Reference Values of Fetal Mechanical PR Interval and Heart Rate-Corrected Fetal Mechanical PR Interval: Influence of Fetal Sex, Heart Rate, Gestational Age, and Maternal Age

Mechanical PR Interval (MPRI) is an important parameter in the fetus as a surrogate for PR interval. Normal values of MPRI have been reported with controversial effects of Fetal Heart Rate (FHR) and Gestational Age (GA). However, the effect of HR-correction on PR interval is unknown.
This study aimed to obtain the reference values of fetal MPRI in a large series of normal fetuses by pulsed-Doppler fetal echocardiography. This was done to determine the influence of fetal sex, Heart Rate (HR), GA, and maternal age on MPRI and to calculate the novel HR-Corrected Mechanical PR Interval (CMPRI) to investigate whether HR-correction of PR interval can decrease the influence of HR.
This cross-sectional study was performed on 516 consecutive normal singleton fetuses. By extrapolation from Bazett’s formula, we corrected the MPRI for HR. Impact of fetal sex, HR, and GA on MPRI and CMPRI was studied. Mean ± standard deviation, 5th, 50th, 95th, and 99th percentiles of MPRI and CMPRI were also calculated. Then, the data were entered into Stata, version 12 and analyzed using t-test, ANOVA, and linear regression.
Reference values of MPPR and CMPRI were provided in four GA groups; i.e., 14 - 18, 19 - 22, 23 - 26, and 27 - 38 weeks. Fetal sex and maternal age had no influence on either MPRI or CMPRI. After adjustment for fetal sex, GA, and maternal age, there was a 0.14-millisecond (ms) decrease in MPRI and a 8.06-ms increase in CMPRI for every single increase in FHR. Additionally, adjusted linear regression model indicated a 0.43-ms increase in MPRI and a 2.53-ms increase in CMPRI per gestational week. The results of paired t-test showed no significant difference between fetal MPRI and neonatal PR interval.
This study provided reference values for MPRI and CMPRI in fetus from 14 to 38 weeks of gestation from the 5th to the 99th percentile. The results also revealed significant correlations between both FHR and GA and MPRI and CMPRI. Furthermore, HR correction of MPRI did not add any advantage in terms of HR-independency of mechanical PR interval.