Distribution of fetal cardiac output: importance of pacemaker location

Abstract

Important questions exist about the relative roles of changes in heart rate versus extent of myocardial shortening in regulating fetal cardiac output, because increases in heart rate created by left atrial pacing have been shown to increase right ventricular output and decrease left ventricular output. Since the pacemaker site could importantly influence foramen ovale flow and, hence, each ventricle's output, changes in individual ventricular outputs were examined when both the right and left atria were paced at a rate of 270 beats/min in five acute and in eight chronically instrumented fetal lamb studies. With pacing of either atrium, total cardiac output was unchanged compared to control values. However, the right ventricle contributed more to total cardiac output with left atrial pacing (73% acute, 65% chronic) than with right atrial pacing (51% acute, 57% chronic). Converse changes were observed in left atrial pacing (27% acute, 35% chronic) as compared to right atrial pacing (49% acute, 43% chronic). Thus the disparity that exists normally in the contributions of the right and left ventricles to total cardiac output is accentuated with left atrial pacing and minimized with right atrial pacing. Pressure measurements demonstrated changes in the atrial pressure relations that would be expected to alter flow across the foramen ovale depending on the chamber initially activated. Previous experimental differences can, therefore, be attributed to changes in the magnitude of shunting across the foramen ovale and depend on pacemaker location.

Wave intensity analysis of right ventricular and pulmonary vascular contributions to higher pulmonary than aortic blood pressure in fetal lambs

“…However, blood gas and blood pressure data were within the normal range reported in unanesthetized, chronically instrumented late-gestation fetal lambs (1,24,28,34,39,41,44,48). Furthermore, as with prior studies in partially exteriorized fetuses where the umbilical cord remained protected within the uterus (3,33), RV output exceeded LV output, which is the pattern observed in chronically instrumented fetuses (1,16,36,41,43,44,48). It is thus unlikely that the qualitative features of our findings were affected by our experimental approach.…”

The Effect of Atrial Pacing-Induced Tachycardia on Left Ventricular Contractile Function in Conscious Newborn and Adult Sheep

“…It is possible that the unusual activation sequence initiated by left atrial pacing may have influenced the results in the newborns by producing shunting through the foramen ovale. In the unanesthetized lamb fetus, it has been demonstrated that whereas pressure is normally higher in the right atrium than the left, left atrial pacing was associated with a higher left atrial pressure throughout most of the cycle (32). If a similar mechanism occurred in the newborn lambs, this might produce left to right shunting, which could have contributed to the decrease of LVEDP that occurred at the most rapid heart rate ( Table 2).…”

Comparative Effects of Metabolic Acidemia and Hypoxemia on Cardiac Output and Regional Blood Flows in Unanesthetized Newborn Lambs

“…An effect of pacemaker location on regional blood flows has been demonstrated in fetal lambs (25), but the use of left atrial pacing should not have had an independent effect on the results in newborn lambs at the ages that were studied because the atrial foramen is functionally closed. However, the reversal of sinus bradycardia to a normal heart rate during acidemia could have prevented a more severe reduction in cardiac output (6), which could have altered the regional blood flow responses.…”