Andersons pediatric cardiology 168

FIG 6.14 Doppler recordings (A) Prograde coronary flow in a fetus with twin-twin transfusion syndrome suggesting a “heart-sparing” response secondary to fetal hypoxemia or distress (B) Bidirectional coronary blood flow in a fetus with pulmonary atresia with intact ventricular septum and a right coronary artery to right ventricle fistula There is normal velocity Doppler suggesting no elevation of right ventricular pressure (C) In contrast to (B), this recording shows high-velocity reversal of flow along the right coronary artery at 2.9 m/s and normal velocity forward flow Animal studies of myocardial flow show that coronary reserve is mediated by nitric oxide and changes during hypoxemia.108 This finding has been termed fetal cardiac sparing.109 Accordingly, visible flow in the coronary arteries is attributed to an increased volume of flow secondary to low fetal arterial content of oxygen Visualization of coronary flow coincides with important increases in the Doppler velocity z-scores in the umbilical artery, inferior caval veins, and venous duct The greatest change was observed in the venous duct z-score occurring 24 hours before visible coronary arterial flow was identified These changes were associated with adverse perinatal outcomes.110 Developmental Changes in Cardiac Function The ability to assess fetal cardiac function has improved as imaging techniques have advanced Moreover, the potential for fetal therapy for certain conditions, such as semilunar valve stenosis, TTTS, and diaphragmatic hernia has stimulated a more comprehensive assessment of fetal cardiovascular function to guide their timing and monitor their success.111 In the early embryo, gradients across the atrioventricular orifices act as a resistance to, and regulate, the flow of blood, thus influencing ventricular development.112 As the ventricular mass becomes trabeculated, so its mass increases and stiffness decreases, thus optimizing ventricular filling and ejection Increasing cardiac efficiency is associated with increasing myocardial mass and competence of the atrioventricular and arterial valves.113 Diastolic Maturation Atrial pressure exceeds ventricular pressure throughout filling, and from early gestation there is a clear distinction between the so-called passive and active filling phases, referred to as the E and A waves, respectively.5 The terms are misleading because we have long recognized that early diastole is an “active” relaxation, but the terms persist in common usage.114 Active velocities are higher than passive velocities in the fetus and in the newborn period, resulting in a ratio between the E and A waves that is less than 1 Transvaginal Doppler ultrasound of the human fetal heart has confirmed that ventricular inflow waveforms are monophasic before 9 weeks of gestation, becoming biphasic by 10 weeks.115 The patterns of ventricular filling change with age, with a relative increase in early diastolic filling, represented by the E wave, compared with the late diastolic component, or A wave, suggesting improved ventricular relaxation.116–118 Reference ranges between 8 and 20 weeks of gestation show a greater volume of flow passing through the tricuspid than mitral valve at all gestational ages The E/A ratio is also highly dependent on preload It cannot provide a load-independent assessment of ventricular function It is therefore a particularly unsuitable measure in fetal life, when direct pressures cannot easily be measured, and there is debate about the interpretation of longitudinal studies Although patterns of ventricular filling— monophasic compared with biphasic—have been proposed as more simple barometers of diastolic function, these have failed to correlate with changes in downstream impedance.119 Maturational changes in ventricular properties in human fetuses accelerate after mid-gestation as diastolic filling increases mainly after 25 weeks They are associated with a decrease in the ratio of the area of the myocardial wall to the end-diastolic diameter of the left ventricle Thus the decrease in left ventricular wall mass related to gestational age may be one important mechanism responsible for the alterations in diastolic properties noted in the fetal heart These are coincident with the reduction in placental impedance associated with normal adaptation of the spiral arteries.29,120 Atrioventricular valvar regurgitation is a common finding from 9 weeks onwards.115 Tricuspid regurgitation is commonly found in the first trimester and is thought to be more common in fetuses suffering from aneuploidy, particularly trisomy 21 It has been incorporated into early screening programs and is used to adjust the age-related risk for trisomy 21.121 It is not clear why tricuspid regurgitation is more common in these fetuses but may be associated with delayed development of the atrioventricular cushions because it resolves spontaneously in most cases and has no physiologic consequences later in pregnancy or after birth Most cases of tricuspid regurgitation reported in later gestation are also transient and trivial They are associated with a normal outcome.122 Systolic Maturation The second half of pregnancy is associated with an increase in mean velocities through the outflow tracts and a decrease in isovolumic relaxation and contraction times The ventricular stroke volume rises, and the afterload reduces, which is more pronounced on the left side of the heart The peak velocities in the ascending aorta are generally higher than in the pulmonary trunk, and the gestational increase is linear in longitudinal studies Cardiac output is traditionally calculated from the right and left ventricles using the velocity time integral of the maximum velocity envelopes through the valves, and a static assessment of valvar diameter measured at the hinge points The mean total