The Equivalence of Fetal Heart Rate Variability and Accelerations in the Interpretation of Non-Stress Tests

Accelerations were initially chosen as the basis for non-stress test (NST) interpretation against a background of significant artifact present in the first-generation, non-autocorrelated fetal monitors that made the interpretation of variability with external fetal heart rate (FHR) monitoring unreliable. Since moderate variability and accelerations are both physiologically similar and equally indicative of a non-acidemic fetus, we hypothesized that interpreting NSTs based on variability rather than accelerations would produce similar results.

We performed a retrospective cohort study of singleton fetuses undergoing antenatal testing in our system between 2011 and 2022. Data regarding the interpretation of NSTs were extracted and non-reactive tests were identified. For patients with non-reactive NSTs, data regarding follow-up testing were collected.

Our cohort included 76,232 total NSTs performed on 22,619 patients at increased risk of fetal demise, of which 1,662 (2%) were found to be non-reactive. Of the non-reactive tests, 1,499 (90%) were interpreted as having moderate variability, 114 (7%) had minimal or absent variability, and in 49 (3%), the variability was uncertain. Data regarding follow-up testing were available for 1,480 of the 1,499 non-reactive tests with moderate variability. In this group, 1,476 (99%) went on to have either reassuring follow-up testing. The four infants (0.3%) who failed to have a reassuring follow-up test all had major anomalies.

In nonanomalous fetuses, moderate variability in an otherwise non-reactive NST was invariably followed by a reassuring test of fetal well-being. These data confirm basic science observations regarding FHR regulation and suggest that, in the presence of moderate variability during an otherwise non-reactive NST, additional follow-up testing may not be necessary. Such an approach would avoid the need for additional testing in 90% of fetuses with non-reactive NSTs.

· A clinical distinction between moderate variability and an acceleration represents historical artifact.. · Nonanomalous fetuses with a nonreactive nonstress test but moderate variability invariably had reassuring subsequent testing.. · These clinical observations confirm known basic science physiology.. · In the antepartum evaluation of fetal well- being, moderate variability and accelerations are clinically equivalent..