Characteristics and physiological basis of falls in ventricular outputs after immediate cord clamping at delivery in preterm fetal lambs.

Key Points: Controversy exists about the physiological mechanism(s) underlying decreases in cardiac output after immediate clamping of the umbilical cord at birth. To define these mechanisms, the four major determinants of ventricular output (afterload, preload, heart rate and contractility) were measured concurrently in fetal lambs at 15 s intervals over a 2 min period after cord clamping and before ventilation following delivery. After cord clamping, right (but not left) ventricular output fell by 20% in the initial 30 s, due to increased afterload associated with higher arterial blood pressures, but both outputs then halved over 45 s, due to a falling heart rate and deteriorating ventricular contractility accompanying rapid declines in arterial oxygenation to asphyxial levels.