Development of Normal Gestational Ranges for the Right Myocardial Performance Index in the Australian Population with Three Alternative Caliper Placements

Abstract

Objectives: To construct gestational age-adjusted reference ranges for the right fetal modified myocardial performance index (RMPI) in an Australian population and to assess the influence of valve click caliper position on constituent time intervals and the RMPI. Methods: A prospective cross-sectional study of RMPI from 235 normal fetuses at 17-38 weeks of gestation was performed. Two Doppler waveforms were obtained: tricuspid and pulmonary valves for ‘a' and ‘b' readings, respectively. The ultrasound machine settings were: Doppler sweep velocity 15 cm/s, angle of insonation <15°, minimal gain, and wall motion filter 300 Hz. The ‘a' and ‘b' intervals were measured at three different caliper positions in each fetus: at the beginning of the original valve clicks (‘original'), at the beginning of the reflected tricuspid and pulmonary closure clicks (‘reflected') and at the peak of valve clicks (‘peak'). RMPI was calculated as (a - b)/b. The three readings were obtained and averaged per examination, with intraobserver repeatability assessed by intraclass correlation coefficient (ICC) and 95% CI. Results: For ‘original', ‘reflected' and ‘peak' RMPI, mean ± SD, ICC (95% CI) were: 0.53 ± 0.10, 0.86 (0.83-0.89); 0.48 ± 0.10, 0.84 (0.81-0.87) and 0.48 ± 0.10, 0.89 (0.87-0.91), respectively. The RMPI increased by approximately 15% as gestation increased and decreased slightly with increasing heart rate. Conclusion: This is the first publication of reference ranges for RMPI based on caliper position. All methods showed good ICC, including the ‘peak' method which we have previously proposed for routine use based on its repeatability and ease of identification when measuring the myocardial performance index.