Myasthenia gravis is an autoimmune disorder of the motor endplate. Medical treatment of myasthenia gravis includes improving neuromuscular transmission by anticholinesterases, suppressing the immune system by corticosteroids and immunosuppressants, and decreasing circulating antibodies by plasmapheresis . Myasthenia gravis is frequently associated with morphological abnormality of the thymus gland. Patients with generalized myasthenia and patients with ocular symptoms poorly controlled by anticholinesterases often benefit from thymectomy . After transsternal thymectomy, postoperative pain, analgesics and residual effects of anaesthetics can adversely affect pulmonary function which is already limited by the myasthenia gravis itself. This report describes the anaesthetic management of two patients with myasthenia gravis undergoing transsternal thymectomy using a high thoracic epidural and total intravenous (i.v.) anaesthesia with propofol and remifentanil.
In the last year two patients with myasthenia gravis were admitted to the Department of Thoracic Surgery for elective transsternal thymectomy. The first patient was a 32-yr-old female. Thoracic computed tomography revealed a thymoma in the thymus gland. The patient had an Ossermann and Genkins classification score of IIa  and was taking 90 mg pyridostigmine daily. The second patient was a 25-yr-old female with hyperplasia of the thymus gland. This patient had an Ossermann and Genkins classification score of IIb and was taking 120 mg pyridostigmine daily. Preoperative blood chemistry, respiratory and thyroid function test were normal in both patients.
On the day of surgery each patient received their usual dose of pyridostigmine and a premedication of diazepam 5 mg orally. Prior to surgery, an i.v. cannula was inserted and an infusion of 10 mL kg−1 of Ringer's lactate started. A radial artery catheter was placed under local anaesthesia. Electrocardiogram (lead II), invasive arterial pressure, pulse oximetry and capnography were continuously monitored, recorded and stored in an IBM-compatible computer. An epidural catheter was placed at the T5-6 level and a test dose of lidocaine 2% 3 mL was given. After 5 min, the patient received bupivacaine 0.5% 5 mL with 50 μg fentanyl via the epidural catheter. Anaesthesia was induced after preoxygenation with an infusion of propofol 6 mg kg−1 h−1, remifentanil 0.5 μg kg−1 min−1 and atracurium 0.2 mg kg−1. Five minutes after induction, the patients were intubated. After intubation the infusion of propofol was decreased to 4.5 mg kg−1 h−1 and the infusion of remifentanil to 0.2 μg kg−1 min−1. The patients were ventilated with a 50% mixture of oxygen and air to maintain end-tidal CO2 between 4 and 4.5 kPa. Patients were haemodynamically stable during the whole surgical procedure. Ten minutes before termination of the surgical procedure, the patients received bupivacaine 0.25% 5 mL via epidural catheter. At the end of surgery, the patients received neostigmine 1.5 mg and atropine 0.5 mg and the infusions of propofol and remifentanil were stopped. Five minutes later the patients were fully orientated, had no pain and were breathing spontaneously. They were extubated and transferred to the intensive care unit (ICU).
In the ICU the patients were haemodynamically stable without any subjective or objective impairment of respiratory function (Table 1). Analgesia was excellent with bupivacaine 0.125% 5 mL and 1 mg morphine epidurally every 6 h. The patients were discharged from the ICU 24 h after surgery.
In patients with myasthenia gravis, the combination of high thoracic epidural analgesia (TEA) and general anaesthesia for transsternal thymectomy is recommended by many authors . Many authors combined TEA with light general anaesthesia using inhalation anaesthetics  or with i.v. general anaesthesia. Lorimer and Hall  used total i.v. anaesthesia with propofol and remifentanil for transsternal thymectomy in myasthenia gravis. The combination allowed excellent control of heart rate and pressor response during surgery, an early return to spontaneous ventilation and extubation within 9 min after termination of anaesthesia. Remifentanil is hydrolysed by non-specific tissue and plasma esterases, and its duration of action is not prolonged in patients with cholinesterase deficiency . As remifentanil is not a substrate for pseudocholinesterase the metabolism of remifentanil is not affected by a cholinesterase inhibitor . Baraka and colleagues  did, however, report a delayed postoperative arousal for 12 h following sevoflurane and remifentanil anaesthesia in a patient with myasthenia gravis undergoing thymectomy. Those authors believed that delayed postoperative arousal was due to prolonged remifentanil metabolism caused by pyridostigmine.
In our two patients we used a combination of high TEA and total i.v. anaesthesia with remifentanil and propofol. To our knowledge this combination has not yet been reported in the literature. High TEA suppresses hormonal and metabolic stress response to pain allowing stable haemodynamics during surgery and excellent postoperative analgesia without compromising pulmonary function. We combined this technique with light i.v. anaesthesia using remifentanil and propofol. In our opinion, although TEA alone offers very stable haemodynamics during sternotomy, spontaneous breathing during this procedure would be most uncomfortable for the patient. Due to the fast elimination of remifentanil it is possible to rapidly alter its concentration in plasma by altering the speed of infusion. This enabled us to adjust the depth of analgesia to the phase of the operation. Since adequate analgesia for surgery was achieved in our patients by the use of TEA, only light general anaesthesia was needed for the patient to tolerate the tracheal tube allowing adequate mechanical ventilation. Therefore we could lower the dose of remifentanil for general anaesthesia in our patients. This may be one of the reasons why we observed no clinically important differences in the duration of remifentanil action in our patients also receiving pyridostigmine in contrast to the experiences described by Baraka and colleagues . Excellent analgesia enabled us also to minimize the dose of atracurium to facilitate tracheal intubation during the induction of anaesthesia. After that, no additional muscle relaxant was given during surgery.
Our experiences show that the combination of high TEA and total i.v. anaesthesia with remifentanil and propofol is an effective technique for transsternal thymectomy providing haemodynamic stability during surgery followed by rapid awakening, a quick transition to spontaneous breathing, excellent postoperative analgesia and an uneventful recovery.
1Department of Anesthesiology, Intensive Care and Pain Management, Maribor Teaching Hospital, Maribor, Slovenia
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