Use of thyroid hormones after cardiopulmonary bypass in children.

IREAD WITH GREAT INTEREST THE PAPER OF Mainwaring and his colleagues concerning the use of supplementation of triiodothyronine following the modified Fontan procedure. The authors found an evident reduction in levels of triiodothyronine in the blood both during and after cardiopulmonary bypass in patients undergoing the Fontan procedure. We have encountered similar findings in 47 consecutive children undergoing surgery for various heart defects, and observed that the changes did not depend on age, body weight, the type of the heart defect, nor on the depth of hypothermia achieved during the operation. Mainwaring et al gave triiodothyronine as a bolus at a dose of 0.2 mcg/kg immediately after the operation, and administered a similar dose over 6 hours in the intensive care unit. They obtained an improved clinical outcome, as well as observing faster recovery of thyroid hormones in the blood following the procedure. This group of patients seems to be most amenable to treatment with triiodothyronine, which combines positive inotropic and vasodilatory effects. My findings in infants with low cardiac output postoperatively were similar, and make me think that triiodothyronine has a moderately positive influence in such circumstances. The optimal dosage to be given after cardiopulmonary bypass, however, has not yet been determined. In animal experiments, the treatment was dose-dependent. I gave triiodothyronine to 14 infants with low cardiac output postoperatively after various operations (arterial switch, total correction of tetralogy of Fallot, correction of complete atrioventricular septal defect, and closure of ventricular septal defect) according to two different protocols. In the first protocol, used in 7 children, triiodothyronine, given as Thyreotardin-inject.N, was administered as an intravenous bolus at a strength of 0.4 mcg/kg, and was followed by an infusion of 0.2 mcg/kg/hr for 8 hours. This dosage was comparable to that used by Rosen and colleagues in children, and by Cimochowski et al in adults, but was lower than the dosage administered by Novitzky and associates to some of their high-risk patients undergoing open heart procedures. In one of our infants with Down's syndrome and tetralogy of Fallot, an unusual complication occurred after administration of triiodothyronine according to the above mentioned regimen, but given for 14 instead of 8 hours. In this patient, we noted persistent hypotension with markedly increased warmth of the arms and legs. Acute renal failure developed despite giving an infusion of norepinephrine. After haemofiltration, which probably eliminated the excessive amount of triiodothyronine, cardiovascular as well as renal functions returned to normal. As far as I know, this is the first complication encountered of this kind, although reduction of systemic vascular resistance requiring administration of vasoconstrictive drugs has been described after higher dosages of triiodothyronine. In our second protocol, we gave triiodothyronine to another 7 infants with low cardiac output postoperatively as a 0.4 mcg/kg bolus followed by a maintenance infusion of 0.4 mcg/kg for 24 hours. No such complications were observed. An interesting alternative for treatment of postoperative cardiac failure in children is the use of thyroxine. This has been given with good results by Andreas Urban at St Augustin's Hospital in Germany for several years (personal communication). The main advantage of treatment with thyroxine is the provision of a more physiological control of the concentrations of triiodothyronine in the serum, this being synthesized in extrathyroid tissue from the ingested thyroxine. This also has a beneficial effect on renal function. In keeping with my own initial experience, therefore, delivery of a 3 mcg/kg bolus of thyroxin (L Thyroxin Henning inject.) followed by continuous infusion of triiodothyronine at a dosage of 0.4 mcg/kg/24 hrs is another promising and safe alternative for treatment of low cardiac output postoperatively.