37. INTRAVENOUS REGIONAL ANAETHESIA (IVRA)

Intravenous regional anaesthesia (Bier’s block) is a method of producing analgesia of the distal part of a limb by intravenous injection, while the circulation to the limb is occluded. It was first described by Bier in 1908.

It is suitable for any procedure on the arm below the elbow or on the leg below the knee that will be completed within 60 minutes (though operations of 6 hours duration have been described).

Intravenous regional anaesthesia is reliable, easily performed (no specific anatomical knowledge is required), safe, has a rapid onset (5 to 10 minutes), controllable duration of action (governed by the time the tourniquet is kept inflated), controllable extent of analgesia, rapid recovery, good motor blockade and no risk of infection.

A tourniquet must be used which introduces several disadvantages including tourniquet pain, being unable to produce analgesia of an entire limb, and the duration of surgery being limited by the time an arterial tourniquet is safe.

There is a risk of toxicity of local anaesthetic if the tourniquet suddenly deflates soon after the local anaesthetic has been injected. Local anaesthetic toxicity mainly affects the central nervous system and cardiovascular system. Because of the toxicity of bupivacaine, it should never be used for IVRA.

Contraindications

Intravenous regional anaesthesia must not be used in diseases for which tourniquets cannot be safely used, for example, severe Raynaud’s or homozygous sickle cell disease. It should be used with caution on limbs which have sustained crush injuries where a further period of hypoxia may theaten viable tissue or where there are extensive infections of the limb.

Intravenous Regional Anaesthesia

The patient should be fasted and the anaesthetist must check that there are no contraindications to IVRA or a local anaesthetic allergy.

Before performing IVRA the patient’s blood pressure should be measured and an intravenous cannula inserted in a distal vein of the other arm in case of any complications. An intravenous cannula should be inserted into the distal end of the limb to be anaesthetised. A pneumatic tourniquet is applied to the upper part of the limb. The cuff size should be 20% wider than the limb diameter (12 to 14 cm wide for the average adult limb). The tourniquet should never be placed on the forearm or lower leg, as adequate arterial compression cannot be obtained. (Ideally a tourniquet with a double cuff is used. The upper cuff is inflated first and IRVA performed. Once anaesthesia is established the lower cuff may be inflated over the now anaesthetised arm and the upper cuff deflated.)

A better block may be obtained if the limb is exsanguinated before the tourniquet is inflated. The usual method is to wrap a bandage snugly up the limb starting, where possible, just proximal to the needle. If the patient is unable to tolerate this, elevation of the limb for 30 seconds while applying firm digital pressure on the brachial (or femoral) artery is acceptable.

The tourniquet is inflated to a pressure 50 mmHg greater than the patient’s systolic blood pressure. Disappearance of a distal pulse will confirm an adequately high inflation pressure.

As the local anaesthetic is slowly injected, the skin usually becomes mottled and the limb may start to feel hot. If sufficient analgesia is not present by 5 to 10 minutes, a further bolus of local anaesthetic may be required.

The cuff must remain inflated for a minimum of 20 minutes. At the completion of surgery the tourniquet is deflated and normal sensation quickly returns. At this time adverse reactions may occur. the patient should be warned about transient generalised paraethesia and sometimes ringing in the ears (tinitus).

Local Anaesthetic Agents

The drug of choice for IVRA is prilocaine (low toxicity, largest therapeutic index). Cloroprocaine is the least toxic local anaesthetic, however it has a high incidence of thombophlebitis. Lignocaine is an acceptable alternative however patients may experience a greater incident of transient tinnitus and general paraesthesia. If prilocaine is not available, then lignocaine is a very safe alternative.

Using 0.5% lignocaine in a dose of 2.5 mg/kg and releasing the tourniquet after only 5 minutes, the maximum levels of lignocaine in venous blood do not exceed 2 µg/ml. (The venous blood level of lignocaine that causes convulsions is 10 µg/ml).

The clinical profile of prilocaine is similar to lignocaine. It has a relatively rapid onset, intermediate duration of action and profound depth of conduction blockade. It causes significantly less vasodilatation than lignocaine so can be used without adrenaline. (In general, the duration of action of lignocaine with adrenaline is equal to plain prilocaine). The main advantage of prilocaine over lignocaine is its significantly decreased potential for producing systemic toxic reactions. Prilocaine is approximately 40% less toxic than lignocaine.

The major deterrent to the use of prilocaine is related to the formation of methaemoglobin. In general, doses of prilocaine of 600 mg are required before the development of clinically significant levels of methaemoglobin. The formation of methaemoglobin is believed to be related to the degradation of prilocaine in the liver to O-toluidine, which is responsible for the oxidation of haemoglobin to methaemoglobin. The methaemoblobin is spontaneously reversible or may be treated by intravenous methylene blue (1 mg/kg).

Bupivacaine is contraindicated. Adrenaline containing solutions must be avoided in IVRA.

Dosage

A suitable dose for anaesthesia of the arm is 40 ml of 0.5% prilocaine (or 0.5% lignocaine). The maximum recommended dose for a 60 to 70 kg patient is 400 mg prilocaine or 250 mg lignocaine.