CEREBRAL HYPOXIA DURING CARDIOPULMONARY BYPASS - A MAGNETIC-RESONANCE-IMAGING STUDY

Background. Neurocognitive deficits after open heart operations have b een correlated to jugular venous oxygen desaturation on rewarming from hypothermic cardiopulmonary bypass (CPB). Using a porcine model, we l ooked for evidence of cerebral hypoxia by magnetic resonance imaging d uring CPB. Brain oxygenation was assessed by T2-weighted imaging, bas ed on the blood oxygenation level-dependent effect (decreased T2-weig hted signal intensity with increased tissue concentrations of deoxyhem oglobin). Methods. Pigs were placed on normothermic CPB, then cooled t o 28 degrees C for 2 hours of hypothermic CPB, then rewarmed to baseli ne temperature. T2-weighted imaging was undertaken before CPB, during normothermic CPB, at 30-minute intervals during hypothermic CPB, afte r rewarming, and then 15 minutes after death. Imaging was with a Bruke r 7.0 Tesla, 40-cm bore magnetic resonance scanner with actively shiel ded gradient coils. Regions of interest from the magnetic resonance im ages were analyzed to identify parenchymal hypoxia and correlated with jugular venous oxygen saturation. Post-hoc fuzzy clustering analysis was used to examine spatially distributed regions of interest whose pi xels followed similar time courses. Attention was paid to pixels showi ng decreased T2 signal intensity over time. Results. T2* signal inten sity decreased with rewarming and in five of seven experiments correla ted with the decrease in jugular venous oxygen saturation. T2 imaging with fuzzy clustering analysis revealed two diffusely distributed pix el groups during CPB. One large group of pixels (50% +/- 13% of total pixel count) showed increased T2 signal intensity (well-oxygenated ti ssue) during hypothermia, with decreased intensity on rewarming. Chang es in a second group of pixels (34% +/- 8% of total pixel count) showe d a progressive decrease in T2 signal intensity, independent of tempe rature, suggestive of increased brain hypoxia during CPB. Conclusions. Decreased T2 signal intensity in a diffuse spatial distribution indi cates that a large proportion of cerebral parenchyma is hypoxic (evide nced by an increased proportion of tissue deoxyhemoglobin) during CPB in this porcine model. Neuronal damage secondary to parenchymal hypoxi a may explain the postoperative neuropsychological dysfunction after c ardiac operations. (C) 1997 by The Society of Thoracic Surgeons.