Fetal Cardiac Effects of Oral Ritodrine Tocolysis

ABSTRACT

The p-sympathomimetic oral tocolytic ritodrine can cause maternal tachycardia and hypotension, and may cross the placenta. A new echocardiographic technique has been developed to explore fetal and placental ritodrine effects. Values in 76 healthy historic controls were compared to 18 studies in 16 patients performed while receiving stable oral ritodrine therapy, measured both at baseline and 30 minutes after a dose. Data collected included maternal pulse and blood pressure (BP), fetal cerebral and umbilical Doppler waveforms, and fetal heart rate. A new index of fetal myocardial contractility, combined ventricular shortening fraction, was derived from two-dimensionally directed M-mode. Maternal pulse and BP, fetal heart rate and heart size, and all Doppler indices were normal, without demonstrable dose-response effects. In the control subjects, combined ventricular shortening fraction fell with increasing gestational age (combined ventricular shortening fraction = -0.27 estimated gestational age + 49; r = 0.27; P ≤ 0.02; standard error of the estimate, 11%). However, combined ventricular shortening fraction in ritodrine patients was abnormally decreased in 72% of cases. The mean index in normal subjects was 43 ± 5%, but in ritodrine patients it was only 31%. We conclude that a history of premature labor or oral ritodrine, or both, is associated with reduced shortening fraction. Since there was no change in placental resistance, cerebral hypoxia, fetal heart rate, or heart size (preload), then low shortening fraction may be due to increased fetal systemic vascular resistance (BP) or decreased myocardial contractility.