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• W2113013616 abstract "The approach to women with suspected preterm labor has changed little over the last 40 years, and a key element to that approach continues to be the inhibition of contractions.1Caritis S.N. Edelstone D.I. Mueller-Heubach E. Pharmacologic inhibition of preterm labor.Am J Obstet Gynecol. 1979; 133: 557-558PubMed Scopus (79) Google Scholar, 2Simhan H.N. Caritis S.N. Prevention of preterm delivery.N Engl J Med. 2007; 357: 477-487Crossref PubMed Scopus (213) Google Scholar This focus on contraction inhibition has led to the development of agents affecting myometrial contractility. Ritodrine, a beta-adrenergic agonist that gained Food and Drug Administration approval in 1980, was the first agent developed specifically for treating episodes of preterm labor.See related article, page 134 See related article, page 134 Agents targeting other mechanisms of myometrial contractility have subsequently been introduced for the treatment of preterm labor including prostaglandin inhibitors, oxytocin antagonists, nitric oxide donors and calcium channel blockers.1Caritis S.N. Edelstone D.I. Mueller-Heubach E. Pharmacologic inhibition of preterm labor.Am J Obstet Gynecol. 1979; 133: 557-558PubMed Scopus (79) Google Scholar, 2Simhan H.N. Caritis S.N. Prevention of preterm delivery.N Engl J Med. 2007; 357: 477-487Crossref PubMed Scopus (213) Google Scholar These agents have been compared with placebo (the gold standard) and with each other. The placebo-controlled studies are few in number and generally suffer from small sample size when considering the selected outcome of interest. The ethical concerns in performing placebo controlled studies in the setting of preterm labor, in contemporary obstetric practices, has led to the comparative effectiveness approach wherein 1 agent of interest (drug A) is compared with another agent that has been shown to be superior to placebo in the treatment of preterm labor (drug B). If drugs A and B are comparable in the outcome of interest (for example, duration of pregnancy maintenance or frequency of side effects), the conclusion implied is that drug A is therefore also superior to placebo. This approach poses considerable risk of judging a treatment effective when it is not. In many comparative studies, effectiveness during the treatment of acute preterm labor is confounded because the trial is not based on an intent-to-treat study design, and other treatments may be used once the primary treatment fails. This leads to potential bias because the decision to deem a treatment as a failure is subjective. This is best illustrated when considering beta adrenergic agents (the most commonly used comparator) in which there is a wide variation on what symptoms constitute failed therapy. Drug discontinuation rates because of side effects from beta agonist therapy range from 6% to 38%.3Beall M.H. Edgar B.W. Paul R.H. Smith-Wallace T. A comparison of ritodrine, terbutaline, and magnesium sulfate for the suppression of preterm labor.Am J Obstet Gynecol. 1985; 153: 854-859Abstract PubMed Google Scholar, 4Hollander D.I. Nagey D.A. Pupkin M.J. Magnesium sulfate and ritodrine hydrochloride: a randomized comparison.Am J Obstet Gynecol. 1987; 156: 631-637Abstract PubMed Google Scholar Thus, in those centers with high rates of discontinuation for side effects, most agents would appear superior to beta agonist therapy. Comparative studies are also problematic in gauging efficacy if the treatment of the acute episode of preterm labor is not distinguished from maintenance therapy. Success rates for pregnancy prolongation of 24-48 hours are quite high, regardless of the agent used, suggesting that most contemporary agents are effective in treating acute preterm labor or that the diagnosis of preterm labor is incorrect. Because of the high success rates, sample size in these comparative trials is often inadequate to demonstrate a difference between treatments (average of 40 subjects/group for nifedipine trials). Comparative trials that evaluate efficacy using longer-term endpoints such as neonatal outcome or prolongation to some future gestational age (ie, 34-37 weeks) are also at risk of bias unless a consistent approach to maintenance labor inhibition therapy is undertaken. The historical focus on contraction inhibition as central to preventing preterm birth continues influencing our thinking of labor inhibiting drugs. Treating contractions and abolishing them for 24-48 hours does not alter the labor stimulus, rather that treatment renders the end organ (myometrium) less responsive to that stimulus. Therefore, in judging the various labor-inhibiting agents, it is unreasonable to assume that a treatment of 24-48 hours will have a long-lasting effect on the parturitional process. As an analogy, treating elevated blood pressure or lipids for 1 month can reduce blood pressure and lipid levels to acceptable levels, but such short-term treatment does little to prevent the long-term consequences of hypertension or hyperlipidemia. Despite the fact that the extant literature suggests that maintenance labor inhibition therapy does not provide any benefit over placebo, it is a confounder that must be considered in any analysis of labor inhibition therapy. In the specific case of ritodrine, which is the primary comparator with nifedipine in the majority of comparative trials, the impact of an inadequate maintenance dose of ritodrine must be considered in judging efficacy.5Caritis S.N. Venkataramanan R. Cotroneo M. Chiao J.P. Pharmacokinetics of orally administered ritodrine.Am J Obstet Gynecol. 1990; 162: 1643-1644PubMed Google Scholar There have been no placebo-controlled trials with nifedipine in treating acute preterm labor; therefore, any assessment of the drug must rely on comparative trials, most of which are small and inadequately powered. Thus, the use of metaanalyses is logical but must be viewed cautiously as noted in previous text. It is in this challenging environment that the metaanalysis of Conde-Aguelo et al is presented. This very well-done metaanalysis suggested that: (1) nifedipine is superior to beta agonists in pregnancy prolongation and side effect profile (16 comparative trials with beta agonists); (2) nifedipine is comparable with magnesium sulfate in pregnancy prolongation but superior in risk profile (5 comparative studies); and (3) nifedipine is comparable with atosiban (1 study). How this information can be used in clinical practice is not clear. The arguments in support of nifedipine are strongest in the area of relative safety, maternal tolerance, and ease of use when compared with beta agonists, particularly in the treatment of acute preterm labor. Nifedipine is an excellent agent for reducing intracellular calcium (and hence contractility), and it has fewer side effects and is better tolerated than beta adrenergic agonists.1Caritis S.N. Edelstone D.I. Mueller-Heubach E. Pharmacologic inhibition of preterm labor.Am J Obstet Gynecol. 1979; 133: 557-558PubMed Scopus (79) Google Scholar, 2Simhan H.N. Caritis S.N. Prevention of preterm delivery.N Engl J Med. 2007; 357: 477-487Crossref PubMed Scopus (213) Google Scholar Therapeutic concentrations can be rapidly achieved with oral administration, precluding the need for intravenous fluids, infusion pumps, and close monitoring by nursing staff. Furthermore, tachyphylaxis is not seen with nifedipine but is universally seen with beta adrenergic agonists. The absolute efficacy of nifedipine, on the other hand, is more difficult to assess in the absence of placebo-controlled trials. Reliance on comparative trials for efficacy determination is problematic because of the issues noted above, especially because most comparative studies are with ritodrine, a drug in which inadequate pharmacologic understanding led to excessive dosing during intravenous therapy and inadequate dosing during oral therapy.5Caritis S.N. Venkataramanan R. Cotroneo M. Chiao J.P. Pharmacokinetics of orally administered ritodrine.Am J Obstet Gynecol. 1990; 162: 1643-1644PubMed Google Scholar, 6Caritis S.N. Venkataramanan R. Darby M.J. Chiao J.P. Krew M. Pharmacokinetics of ritodrine administered intravenously: recommendations for changes in the current regimen.Am J Obstet Gynecol. 1990; 162: 429-437Abstract Full Text PDF PubMed Scopus (33) Google Scholar Metaanalysis relies on the component trials that compose it. If systematic biases exist in several of the included trials, these biases may be accentuated by metaanalysis. Other agents may be useful in suppressing uterine contractility through other pathways relevant to parturition (ie, prostaglandin inhibitors); therefore, the concept of identifying a best or first-line agent is not necessary. Once a decision is made to inhibit uterine contractility, the choice of agent should be based on its ability to stop myometrial contractions acutely, its safety for mother and fetus, and the experience and comfort of the attending provider. Nifedipine in the management of preterm labor: a systematic review and metaanalysisAmerican Journal of Obstetrics & GynecologyVol. 204Issue 2PreviewTo determine the efficacy and safety of nifedipine as a tocolytic agent in women with preterm labor. Full-Text PDF" @default.
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