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of normal fetuses studied from 20 to 30 weeks of gestation, and an increase in the proportion of the cardiac output from the right side to 60% at term.95 Pulmonary vascular resistance increases during the last trimester This again changes the balance of the cardiac output, increasing flow into the systemic circulation The Doppler waveform of the branch pulmonary arteries is characterized by a sharp systolic upstroke with a short acceleration time and reversal of flow in late systole (Fig 6.13) Studies have correlated these findings with pressure and circulatory events in fetal lambs and attribute the reversal of flow to the low compliance of the branch pulmonary arteries and high pulmonary vascular resistance in the fetus.93 FIG 6.13 Doppler waveform in the branch pulmonary arteries is characterized by a sharp systolic upstroke, a short acceleration time and a notch, or reversal of flow in late systole Optimal perinatal planning requires the prediction of important fetal pulmonary hypoplasia, both to aid counseling of parents and to prepare the resources required for neonatal resuscitation and support Early echocardiographic ratios, such as the ratio of sizes of the lungs and head, were derived to predict pulmonary hypoplasia in cases with diaphragmatic hernia A ratio less than 0.6 was associated with poor outcome, whereas a ratio greater than 1.4 predicted survival.96 Alternative indexes of the ratio of fetal lung volume to fetal body weight using MRI in combination with ultrasonography have been devised.97 Others have used MRI alone to assess the total lung volume by comparing the signal intensity of lung to that of spinal fluid.98 MRI and ultrasound are complimentary imaging methods for predicting fetal pulmonary hypoplasia and the prognostic accuracy of observed-to-expected MRI fetal lung volume and observed-to-expected ultrasound lung-to-head ratio have been published.99 Both methods have shown comparable ability to assess lung volumes in left sided diaphragmatic hernia, particularly before 32 gestational weeks, although the ultrasound method overestimates lung size by 1.5 to 1.8 compared with MRI methods However, neither were useful predictors of outcome in right-sided lesions.99 Doppler parameters such as the pulsatility index have not been found discriminatory for pulmonary hypoplasia100 and prompted investigation of physiologic testing of the fetal pulmonary bed Maternal hyperoxia testing studies have attempted to determine whether the near-term fetus with certain congenital defects such as diaphragmatic hernia or hypoplastic left heart syndrome can respond to a 15-minute administration of 100% oxygen delivered to the mother via facemask by increasing pulmonary blood flow.101,102 Interpretation of the results of acute administration of oxygen are made difficult by technical considerations such as ensuring accuracy of placement of the Doppler sample volume so that the same part of the pulmonary tree is sampled before and after oxygen administration This imaging is particularly challenging in the setting of diaphragmatic hernia Much uncertainty exists around the role and efficacy of chronic administration of oxygen to the pregnant woman Long-term maternal administration of oxygen by facemask for 6 or 8 hours a day over several weeks is thought to increase pulmonary venous return in responsive fetuses, and this in turn may promote growth of left-sided heart structures in borderline left hearts or those with aortic coarctation Although a plausible theory, outcomes have been less than convincing, and more research is required to establish, for example, the effects of chronic oxygen administration on the fetal retina, and to provide a robust evaluation of its cardiovascular effects above natural history outcomes Little is yet known of the determinants of responsiveness in the second and third trimester fetus.103–105 Coronary Arterial Flow In normal fetuses, flow in the coronary arteries is not usually seen until the third trimester Reference ranges for velocities have been described and do not appear to change with gestational age in the structurally normal heart.106 Visible flow was first described in terminally sick fetuses and proposed as an additional predictor of adverse outcome (Video 6.11).107 Fetal ultrasound may also demonstrate visible coronary arterial flow in conditions associated with restriction of growth, anemia, constriction of the arterial duct, twin-twin transfusion syndrome (TTTS), and bradycardia, thus demonstrating short-term autoregulation and long-term alterations in myocardial flow reserve in the human fetus It can be demonstrated in growth-restricted fetuses earlier in gestation than in appropriately grown fetuses and at higher velocity Fetuses with anemia show the highest velocities in the coronary arteries, perhaps reflecting increased left ventricular output due to a reduction in cerebral impedance in response to both pathologic situations (Fig 6.14) Coronary arterial flow is no longer visible once the underlying cause has been treated, for example by intrauterine fetal transfusion for anemia, or by stopping any causative medication such as indomethacin in the case of constriction of the arterial duct