OBJECTIVE:To assess the clinical utility of the fetal myocardial performance index (MPI) in assessment and management of the small-for-gestational-age (SGA) fetus/growth-restricted fetus (FGR). METHODS:This was a prospective cohort study in metropolitan Australia of patients referred in the period June 2012 to March 2015 to fetal medicine services at 24-38 weeks' gestation for suspected singleton SGA/FGR (estimated fetal weight (EFW) < 10th centile with or without abnormal umbilical artery (UA) Doppler) pregnancy. Patients had MPI assessed in addition to routine measures, and were followed through to birth. We compared MPI values against those of a local reference population and gestational age-matched controls, and assessed the correlation with perinatal outcome and other Doppler measures. RESULTS:Fifty-two cases were included, 38 diagnosed < 32 weeks and 14 diagnosed ≥ 32 weeks. None demonstrated significantly elevated left, right or delta MPI compared with the reference population or with gestational age-matched controls at the time of first MPI evaluation. There were no consistent longitudinal patterns in MPI that would suggest its clinical utility. The mean ± SD gestational age at delivery was 34.6 ± 3.8 weeks and birth weight was 1.7 ± 0.6 kg, and the median neonatal hospital admission time was 27 days, confirming a pathological cohort. There were no significant correlations between left, right or delta-MPI and perinatal outcome, although there were significant correlations between UA, middle cerebral artery (MCA) and ductus venosus (DV) Doppler and perinatal outcome (birth weight, gestational age at birth and length of neonatal hospital stay). Exploratory subgroup comparisons (EFW < 3rd vs 3rd -10th centile; early- vs late-onset; abnormal vs normal UA Doppler) found only minor differences in MPI, reaching statistical, but not clinical, significance, only in the EFW < 3rd vs 3rd -10th centile comparison. CONCLUSIONS:MPI did not demonstrate clinical utility in either triage or longitudinal follow-up of an SGA/FGR cohort presenting to fetal medicine services. Given that prior research suggesting its utility originates from single-center cohorts, while multicenter, large cohorts have suggested little utility or no additional utility if routine UA/MCA/DV Doppler is performed, publication bias may have affected previous reports. It seems unlikely that MPI has clinical utility in assessment and management of SGA/FGR fetuses. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

The aortic isthmus (AoI) is a unique fetal watershed with a waveform reflecting its complex haemodynamic physiology. The systolic component represents left and right ventricular systolic ejection, and the diastolic component represents comparative downstream vascular impedance between the brachiocephalic and subdiaphragmatic fetal circulations. Several indices have been devised to quantify different components of the waveform, including the pulsatility index, resistance index, isthmic flow index, and recently the isthmic systolic index. There have been promising preliminary studies applying these indices to both cardiac (congenital) and extracardiac pathologies, including intrauterine growth restriction and twin-twin transfusion syndrome. However, the waveform's multifactorial origin has proven to be challenging, and the difficulty in separating various components of the waveform could explain that AoI evaluation does not have a clear clinical utility. Further research is underway to realise the full potential of this vessel in fetal cardiac and haemodynamically compromised pathological conditions. In this review article we outline the physiological origin of this Doppler waveform, describe in detail the various published indices, summarise the published literature to date, and finally outline potential future research and hopefully clinical applications.

To assess MPI in a prospective cohort of singleton SGA fetuses, both early-onset (diagnosis <32 weeks) and late-onset (diagnosis 32 + weeks), compared to a cohort of uncomplicated pregnancies. 58 women with ultrasonically-diagnosed SGA fetus (< 10th centile) were prospectively followed to birth and compared to contemporaneous reference interval patients. Biometry, UA, MCA and DV Dopplers, AFI, biophysical profile and left, right and delta MPI (LMPI, RMPI, DMPI) studies were performed. 51/58 women with SGA fetus (37 early-onset diagnosis, 14 late-onset diagnosis) were included after exclusion of babies with birthweight > 10th centile. Mean EFW Z-score of SGA cohort was −1.8 ± 0.7, median GA at birth 33.0 weeks (IQR 31.0–37.4) and mean birthweight 1.6 ± 0.6 kg. There was one stillbirth. MPI data is shown in table 1, with few differences seen either between the reference and SGA cohorts or early- vs. late-onset SGA. There were no significant correlations between EFW Z-score and MPI values or MPI components, or between MPI and GA at birth, birthweight or cord pH. As expected for a true SGA cohort EFW Z-score was significantly negatively correlated with UA PI and significantly positively correlated with cerebroplacental ratio. In both early and late-onset SGA, MPI values were similar to reference values and did not correlate with birth gestation or birthweight. MPI is likely to be of limited value for in-utero monitoring or prediction of immediate neonatal outcome in SGA fetuses. 1 = p < 0.05 across groups 2 = p < 0.01 across groups.

Fetal cardiac strain may manifest unilaterally in IUGR and TTTS, as the right ventricle is affected earlier and to a greater degree than the left. No gold standard exists for monitoring fetal differential hemodynamic compromise, though two such indices have been proposed: Delta-MPI (Right mod-MPI (RMPI)–Left mod-MPI (LMPI)) and aortic isthmic systolic index (ISI) (nadir systolic velocity/peak systolic velocity). We aimed to investigate the correlation between Delta-MPI and ISI. Prospective cross-sectional study of 41 uncomplicated singleton fetuses, 16 singleton IUGR fetuses and 5 TTTS pregnancies between 17–38 weeks gestation. During a single examination, LMPI and RMPI were measured with three waveforms by the “peak” valve click technique to calculate Delta-MPI and additionally aortic isthmic Doppler was acquired for ISI calculation. Delta-MPI and ISI correlation was assessed in the combined cohort and pathological subgroups with Spearman's correlation. Delta-MPI, ISI, LMPI and RMPI were compared between uncomplicated versus TTTS donor, TTTS recipient and IUGR fetuses (Mann–Whitney U test). There was no significant correlation between Delta-MPI and ISI for total cohort or any subgroups (Spearman's ρ = −0.60, p = 0.26). There were no significance differences between ISI values in uncomplicated fetuses versus IUGR, TTTS Donor or TTTS recipient fetuses (p = 0.56; 0.22; 0.21 respectively) and similarly for Delta-MPI (p = 0.32; 0.36; 0.19 respectively). Statistical significance was found for LMPI in uncomplicated fetuses versus TTTS Donor or recipient fetuses (p < 0.01, p < 0.01) and similarly for RMPI (p < 0.01, p = 0.03). No correlation was found between delta-MPI and ISI, indicating that delta-MPI and/or ISI are unlikely to both be solely measuring differential cardiac strain. Preload, peripheral resistance and cardiac contractility may impede their correlation. Further exploration into the interrelationships of these factors is underway.