Sciatic nerve block in amputees

We read with interest the letter ‘problems performing a sciatic nerve block in an amputee’ (Lok & Kirk. Anaesthesia 2003; 58: 289–90) and the subsequent response (MacLeod et al. Anaesthesia 2003; 58: 612–13). We recognise the authors' experiences of technically difficult blocks in amputees. In such cases, we have utilised the motor response in the proximal calf of a below-knee amputee as an end-point for successful sciatic nerve block. We describe one example of this technique. A 54-year-old female was scheduled for refashioning of left below knee amputation stump as an elective procedure on a plastic surgery operating list. Her past medical history included myocardial infarction, seven years previously, and stable angina. She was on aspirin and clopidogrel, both of which she stopped taking a week pre-operatively. She had had previous general anaesthetics for left below knee amputation for peripheral vascular disease, and a revision of amputation. On both occasions, she suffered from severe postoperative nausea and vomiting (PONV). Our initial plan was to site a lumbar epidural awake, and a general anaesthetic. However, the patient declined an epidural because of concerns regarding epidural haematoma. She also refused peripheral nerve blocks whilst awake. Our aim was to provide her with optimum postoperative analgesia with minimum risk of PONV. After due consideration, it was agreed that the most suitable option was a general anaesthetic and a femoral and sciatic nerve block whilst under general anaesthesia. After full and frank discussion about the potential risks and benefits of the nerve block while under general anaesthesia, the patient consented to this approach. The general anaesthesia was provided by fentanyl, propofol and spontaneous ventilation with a size 4 laryngeal mask using oxygen, air and sevoflurane. The sciatic nerve block was performed by a posterior approach with the patient in the right lateral position. Under aseptic precautions, we used a 150-mm, 21-gauge pencil tip needle (Polymedic UPA) and a Polymedic Polystim nerve locator to perform the sciatic nerve block. We were seeking contractions of the proximal calf muscle to aid localisation of the sciatic nerve. Initially, we set a current of 3 mA and reduced to 0.7 mA to locate the nerve. After obtaining visible motor response in the proximal calf and negative aspirations for blood, 20 ml of ropivacaine 0.75% was injected in 5 ml aliquots. Femoral nerve block was performed with 10 ml of ropivacaine 0.375% using the nerve locator. The patient did not require subsequent opioid analgesia intra-operatively. She was also provided with patient-controlled analgesia (PCA) at the end of the procedure to cover the possibility of failure of the peripheral nerve block. We interviewed the patient 24 h later to assess the quality and duration of postoperative analgesia. She was virtually pain free during this period, and had not used her PCA at all. She was also free of PONV.