To the Editor:-The practice of using nitroglycerin (NTG) to reduce uterine smooth muscle tension acutely, has gained popularity in the past several yr. Unfortunately, we still lack reproducible quantitative data demonstrating that this intervention is effective. Further, the intravenous administration of NTG definitely reduces maternal blood pressure which may impact negatively on uterine blood flow at times when fetal oxygenation is challenged.
A recent Letter to the Editor by Bell et al., describing a case of NTG induced acute relaxation of the uterus, included data suggesting that NTG might have a measurable effect on uterine contraction quantifiable with an intrauterine catheter. The maximal pressure generated during uterine contraction in this patient was clearly reduced after the NTG administration. Although frequency and resting tone appeared to be unaltered, the patient described was initially hyperstimulated by pitocin (probable half-life 3-5 min) which was discontinued at about the same time NTG was administered clouding the cause of this effect.
Work in gravid rabbits and ewes in our laboratory has failed to document a reduction in the frequency of contraction, resting tone, or the maximal force generated during spontaneous or induced labor. In-vitro studies by Shin demonstrated that uterine smooth muscle harvested from term pregnant rats responded to NTG only at pharmacologic concentrations. A study by Kumar in 1965 which examined the effect of amyl nitrate on uterine tension as examined by tocodynometry similarly failed to demonstrate a measurable response to nitrosovasodilators. In that study, intrauterine pressure was also monitored in women induced to labor with pitocin.
Recent data from a very different in-vitro model using uterine smooth muscle strips from primigravida rabbits at term show that the compliance of the uterus is altered by NTG. Prior to Dr. Bell's case all reports of the efficacy of NTG involved the application of an external force to the uterus by the obstetrician. In a hypercontractile state, the sustained generation of active tension would be analogous to application of an external force. Hence, a change in compliance would be expected to appear as a decrease in maximal tension during sustained contraction. Dr. Bell's findings then are consistent with this recent data from our laboratory. Although Dr. Bell's report only describes a single patient, it presents an appealing model to consider (ie, the hyperstimulated uterus may be a model in which a change in compliance resulting from the administration of NTG can be reproducibly demonstrated).
Paul B. Langevin, M.D.
Department of Anesthesiology; University of Florida College of Medicine; P.O. Box 100254; Gainesville, Florida 32610-0254
(Accepted for publication May 21, 1997.)