In the patient who is already receiving analgesia in labour or who is having a caesarean section under regional anaesthesia, uterine relaxation may be required for a variety of reasons. Sometimes, the need may arise suddenly (eg. difficulty encountered during delivery of an aftercoming head in assisted breech delivery). Pharmacological agents which can render uterine muscle quiescent and, in higher concentrations, paralysed and flaccid, include:
Inhalationed General Anaesthetics:
halothane (1, 2, 3, 4, 5, 6),
methoxyflurane (5),
enflurane (2, 5, 6),
isoflurane (2),
sevoflurane (8),
desflurane,
Nitrates:
(intravenous) nitroglycerine (GTN/NTG),
(inhaled) amyl nitrate,
Beta adrenergic agents:
(intravenous) ritodrine,
(intravenous) salbutamol,
(intravenous) terbutaline.
Indications for these regimens:
Situations where uterine relaxation may be necessary in the parturient with an epidural block in the peridelivery period include:
1. manual removal of a retained placenta,
2. breech delivery: obstructed aftercoming head or shoulders,
3. multiple pregnancy: second twin in transverse-lie,
4. inadvertent oxytocic overdose prior to delivery,
5. uterine constriction ring,
6. inverted uterus, and
7. (some cases of) fetal distress.
My preferred technique for use in conjunction with regional analgesia is IV nitroglycerine and I attempt to produce adequate uterine muscle relaxation without the need for general anaesthesia.
Drug Dosage:
Presentation: Use the 50mgs in 10mls (5mg/ml) ampoule of glyceryl trinitrate.
Final dosage required for bolus injection: 50 micrograms/ml.
Dilution:
1. Dilute 1 ampoule (50mgs) in a 1000ml (1 Litre) bag of N/Saline or 5% Dextrose to produce a solution of GTN 50 mcg/ml.
2. Withdraw 10mls into a 10cc syringe and label it: 'GTN 50mcg/ml'.
[Do not forget to discard the 1000ml bag containing the remaining GTN.]
Treatment:
1. Preload the patient with Hartmann's solution (compound sodium lactate) or colloid as appropriate until the patient is euvolaemic.
2. Into a free-flowing IV, inject 1 ml (50mcg) of the diluted (50 mcg/ml) GTN solution.
3. Titrate the injection to effect at intervals of a minute or so, up to a maximum of 10 doses (500 mcg).
An alternative dosage regimen is contained in Table 14.7.
Management of Side Effects:
1. Hypotension is uncommon in euvolaemic patients and can be treated either with increments of ephedrine 3mg intravenously and/or volume replacement.
2. The effect of this dose of GTN on uterine, cervical and vaginal muscle is very transient, and any persisting loss of uterine muscle tone can be reversed with oxytocics (syntocinon +/- ergometrine).
Monitoring:
1. Repeated blood pressure monitoring.
2. Continuous fetal heart monitoring and maternal ECG or oximetry (where appropriate).
Contraindications:
1. Significant blood loss.
2. (Some cases of) valvular heart disease.
Effect:
With GTN, uterine relaxation should be achievable in 75 to 90 seconds and is maintained for less than 10 minutes (without the need for general anaesthesia).
References:
1. Miller J, Stoelting V, Stander R, et al: In vitro and in vivo Responses of the Uterus to Halothane Anesthesia, Anesth Analg 45:583-589, 1966.
2. Munson E, Embro W: Enflurane, Isoflurane and Halothane and Isolated Uterine Muscle. Anesthesiol 46:1:11-14, 1977.
3. Naftalin N, McKay D, Phear W, et al: The Effects of Halothane on Pregnant and Non-pregnant Human Myometrium. Anesthesiol 46:1:15-19, 1977.
4. Crawford JS: The Place of Halothane in Obstetrics. BJA 34:386-390, 1962.
5. Paull J, Ziccone S: Halothane, Enflurane, Methoxyflurane, and Isolated Human Uterine Muscle. Anaesth Int Cr 8:4:397-401, 1980.
6. Marx G, Kim Y, Lin C, et al: Postpartum Uterine Pressures Under Halothane or Enflurane Anesthesia. Obstet Gynecol 51:695, 1978.
7. Gatt S. Intravenous Nitroglycerine (GTN/NTG) for Utero-relaxation. Protocol 77, Department of Anaesthesia & Acute Care, Royal Hospital for Women, August 1994.
8. Gambling D, Sharma S, White P, et al. Use of sevoflurane during elective cesarean birth: A comparison with isoflurane and spinal anesthesia. Anesth Analg 81:90-95, 1995.