Local Anaesthetic and Diagnostic Blocks

A diagnostic block uses the simple principle that anaesthetising the source of pain should relieve it. A small volume of local anaesthetic is placed accurately into the suspected structure, such as a joint, bursa or nerve territory, and if the pain settles for the expected duration of the anaesthetic, that structure is likely to be contributing to the pain. This is especially useful when the source is uncertain, for example distinguishing pain arising from a particular joint from pain referred from elsewhere.

Interpretation is where judgement is needed. A positive block supports the diagnosis but does not prove it, because the anaesthetic can spread to adjacent structures, a placebo effect can produce apparent relief, pain can be referred from another site, and an incomplete block can give a misleading partial response. Accuracy is improved by image guidance, particularly ultrasound or, for spinal targets such as facet joint and medial branch blocks (which are specialist, image-guided pain procedures rather than routine clinic blocks), fluoroscopy, so that the anaesthetic reaches the intended structure and not its neighbours. Common examples include a subacromial block at the shoulder, a block at the greater trochanter, and an intra-articular diagnostic block. The choice of agent affects timing: lidocaine has a fast onset and short duration, which suits an immediate test, while bupivacaine, levobupivacaine and ropivacaine have a slower onset and a longer duration, which can be useful when a longer period of assessment is wanted.

The agents used are amide local anaesthetics: lidocaine, bupivacaine, levobupivacaine and ropivacaine. Adrenaline is sometimes added to prolong the effect and reduce systemic absorption; caution is traditionally advised near end-arterial areas such as the fingers, particularly where the peripheral circulation is impaired, although dilute adrenaline has been used safely in selected healthy digits, and local protocols are followed. Each agent has a maximum safe dose that is calculated by body weight and must not be exceeded. As an approximate guide, the maximum dose of lidocaine is in the region of three milligrams per kilogram when used plain and about seven milligrams per kilogram when combined with adrenaline, and the maximum for bupivacaine or levobupivacaine is in the region of two milligrams per kilogram. These figures are approximate and not universally applicable: the safe maximum varies with the exact product and concentration, the route and site, any combined local anaesthetics, and patient factors such as age, frailty, pregnancy, and cardiac or hepatic disease, so the current Summary of Product Characteristics or British National Formulary is checked before use.

Safe delivery reduces the risk of the main serious complication, which is local anaesthetic systemic toxicity. The key steps are to inject slowly and in small increments while watching the patient, to draw back before and during injection as one check for a vessel (recognising that a negative aspirate does not reliably exclude an intravascular position), to use the lowest effective dose, to stay within the maximum dose, and to use ultrasound guidance where appropriate. The clinician keeps talking to the patient and watches for early warning symptoms during and after the injection, and ensures that resuscitation equipment and lipid emulsion are available where potentially toxic doses are used. Careful attention to dose and technique means that toxicity is rare, but every clinician who injects local anaesthetic must know how to recognise and treat it.

Local anaesthetic systemic toxicity (LAST) results from too high a concentration of local anaesthetic in the blood, usually from an inadvertent intravascular injection or an excessive dose. It is recognised by a progression of features. Early symptoms affect the nervous system: numbness or tingling around the mouth, a metallic taste, ringing in the ears, light-headedness, agitation and confusion. As toxicity increases, seizures can occur, followed by cardiovascular effects including abnormal heart rhythms, low blood pressure, and in severe cases cardiovascular collapse and cardiac arrest. This sequence is not fixed: toxicity can present atypically, sometimes with sudden cardiovascular collapse and no warning neurological features, and it can be delayed rather than immediate. Because the early symptoms can be subtle, any unexpected neurological symptom during or shortly after a local anaesthetic injection is treated as possible toxicity.

Management follows a clear sequence. The injection is stopped immediately and help is called. Standard resuscitation is started, with attention to the airway, breathing and circulation, high-flow oxygen and effective ventilation, and seizures are controlled, usually with a benzodiazepine. The specific treatment is intravenous twenty per cent lipid emulsion, given as an initial bolus of about 1.5 millilitres per kilogram over one minute followed by an infusion, with further boluses if the circulation does not recover; the doses follow the national guideline, which is displayed with the lipid emulsion in clinical areas. Resuscitation may need to be prolonged, since recovery can take a considerable time, and it is modified to avoid giving further local anaesthetic and large doses of adrenaline, following the displayed current toxicity checklist; lidocaine is not used as an anti-arrhythmic drug in this setting. Prevention, through careful dosing and technique, remains the most important measure.

• •A diagnostic block supports, but does not prove, that the injected structure is the source of pain, because spread, placebo, referred pain and an incomplete block can all affect the response.
• •Accuracy is improved by image guidance; ultrasound is used for many targets and fluoroscopy for spinal facet and medial branch blocks.
• •Lidocaine has a fast onset and short duration; bupivacaine, levobupivacaine and ropivacaine have a slower onset and longer duration.
• •Local anaesthetics have a maximum dose by body weight (approximately three milligrams per kilogram for plain lidocaine, about seven with adrenaline, and about two for bupivacaine or levobupivacaine), but these are approximate and vary with the product, site and patient, so check the current Summary of Product Characteristics or British National Formulary; inject slowly and incrementally, use the lowest effective dose, and treat aspiration as only one imperfect check.
• •Local anaesthetic systemic toxicity often progresses from perioral numbness, tinnitus and agitation to seizures and then cardiovascular collapse, but it can present atypically or with sudden cardiovascular collapse and no warning symptoms.
• •Manage toxicity by stopping the injection, calling for help, giving oxygen with effective ventilation, controlling seizures, and giving twenty per cent lipid emulsion per the displayed checklist; use modified resuscitation, avoiding further local anaesthetic and large doses of adrenaline, and do not use lidocaine as an anti-arrhythmic.