand, both values decreased when the CO2 was removed, to respective final values of 55.8 and 34.4 mmHg. Mean arterial pressure, haematocrit and PaO2 didn't modify significantly during this period.

Nowadays, NIRS is usually used in cardiac surgery to reduce possible risks of neurological damage. The importance of the role of pCO2 in the cerebral vascular resistance and in cerebral blood flow has already been proven. This research demonstrates a relationship between pCO2 and the NIRS value. This paper could introduce an important correcting tool when an inadequate NIRS value occurs although the level of oxygenation, haematocrit and mean arterial pressure are acceptable and the arterial line is accurately positioned.