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 released as mediators of this response. The placental dysfunction could be blocked by the administration of indomethacin, allowing longer fetal survival. This unmasked a more subacute (but fatal) problem: fetal surgical stress resulted in diminished fetal cardiac output and progressive metabolic acidosis and contributed to the placental vasoconstriction. In acute studies, when indomethacin was given and the stress response was inhibited by the use of total spinal anesthesia, the fetus maintained normal blood gas levels, cardiac output, placental blood flow, and acid-base status for several hours after bypass. We hypothesized that beyond this point, no further fetal or placental compromise would occur and that this management technique would thus allow long-term fetal survival. With the use of total spinal anesthesia and sterile technique for long-term study, 12 fetal lambs at 120 days (80%) gestation underwent exposure, line placement, and cannulation for fetal cardiac bypass. 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 and fetus were allowed to recover. There was one maternal death (pneumonia) and one early abortion (of twins); the remaining 10 ewes progressed to term. At term, five healthy lambs that had undergone fetal cardiac bypass were delivered (including one twin), four ewes delivered a mummified study fetus and one or two healthy siblings, and one delivered a dead term fetus. With the use of techniques that inhibit fetal stress and block placental vasoconstriction, cardiac bypass can be performed 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.