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over one cardiac cycle The aortic fractional area change (%) is defined as the difference between minimum and maximum area divided by minimum area (From Wohlmuth C, Osei FA, Moise KJ, et al Aortic distensibility as a surrogate for intertwin pulse pressure differences in monochorionic pregnancies with and without twin-twin transfusion syndrome Ultrasound Obstet Gynecol 2016;48[2]:193–199.) Pressure-Volume Loops Pressure-volume loops remain the gold standard for measuring ventricular function independently of load, but their measurement is invasive It is possible to construct pressure-volume loops in the chick embryo (Fig 6.22).1 Physiologic differences have been demonstrated between right and left ventricular function in the postnatal dog.198 The shape of the pressure-volume relationship of the right ventricle differs in several ways from that of the left ventricle Right ventricular ejection occurs long after peak pressure has been achieved, and, for a given intraventricular volume, the pressure is less in the right ventricle.199 The instantaneous pressure-relationship at end-systole is linear in both ventricles, but the correction volume, as defined by the intercept of this line with the x-axis, is constant in the left ventricle It does not change with contractile state, in contrast to that of the right ventricle.198 FIG 6.22 Pressure-volume loops in a stage 21 chick embryo measured during preload infusion (From Keller BB Maturation/coupling of the embryonic cardiovascular system In: Clarke EB, Markwald RR, Takao A, eds Developmental Mechanisms of Heart Disease Armonk, NY: Futura;1995:375.) Similar assessment of the pressure-volume relationships, independent of load, is not yet technically possible in the healthy human fetus However, noninvasive insights into normal cardiovascular physiologic development have been obtained using echo-tracking equipment and Doppler ultrasound tissue imaging Although the right ventricle deals with a greater volume load in fetal life than does the left ventricle, minor axis ventricular systolic function and simultaneous direct pressure measurements are similar in right and left ventricles before birth.195 Studies of fetal long axis function have reported that myocardial velocities and amplitude of motion are increased in the right ventricular free wall compared with the left or the ventricular septum.129,135–137 This may be in response to the increased volume loading of the right ventricle compared with the left ventricle during normal maturation, and the increased number of myocytes aligned in longitudinal fashion The relative volume loading of the right ventricle during fetal life may alter the deposition, or cause the reexpression of, essential cytoskeletal and certain heat shock proteins such as desmin, the cytokeratins, vimentin, and HSP-72 These have been described in conditions of volume and pressure loading of ventricles postnatally and may act adversely, altering responses in the postnatal situation, thus permitting the ventricle to dilate more readily in response to volume and pressure loads, and so further prejudice its function Heart Rate Variability The variability in heart rate is determined by the maturation of the autonomic system There are major differences between species in the time at which the balance between neurotransmission of the sympathetic and parasympathetic system is accomplished This is related to the independence of the individual species before and immediately after birth Maladaptation, or immaturity, of neural control may manifest in acute life-threatening events in infancy Its antenatal assessment is more challenging because existing technology such as the cardiotocograph fails to measure beat-to-beat variability Although the full electrocardiogram can be recorded by use of scalp electrodes, this is feasible only once rupture of membranes has occurred Nevertheless, this has provided useful information during labor by analyzing the ST waves.200 Noninvasive recordings of the full fetal electrocardiogram have been reported from 15 weeks of gestation in the human fetus using blind signal separation of signals obtained from electrodes placed on the maternal abdomen and from magnetocardiography (Figs 6.23 and 6.24) Reference ranges for time intervals in the normal fetus have been published using these techniques,201–203 and magnetocardiography has permitted a more detailed analysis of fetal arrhythmias, particularly in the diagnosis of emerging heart block and long QT syndrome in the fetus.204–208 There are considerable computational challenges to separating the fetal from maternal signals,209–211 with continuous telemetric recordings of the fetal ECG in the ambulatory pregnant woman remaining the “holy grail” of fetal monitoring.212