Fenoterol Hydrobromide

Uterine contractions cause hypoxic stress to human fetuses by repeatedly occluding maternal spiral arterioles which feed the placental bed and/or compressing the loops of the umbilical cord, interrupting blood flow through the umbilical vessels. For some fetuses, even such transient and repeated interruptions of oxygenation due to ongoing uterine contractions may increase the risk of decompensation in the "high priority" central organs (i.e. heart and the brain). The onset of anaerobic metabolism and resultant production of lactate in the central organs may lead to increased likelihood of fetal neurological injury and/or perinatal death. Therefore, an immediate relaxation of the myometrium by abolishing ongoing uterine contractions may help to rapidly restore oxygenation to fetal central organs. Such timely administration of acute tocolytics would help maintain aerobic metabolism in the high-priority fetal central organs, avoiding the onset of neurological injury and/or perinatal death. Commonly used acute tocolytics include beta-sympathomimetics, nitric oxide donors, oxytocin antagonists, which have different mechanisms of actions, and maternal side-effect profile. The indications include elimination of uterotonic-induced excessive uterine contractions to facilitate normalization of the fetal heart rate so as to allow continuation of labor in anticipation of vaginal birth and for rapidly improving the fetal condition immediately prior to an emergency cesarean section. The latter includes umbilical cord prolapse or chronic hypoxia when a delay in birth is anticipated. This review addresses the indications for acute tocolytics (why), the recommended timing of administration (when), ideal tocolytic (what), route of administration (how), side effects and contraindications (why-not). Based on current evidence, and pharmacokinetics, 250 mcg of subcutaneous terbutaline (or another beta-sympathomimetic such as intravenous fenoterol) is the recommended first line tocolytic, unless there are specific maternal contraindications. In the absence of maternal hypotension, 100 mg of intravenous glyceryl trinitrate (GTN) may be administered as an alternative. Acute tocolytics are not recommended to treat myometrial irritability observed in chorioamnionitis or in acute feto-maternal hemorrhage.