Effects of external constraint on the fetal left ventricular function curve

Area of Science:

• Cardiovascular Physiology
• Fetal Development
• Cardiac Mechanics

Background:

• The fetal ventricular function curve describes how the fetal heart's output changes with filling pressure.
• A plateau in this curve suggests limitations in the fetal heart's ability to increase stroke volume.
• Myocyte (heart muscle cell) immaturity has been proposed as a cause for this plateau.

Purpose of the Study:

• To investigate whether external ventricular constraint limits fetal left ventricular (LV) stroke volume.
• To determine if this constraint explains the plateau observed in the fetal ventricular function curve.
• To differentiate the effect of constraint from potential myocyte immaturity.

Main Methods:

• Studied nine fetal lambs (142-144 days' gestation) under halothane anesthesia.
• Measured LV stroke volume, LV end-diastolic pressure, and external ventricular constraint (intrapericardial pressure).
• Compared measurements under closed chest/pericardium versus open chest/pericardium conditions.

Main Results:

• Stroke volume was significantly higher with an open chest and pericardium compared to a closed chest (p < 0.01).
• Reduced external ventricular constraint increased LV transmural pressure (preload).
• Increased preload substantially elevated fetal LV stroke volume.

Conclusions:

• External ventricular constraint significantly limits fetal left ventricular stroke volume by reducing preload.
• This constraint is a primary factor responsible for the plateau in the fetal ventricular function curve.
• The plateau is likely not due to myocyte immaturity but rather mechanical limitations.