Fetal heart block: a new experimental model to assess fetal pacing.

Epicardial fetal pacing via thoracotomy has the potential of being a safer and more reliable procedure to treat congenital complete heart block (CHB) associated with fetal hydrops refractory to medical therapy. To assess the acute electrophysiological characteristics of two ventricular epicardial leads, a new experimental model of fetal heart block induced by cryosurgical ablation of the AV node without the need for fetal cardiac bypass was performed in 12 pregnant ewes at 110-115 days gestation. A modified screw-in lead (1 1/2 turns) was used in six fetal lambs and a stitch-on lead in the other six lambs. CHB was achieved in 100% of the fetal lambs, with no ventricular escape rate noticed in any of the lambs. The acute stimulation thresholds were consistently low for both leads, with lower values for the screw-in lead at pulse duration below 0.9 msec (P < 0.03). Current measured at voltage threshold with pulse width below 0.5 msec was lower for the screw-in lead (P < 0.048). Stimulation resistance, measured during constant-voltage pacing, was not statistically different between the two leads (441.8 +/- 13.7 omega for the screw-in lead vs 480.2 +/- 59.2 omega for the stitch-on lead). No significant differences (P > 0.20) were found in R wave amplitude between the two electrodes. Slew rates were significantly higher in the screw-in group than in the stitch-on group (1.40 +/- 0.2 vs 0.62 +/- 0.2 V/sec, P = 0.04). This model of CHB is a simple and reproducible method to assess fetal pacing. We find the screw-in electrode to be a better option when fetal pacing is indicated.