Labour is well recognised as a painful process, and is probably one of the most painful experiences reported by many women.
The different stages of labour have different mechanisms for pain.
The first stage of labour has pain arising from uterine contractions. The intensity of contraction results in a degree of myometrial ischaemia, with release of pain mediators such as bradykinin and serotonin. There is also stimulation of mechanoreceptors as the lower uterus and cervix are stretched. These stimuli transmit nociceptive impulses in sensory fibres that accompany the sympathetic nerve fibres, ultimately reaching the spinal cord at levels T10 - L1.
The second stage of labour involves more involvement of the perineum, as this is stretched. Somatic sensory nerve fibres transmit the nociceptive signals, being transmitted via the pudendal nerve and ultimately joining the spinal cord at the S2-4 roots.
Pain also has a strong psychological component and there are multiple factors which contribute to this pain in labour, which should be understood. Factors which may impact on pain include:
Psychological preparation and expectations
Induction/augmentation of labour
An occipital-posterior lie of the foetus
There are several common pharmacological options available for labour analgesia:
Regional anaesthesia (epidural, spinal or combined)
There are also a number of non-pharmacological options that may be encountered. The evidence that these are effective is poor, and they will not all be discussed in detail, but are worth being aware of:
A 50/50 combination of gas and nitrous oxide that has been commonly used in labour. It is most commonly used as a patient controlled technique rather than continuous - again the control that this provides appears to confer additional analgesia/satisfaction. Analgesia onset occurs at a maximum after 45 to 60 seconds, and thus inhalation needs to begin at the start of a contraction, and stop at its peak. The analgesia provided is often significant but never complete. Side effects can include nausea, dizziness, and dysphoria, and its effectiveness does have some variability amongst patients. There are also some theoretical concerns about the risk to staff from the lack of scavenging, and thus frequent exposure.
Transcutaneous electrical nerve stimulation is thought to help block nociceptive impulses at the low thoracic level, by a ‘gate control theory’ mechanism. It is also postulated to trigger release of endorphins to provide central analgesia. There is little evidence that it works.
Opioids are the main class of systemic analgesia used in labour. They have the commonly associated side effects associated with opioids, including nausea, dysphoria, drowsiness, and respiratory depression. Due to their physicochemical properties they all cross the placenta and can cause effects in the newborn baby, of most interest being respiratory depression. Opioids used can include remifentanil, fentanyl, pethidine, and diamorphine. The more specific details of opioid pharmacology are discussed elsewhere.
Remifentanil This is a particular opioid that is worth discussing in further detail. Remifentanil has several pharmacological properties that make it well suited to labour analgesia:
Potent mu receptor agonist
rapid onset (30-60s)
Rapid breakdown, even in the foetus via tissue esterases
Context insensitive half life (approx 3 mins)
The downside also relates to its high potency, particularly its risk of respiratory depression. It has been used in different ways as a labour analgesia agent, including as an infusion, and as a PCA. The PCA option is the most commonly used option. It is quoted as having a better analgesia profile than other opioids. The control that a PCA brings, also contributes to increased satisfaction with this method.
The theory behind a remifentanil PCA is that the rapid onset and offset profile of the drug can allow it to be targeted to just the period of severe pain, i.e. during the contractions. Careful set -and conduct of the technique is important to maintain safety.
Clear explanation and consent process with the patient
Button needs to be pressed right at the start of a contraction
Provides pain relief, not complete removal of pain
Close monitoring is required
One-to-one observation (midwife) must be established
Dedicated IV access
No opioids within the preceding 4 hours
Continuous SpO2 monitoring
Nasal oxygen if SpO2 is <94% on room air
The standard PCA set-up is of a 40 microgram bolus with a 2 minute lockout
Pethidine Also known as meperidine, it is commonly used drug from a historical perspective (made legally available for midwives to use independently in 1950). A synthetic opioid closely related to fentanyl. Commonly given at a dose of 1mg/kg (up to a max of 150mg). It hasn’t remained a popular analgesia agent and probably provides more sedation than analgesia. Its efficacy is rated higher by midwives that by patients. As well as the normal adverse effects of opioids, on of the metabolites lowers the seizure threshold, and shouldn’t be used in patients with pregnancy induced hypertension. It crosses the placenta, and because of the lower pH of the foetus has a predisposition to accumulate there, resulting in increased drowsiness and respiratory depression in the neonate.
Diamorphine The theoretical benefits of diamorphine compared to morphine, are a faster speed of onset and rapid elimination by the placenta. There is some impression that it has improved efficacy compared with pethidine or entonox.
Several options are available for providing regional analgesia in the obstetric patient. The advantages of regional technique include the minimisation of a depressant effect on the mother or foetus, which is a common adverse effect of nearly all the other options.
The anaesthetic options include:
Lumbar sympathetic block (rare)
Regional techniques that the obstetric team may employ, include:
Local perineal infiltration
The process is similar to in many other cases of regional anaesthesia, with a few variations due to pregnancy.
Obstetric disease where it can be beneficial e.g. pre-eclampsia
Maternal disease where it can be beneficial e.g. some forms of cardiovascular disease
Conditions where a GA is particularly risky e.g. severe obesity, significant respiratory disease
Anticipated operative delivery e.g. multiple pregnancy
Many of the benefits that come from the use of the regional technique in disease states, is from the obtundation of the high sympathetic activity that is associated with the pain and stress of labour.
INR > 1.4
Platelets < 80 x 10^9/L
Still a clinical decision, given the well known discrepancy between lab values and actual clotting capacity
Allergy (though true allergy is rare)
High risk of significant haemorrhage
Untreated systemic infection (concern of propagation local infection)
Previous back surgery (may result in impaired epidural flow of local anaesthesia if there is scarring)
Specific cardiovascular disease e.g. aortic stenosis (although may be used in careful, skilled hands)
The use of lumbar epidural analgesia has a well recognised role in labour analgesia. When working well, ts advantages include:
High quality analgesia (the most effective form)
An option for rapid conversion to anaesthesia e.g. for c-section
Minimal motor block
Reduction in maternal sympathetic ANS activity
A range of contraindications (though the majority being relative)
Technically challenging in some cases
Inadequate function in some patients.
The Process Preparation is important. Gaining adequate consent from the patient is important. Ideally, the benefits and risks of an epidural will have been discussed with the patient prior to labour, as the pain of labour can make effective and informed consent difficult. However, every effort must be made to optimise this. A focused history is important to ensure that there are no contraindications or other risks to conducting the procedure. Very careful attention must be paid to sterility throughout the procedure.
Other important checks before proceeding include:
Patient monitoring ready - NIBP, HR, SpO2
Foetal heart rate monitoring
Optimising positioning - sitting is often easier
The details of inserting an epidural catheter are found elsewhere. The key steps are.
Aseptic preparation - hat, mask, meticulous hand-washing, gown, gloves.
Skin sterilisation - 0.5% chlorhexidine is recommended - must be left to dry and contact with gloves avoided (due to its neurotoxicity)
Application of sterile drapes
Identification of landmarks (midline, spinous processes, level)
Local anaesthesia of skin and deeper tissues - usually with 1% lidocaine
Identification of the epidural space
Use of a loss of resistance technique. Saline is recommended
A note made of the depth (by cm marking on the tuohy needle)
Introduction of 4-5cm of epidural catheter into space (depth of space + 5cm)
This length is associated with reduced risk of unilateral block (if too far) and reduced risk of displacement (if too short)
First check of position through ‘falling meniscus’ test.
Relative negative pressure of epidural space causes the saline in the catheter to drop as the end is lifted
Aspirate catheter for blood or CSF
Administration of a ‘test dose’.
Further test to minimise risk of subarachnoid placement of catheter
Should be a suitable anaesthetic dose to induce a ‘spinal anaesthetic’ e.g. 10ml 0.1% bupivacaine, which can act as the loading dose too.
Assessment of vital signs and for any motor or sensory block that may indicate a subarachnoid catheter
Suitable affixation of catheter
Commencement of maintenance therapy
Further initial loading may be needed to establish analgesia
The common local anaesthetic agent is ‘low dose’ 0.1% bupivacaine with 2 mcg/ml fentanyl
Maintenance There are several options available to maintain analgesia once an epidural is established:
Patient controlled epidural analgesia (PCEA)
Intermittent bolus by midwives
PCEA is a commonly used option. A PCA pump button is given to the patient, giving an epidural bolus of 5ml of the chosen epidural local anaesthetic agent, usually with a 20 minute lockout. This can also involve a continuous low rate background infusion as well, commonly 5ml/hr. The control that this gives the patient is a positive factor for satisfaction with labour analgesia. Ideally, the titration can allow optimisation of the analgesic effects, with minimisation of the non-nociceptive nerve blockade e.g. motor.
Continuous infusions are also an option, as is more traditional with postoperative epidural analgesia. This may be adjusted based on analgesia. A starting rate may be 5ml/hr of 0.1% bupivacaine.
Complications These should be discussed with the patient as part of the consent process. The Obstetric Anaesthetists’ Association (OAA) have very useful information sheets that provide a numerical rating to this risk : http://www.labourpains.com/UI/Content/Content.aspx?ID=43 This information sheet can be provided to the patient to read if there is time. The important side effects and risks to discuss include:
Failure to work fully - 1 in 8
Hypotension - 1 in 50
Severe headache - 1 in 100
Temporary 1 in 1000
Permanent 1 in 13,000
Severe (e.g. paralysis) - 1 in 250,000
Epidural abscess 1 in 50,000
Meningitis 1 in 100,000
Bleeding with haematoma - 1 in 170,000
Unconsciousness - 1 in 100,000
It can also be important to note these important misunderstandings when thinking about complications (e.g. in the consent process)
No increase in the risk of cesarean section
No association with long term back pain
Failure The technical skill of epidural catheter insertion can be difficult, particularly in patients with increased adiposity that may obscure the relevant landmarks. Whilst this can lead to failure, due to considering another analgesia technique, there are also a number of epidurals which have inadequate analgesia, despite apparent successful insertion. In general, if there is complete failure of the epidural with no detectable block despite an adequate loading dose (10ml 0.25% bupivacaine or equivalent), then the catheter is completely misplaced and needs removing and the procedure repeating.
However, there are commonly situations of partially working epidurals which require manipulation to get analgesia. Assessment should be a clinical review of the effect of block by testing motor power, warmth of skin (vasomotor inhibition) and loss of cold touch. This may reveal a particular pattern of failure:
Unilateral - can try:
Top up with spared side in dependent position
Withdraw catheter 2-3cm
Missed segment - rare, and usually actually unilateral
Top up with 50 - 100 mcg of fentanyl
Perineal pain - due to more sacral root transmission
May need catheter
Top up in sitting position
There can also be particularly bad back pain when the baby has an abnormal lie e.g. OP. This can be difficult to get full analgesia, and may require a fairly dense block.
Dural puncture This is a recognised complicated of neuraxial anaesthesia and can lead to a post dural puncture headache (PDPH). The size of the needle used for introducing the epidural catheter (16g) means that inadvertent dural puncture has a very high chance of leading to PDPH.
The pathophysiology is one of ongoing CSF leak which exceeds CSF production. As the CSF pressure falls, there is repositioning of the brain and stretch of the meninges. The headache classically has the following features:
Positional (worse on standing) - a key differentiating factor
They will often start in the first 24-48 hours after the dural puncture, and can last for up to 10 days.
Immediate management If noted at the time of epidural insertion, it may be obvious as there will be CSF pouring out of the tuohy needle. The goal is to still provide analgesia without resulting in further complication (although the dural tear has now happened). A good option is to proceed with an intrathecal catheter:
Insert the catheter along the needle into the subarachnoid space
Use the catheter for intermittent top ups
E.g. 1ml 0.25% bupivacaine +/- 5-10mcg fentanyl
Only the anaesthetist can perform this
The catheter must be clearly labelled as intrathecal
This will almost certainly provide excellent analgesia. If the catheter is left in for over 24 hours then it may actually reduce the rate of PDPH.
Another option is to remove the needle and retry insertion of an epidural catheter at a different space. There may be risk of intrathecal seepage of the infused epidural anaesthesia and so careful monitoring must be taken.
Subsequent management Simple advice can be given in the event of a PDPH:
If persistent and the symptoms severe, the management of choice is an epidural blood patch.
Epidural Blood Patch This is a technique for treating PDPH. If performed at 48 hours it has a quoted success rate of 60-90%. It involves injecting a sample of the patients blood into the epidural space around the puncture site. Theoretically this will clot, blocking the hole, as well as raising CSF pressure due to the effect of the blood volume compressing the subarachnoid space (and thus improving symptoms). The patient must be apyrexial and without a raised WBC count.
This requires two operators:
Consent of the patient and a clear explanation beforehand
Strict aseptic maintained by both operators throughout.
This epidural space is identified by the usual technique
The level is usually at the same or level below the initial puncture (as the blood tends to spread cephalad)
Then 20ml of blood is taken from the patient at a peripheral site
This blood is carefully injected into the epidural space
The aim is for the whole 20ml
If pain develops (usually back or legs), stop injecting, and can try continue if the pain resolves
Patients must have bed rest for 2 hours and avoid excessive activity for 48 hours
Usually just for 48 hours, but can be several weeks
Neurological deficit (rare)
Links & References
Birnbach, D. Browne, I. Anaesthesia for obstetric, in: Miller’s Anaesthesia (7th ed).
Allman, K. Wilson, I (eds). Oxford handbook of anaesthesia (3rd ed). 2012. Oxford University Press