3-Agonist drug infusion has resulted in serious and even fatal maternal side efects. Pulmonary edema is a special concern, and its contribution to morbidity is discussed in Chapter 47 (p. 917). In one early study, tocolysis was the third most common cause of acute respiratory distress and death in pregnant women during a 14-year period in Mississippi (Perry, 1998). The cause of pulmonary edema is multifactorial. isk factors include tocolytic therapy with 3-agonist drugs, multifetal gestation, concurrent corticosteroid therapy, tocolysis for more than 24 hours, and intravenous infusion of large volumes of crystalloid. 3-Agonist agents cause retention of sodium and water, and with time-usually 24 to 48 hours-these can cause volume overload (Hankins, 1988). The drugs have been implicated in increased capillary permeability, cardiac rhythm disturbances, and myocardial ischemia.
Terbutaline is commonly used in the United States to forestall preterm labor. Like ritodrine, it may cause pulmonary edema (Angel, 1988). Low-dose terbutaline can be administered longterm by subcutaneous pump (Lam, 1988; Perry, 1995). But, randomized trials have shown no beneit for terbutaline pump therapy (Guinn, 1998; Wenstrom, 1997). Oral terbutaline given to prevent preterm delivery is also inefective (How, 1995; Parilla, 1993). In one trial, 203 women with arrested preterm labor at 24 to 34 weeks' gestation were randomly assigned to receive 5-mg terbutaline tablets or placebo every 4 hours (Lewis, 1996). Of outcomes, delivery rates at 1 week, median latency duration, mean gestational age at delivery, and incidence of preterm labor relapse were similar in both groups. Because of reports of serious maternal side efects, the FDA (2011) issued a warning regarding the use of terbutaline to treat preterm labor. The American College of Obstetricians and Gynecologists (2016b) recommends only short-term inpatient use of terbutaline as a tocolytic or as acute therapy of uterine tachysystole. Subcutaneous dosages of 0.25 mg are commonly used for the latter indication. T erbutaline, used as a tocolytic prior to external cephalic version, is discussed in Chapter 28 (p. 550).
Ionic magnesium in a suiciently high concentration n alter myometrial contractility. Its role is presumably that of a calcium antagonist, and when given in pharmacological doses, it may inhibit labor. Intravenous magnesium sulfate, given s a 4-g loading dose and followed by a continuous inusion of 2 g/hr, usually arrests labor (Steer, 1977). Like 3-mimetic agents, magnesium treatment can cause pulmonary edema (Samol, 2005). However, this has not been our experience at Parkland Hospitl in the treatment of tens of thousands of preeclamptic women with intravenous magnesium sulfate. Pharmacology and toxicology of magnesium are considered in more detail in Chapter 40 (p. 737).