Vagal nerve stimulators and anaesthesia: 1

We read the recent letter (Tovey & Griffiths. Anaesthesia 2001; 56: 703–4) with great interest. Over 11 000 devices have now been implanted worldwide, so anaesthetists will increasingly encounter them. Five previous studies have shown a low incidence of early postoperative complication with one pneumonia and one aspiration pneumonitis causing death being reported [1]. One study has shown an incidence of coughing and dyspnoea, one fatal seizure, two complaints of hyperaesthesia over the stimulator box, a wound infection, one wound resuturing and a Horner's syndrome [2]. However, little regard has been taken of the immediate peri-operative phase. We would like to describe the experience of King's College Hospital (London) anaesthetic department with neuroscience patients undergoing vagal nerve stimulator (VNS) procedures. We have records of 125 general anaesthetics administered for VNS surgery over a period of 6 years. The experience of the neurosurgeons of King's totalled 106 patients undergoing VNS insertion. During this period, we have encountered one serious incident associated with anaesthesia, an episode of asystole, from which the patient recovered uneventfully after stopping the stimulator. Also, a case of periodic apnoea encountered in the outpatient clinic, which recovered after deactivating the device. The stimulator is tested and then programmed once implanted under the skin of the chest wall, the left vagus nerve is stimulated through the pair of electrodes in the neck. The electrical signal generated is of variable intensity; 0.3-ms pulses of 30 Hz delivering a current of 1.0–2.75 mA has been studied [3]. Less than 1.5 mA may be desirable for chronic treatment in epilepsy. A recommended regimen has been for a 30-s stimulation period and then 3 min without stimulation. The clinical response may be improved by further adjustment [4]. Stimulation at various frequencies has no noticeable effect on electroencephalographic (EEG) activity regardless of whether the patient was under general anaesthesia, awake or asleep [5]. In spite of voltage in the neck, which can cause muscle paralysis locally, monitoring artefact has not been encountered as a problem during anaesthesia. A magnet placed over the unit deactivates it, with the current recommended implant techniques; the magnetic field of an MRI scanner will also deactivate the pulse generator [6, 7]. It is recommended that insertion of central venous catheters in the left internal jugular vein, and the use of unipolar diathermy is avoided. Should external cardiac defibrillation be necessary, similar precautions as with a cardiac pacemaker are probably appropriate. The battery life quoted for devices is 4–6 years, after which it needs to be changed. Current devices may have a longer life (10 years) and can register the remaining battery energy. The vagal nerve stimulator is considered a safe treatment for refractory seizures. From an anaesthetic viewpoint, however, the anticonvulsant therapy may be changed little and is still an essential part of peri-operative management.