cardiac output calculated in this way is approximately 550 mL/min per kg body weight However, the range considered to be normal is so wide as to make its interpretation difficult, except in serial assessment.123 Various investigations in humans, supported by animal data, have shown right cardiac output to be greater than left by at least two-fifths.51,52 The major source of error in the calculation of cardiac output results from measurement error of the diameter of the vessel, particularly for clinical studies, and failure to account for pulsatility of the vessel in the equation In one study, the upper 95% confidence limits for intraobserver variation were reduced to 0.04 and 0.09 mm for diameters of 0.6 and 6 mm, respectively, by making six repeated measurements of the vessel.124 Functional Analysis of the Fetal Heart The previously discussed limited methods used to assess function of the fetal ventricles have been replaced in most studies by measurement of long axis function of the fetal heart using amplitude of displacement of the atrioventricular ring, and Doppler tissue velocities using spectral or color methods These methods have been incorporated into routine clinical practice, whereas more research-based tools include offline assessment of strain, strain-rate, and ventricular twist and torsion Long Axis Function M-Mode Measures of Displacement of the Atrioventricular Ring The pattern of arrangement of myocytes differs in the right and left ventricles, with the right ventricle lacking myocytes aggregated in circular fashion.125 The myocytes aggregated in longitudinal fashion lie predominantly in the ventricular subendocardium and are affected first by ischemia Thus reduced displacement of the atrioventricular ring may indicate early myocardial ischemia There are additional benefits because the geometry of the right ventricle makes it difficult to assess right ventricular function in the minor axis because of poor detection of the endocardial borders Studies in adults with heart failure have reported that assessment of the amplitude in the long axis predicts exercise tolerance and survival.126,127 Displacement of the atrioventricular ring is assessed using Mmode techniques and reflects shortening of the myocytes aggregated in longitudinal fashion toward the apex of the ventricle in systole and their active lengthening in diastole.114 The methodology is simple and reliable, and offline assessment using anatomic M-mode provides comparable values when fetal lie is transverse Normal reference ranges have been described in the fetus and adult.128–130 A comparison with postprocessing M-mode measurements of displacement derived from 3D volumes has shown good correlation.131 Nomograms show agerelated increases in amplitude of displacement in the fetus, confirming right ventricular dominance as the right ventricular free wall shows increased displacement compared with the left or the ventricular septum.130,131 Doppler Tissue Imaging Doppler tissue imaging can be performed using two modalities, spectral and color Doppler imaging Spectral imaging produces higher values than those obtained with color Technologic differences may explain some of the reported differences in values.132,133 Color Doppler imaging produces information from the whole imaging field and not just the tissue selected by placing a Doppler sample volume This allows assessment of multiple sampling points in the same cardiac cycle Furthermore, it produces velocity vectors that have the potential to be manipulated by automated programs Spectral Doppler allows the placement of a sample volume, but the sampling usually produces a spectrum of velocities, depending on the gain settings and this can influence the peak systolic velocity measured.134 Ventricular long-axis shortening velocities and amplitude correlate with overall ventricular function as assessed by ejection fraction, and early and late diastolic lengthening velocities correlate with ventricular filling velocities assessed by Doppler Maturational changes have been characterized in the normal fetus and gestationally related values show similar increases.129,133,135–138 Long-axis fetal cardiac function has been studied using M-mode and tissue Doppler in pregnancies complicated by maternal disease, such as diabetes mellitus and fetal growth restriction and altered function has been described.139,140 The inconsistencies in reported results are due in part to the technical limitations in applying these measurements, developed in adult hearts, to the smaller fetal heart There is a lack of electronic gating, and smaller tissue volumes appear to be the most important In addition, all Doppler modalities require high frame rates, with 200 Hz being ideal (although velocities nearer to 100 Hz are more usual using ultrasound machines with obstetric platforms) and must be closely aligned parallel to the mural motion, ideally less than 20 degrees Accuracy of measurement requires that the sample volume is small, and velocity limits reduced to optimize the trace The technical differences in these methods and in recordings made on different equipment lead to different results, and this must be recognized as a limitation when comparing different studies or