Peripheral vascular disease
Acute vascular disorders: post-traumatic vasospasm; acute arterial or venous occlusion; cold injury; inadvertent intra-arterial injection of drugs - for example, thiopentone or contaminated drugs of abuse.
Chronic vasospastic conditions: Raynaud’s syndrome; acrocyanosis; livedo reticularis; and sequelae of spinal cord injury or disease (e.g. polio).
Chronic obliterative arterial diseases: thromboangiitis obliterans (Buerger’s disease); and atherosclerosis.
Perioperative purposes: microvascular surgery; arteriovenous fistula formation for dialysis.
Mechanism Of action
Guanethidine (sympathetic blocking drug) is rapidly taken up from the venous system by the peripheral nerves during the 20 minutes that a tourniquet is applied to the affected limb. It has an immediate action locally on the peripheral sympathetic nerves. During the next 24 hours, the guanethidine is transported by the peripheral nerves to the dorsal root ganglion in the spinal cord, where it contributes further to the sympathetic block centrally.
The block should be performed in an area with suitable resuscitation equipment, and where there are trained assistants available. The patient should be fasted for 4 hours beforehand. Two intravenous lines are inserted, one into the affected limb, and one in the back of a hand that is spare. The first line is used to inject the guanethidine/local anaesthetic mixture, and the second line is there to act as emergency venous access, should the need arise. Vital signs monitoring is used throughout to measure heart rate, blood pressure and oxygen saturation. Intravenous sedation is used for patient comfort, and can be supplemented with Entonox. The upper part of the affected limb is wrapped in gauze to protect it, and a tourniquet applied. Excessive blood is gently squeezed out of the affected limb by using an exsanguinator. The tourniquet is inflated to about 100 mmHg higher than the patient's systolic blood pressure and held at that pressure by a gas-powered machine.
The guanethidine mixture is injected slowly into the exsanguinated arm via the intravenous line, and the tourniquet is kept in place for 20 minutes.
Commonly used mixtures
Arm - guanethidine 20 mg + 25 ml 0.5% lidocaine or 0.5% prilocaine
Leg - guanethidine 30 mg + 30 ml 0.5% lidocaine or 0.5% prilocaine.
While the touniquet is inflated, the affected limb will go slowly numb from the combination of the local anaesthetic and the lack of blood flow. After 20 minutes, the tourniquet is slowly deflated, allowing the blood to return to the affected limb. During this time, all the guanethidine and local anaesthetic has been absorbed by the peripheral nerves.
Should the tourniquet deflate before 20 minutes, and before all the local anaesthetic and guanethidine has been absorbed, then significant amounts of both drugs can escape into the systemic circulation. Depending on the dose, this can cause hypotension (guanethidine) or cardiac arrest (lidocaine).
Should the injection pressure rise too high when the mixture is initially injected, some of the drug can escape into the systemic circulation through blood vessels that travel through bone, and therefore not be prevented by the tourniquet. Thus, it is important not to inject too quickly or with too high a volume.
Unstable asthmatics can develop severe wheezing on tourniquet deflation, due to the effects of traces of guanethidine leaking into the circulation and causing bronchoconstriction of the airways. This is usually managed with intravenous salbutamol and steroids.
Allergic reactions can occur to guanethidine, causing an itchy rash in the affected limb, mostly settling spontaneously, but with some requiring treatment with intravenous steroids and antihistamines.