Preinsertion of multiple osmotic dilators 6 to 24 hours prior to amnio-infusion may reduce the time from induction to completion, reduce the risk of cervical laceration, and almost eliminate posterior wall rupture.
Prostaglandins (PGF2α, PGE1, PGE2) can be used for cervical ripening but may cause unsupervised delivery.
Another form of cervical preparation is the insertion of an Atad double balloon catheter; this method may also be used as a mechanical method of labour induction with similar end points (induction interval and complete abortion rates) to techniques with prostaglandins.
Techniques for Amniocentesis
The optimal site for amniocentesis is approximately 2.5 cm below the most prominent part of the uterus as palpated through the abdominal wall. The amniocentesis can be performed with a number 16 teflon extracath. Once the amniotic cavity is entered and the amniotic fluid is identified, the rigid central needle should be removed and drainage of amniotic fluid should continue. A pH test on the fluid will confirm its amniotic origin. The induction agent is then injected into the amniotic cavity. Ultrasound guidance may be necessary under some circumstances (e.g., if the patient is obese).
(a) Prostaglandin F2α (PGF2α) Amnio-Infusion
Intra-amniotic installation of PGF2α is an effective technique but may be associated with the need for a second injection, transient fetal survival, failure, and significant GI side effects. Pretreatment with laminaria tents can decrease the need for repeat injection, and pretreatment with prochlorperazine 10 mg IM or IV, or another antiemetic with loperamide (4 mg) orally one hour prior to amnio-infusion, will reduce the GI side effects.
A test dose of 5 mg of PGF2α is given over one minute to detect sensitivity to the drug or accidental intravascular placement of the needle. Then a total of 40 to 50 mg is given over a few minutes. Constant confirmation of the catheter’s position in the amniotic cavity must be assured. An additional 20 to 40 mg may be required if the membranes are intact or if there is poor cervical effacement or inadequate uterine activity.
The absence or a decrease in the amniotic fluid volume as a result of ruptured membranes or genetic abnormalities may require an alternative route of prostaglandin administration.
(b) Hyperosmolar Urea Amnio-Infusion
Intra-amniotic hyperosmolar urea (80 - 90 gms/100 mL 5% dextrose in water for a 59.7% concentration) is effective for labour induction, but injection to abortion times are prolonged. Augmentation with PGF2α (5, 10, or 20 mg) administered with or immediately after the urea infusion decreases the interval from induction to abortion.
Following successful amniocentesis and the removal of 200 mL of amniotic fluid, the solution is slowly infused by gravity feeding.