Physiological Determinants of PR Interval in Healthy Fetuses: Insights from Correlation and Regression Modeling

Background: The fetal mechanical PR interval (mPR), measured using pulsed-wave Doppler, is a widely used parameter to assess atrioventricular conduction in fetuses, particularly in cases at risk of developing atrioventricular (AV) block. However, the physiological factors that influence mPR readings are not fully understood. This study aimed to identify determinants affecting the measurement of the mPR interval using the mitral valve/aorta (MV/Ao) Doppler method in a cohort of structurally normal fetuses. Methods: We retrospectively analyzed 925 fetuses with normal echocardiographic findings and no structural cardiac or extracardiac anomalies. Correlation analysis, group comparisons, trend testing, and multivariable modeling were performed to assess the impact of biometric and Doppler parameters on mPR interval measurements. Results: The median mPR interval across the cohort was 116 ms (interquartile range: 108-123 ms). Fetuses were categorized into four gestational age groups (≤19 weeks, 20-23 weeks, 24-27 weeks, and ≥28 weeks). Significant differences in mPR were observed between gestational age groups (p < 0.01), with a positive trend across increasing gestational age (p < 0.0001). The strongest correlation was an inverse relationship between mPR and fetal heart rate (FHR) (ρ = -0.256, p < 0.01). Multivariable regression identified five independent predictors of mPR: lower FHR, greater biparietal diameter (BPD), larger pulmonary valve diameter (PVD), increased fronto-occipital diameter (FOD), and lower umbilical artery pulsatility index (UA PI). The final model explained approximately 9.9% of the variance in mPR interval (R² = 0.099). Conclusions: The fetal mPR interval increases with gestational age and is primarily influenced by fetal heart rate, even after adjusting for other factors. Certain biometric and Doppler parameters also contribute modestly to mPR variation. These findings highlight the importance of accounting for physiological variability when interpreting mPR measurements in clinical fetal cardiology.