Amnioinfusion Solution and Rate: Lactated Ringer's vs. Normal Saline
Lactated Ringer's solution is recommended for amnioinfusion at an initial bolus of 250-500 mL followed by a maintenance rate of 100-200 mL/hour. This recommendation is based on the most recent and highest quality evidence showing that Lactated Ringer's solution most closely resembles the biochemical properties of amniotic fluid 1.
Solution Choice: Lactated Ringer's vs. Normal Saline
Evidence for Lactated Ringer's:
- Lactated Ringer's solution approximates amniotic fluid most closely in both electrolyte composition and pH 1
- Using Lactated Ringer's results in minimal changes in fetal electrolytes and acid-base balance 2
- Recent in vitro research shows that both normal saline and Lactated Ringer's can cause amniotic membrane damage, but Lactated Ringer's causes less cell death (80% viability vs 68% with normal saline) 3
Evidence Against Normal Saline:
- Normal saline amnioinfusion can significantly alter fetal plasma electrolyte concentrations and blood pH 2
- Normal saline causes a decrease in fetal arterial pH (-0.015 units) that is linearly related to increases in plasma chloride concentration 2
- Normal saline has been associated with hyperchloremic metabolic acidosis in other clinical contexts 4
Amnioinfusion Rate Protocol
Initial Bolus:
- 250-500 mL of Lactated Ringer's solution
Maintenance Rate:
- 100-200 mL/hour
Duration:
- Continue as clinically indicated based on the specific indication for amnioinfusion (meconium, variable decelerations, oligohydramnios)
Clinical Considerations
Monitoring During Amnioinfusion:
- Continuous fetal heart rate monitoring
- Uterine pressure monitoring to avoid polyhydramnios and potential uterine overdistention
- Monitor maternal vital signs
- Consider periodic assessment of amniotic fluid volume via ultrasound
Potential Complications to Watch For:
- Uterine overdistention
- Umbilical cord prolapse
- Uterine rupture (rare)
- Amniotic fluid embolism (rare)
Special Situations
Meconium-Stained Amniotic Fluid:
- Amnioinfusion may help dilute meconium and reduce the risk of meconium aspiration syndrome
Variable Decelerations:
- Amnioinfusion can relieve umbilical cord compression by increasing amniotic fluid volume
Oligohydramnios:
- Amnioinfusion may improve the amniotic fluid index and reduce the risk of cord compression
Important Caveats
While both normal saline and Lactated Ringer's have been used clinically without significant adverse neonatal outcomes 5, 6, the biochemical evidence favors Lactated Ringer's for its closer resemblance to amniotic fluid composition.
Total volume and duration of infusion correlate with neonatal chloride levels 5, so careful monitoring of total infused volume is important.
Recent research suggests that even better solutions might be developed in the future that more closely mimic natural amniotic fluid 3, but Lactated Ringer's remains the best currently available option.