The cardiac output of the near-term fetus is 450-500 ml/kg/min; the combined outputs of right ventricle and left ventricle. Recall that in utero the output of the right ventricle is about double that of the left ventricle. After one postnatal week the outputs of the right ventricle and the left ventricle are each 425 ml/kg/min. This represents approximately a 30% increase for the right ventricle, and a 250% increase for the left ventricle. Due to this high flow rate, the increased pressure (i.e. afterload) that the left ventricle is required to pump against, and the relatively modest muscle mass, there is little reserve capacity.
Rudolph carried out studies in neonatal sheep where hearts were volume loaded in order to simulate varying degrees of shunt at intervals post-birth (Figure 4.12). He found, for example, that a 35% shunt is the maximal allowable 1 week after birth which will maintain adequate systemic blood flow. By 3-6 weeks a 50% shunt can barely be maintained, while in adulthood a 50% shunt is well tolerated with plenty of reserve capacity.
Go to Next Page
Return to Previous Page
Return to Index