Labor Position and Role of TENS in Pain Relief

PART A: MATERNAL POSITIONS IN LABOR

Introduction

Classification of Positions

Role/Advantages of Positions During Labor

First Stage of Labor

1. Upright & ambulating positions
   • Harness the force of gravity to aid fetal head descent and engagement.
   • Improve uterine contraction efficiency and frequency.
   • Decrease the duration of the first stage.
   • Reduce the need for oxytocin augmentation.
   • Associated with less labor pain compared to supine.
2. Lateral (left lateral decubitus)
   • Relieves aortocaval compression by the gravid uterus, improving placental blood flow and fetal oxygenation.
   • Useful when continuous fetal monitoring is required and ambulation is not possible.
   • Recommended after epidural analgesia placement.
3. All-fours / hands-and-knees
   • Particularly effective when the fetus is in the occiput posterior (OP) position - the anterior position allows gravity to encourage spontaneous rotation of the fetal head.
   • Reduces back pain in labor (back labor).
   • "Any maternal position that causes her to curl forward from the hips is felt to be helpful" for OP rotation (Pfenninger & Fowler).
4. Squatting
   • Widens the pelvic outlet (increases bi-ischial diameter by ~1-2 cm).
   • Beneficial when fetal head is low and pushing is imminent.

Second Stage of Labor

1. Squatting
   • Shortens the second stage by ~9 minutes compared to supine in both primiparas and multiparas.
   • Utilizes gravity and maximizes pelvic outlet dimensions.
   • Disadvantage: increased risk of obstetric anal sphincter injury (OASIS) due to difficulty controlling extension of the fetal head.
2. Sitting on birth seat
   • Significantly shorter second stage compared to lateral, supine, and standing positions (RCT evidence).
   • Less synthetic oxytocin needed for augmentation.
3. Lithotomy
   • Traditional in many hospital settings.
   • Allows the provider good perineal visibility and access.
   • Loses the advantage of gravity; associated with more perineal trauma.
   • Risk of supine hypotension syndrome (aortocaval compression).
4. Lateral (Sims')
   • Useful for slow, controlled delivery of the fetal head.
   • Good when perineal protection is the priority.
   • Less effective use of gravity.
5. Kneeling / all-fours
   • Used to manage shoulder dystocia (Gaskin maneuver).
   • Helps rotate a persistent OP fetus.

Management of Persistent Occiput Posterior (OP) Position

• The OP position prolongs labor by ~1 hour in multiparas and ~2 hours in nulliparas.
• Position changes to encourage rotation: squatting, ambulating, hands-and-knees, lateral Sims', or back-arched positions.
• Manual rotation if positional changes fail: mother in lithotomy, lateral Sims', or hands-and-knees; hand placed in posterior pelvis behind occiput; rotation during a contraction while mother pushes. (Pfenninger & Fowler)

Supine Hypotension Syndrome (Aortocaval Compression)

• Supine position allows the gravid uterus to compress the inferior vena cava and aorta, reducing venous return and uteroplacental perfusion.
• Can cause fetal asphyxia and maternal hypotension.
• Prevention: Avoid the supine position; use left lateral tilt (15°) during all monitoring and procedures.

PART B: ROLE OF TENS IN LABOR PAIN

What is TENS?

Mechanism of Action

1. Gate Control Theory (primary mechanism)
   • Pain signals (nociception) are transmitted via small, slow C-fibers and Aδ-fibers to the dorsal horn (T-cells), which convey information to higher brain centers.
   • TENS activates large-diameter, fast Aβ fibers, which stimulate inhibitory interneurons in the substantia gelatinosa of the dorsal horn.
   • These interneurons "close the gate" - they presynaptically inhibit the transmission of pain impulses to the brain.
   • "In a mechanism called the gate control theory, sensory inputs from large-diameter nonpain Aβ fibers reduce pain transmission through the dorsal horn. Thus, TENS devices work to reduce chronic pain by activating Aβ fibers." - Neuroanatomy through Clinical Cases
2. Endogenous Opioid Release
   • Low-frequency TENS (<10 Hz) stimulates the release of endorphins and enkephalins from the periaqueductal gray, rostral ventromedial medulla, and spinal cord dorsal horn.
   • These endogenous opioids modulate pain at central and spinal levels.
3. Local Vasodilation
   • Direct local vasodilation may reduce relative ischemia in the painful area (e.g., uterine ischemia contributing to labor pain), thereby reducing pain.

TENS Parameters

• Conventional settings for labor: high-frequency, narrow pulse width, intensity set to strong paresthesia without motor stimulation.
• Electrodes placed over the lower back (T10-L1 dermatomes) - the primary pain pathway for first-stage labor (uterine contractions).

Application in Labor

• Indication: Obstetric pain in labor (after the first trimester).
• Electrode placement: Lower back bilaterally, at the level of T10-L1 for first-stage pain; additional sacral (S2-S4) placement for second-stage perineal pain.
• TENS allows the mother to self-administer stimulation, boosting her sense of control over labor pain.
• Can be used alongside other non-pharmacological methods (hydrotherapy, Lamaze, massage, aromatherapy).

Evidence and Effectiveness in Labor

• Results of studies using TENS in labor are inconsistent.
• A systematic review and meta-analysis concluded: "Labor pain does not appear to be lessened by TENS, nor does TENS lower the use of other analgesic modalities." (Barash, Clinical Anesthesia, 9e)
• However, TENS may improve maternal satisfaction and sense of control, and may reduce anxiety.
• It is safe, non-invasive, and carries no pharmacological risk to the fetus, making it an attractive first-line option in non-pharmacological pain management.

Contraindications to TENS in Obstetrics

• First-trimester pregnancy (absolute)
• Patients with demand-type pacemakers
• Known cardiac dysrhythmias
• Undiagnosed pain syndrome
• Mentally incompetent or uncooperative patients
• Application over carotid sinuses, chest (cardiac history), head/neck (epilepsy history), eyes, or broken skin

Other Non-Pharmacological Methods (Context)

Summary Table

• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Nonpharmacologic Methods of Labor Analgesia
• Pfenninger & Fowler's Procedures for Primary Care, 3e - TENS technique and obstetric application
• Neuroanatomy through Clinical Cases, 3e - Gate control theory of pain
• PMC6839002 - Review and comparison of common maternal positions during the second stage of labor

DIAGRAM 1 — Gate Control Theory of Pain (Kandel's Principles of Neural Science)

DIAGRAM 2 — Gate Control Theory (Spinal Cord Cross-Section - Neuroscience: Exploring the Brain)

DIAGRAM 3 — Descending Pain Modulation Pathway (PAG → Raphe Nuclei → Dorsal Horn)

DIAGRAM 4 — TENS Unit Applied to the Back

DIAGRAM 5 — TENS Electrode Placement for Labor Pain (Spinal Levels)

DIAGRAM 6 — Maternal Labor Positions

• Upright/Ambulating - gravity aids descent, shortens first stage
• Squatting - widens pelvic outlet, shortens second stage by ~9 min
• All-fours - best for OP rotation and back labor
• Left lateral (Sims') - prevents aortocaval compression, controlled delivery
• Lithotomy - clinical access, but no gravity advantage
• Sitting (birth ball) - significantly shorter second stage vs supine (RCT evidence)