Amnioinfusion is a medical procedure used primarily during labor and delivery to introduce sterile fluid into the amniotic cavity, the fluid-filled sac surrounding the fetus. The name itself breaks down the process: “amnio” refers to the amniotic fluid, and “infusion” means the introduction of a substance into the body. This intervention is used when conditions arise that threaten the fetus’s well-being, most often related to a low volume of surrounding fluid. By restoring the fluid volume, the procedure aims to improve the intrauterine environment and help ensure a safer delivery.
The Medical Necessity of Amnioinfusion
The procedure is typically initiated when a patient is experiencing oligohydramnios, a condition characterized by an abnormally low amount of amniotic fluid. This fluid acts as a cushion and is essential for protecting the fetus and the umbilical cord from external pressure. When the volume is too low, the protection is lost, leading to potential complications.
A common indication for amnioinfusion is the presence of variable decelerations in the fetal heart rate tracing during labor, which are sudden drops in heart rate. These decelerations often occur because the umbilical cord is temporarily compressed between the fetus and the uterine wall during contractions. The lack of a fluid cushion means the cord is easily squeezed, interrupting the flow of oxygenated blood to the fetus.
By infusing a sterile solution, the procedure artificially increases the fluid volume, thereby recreating the protective buffer around the umbilical cord. This added cushion helps prevent the cord from being compressed during contractions, which consequently improves the blood and oxygen flow to the fetus.
Amnioinfusion may also be considered in cases where the amniotic fluid is heavily stained with meconium. The procedure works to dilute the thick meconium, theoretically reducing the risk of the fetus inhaling the material into the lungs, a condition known as meconium aspiration syndrome. The primary benefit, however, remains the relief of umbilical cord compression and the resulting improvement in the fetal heart rate.
Logistics of the Procedure and Monitoring
For amnioinfusion to be performed, the patient’s membranes must be ruptured to allow direct access to the amniotic sac. The procedure is most commonly performed using a transcervical approach, where a thin tube is guided through the cervix and into the uterus. This device is typically an Intrauterine Pressure Catheter (IUPC), which is used for both fluid delivery and monitoring.
The IUPC allows for the instillation of the fluid and provides a means to accurately measure the pressure inside the uterus. The solution used is an isotonic fluid, usually warmed normal saline or Lactated Ringer’s solution, which closely matches the body’s natural fluids. This warmed solution is connected to the IUPC tubing and is administered to the patient.
Administration begins with an initial rapid infusion, called a bolus, often between 250 to 500 milliliters, delivered over a period of 20 to 30 minutes to quickly build the fluid volume. Following the bolus, a slower maintenance infusion, typically ranging from 60 to 180 milliliters per hour, is continued to replace lost fluid and maintain the volume. The volume infused is generally limited to prevent overdistension of the uterus.
Constant monitoring is required throughout the process to ensure the safety of both the mother and the fetus. The fetal heart rate is continuously assessed to determine if the variable decelerations are resolving, indicating the procedure is working. Additionally, the uterine resting tone (the pressure inside the uterus between contractions) is closely checked via the IUPC to prevent uterine overdistension, which could impair blood flow or cause uterine hyperstimulation.
Potential Complications and Outcomes
While amnioinfusion is a straightforward procedure with a generally favorable safety profile, practitioners must be aware of potential complications. Uterine overdistension occurs if too much fluid is infused too quickly, leading to increased uterine tone. This can sometimes result in painful contractions or, in rare circumstances, compromise placental blood flow or lead to uterine rupture, particularly in women with a prior cesarean scar.
Another potential risk is infection, specifically chorioamnionitis, which is an infection of the amniotic fluid and membranes, though the incidence is low given the use of sterile technique. Extremely rare but serious complications include an amniotic fluid embolism, where amniotic fluid enters the mother’s bloodstream.
The successful cushioning of the umbilical cord leads to an improvement in the fetal heart rate pattern, with a reduction in the number and severity of variable decelerations. This physiological improvement can prevent the progression to a non-reassuring fetal status that would otherwise necessitate an immediate operative delivery.
By stabilizing the fetus’s condition, amnioinfusion is associated with a lower rate of cesarean sections performed specifically for fetal distress. The procedure effectively buys time and improves the intrauterine environment, often allowing labor to progress safely toward a vaginal delivery.