Plasma levels and cardiovascular effect of nitroglycerin in pregnant sheep

Glyceryl trinitrate and acute uterine relaxation: a literature review

The use of nitro-vasodilators for achieving rapid uterine relaxation in the resolution of obstetric emergencies has been documented for nearly 120 years. Glyceryltrinitrate (GTN) is the most commonly used nitro-vasodilator for this purpose, with the presumed mechanism of action being via nitric oxide and cyclic guanosine monophosphate (cGMP) mediated processes. GTN is known to release nitric oxide to effect smooth muscle relaxation and some dose response data is available for its vasodilator activity. Human myometrium is known to synthesize and respond to nitric oxide, with changes in the production of and sensitivity to nitric oxide being subject to the cyclical and gestational state of the uterus. Experimental data on the efficacy of GTN in reliably producing uterine relaxation is conflicting and inconsistent. A total of 32 studies and case reports on the use of GTN in achieving rapid uterine relaxation have appeared in the English language literature. Case reports are subject to reporting bias and prospective randomized controlled trials are not without design flaws. Indications for the use of GTN in achieving rapid uterine relaxation cover the antepartum, intrapartum and postpartum periods. The safety of GTN during obstetric emergencies appears high, with no adverse maternal or neonatal outcomes. To establish the efficacy of GTN in reliably achieving uterine relaxation, well designed randomized controlled trials in labouring women are required.

Nitroglycerin application during cesarean delivery: plasma levels, fetal/maternal ratio of nitroglycerin, and effects in newborns

OBJECTIVE

We sought to investigate maternal and fetal nitroglycerin metabolization and to assess the clinical condition of neonates after intravenous nitroglycerin application during cesarean delivery.

STUDY DESIGN

At the time of the uterine puncture incision, either 0. 25 mg or 0.5 mg nitroglycerin or a physiologic sodium chloride solution was administered as an intravenous bolus. Plasma concentrations of nitroglycerin and its metabolites were measured in maternal venous blood and in umbilical blood samples taken immediately after cord clamping. Arterial blood pressure, pulse rates, and Apgar scores were recorded for the neonates 1, 5, and 10 minutes after birth.

RESULTS

Sixty-two patients were included in the pharmacokinetic study. Median maternal plasma levels 1 and 5 minutes after injection of 0.5 mg nitroglycerin were 80 and 3.2 ng/mL, respectively; median maternal plasma levels 1 and 5 minutes after injection of 0.25 mg nitroglycerin were 38 and 1.2 ng/mL, respectively. In the umbilical vein 1 minute after application of 0. 5 mg or 0.25 mg nitroglycerin, the plasma levels were 0.41 and 0.09 ng/mL, respectively, and in the umbilical artery they were 0.03 and 0.008 ng/mL, respectively. Circulatory parameters and Apgar scores in the neonates did not differ significantly from those found in the placebo group.

CONCLUSION

The level of nitroglycerin in umbilical plasma was two to three orders of magnitude lower than that found in maternal plasma and clearly in a subtherapeutic range. There was no indication that prenatal application of nitroglycerin to facilitate obstetric management is hazardous for neonates.

Effects of nitroglycerin and indomethacin on fetal-maternal circulation and on fetal cerebral blood flow and metabolism in sheep

OBJECTIVE

Our purpose was to evaluate the effects of maternal administration of nitroglycerin and indomethacin on maternal and fetal hemodynamics and on fetal cerebral blood flow and metabolism in sheep.

STUDY DESIGN

Invasive vascular and fetal carotid flow monitoring was established in 12 gravid ewes. Isotonic sodium chloride solution, nitroglycerin, and indomethacin were infused maternally, and maternal and fetal heart rate, blood pressure, blood gas values, fetal carotid blood flow, and flow variability were measured. Fetal cerebral uptake of oxygen, glucose, and lactate were calculated.

RESULTS

Nitroglycerin infusion caused a significant increase in maternal and fetal heart rate and a significant decrease in maternal and fetal mean arterial pressure at a dosage of 10 microram/kg per minute, without a change in blood gas values. Neither drug had any effect on fetal carotid blood flow, flow variability, or cerebral substrate metabolism.

CONCLUSION

Maternal administration of nitroglycerin and indomethacin caused no adverse maternal or fetal circulatory changes and did not alter fetal carotid blood flow or substrate metabolism.

Use of nitroglycerin for uterine relaxation

Data from human and experimental animal research indicate that nitric oxide (NO), a novel messenger, formed during the nitric oxide synthase-catalyzed oxidation of L-arginine to L-citrulline, is involved in maintaining normal uterine tone during gestation. There are demonstrated and potential benefits of manipulating the L-arginine-NO system during pregnancy. Several recent case reports and case series have described the effective use of nitroglycerin (GTN), a NO donor compound, antenatally, intrapartum, and postpartum for acute uterine relaxation. Therapeutic indications for GTN range from facilitating external cephalic version, difficult vaginal or cesarean section delivery, and manual exploration of the uterus, to its use as a tocolytic. The intravenous regimen of GTN required to obtain the desired degree of uterine relaxation is extremely variable; intravenous bolus doses of 50 micrograms to 500 micrograms GTN with up to three repeated injections of 50 micrograms to 250 micrograms have been reported. Other methods of GTN administration include transdermal patches and sublingual spray. GTN, when used in low doses, may provide safe and effective uterine relaxation with no clinically apparent fetal or maternal adverse effects. However, clinical trials with use of objective methods of evaluating uterine tone and comparing GTN to other tocolytic agents are required before widespread use in advocated.

Fetal and maternal hemodynamic and metabolic effects of maternal nitroglycerin infusions in sheep

OBJECTIVES

We previously demonstrated effective tocolysis with nitroglycerin in laboring sheep. In this study we evaluated maternal and fetal circulatory effects of nitroglycerin.

STUDY DESIGN

Vascular catheters and an umbilical Doppler flow transducer were placed in eight ewes. Nitroglycerin was infused maternally over 30 minutes at 1.5, 2.5, 5.0, and 10 microg x kg maternal weight(-1) x min(-1). We recorded maternal and fetal variables before, at the end of each 30-minute infusion period, and 30 minutes later. Fetal regional flow distribution was measured with microspheres. Fetal oxygen, glucose, and lactate uptake were calculated. Data were analyzed by analysis of variance.

RESULTS

There were no changes in fetal blood gas values, oxygen or substrate uptake, umbilical flow, output distribution, pressures, or heart rate. Maternal mean pressure fell and heart rate increased concomitantly. Maternal blood gases were unchanged.

CONCLUSION

Nitroglycerin at tocolytic and greater doses had no adverse effects on fetal circulatory function during a 2-hour infusion.

Intravenous nitroglycerin for intrapartum internal podalic version of the second non-vertex twin

OBJECTIVE

Authors report their experience of intravenous nitroglycerin as uterine relaxing agent for managing successfully internal podalic version of the second twin.

METHODS

From a retrospective study including nine observations of internal podalic version of the second non vertex twin performed with administration of intravenous nitroglycerin, between August 1994 and February 1996, authors compare their results with those reported elsewhere.

RESULTS

Two failures of internal podalic version with nitroglycerin have been observed. But one failure is not considered to be due to the NTG: it was a patient, who had a panic attack necessitating a general anesthesia for sedative purpose. The internal podalic version succeeded. The true failure of NTG needed an emergency cesarean due to acute fetal distress and a non relaxing uterus. One internal podalic version was complicated by hemorrhage. The intravenous NTG used to induce uterine atonia associated with epidural-analgesia to relief pain avoiding general anesthesia makes internal podalic version easier.

CONCLUSION

Our results confirmed those already reported. That intravenous nitroglycerin (NTG) injection induces a transient and prompt uterine relaxation required for internal podalic version without affecting maternal and fetal prognosis.

Utilization of intravenous nitroglycerin for obstetrical emergencies

OBJECTIVE

The authors investigated, by reviewing publications and their personal experience, opportunities for the use of nitroglycerin to manage several obstetric emergencies.

METHODS

A retrospective study of 10 observations of internal podalic versions of the second non-vertex twin, with administration of intravenous nitroglycerine, performed between August 1994 and March 1996, was undertaken. The authors compared their results with those reported elsewhere. The indications of nitroglycerin have been extended to intrapartum external version of the second twin, retained placenta, and uterine inversion.

RESULTS

Only one failure of internal podalic version due to failure of uterine relaxation was observed.

CONCLUSION

From both experience and published reports it was confirmed that intravenous nitroglycerin (NTG) injection induces a transient and prompt uterine relaxation without affecting maternal and fetal prognosis.

Endothelial dysfunction in preeclampsia. Part II: Reducing the adverse consequences of endothelial cell dysfunction in preeclampsia; therapeutic perspectives

Next to low-dose Aspirin there appear to be several new and promising pharmacologie approaches for reducing the adverse consequences of endothelial cell dysfunction in preeclampsia. Among these are selective thromboxane-A2 synthetase and/or thromboxane-A2 receptor antagonists, stable prostacyclin analogues, selective S(erotonin)2-receptor blockers, nitrovasodilators, glycoprotein IIb/IIIa antagonists, hirudin, and ticlopidine. Early-onset preeclampsia appears to be associated with certain disorders that are likely to provoke an arterial thrombotic process by impairing the normal endothelial cell-platelet interactions. Especially heterozygous hyperhomocysteinemia, protein S deficiency and anticardiolipin antibodies appear to be fairly common. The management of these 3 separate disease entities will be discussed.

The use of intravenous nitroglycerin for emergency cervico-uterine relaxation

A patient with a twin pregnancy was in preterm labour after 25 gestational weeks when, during vaginal delivery of the second twin, severe spasm of the cervix and fetal bradycardia ensued. Induction of general anesthesia did not relax the cervix. After bolus doses of nitroglycerin 100 + 50 micrograms i.v., prompt cervico-uterine relaxation was obtained allowing manual extraction of the baby. A short review of the literature and a summary of our experience in the administration of nitroglycerin i.v. in obstetrics are presented.

Antepartum uterine relaxation with nitroglycerin at caesarean delivery

In this case report, NTG provided sufficient uterine relaxation to allow difficult twin extraction at Caesarean delivery. Administration of NTG in this situation was controversial, and if it had not been effective, time may have been lost before general anaesthesia could have been induced. Nitroglycerin merits further evaluation as an antepartum uterine relaxant, but the initial experience is encouraging.