The Regional Distribution and Determinants of Myocardial Blood Flow during Asphyxia in the Fetal Lamb

Abstract

SummaryThe syndrome of hypoxemia-related myocardial dysfunction in the newborn is generally associated with severe intrapartum asphyxia. We investigated the changes in total and regional distribution of myocardial blood flow (MBF) that occur during asphyxia and the factors that regulate MBF, in the chronically prepared, near term, fetal lamb. Studies were done in the awake, physiologically stable state 36-72 h after surgical preparation.In utero asphyxia was produced by partial cord occlusion and blood flow to fetal organs was measured by the radioactive microsphere technique. A complete set of control measurements was made, and then an occlusion loop was inflated to partially occlude the umbilical vessels. After 30-60 min of partial occlusion, all measurements were repeated. The third set of measurements was made after release of the occlusion loop. Six animals were studied and 17 sets of measurements obtained.Partial umbilical cord constriction produced a progressive asphyxia and acidosis. Cardiac output was severely depressed, although the increase in percentage of cardiac output directed to the myocardium (%F) was very significant. Release resulted in improved cardiac output and O2 and CO 2 exchange, although the metabolic acidosis was worse. No change occurred in the intramyocardial distribution of MBF during asphyxia or recovery. Total MBF was not significantly increased during asphyxia.Multiple linear regression analysis indicated that arterial oxygen saturation (Sa02) and heart rate were statistically significant predictor variables for absolute MBF, accounting for about 45% of the variation in MBF. Further analysis determined that mean arterial blood pressure, pH, P02, and Sa02 were all statistically significant univariate predictors of MBF as %F, but only Sa02 was a statistically significant multivariate predictor, accounting for about 82% of the variation in %F.

Simultaneous pulmonary trunk and pulmonary arterial wave intensity analysis in fetal lambs: evidence for cyclical, midsystolic pulmonary vasoconstriction

“…Finally, one potential limitation of our study was that it was performed under general anesthesia and open-chest conditions, an approach necessary because of the extent of surgical instrumentation required. However, blood gas data were comparable to those of unanesthetized fetuses, while the average level of mean arterial blood pressure in our study (59 -60 mmHg) was at the upper end of the normal range reported in chronically instrumented late-gestation fetal lambs (1,6,9,19,27,30,33,39,40,42). In addition, LPA flow (70 ml/min) was close to the range of 71-87 ml/min measured with transit-time flow probes in unanesthetized and anesthetized late-gestation fetuses (14,19,33).…”

Ductus arteriosus wave intensity analysis in fetal lambs: midsystolic ductal flow augmentation is due to antegrade pulmonary arterial wave transmission

“…However, blood gas and blood pressure data were within the normal range reported in unanesthetized, chronically instrumented late-gestation fetal lambs (1,5,19,22,26,30,33,35,38). It is thus unlikely that the qualitative features of our findings were affected by our experimental approach.…”

In vitro validation of Doppler indices using blood and water.

“…Maintenance of blood pressure is known to occur with chronic growth retardation in animal models, J and an alteration in the distribution of cardiac output and an increase in systemic blood pressure have been described in response to acute occlusion of the umbilical artery in fetal sheep. 4 Our results show that a parallel reduction in both systolic and diastolic shifted-frequencies will produce the expected effect of a relative increase in Doppler indices because D becomes a smaller proportion of S as the values of each approach zero.…”