The purpose of the epidural test dose is to detect intravenous or subarachnoid placement of the catheter. As such, it must be formulated to produce an easily detected result when in one of these two situations, without compromising the safety of the mother or the fetus.
Problems and special considerations
As with all screening procedures, the sensitivity and specificity of the test dose are allimportant when assessing its ability to protect the mother from the unwanted effects of intravenous or subarachnoid administration. Sensitivity is the ability of the test dose to detect accurately a misplaced catheter; failure to do so represents a false-negative result. Specificity is the ability of the test dose not to produce a false-positive result, i.e. incorrectly alerting the anaesthetist when the catheter is actually in the right place.
Accidental catheterisation of the epidural veins is common in obstetric practice, since these vessels act as collateral conduits of venous blood from the lower limbs to the heart and are therefore dilated when the inferior vena cava is compressed by the gravid uterus. Venous cannulation occurs about 1 in 20 times and is usually detected before the test dose is given, by the ability to aspirate blood.
Subarachnoid puncture occurs about 1 in 100 times, with the incidence decreasing as the practitioner becomes more experienced. Again, the vast majority of dural punctures are detected by the free flow of cerebrospinal fluid through the needle or catheter.
It follows that it is very rare for a test dose to be used in a situation where the catheter is actually intravenous or subarachnoid. This means that, in addition to the sensitivity and specificity of the test, it is also important to know what proportion of positive test results will actually indicate misplacement of the catheter; this is known as the positive predictive value of the test, and it is often lower than might be expected from the sensitivity and specificity.
A test dose, designed to improve safety, should not in itself compromise the safety of the mother or fetus. This is a particular problem when trying to detect intravenous placement, as many tests rely on the use of adrenaline to produce tachycardia in these circumstances. Intravenous adrenaline may stress the maternal cardiovascular system and temporarily reduce placental perfusion, so doses must be carefully chosen.
Although initial confirmation of the correct placement of the epidural catheter is particularly important, the possibility of later catheter migration should not be forgotten. Every top-up given down an epidural catheter that might be dangerous if accidentally injected intravenously or intrathecally should really be regarded as a test dose and should be fractionated if time permits, so as not to produce systemic toxicity or a dangerously high block.
It is difficult - if not impossible - to design a test dose that will detect subdural, extraarachnoid placement with any reliability. The possibility of subdural catheterisation must always be borne in mind and suspected when an unusual block pattern emerges.
Management options
Subarachnoid placement
This is usually relatively easy to detect, and it is only necessary to choose a dose of local anaesthetic that will produce a recognisable but safe block. Lidocaine 45-60 mg or bupiva- caine 7.5-12.5 mg is suitable; lidocaine has the advantage of a slightly faster onset of block, but many practitioners prefer bupivacaine on the grounds that this will be the drug used for the main dose. Signs of sensory block in the lower lumbar segments and motor block of the legs should be sought after 3 minutes with lidocaine or 5 minutes with bupivacaine. This test is regarded as close to 100% specific and sensitive.
Intravenous placement
Most tests rely on the use of sympathomimetic drugs to produce changes in maternal heart rate and/or blood pressure when administered intravenously. Adrenaline 15 µg has been recommended for this purpose. Intravenous injection of adrenaline has been shown to have a low positive predictive value (55-75%) and it has been associated with significant side effects. Even when an adrenaline-free bupivacaine test dose is used, 12% of patients will demonstrate a rise in heart rate that would count as a positive response. The sensitivity is also low but can be improved by ensuring that the heart rate is measured between contractions and by using the change in peak heart rate (as measured over a 2-minute period before and after the test injection), rather than a simple change from a random baseline rate, as the basis of the test.
Subjective symptoms have also been used to detect intravenous placement, and some test-dose regimens rely on this. Fentanyl 100 µg produces obvious sensations of drowsiness within 1 minute when given intravenously, and lidocaine 45-60 mg may cause tingling in the perioral region. These methods are far from reliable, and the increased sensitivity of pregnant women to local anaesthetic toxicity should always be borne in mind when using the systemic effects of lidocaine as a test.
Best practice
It is important to consider the possibility of a misplaced epidural catheter before every top-up. Careful aspiration via the catheter before administering a dose of local anaesthetic and a continuous high index of suspicion are vital adjuncts to an effective testdose policy.
Since no single regimen has been proven to be superior, many obstetric anaesthetists therefore recommend abandoning a formal test dose, instead fractionating all epidural doses. The current practice of using low concentrations of local anaesthetic (usually 0.1-0.125% bupivacaine) means that accidental intravenous or subarachnoid administration is unlikely to produce any serious adverse maternal or fetal effects.
Midwives should be trained to recognise the early signs of local anaesthetic toxicity and fractionate all doses, to ensure safety if the catheter migrates through a dural tear.
Key points
• A test dose should be safe, with high sensitivity and specificity; high specificity is difficult to achieve when the condition being sought is uncommon.
• Accidental subarachnoid placement is relatively easy to detect with a test dose, while intravenous placement is more difficult to detect.
• Subdural placement cannot be reliably detected with a test dose.
• Catheter migration can occur, and every epidural top-up should be regarded as a test dose.
Further reading
Camorcia M. Testing the epidural catheter. Curr Opin Anaesthesiol 2009; 22: 336-40.
Guay J. The epidural test dose: a review. Anesth Analg 2006; 102: 921-9.