LONG-TERM SURVIVORS OF FETAL CARDIAC BYPASS IN LAMBS

The initial experience with cardiac bypass in fetal lambs resulted in early fetal death from placental insufficiency. Subsequent work in our laboratory indicated that vasoactive cyclooxygenase products were rel eased as mediators of this response. The placental dysfunction could b e blocked by the administration of indomethacin, allowing longer fetal survival. This unmasked a more subacute (but fatal) problem: fetal su rgical stress resulted in diminished fetal cardiac output and progress ive metabolic acidosis and contributed to the placental vasoconstricti on. In acute studies, when indomethacin was given and the stress respo nse was inhibited by the use of total spinal anesthesia, the fetus mai ntained normal blood gas levels, cardiac output, placental blood how a nd acid-base status for several hours after bypass. We hypothesized th at beyond this point, no further fetal or placental compromise would o ccur and that this management technique would thus allow long-term fet al survival. With the use of total spinal anesthesia and sterile techn ique for long-term study, 12 fetal lambs at 120 days (80%) gestation u nderwent exposure, line placement, and cannulation for fetal cardiac b ypass. Indomethacin was given intravenously on obtaining venous access . After 20 minutes of normothermic cardiac bypass at flow rates of 250 to 300 ml/kg/min, the fetus was weaned from bypass, the cannulas and lines were removed, the uterus and abdomen were closed, and the ewe an d fetus were allowed to recover. There was one maternal death (pneumon ia) and one early abortion (of twins); the remaining 10 ewes progresse d to term. At term, five healthy lambs that had undergone fetal cardia c bypass were delivered (including one twin), four ewes delivered a mu mmified study fetus and one or two healthy siblings, and one delivered a dead term fetus. With the use of techniques that inhibit fetal stre ss and block placental vasoconstriction, cardiac bypass can be perform ed in single-gestation fetal lambs with a high degree of recovery and survival (80% in this study). The cause of the elevated abortion rate associated with twin gestation is unclear.