Influences of different bypass modalities and body core temperatures during extracorporal circulation (ECC) on pathomorphological and immunohistochemical changes in the pig brain

Aim

To assess influences of different bypass modalities and core temperatures during ECC on brain pathomorphology and immunohistochemistry in an animal model of cardiac surgery.

Methods

Twenty-one pigs, aged 4 months, were assigned to the following temperature- and flow groups: I: 37°C–2.7 l/min/m²-(7), II: 28°C–1.6 l/min/m²-(9), III: 20°C–1.3 l/min/m²-(5). Duration of ECC was 120 min, including cooling and rewarming 30 min each. Six hours after ECC, brains were harvested for histology and immunohistochemistry. Coronal sections from different brain regions were stained with hematoxylin/eosin and antibodies against glial fibrillary acid protein (GFAP), astroglial cell protein S-100 (PS-100) and neuron-specific enolase (NSE) for standard light microscopy. Ischemic lesions were evaluated by a quantitative histological score with respect to tissue stratification and vacuolization, ganglion and glial cell densitiy and glial fiber reaction. Apoptosis was identified by in situ DNA fragmentation (TUNEL). In addition, perioperative PS-100 serum kinetic from jugular venous bulb blood was evaluated.

Results

In frontal lobe, the most severe ischemic lesions were observed in group III showing frequent and distended loss of histologic stratification between outer molecular and pyramidal cell layers, caused by necrosis of pyramidal cells and vacuolization within the outer molecular cell layer. In group II, these lesions were less and only focal, in group I hardly present. In all groups, apoptosis was not apparent in the neocortex. In hippocampus, pyramidal cell damage was severe in III, moderate in II, whereas slight in I. In cerebellum, Purkinje's cells were found markedly reduced in III, moderately reduced in II, and slightly reduced in I. Striate body and medulla oblongata were free from ischemic lesions in all groups. Signs of inflammation or edema were not observed. In addition, marked expression of PS-100 in perivascular glial cells as well as significant elevation of serum PS-100 values at the end of ECC and return to preoperative values 6 h after ECC were assessed in all groups.

Conclusions

Ischemic lesions with loss of ganglion cells in sensitive brain regions as cortex, hippocampus and cerebellum are most marked after deep-hypothermic low-flow bypass, lowest-graded after normothermic full-flow bypass, and intermediate after moderate hypothermic low-flow bypass. Reduction of flow-rate and hypoxia during ECC rather than inflammation are assumed to contribute to the histopathological findings. Serum PS-100 elevation after ECC is, independent of the bypass modalities, supposed as the result of a temporary disturbance of the blood-brain barrier with respect to tight relations between astroglial and vascular endothelial cells.