Obstetrics and Gynaecology chapters
Obstetric anaesthesia and analgesia
Pain relief in labour from breathing exercises to opioids and nitrous oxide, how an epidural and a spinal are performed, their indications, contraindications and complications, and the risks of general anaesthesia in obstetrics.
- Epidural analgesia is the most reliable pain relief in labour: dilute local anaesthetic with an opioid at L2-L3 to L4-L5, needing intravenous access and continuous fetal monitoring.
- Main problems are hypotension, accidental dural puncture with spinal headache, bladder distension and, rarely, total spinal block.
- A spinal is faster and denser and is used for caesarean section; general anaesthesia is kept for when regional is contraindicated or too slow, and carries the risk of failed intubation and acid aspiration.
On this page
Overview
Labour pain relief runs from support and breathing exercises through opioids and inhaled nitrous oxide to regional blocks. Regional anaesthesia also covers caesarean section, instrumental delivery, manual removal of the placenta and repair of difficult tears. General anaesthesia is now unusual in obstetrics because of airway and aspiration risk, but it is still needed when regional block is contraindicated, has failed, or when delivery must be immediate.
Non-drug methods
- One-to-one midwifery care with a supportive birth partner lowers how much analgesia is needed.
- Relaxation and controlled breathing help some women, though overbreathing for too long brings on dizziness and alkalosis.
- Warm water immersion in the first stage improves coping; water temperature should stay at or below 37.5 degrees C.
- Transcutaneous electrical nerve stimulation blocks pain fibres in the posterior horn by stimulating small afferents (the gate theory). It may suit the latent phase but has not been shown to reduce pain scores in established labour. It is harmless.
- Acupuncture, hypnosis and homeopathy have not shown a clear reduction in pain scores.
Systemic opioids
Pethidine and diamorphine are widely used because a midwife may give them without a doctor. They give limited relief and have real side effects.
| Problem | Comment |
|---|---|
| Nausea and vomiting | Always give with an antiemetic |
| Maternal sedation | Drowsiness and poor cooperation |
| Delayed gastric emptying | Raises the hazard of general anaesthesia later, so antacids are given |
| Neonatal respiratory depression | Short-lived; naloxone may be needed |
| Interference with feeding | Early breastfeeding may be harder |
A patient-controlled device, subcutaneous or intravenous, is an alternative; with a very short-acting opioid the doses can be timed to contractions. This suits women who cannot have an epidural.
An opioid in labour is not harmless bookkeeping. It makes the stomach slower, so if general anaesthesia follows, the aspiration risk is higher, and light diet is no longer acceptable.
Inhalational analgesia
Entonox is an equal mixture of nitrous oxide and oxygen. Onset and offset are quick and it works better than pethidine. It causes light-headedness and nausea, and prolonged use from early labour causes overbreathing with a low carbon dioxide level, dizziness and, occasionally, tetany with fetal hypoxia. It suits late labour or the wait for an epidural.
Epidural analgesia
Indications and contraindications
| Indications | Contraindications |
|---|---|
| Effective pain relief (the main one) | Coagulation disorder including low platelets |
| Prolonged labour or oxytocin augmentation | Local or systemic sepsis |
| Maternal hypertensive disease | Hypovolaemia |
| Multiple pregnancy | Lack of trained anaesthetic and midwifery staff |
| Selected maternal medical disease | - |
| High chance of operative delivery | - |
Coagulopathy is the classic absolute contraindication to an epidural, because of the risk of a spinal haematoma.
Advanced cervical dilatation is not in itself a bar to an epidural, and severe pain even in the latent phase should not be refused. What counts is how fast labour is progressing, how long delivery is likely to take and what sort of delivery is expected.
Technique
- Position: extreme left lateral, or sitting and leaning forward, with the upper spine and hips flexed to open the interspaces.
- Aseptic technique, skin infiltrated with local anaesthetic.
- Catheter placed into the epidural space at L2-L3, L3-L4 or at L4-L5, which holds fat, vessels and nerve roots.
- The needle passes skin, then interspinous tissue, then the ligamentum flavum, the last resistance before the space.
- Aspirate for blood or cerebrospinal fluid, then give a test dose of dilute local anaesthetic. Leg weakness, sensory block or peripheral vasodilatation after the test dose means the catheter is in the subarachnoid space; a full dose there would paralyse the legs and the breathing muscles.
- If there is no effect 5 minutes after the test dose, the loading dose is given. The solution is a low concentration of bupivacaine with an opioid such as fentanyl; adding the opioid lets you use less local anaesthetic, so there is less motor block and less hypotension.
- Keep the mother on her right or left side, and check blood pressure every 5 minutes for 15 minutes.
- Maintain with intermittent boluses, an infusion, or patient-controlled epidural analgesia. Check the sensory level hourly with cold spray to catch a block creeping too high.
- Continue the block until the third stage and any perineal repair are complete.
Effects on labour
Epidural analgesia does not raise the caesarean section rate, but the second stage is longer and instrumental delivery is more likely. A longer passive second stage, waiting for the urge to push, reduces that. Sometimes an epidural helps a vaginal birth by relaxing the woman and giving the head time to descend and rotate. The woman should be warned about temporary loss of leg sensation and movement, the drip, and the need for continuous monitoring.
Complications
| Complication | Mechanism and management |
|---|---|
| Hypotension | Sympathetic block and vasodilatation; fluid bolus, left lateral position, ephedrine if needed. May cause fetal bradycardia |
| Accidental dural puncture | Should be under 1%; cerebrospinal fluid leaks and causes a spinal headache, worse sitting up, relieved lying flat. Severe or persistent: epidural blood patch at the puncture level, the clot sealing the leak |
| Bladder dysfunction | The woman cannot feel a full bladder; overdistension can damage the detrusor permanently, so catheterise if she is not passing good volumes |
| Total spinal block | Epidural dose into the subarachnoid space: profound hypotension, respiratory failure, loss of consciousness, death if untreated. Needs intubation, ventilation, fluids, vasopressors, left lateral position; urgent delivery may be needed to relieve aorto-caval compression |
| Local anaesthetic toxicity | Catheter in a vessel; detected by aspiration before injection |
| Neonatal respiratory depression | Modern solutions contain opioid, which reaches the maternal circulation and crosses the placenta |
| Spinal haematoma, neurological damage | Rare, usually with bleeding disorders |
The mother with a regional block must never lie supine: aorto-caval compression drops cardiac output and uterine perfusion.
Spinal anaesthesia
A fine atraumatic needle passes through the epidural space and dura into the subarachnoid space, a small dose of local anaesthetic goes in and the needle comes straight out.
| Feature | Epidural | Spinal |
|---|---|---|
| Site of drug | Epidural space | Subarachnoid space (cerebrospinal fluid) |
| Dose | Larger, dilute | Very small |
| Onset | Slower | Rapid |
| Density of block | Less dense, can be topped up through a catheter | More effective and denser |
| Hypotension | Common | More common |
| Main use | Analgesia through labour | Caesarean section, instrumental delivery in theatre, manual removal of the placenta, repair of difficult tears |
Combined spinal-epidural gives the rapid onset of a spinal with the prolonged top-up ability of an epidural; because the spinal dose is low it is usable for labour analgesia. Plain spinals are not used for routine labour analgesia.
Local infiltration and pudendal block
- Perineal infiltration with local anaesthetic covers episiotomy and simple tear repair. Third- and fourth-degree tears need regional anaesthesia in theatre.
- Pudendal nerve block: the pudendal nerve passes just behind and below each ischial spine, and local anaesthetic is placed at that point, usually with perineal infiltration, for vacuum or forceps delivery. For a rotational forceps delivery an epidural or spinal is preferred.
"Spine of the ischium, spine of the needle": the ischial spine is the landmark for the pudendal block and also the marker of station zero, below which instrumental delivery becomes possible.
General anaesthesia in obstetrics
Most planned caesarean sections use a spinal with the mother awake; an epidural already in place can be topped up for an emergency section. General anaesthesia is reserved for contraindicated or failed regional block, or extreme urgency.
Extra hazards in a pregnant woman:
- Difficult or failed intubation from airway oedema, large breasts and weight gain.
- Acid aspiration (Mendelson syndrome): suspect it when there is a spiking temperature with wheeze, breathlessness or hypoxia after a general anaesthetic. Reduce risk with antacids, an H2 blocker or proton pump inhibitor, no solid food where general anaesthesia is likely, cricoid pressure and a rapid sequence induction.
- Aorto-caval compression: a left lateral tilt reduces hypotension and protects placental perfusion.
- Uterine atony: the halogenated volatile agents relax the uterus in a dose-dependent way, useful for intrauterine manipulation or a retained placenta but a cause of heavier bleeding after delivery. Nitrous oxide has little effect on the uterus.
- Drugs that cross quickly, such as thiopental, can sedate the baby; highly ionised relaxants such as suxamethonium barely cross.
- Antacids should also be given to any woman with risk factors or who has had opioid analgesia.
- In asthma, regional is preferred to general anaesthesia, and ergometrine and prostaglandin F2 alpha are avoided because they constrict bronchi.
In severe pre-eclampsia with normal clotting, epidural anaesthesia is positively useful because it helps control blood pressure; if the platelet count is low, regional block is avoided and delivery is managed another way.
Expect: the ligamentum flavum as the layer before loss of resistance and L2-L3 to L4-L5 as the insertion level; the absolute contraindications of coagulopathy, sepsis and hypovolaemia; the purpose of the test dose and what a positive one means; management of epidural hypotension with fluids, left lateral tilt and ephedrine; spinal headache features and the blood patch; total spinal as an emergency; bladder care; epidural lengthening the second stage and raising instrumental delivery without raising caesarean rates; Entonox as an equal nitrous oxide and oxygen mixture; pethidine side effects including delayed gastric emptying; the ischial spine as the pudendal block landmark; spinal as the standard for caesarean section; and failed intubation with acid aspiration as the dangers of general anaesthesia.
Quick revision
- Epidural is the most reliable labour analgesia; dilute bupivacaine plus an opioid.
- Insert at L2-L3 to L4-L5; ligamentum flavum is the final resistance.
- Absolute contraindications: coagulopathy, sepsis, hypovolaemia.
- Test dose first; leg weakness means the catheter is subarachnoid.
- Hypotension: fluids, left lateral, ephedrine; never let her lie supine.
- Dural puncture gives a postural headache; treat severe cases with a blood patch.
- Catheterise to avoid permanent detrusor damage.
- Spinal is faster, denser, smaller dose; it is the standard for caesarean section.
- Combined spinal-epidural gives fast onset plus top-up.
- Pudendal block is placed behind and below the ischial spine.
- General anaesthesia risks failed intubation and acid aspiration; volatile agents relax the uterus.
Written by The House Job from standard references (Obstetrics by Ten Teachers; Basic and Clinical Pharmacology by Katzung).
FAQ
Why is an opioid plus local anaesthetic used in an epidural rather than local anaesthetic alone?
The opioid allows a lower concentration of local anaesthetic, which means less leg weakness and less hypotension while keeping good pain relief.
How is an epidural blood patch supposed to work?
A small volume of the woman's own blood injected into the epidural space clots and seals the dural leak, stopping the loss of cerebrospinal fluid that causes the headache.
Does having an epidural mean a caesarean section is more likely?
No. The evidence shows no rise in caesarean rates, though the second stage is longer and instrumental delivery is more likely.
Why is a left lateral tilt used at caesarean section?
It takes the weight of the uterus off the great vessels, so cardiac output and placental perfusion are maintained and hypotension is less severe.