Does hypothermia prevent cerebral ischaemia during cardiopulmonary bypass?

Abstract

It is believed that moderate hypothermia (25–32°C) during cardiopulmonary bypass provides cerebral protection by reducing the cerebral metabolic rate (CMRO 2). Nevertheless episodes of ischaemia do occur and thus it has been suggested that cerebral oxygenation should be monitored by jugular venous oximetry. However, this technique is cumbersome and invasive. Near infrared spectroscopy (NIRS) provides a non-invasive assessment of cerebral oxygenation and this was used together with continuous jugular venous oximetry in 21 patients undergoing hypothermic cardiopulmonary bypass. During the hypothermic period, jugular venous oximetry indicated reduced oxygen extraction consistent with a reduction in CMRO 2 (increase from 61±2.5% to 74±2.5%). In contrast, near infrared spectroscopy demonstrated increased oxygen extraction (HbO 2 −11.5±1 μM, HHb +3.2±0.3 μM) and a fall in the cerebral concentration of oxidized cytochrome oxidase (−1.7±0.3 μM) indicating ischaemia. These results suggest that cerebral ischaemia occurs during hypothermic cardiopulmonary bypass with a spurious rise in jugular venous oxygen saturation, which represents arterio-venous shunting. Thus if hypothermia does facilitate cerebral protection it does not appear to be a direct result of a reduction in CMRO 2 and oxygen requirement.