5.4 Pain Management in Labour: Non-Pharmacological & Pharmacological Analgesia

Pain Management in Labour: Non-Pharmacological & Pharmacological Analgesia

Labour pain is a complex, subjective, multidimensional physiological experience shaped by neurobiological, emotional, cultural, and environmental factors. Unlike pathological pain, which signals tissue injury or disease, physiological labour pain is a functional, adaptive biological process that guides maternal posture, stimulates endogenous neuroendocrine cascades, and facilitates birth. The midwife's role is not to impose analgesia, but to support the woman's informed choice, optimize physiological coping, and provide safe, evidence-based pharmacological interventions when desired or indicated.

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Neurophysiology and Pathways of Labour Pain

The anatomical origin and transmission pathways of labour pain change fundamentally as labour transitions from the first stage to the second stage.

                 NEUROLOGICAL DUAL PATHWAYS OF LABOUR PAIN
  ┌───────────────────────────────────┬───────────────────────────────────┐
  │     FIRST STAGE: VISCERAL PAIN    │    SECOND STAGE: SOMATIC PAIN     │
  ├───────────────────────────────────┼───────────────────────────────────┤
  │ • Mechanical cervical dilatation  │ • Distension of pelvic floor      │
  │ • Myometrial cellular ischemia    │ • Stretching of vagina & perineum │
  │ • Lower uterine segment distension│ • Direct nerve compression        │
  │ • Transmitted by SYMPATHETIC      │ • Transmitted by SOMATIC          │
  │   visceral afferents (T10 - L1)   │   pudendal nerve fibres (S2 - S4) │
  │ • Diffuse, aching, poorly local   │ • Sharp, localized, intense       │
  │ • Referred to abdomen, groin, back│ • Localized to introitus & rectum │
  └───────────────────────────────────┴───────────────────────────────────┘

1. First Stage: Visceral Pain (T10 to L1)

During the first stage of labour, nociceptive impulses originate primarily from:

• Mechanical stretching and distension of the cervix during effacement and dilatation;
• Mechanical distension of the lower uterine segment; and
• Ischemia of myometrial smooth muscle myocytes during contractions, caused by local arteriole compression.

These visceral nociceptors generate slow, unmyelinated C-fibre and lightly myelinated A-delta fibre impulses that accompany sympathetic nerve trunks through the inferior, middle, and superior hypogastric plexuses. They traverse the lumbar and lower thoracic sympathetic chain and enter the dorsal horn of the spinal cord at the T10, T11, T12, and L1 spinal segments.

Because visceral afferents converge with somatic dermatomal afferents in the dorsal horn, first-stage pain is characteristically diffuse, cramping, and referred. Pain is perceived along the dermatomes corresponding to T10-L1: across the lower abdominal wall, the lumbosacral region, the iliac crests, and the anterior upper thighs.

2. Second Stage: Somatic Pain (S2 to S4)

In the second stage of labour, uterine contractions continue, but the primary source of pain shifts to the mechanical descent of the presenting part:

• Marked distension, stretching, and microtrauma of the pelvic floor musculature (levator ani complex);
• Extreme stretching of the vaginal vault, vulval introitus, and perineal body; and
• Direct compression of pelvic somatic structures and the rectum.

These sensory nociceptive impulses are transmitted via heavily myelinated, rapid somatic sensory nerve fibres within the pudendal nerve, which enters the spinal cord at sacral segments S2, S3, and S4.

Unlike visceral pain, somatic pain is sharp, intense, lancinating, and exquisitely well-localized to the perineum, rectum, and vagina. Somatic pain triggers the involuntary maternal bearing-down urge (Ferguson reflex) as the fetal head exerts direct mechanical pressure on the pelvic floor.

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Non-Pharmacological Comfort Measures

Non-pharmacological pain strategies support physiological birth by activating descending endogenous inhibitory pathways, reducing maternal anxiety, and preventing excess catecholamine secretion.

Continuous One-to-One Midwifery Support

Large Cochrane systematic reviews (e.g., Hodnett et al.) establish that continuous one-to-one emotional and physical support from a midwife or trained birth companion is one of the most effective intrapartum interventions in modern maternity care:

• Significant reduction in the use of regional epidural and systemic opioid analgesia;
• Shortened duration of labour;
• Statistically significant reduction in instrumental deliveries and emergency caesarean births;
• Lower rates of low 5-minute Apgar scores; and
• Substantially higher rates of maternal satisfaction and psychological well-being.

Birth Environment Optimization

The intrapartum environment profoundly affects maternal neuroendocrinology. Exposure to bright lights, loud noises, unfamiliar clinical equipment, or unannounced interruptions activates the maternal sympathetic nervous system, precipitating a "fight-or-flight" adrenaline and noradrenaline surge. High circulating catecholamines:

• Inhibit hypothalamic oxytocin pulsatility, slowing or arresting labour progress;
• Cause peripheral and uteroplacental vasoconstriction, reducing fetal oxygenation; and
• Amplify fear and pain perception.

Midwives optimize the birth space by dimming lights, ensuring warmth, minimizing ambient noise, maintaining continuous privacy, and speaking in calm, reassuring tones, thereby promoting parasympathetic dominance, endogenous oxytocin release, and beta-endorphin secretion.

Hydrotherapy and Warm Water Immersion

Immersing in a deep birth pool during the active first stage provides substantial physiological and psychological benefits:

• Thermal Effect: Warm water promotes generalized vasodilation, reduces arterial blood pressure, and induces deep muscular relaxation.
• Buoyancy: Buoyancy reduces gravitational forces on the maternal pelvis, relieves axial pressure on the spine and joints, and allows effortless maternal postural changes.
• Analgesic Efficacy: Water immersion significantly reduces maternal perception of contraction pain and substantially reduces uptake of regional epidural analgesia.
• Water Temperature Standard: Midwives must monitor and record water temperature hourly, ensuring it is maintained at or below 37.5°C. Excessive water temperature (>37.5°C) leads to maternal pyrexia, which causes maternal dehydration, fetal hyperthermia, and persistent fetal tachycardia on auscultation.

Transcutaneous Electrical Nerve Stimulation (TENS)

TENS units are handheld, battery-powered devices delivering pulsed electrical currents through four adhesive dermal electrodes applied to the maternal back.

                  TENS ELECTRODE ANATOMICAL PLACEMENT
                       ┌─────────────────────────┐
                       │       SPINAL CORD       │
                       └────────────┬────────────┘
                                    │
         ┌──────────────────────────┴──────────────────────────┐
         ▼                                                     ▼
┌────────────────────────────────┐    ┌────────────────────────────────┐
│      UPPER PAIR (T10 - L1)     │    │      LOWER PAIR (S2 - S4)      │
│ • Applied paravertebrally over │    │ • Applied over the sacrum      │
│   lower thoracic / upper lumbar│    │   above the natal cleft        │
│ • Modulates first-stage        │    │ • Modulates second-stage       │
│   visceral uterine afferents   │    │   somatic pelvic afferents     │
└────────────────────────────────┘    └────────────────────────────────┘

Mechanism of Action

1. Gate Control Theory of Pain (Melzack & Wall): TENS delivers non-noxious electrical stimulation that excites large-diameter, myelinated A-beta sensory nerve fibres. In the substantia gelatinosa of the spinal dorsal horn, these A-beta inputs activate inhibitory interneurons, effectively "closing the presynaptic neural gate" and blocking the upward transmission of slow, unmyelinated C-fibres carrying visceral contraction pain.
2. Endorphin Release: Low-frequency pulsed currents stimulate the central nervous system to release endogenous opioid peptides (beta-endorphins and enkephalins) into the cerebrospinal fluid, elevating maternal pain tolerance.

Clinical Use and Safety

• NICE NG235 advises that TENS devices are not provided by the NHS and that there is very little evidence of effectiveness in established labour, but no evidence of harm. If a woman wants to use TENS, her choice is supported and other pain relief can be used alongside it. Women who find it helpful usually start it early, in the latent phase.
• The woman retains complete autonomous control of the handheld unit, adjusting the baseline pulse frequency and activating a "boost" button during contractions.
• Safety & Contraindications: TENS cannot be used in water (pools, showers) and is contraindicated in women with cardiac pacemakers or implantable defibrillators.

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Pharmacological Analgesia: Inhalational & Systemic

When non-pharmacological methods do not provide sufficient comfort, or when a woman requests medical pain relief, midwives provide evidence-based pharmacological analgesia.

1. Inhalational Analgesia: Entonox

Entonox is a pre-mixed medicinal gas composed of 50% nitrous oxide ($N_2O$) and 50% oxygen ($O_2$). It is the most widely utilized pharmacological agent in UK intrapartum care.

Pharmacokinetics and Timing of Administration

• Mechanism: Nitrous oxide induces analgesia and mild euphoria by stimulating the release of endogenous encephalins and modulating spinal NMDA and GABA receptors.
• Onset and Peak: Inhaled Entonox diffuses rapidly across alveolar capillary membranes into the cerebral circulation. Analgesic onset occurs within 20 to 30 seconds, reaching peak concentration and maximum clinical efficacy at 45 to 60 seconds after commencing inhalation.
• Clinical Coaching Technique: Because a typical labour contraction lasts 45 to 60 seconds, timing of inhalation is critical. The midwife must teach the woman to begin deep, rhythmic inhalations through the mouthpiece or mask at the very earliest warning twinge of an oncoming contraction. This ensures that the peak drug effect coincides synchronously with the acme (peak) of the contraction. The woman should stop inhaling as the contraction fades to avoid dizziness between contractions.
• Autonomous Demand Valve: Entonox is self-administered using a two-stage demand valve. The woman must hold the mask or mouthpiece herself. Partners and midwives must never hold the mask for the woman. If the woman becomes drowsy, her hand naturally falls away from her face, releasing the valve, ceasing delivery, and preventing overdose.
• Maternal Side Effects: Lightheadedness, dry mouth, nausea, vivid dreams, mild disorientation, and tingling sensations (paresthesia) caused by hyperventilation.
• Absolute Contraindications: Nitrous oxide diffuses rapidly into closed, air-filled cavities 34 times faster than nitrogen can leave, causing rapid gas expansion. Entonox is strictly contraindicated in:
  • Known or suspected pneumothorax;
  • Bowel obstruction or paralytic ileus;
  • Decompression sickness (recent scuba diving within the preceding 24 hours);
  • Air embolism;
  • Severe maxillofacial injury or fractured skull base with altered consciousness.

2. Systemic Opioids: Diamorphine and Pethidine

Intramuscular systemic opioids provide systemic analgesia, sedation, and relief from emotional distress during the active first stage of labour.

Pharmacological Profiles and Dosing

• Diamorphine Hydrochloride: The opioid of choice in many UK maternity units. Highly lipophilic, diamorphine crosses the blood-brain barrier rapidly, offering superior analgesia, less emesis, and greater maternal relaxation compared to pethidine. Administered as an intramuscular dose of 5 to 10 mg (typically 7.5 mg IM).
• Pethidine Hydrochloride: A synthetic opioid widely utilized across UK units. Administered as an intramuscular dose of 50 to 100 mg IM (every 2-4 hours, maximum 400 mg in 24 hours). Metabolized by the liver into norpethidine, an active toxic metabolite with a long neonatal elimination half-life (up to 60-70 hours) that exerts central neurotoxic and pro-convulsant effects.
• Antiemetic Co-Administration: Opioids directly stimulate the chemoreceptor trigger zone (CTZ) in the area postrema of the medulla. An intramuscular antiemetic—most commonly cyclizine (50 mg IM)—should be prescribed and administered concurrently to prevent severe nausea and vomiting.

Maternal and Neonatal Risks

• Maternal Effects: Central sedation, respiratory depression, nausea, orthostatic dizziness, and delayed gastric emptying (which heightens the risk of pulmonary aspiration [Mendelson's syndrome] should emergency general anaesthesia be required).
• Fetal Effects: Opioids readily cross the placenta. Fetal heart rate monitoring frequently exhibits a temporary loss of baseline variability and absence of accelerations (a flattened CTG pattern), which typically persists for 60 to 90 minutes post-administration.
• Neonatal Respiratory Depression: If an opioid is administered within 2 to 4 hours prior to delivery, the neonate is at high risk of delivery-room respiratory depression, poor muscle tone (hypotonia), hypothermia, and delayed, impaired establishment of breastfeeding.

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Regional Analgesia: Epidural Anaesthesia

Epidural analgesia is the most effective method of intrapartum pain relief, providing complete or near-complete sensory blockade while preserving maternal consciousness.

Pharmacology and Administration

• Technique: An anaesthetist introduces a Tuohy needle into the lumbar epidural space (typically at the L2-L3 or L3-L4 interspace), passing through the ligamentum flavum. A fine polyethylene epidural catheter is threaded into the space, and the needle is removed.
• Medication: Modern UK protocols utilize low-dose combined infusions consisting of a local anaesthetic (e.g., bupivacaine 0.0625% to 0.1%) and a lipid-soluble synthetic opioid (e.g., fentanyl 2 mcg/mL or sufentanil). This low-concentration regimen achieves effective sensory analgesia while preserving motor function ("mobile epidural"), allowing maternal leg movement.

Clinical Management Protocol and Midwifery Care