NEUROLOGICAL FUNCTION AFTER DEEP HYPOTHERMIC CIRCULATORY ARREST IN THE RAT

Background-Integrated neurological function, behavior, and somatic rec overy were studied in 35 rats undergoing 5 to 80 minutes of hypothermi c circulatory arrest (HCA). Methods and Results-A closed extracorporea l circulation system (ECC) consisting of a miniature oxygenator and he at exchanger, primed with 6 mt of asanguinous solution, was connected to a closed-chest rat with cannulae in the right atrium for venous dra inage (ID=1.7 mm) and in the ascending aorta for arterial return (ID=1 .0 mm). The rat was surface- and core-cooled until rectal temperature reached 18 degrees C, when ECC was stopped and cardioplegic solution d elivered, After 5, 10, 20, 40 (each n=5), and 80 minutes (n=15) of HCA , the rat was reperfused, weaned from ECC, and followed with behaviora l scoring, passive avoidance tasks, and cardiopulmonary exercise testi ng until euthanized for morphological study. Every rat resumed weight gain in the first week after HCA and regained preoperative exercise ca pacity by the fourth week. Only rats undergoing 80 minutes of HCA show ed behavioral abnormalities such as stereotypy and incomplete righting reflex, which eventually disappeared in the fourth week. Learning abi lity was preserved in all except for rats after 80 min of HCA, who fai led to acquire new memory to avoid electric stimuli (n=10) up to 3 mon ths after HCA, when pyramidal cells were partly replaced by astroglia in the cerebral cortex and CA1 sector of hippocampus. Nonetheless, old memory established before HCA was preserved even after 80 minutes of HCA and allowed rats (n=5) to avoid electric stimuli. Conclusions-Homo geneity of animals, miniature ECC system, and an established testing s ystem allowed evaluation of rats after HCA, which disclosed learning d isability (functional disorder) and pyramidal cell loss (organic defec t) after 80 minutes of HCA despite recovery of somatic function, behav ior, and growth.