EFFECTS OF CARDIOPULMONARY BYPASS AND DEEP HYPOTHERMIC CIRCULATORY ARREST ON THE THYROID AXIS DURING AND AFTER REPAIR OF CONGENITAL HEART-DEFECTS - PRESERVATION BY DEEP HYPOTHERMIA

Thyroid function is altered by cardiopulmonary bypass (CPB) in childre n. To better understand the cause of altered thyroid hormone levels, w e compared the effects on the pituitary-thyroid axis of CPB in 23 chil dren undergoing elective repair of congenital heart defects. Twelve pa tients underwent CPB with moderate hypothermia without a period of dee p hypothermic circulatory arrest (DHCA), and eleven underwent CPB with DHCA. Nine blood samples were collected from each patient before, dur ing, and after CPB. Free T3 (FT3), free T4 (FT4), total T3 (TT3), tota l T4 (TT4), thyrotropin (TSH), and albumin were measured; concentratio ns of each decreased significantly with the onset of CPB (P < 0.05). T here was a greater decline in hormone than in albumin concentrations, which suggests that factors in addition to hemodilution were present ( P < 0.05). TSH concentrations in the DHCA group began to increase duri ng cooling, exceeding baseline values after rewarming and after separa tion from CPB. Patients undergoing CPB without DHCA had persistently l ow TSH concentrations (P < 0.05). By Postoperative Days 1 and 2, TSH c oncentrations in both groups were similar and significantly lower than baseline values (P < 0.001). FT3, FT4, TT3, TT4, and albumin all incr eased during CPB after an initial decrease. Of these, only albumin and FT4 recovered to their baseline values after the initial decrease. Ne vertheless, by Postoperative Day 1, both groups demonstrated the ''sic k'' euthyroid syndrome and could not be distinguished from one another . This study demonstrates greater pituitary release of TSH in children undergoing repair of congenital heart defects with DHCA compared with CPB alone, the cause of which could not be determined in this study. However, despite the increase in TSH in the DHCA group, the thyroid ho rmone concentrations failed to increase appropriately. Implications: E arly after deep hypothermia circulatory arrest, thyrotopin concentrati ons increase appropriately, responding to decreased concentrations of T3; however, all children undergoing cardiopulmonary bypass eventually develop a ''sick'' euthyroid syndrome by Postoperative Day 1. Whether this difference represents better protection of neuroendocrine functi on by deep hypothermic circulatory arrest (relative to cardiopulmonary bypass alone) remains speculative.