Development of Australian reference ranges for the left fetal modified myocardial performance index and the influence of caliper location on time interval measurement.

OBJECTIVES

To construct gestational age-adjusted reference ranges of the left fetal modified myocardial performance index (Mod-MPI) in the Australian population and assess the influence of valve click caliper position on constituent time intervals and the Mod-MPI.

METHODS

This is a prospective longitudinal study of 117 normal singleton fetuses undergoing 318 ultrasound scans at 4-6 weekly intervals between 18 and 38 weeks of gestation. The isovolumetric contraction time (ICT), isovolumetric relaxation time (IRT), and ejection time (ET) were measured at 3 different caliper positions in each fetus: beginning of the original valve clicks ('original'), beginning of the reflected valve clicks ('reflected'), and peak of valve clicks ('peak'). The Mod-MPI was calculated as (ICT + IRT)/ET.

RESULTS

The Mod-MPI increased throughout gestation with means ± SD of 0.42 ± 0.05 'reflected' and 'peak' versus 0.49 ± 0.03 'original' at 19 weeks, and means of 0.46 ± 0.05 'reflected and peak' versus 0.51 ± 0.08 'original' at 36 weeks. Throughout gestation, ICT remained fairly constant and IRT increased, while ET decreased with 'original' click and remained constant for 'reflected' and 'peak' clicks. A modest increase in Mod-MPI was seen with increasing fetal heart rate. Analysis of repeatability for the 3 methods showed the following ICCs: 'original', 0.797 (95% CI 0.762-0.829); 'reflected', 0.809 (95% CI 0.775-0.839), and 'peak', 0.799 (95% CI 0.764-0.831).

CONCLUSIONS

Detailed exploration of the morphology of mitral and aortic valve closure and opening clicks shows how selection of different phases of these clicks may significantly influence the Mod-MPI. We recommend that the peak of the valve clicks be standardized between research groups.