Fetal heart rate regresses toward the mean in the third trimester

Abstract

The purpose of this study was to investigate the feasibility of different fetal heart rate (FHR) ranges in the nonstress test (NST) and to better understand the meaning of mild bradycardia and/or tachycardia without non-reassuring patterns. We employed the heredity to show that mild bradycardia (100-119 beats per minute, bpm) and mild tachycardia (161-180 bpm) regressed to the normal FHR range (120-160 bpm). We used linear regression to analyze FHR data from FHR tracings recorded 10 min before (NST, as the predictor) and 10 min after vibroacoustic stimulation testing (as the dependent variable). Acceleration for 15 bpm-15 seconds (Acc1515) and deceleration for 15 bpm-15 seconds (Dec1515) in the NST were also analyzed for each group. The slope of the best-fit line was the largest in the mild bradycardia group and the smallest in the normal range group. Dec1515 was most prominent in mild tachycardia and both the mild bradycardia and tachycardia groups regressed towards the mean FHR range. Therefore, we propose that both mild bradycardia and tachycardia of FHR in non-acute situations (range between 100 and 180 bpm) are not regarded a pathologic signal for clinical use.

Keywords: Bradycardia; Fetal Heart Rate Range; Regression; Tachycardia.

Fig. 1

Time series plot of fetal heart rate (FHR) for a normal fetus at 34 weeks of gestation. (A) mild tachycardia, (B) normal range, and (C) mild bradycardia. FHR tracings were recorded for a total of 50 min including the first and second 20 min of the NST and 10 min of the VAS test. However, only data collected during the 30-40 min interval of the NST and the 40-50 min interval of the VAS were analyzed. (1) Baseline FHR (bpm, NST vs VAST): (A) 177 bpm vs 176 bpm (B) 148 bpm vs 154 bpm and (C) 109 bpm vs 132 bpm. (2) Number of acceleration [deceleration] for 15 bpm-15 seconds (NST vs VAST): (A) 2[2] vs 2[5] (B) 2[2] vs 3[3] and (C) 0[0] vs 7[3]. NST, nonstress test; VAST, vibroacoustic stimulation test; bpm, beats per minute.

Fig. 2

Change in fetal heart rate (FHR) after VAS testing according to FHR range. The rate of maintaining FHR range was 26.1%, 66.0%, and 92.1% in the mild bradycardia, normal range, and mild tachycardia groups, respectively (P < 0.001). An increased FHR range was 73.9%, 27.5%, and 0.0% in the mild bradycardia, normal range, and mild tachycardia groups, respectively (P < 0.001).

Fig. 3

Regression line between FHR_NST (bpm) and FHR_VAST (bpm), (correlation coefficients = 0.68, N = 4,589). The lines in the plot are the best-fit straight lines representative of all the data (slope = 0.74 and FHR_AST-intercept = 42.88). Lines 1-3 (slope 1-slope 3) = the best-fit straight lines for the mild bradycardia (100-119 bpm, N = 46), the normal range (120-160 bpm, N = 4,404), and the mild tachycardia (161-180 bpm, N = 139) groups, respectively. The best-fit straight lines according to gestational age; FHR_VAST = 0.81 × FHR_NST + 30.81 (30-33 weeks, N = 568), FHR_VAST = 0.73 × FHR_NST + 44.97 (34-37 weeks, N = 1,979), FHR_VAST = 0.71 × FHR_NST + 47.90 (38-42 weeks, N = 2,008). FHR, fetal heart rate; NST, nonstress test; VAST, vibroacoustic stimulation test; bpm, beats per minute.

Fig. 4

Change in Acc1515 and Dec1515 based on fetal heart rates. Acc1515 [Dec1515] = number of accelerations [decelerations] for 15 bpm-15 seconds in a 10-minute period (NST), bpm = beats per minute. ^*significant at the level α = 0.001 by the general linear model; Acc1515 = 0.94 (standard deviation, 0.43) vs 1.35 (0.04) vs 1.25 (0.16) (P = 0.674). Dec1515 = 0.22 (0.14) vs 0.41 (0.02) vs 1.22 (0.16) (P < 0.001).