Production of complete heart block and utero cardiac pacing in fetal lambs.

We produced a complete heart block in 5 fetal lambs at 120 to 135 days of gestation by injecting 10% formaldehyde into the His bundle. We obtained acute experimental data in 5 fetuses and chronic experimental data in 2 fetuses. Immediately after production of the complete heart block, cardiac output was acutely reduced to 43 +/- 9% when compared with the cardiac output at sinus rhythm. It was possible to restore the cardiac output to 80 +/- 7% of the preoperative levels by atrioventricular pacing at the same rate as had been present before the heart block was produced. When the heart rate decreased below 100 beats per minute, further increase of stroke volume did not occur, and output was markedly diminished. This model allows us to study the pathophysiological response of a complete heart block, and offers basic information useful for understanding fetal cardiac functioning.