Pulmonary Transition at Birth

Lung aeration triggers the cardiovascular transition at birth by decreasing pulmonary vascular resistance, thereby increasing ventricular preload. As left ventricular (LV) preload is primarily from the umbilical circulation before birth, cord clamping before lung aeration causes a large reduction in LV preload; lung aeration before cord clamping allows the source of LV preload to immediately switch from the umbilical to the pulmonary circulation, minimising changes in cardiac output at birth. If birth occurs as a result of caesarean section without labour, the absence of the intra-partum mechanisms for airway liquid clearance will likely increase the volume of liquid that must be cleared into lung tissue. The resulting increase in interstitial pressures must increase the tendency for liquid to re-enter the airways, potentially resulting in transient tachypnoea of the newborn. Many unanswered questions remain regarding physiological and molecular mechanisms underlying the cardio-pulmonary transition at birth.