Hemodynamic effect of rapid atrial pacing in fetal lambs

To determine the threshold at which rapid atrial pacing brings on fetal circulatory failure, we made a fetal supraventricular tachyarrhythmia model and measured the central venous pressure, aortic pressure, and right and left ventricular outputs in five fetal lambs. Under maternal anesthesia, the uterus was opened, and under local anesthesia, polyvinyl catheters were inserted into the fetal superior vena cava and ascending aorta through a neck incision. Pacing leads (Medtronic model 6492) were then sutured onto the fetal right atrial appendage via right thoracotomy. Ventricular output was estimated using a Toshiba SSH-65A echocardiography by a transuterine approach. Fetal hemodynamics were observed without pacing (control), and at the atrial pacing rates of 200, 300, 350, and 400/min. Central venous pressure (CVP) increased and the aortic pressure decreased when the right atrium was paced at 350/min or more. Right ventricular output decreased when the right atrium was paced at 300/min or more. The left ventricular output, however, remained constant. The right ventricular output was 382 +/- 106 mL/kg/min at control, and 391 +/- 117 mL/kg/min when paced at 200/min, but decreased to 210 +/- 138 mL/kg/min when paced at 300/min, to 223 +/- 102 mL/kg/min when paced at 350/min, and to 186 +/- 86 mL/kg/min when paced at 400/min. Fetal circulatory failure occurred when the right atrium was paced at 300/min or more.