Pressure-volume relationship of the fetal lamb heart

In contrast to the adult heart, the fetal heart reportedly has little functional reserve. With increased clinical emphasis on fetal cardiac diagnosis, neonatal surgery, and the potential for future fetal cardiac intervention, it is essential that we better understand fetal cardiac function. Therefore, to demonstrate the extent of fetal cardiac preload reserve, we studied 10 fetal lambs using an isolated, isovolumic, blood-perfused heart preparation. We maintained constant afterload, inotropic state, coronary blood flow, heart rate, and perfusate blood gas values. As left ventricular (LV) volume (preload) was incrementally increased, LV end-diastolic pressure and LV peak systolic pressure were recorded. Linear regression analysis demonstrated that increases in LV developed pressures were predicted by the LV volume, demonstrating the presence of the Frank-Starling mechanism in each case. The plateau of the Starling pressure-volume curve occurred at an LV end-diastolic pressure of 12.5 +/- 4.79 mm Hg (95% confidence interval, 9.07 to 15.9 mm Hg), lower than the plateau expected in the adult heart. This implies that, in the management of fetal and immature neonatal hearts, preload reserve plays an important but limited role in cardiac reserve.