Fetal left modified myocardial performance index: technical refinements in obtaining pulsed-Doppler waveforms.

OBJECTIVE

To assess the influence of machine settings (wall motion filter (WMF), angle of insonation, Doppler aliasing) and phase of valve clicks on repeatability of measurement of the fetal left modified myocardial performance index (Mod-MPI).

METHODS

Fetal left Mod-MPI was evaluated prospectively in 157 morphologically normal fetuses at 19-36 weeks' gestation. In a baseline cohort, a previously published technique and settings were used for measurement of Mod-MPI. In a second cohort, the influence of WMF, angle of insonation, Doppler aliasing and selection of the phase of the valve clicks on repeatability of measurement of Mod-MPI was assessed.

RESULTS

The intraclass correlation coefficient (ICC) for measurement repeatability in the baseline cohort was 0.22. Increase in WMF to 300 Hz or 500 Hz was associated with an increase in ICC to 0.60 and 0.55, respectively. An angle of insonation of < 15° was associated with an increase in ICC to 0.79 and 0.78 at a WMF of 300 and 500 Hz, respectively. A WMF of 300 Hz, angle of insonation of < 15° and absence of aliasing was associated with an increase in ICC to 0.85 and 0.87 at the beginning and peak of the valve click, respectively. Mod-MPI ranged from 0.35 to 0.48. An increase in calculated MPI was associated with increasing WMF, selection of the beginning vs. peak of the valve clicks and increase in angle of insonation. The presence or absence of aliasing had no effect.

CONCLUSIONS

Refinement of machine settings improves repeatability of Mod-MPI, as does selection of the peak of the valve click. We suggest a consensus be reached as to the precise measurement of MPI, but for the moment would suggest: WMF, 300 Hz; angle of insonation, < 15°; avoiding Doppler aliasing; and selection of valve click peak. Systematic variation in measurement of time intervals may be responsible for the widely varying published normal ranges for Mod-MPI.