Can Labor Pain be Reduced by Open Placebo Intervention? A Bayesian Account for Labor Pain

Aims and Hypotheses

Overall Aim:

To assess whether the Bayesian model can explain changes in pain perception during labor following open-placebo administration.

Specific Aims and Hypotheses:

Aim 1: To investigate whether open placebo reduces pain and analgesic consumption during labor.

Hypothesis 1: As most evidence supporting the efficacy of open placebo originates from chronic pain contexts, and given the intensity of labor pain, this hypothesis is bidirectional-open placebo may or may not modulate pain and analgesic consumption during labor at the group level.

Aim 2: To examine whether FAST outcomes predict changes in pain and analgesic consumption during labor following open-placebo administration.

Hypothesis 2: FAST outcomes will predict changes in pain and analgesic use. Lower within-subject variability in pain reports during FAST will be associated with smaller changes in pain and analgesic consumption following open placebo.

Aim 3: To explore the role of expectations and personality traits in placebo analgesia and their potential moderating effects on the relationship between FAST outcomes and the placebo response during labor.

Hypothesis 3.1: Personality traits associated with the prior function will moderate the relationship between FAST outcomes and the placebo response.

Hypothesis 3.2: Personality traits associated with the likelihood function will not moderate this relationship.

Methods

1. Participants:

Participants will include pregnant women admitted to Hillel Yaffe Medical Center. Based on power analysis (t-test, medium effect size = 0.5, α = 0.05, power = 0.8, allocation ratio = 0.666), a sample size of 106 is required. To compensate for potential dropouts, 120 participants will be recruited.

Inclusion criteria:

Singleton pregnancy after 24 weeks of gestation

No contraindications for analgesia or vaginal birth

Exclusion criteria:

Obstetric emergencies such as intrauterine death, suspected placental separation, uterine rupture, placenta previa, or scheduled cesarean section 2. Tools and Procedures 2.1. Pain Assessment Pain will be measured using the Visual Analogue Scale (VAS), a unidimensional tool asking participants to rate pain from 0 ("no pain") to 10 ("worst imaginable pain"). Assessments will occur every 60 minutes throughout labor.

2.2. Analgesic Consumption Assessment Analgesia at Hillel Yaffe Medical Center includes intravenous Pethidine and epidural anesthesia. Nitrous oxide is not available due to infrastructure limitations. Analgesic usage will be tracked via medical records and epidural pump data, including usage duration, PCA button presses, and administered doses.

2.3. FAST Assessment The Focused Analgesia Selection Test (FAST) evaluates variability in response to noxious thermal stimuli of varying intensities, applied to the ventral surface of the non-dominant arm using a thermal sensory analyzer. Pain ratings (0-100 scale) are recorded after each stimulus delivered every 20 seconds. Variability is quantified using R² and ICC values. Lower variability indicates higher confidence in sensory input. Test duration is approximately 20 minutes. FAST outcomes are expected to positively correlate with changes in clinical pain.

2.4. Open Placebo Intervention

Upon admission to the delivery room, participants receive standard Ringer solution. Participants will be randomly assigned to one of two groups:

Open Placebo Group: The midwife will explain that although the infusion contains no medication, previous research has shown it may reduce pain.

Control Group: The standard explanation will be provided, stating the fluids are intended only to maintain venous access.

Placebo response will be assessed through:

Differences in pain intensity (area under the curve of VAS scores over time) between groups

Differences in analgesic consumption, particularly fentanyl dosage via PCA, between groups

Analyses will also consider placebo response during distinct labor stages.

2.5. Expectations Assessment

Expectations will be measured using two questionnaires:

A short-form expectation questionnaire with items on treatment expectations and hopes.

A childbirth-specific questionnaire measuring fear and expectations surrounding birth, using a 6-point Likert scale, with higher scores indicating greater fear.

2.6. Personality Characteristics Assessment

Suggestibility:

Measured using the Short Suggestibility Scale (SSS), consisting of 21 items. Higher scores indicate greater suggestibility.

Anxiety:

Assessed with the GAD-7, a 7-item scale ranging from 0 to 3 per item, with a total score from 0 to 21. Higher scores reflect more severe anxiety symptoms.

Interoception:

Evaluated with the Multidimensional Assessment of Interoceptive Awareness (MAIA), comprising 32 items across eight subscales. Items are rated on a 6-point Likert scale.

• Assessment of expectations from the treatment [Срок оценки: At baseline]
• Fear of childbirth [Срок оценки: At baseline]
• Suggestibility [Срок оценки: At baseline]
• Anxiety [Срок оценки: At baseline]
• Interoception [Срок оценки: At baseline]
• Body Awareness [Срок оценки: At baseline]
• Self-consciousness [Срок оценки: At baseline]
• Questionnaire for the prediction of the placebo response [Срок оценки: At baseline]