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Subtotal thyroidectomy for thyroid goitre is a commonly performed surgery. Usually general anaesthesia for such cases does not pose any problems for the anaesthesiologist, but occasionally thyroid gland of a large size, retrosternal extension or a malignant thyroid may compress or push the underlying trachea, causing narrowing of its lumen and resulting in a compromised airway [1, 2]. If this goes undiagnosed preoperatively, the anaesthesiologist would undoubtedly face problems during tracheal intubation with dangerous consequences.
This report describes a case of medullary carcinoma of the thyroid where difficulty in securing airway during surgery under general anaeshesia (GA) was observed.
A 26-year-old male patient, a case of medullary carcinoma thyroid was listed for near total thyroidectomy. The patient was asymptomatic and ambulatory. Manipulation of the swelling would induce a bout of cough. The trachea appeared central in position. Systemic examination was normal. Airway was Malampetti class I . All the laboratory investigations were normal. Radiograms of neck and chest were normal. Indirect laryngoscopy revealed normally moving vocal cords. He was accepted for near total thyroidectomy under GA in ASA grade I.
The patient was premedicated with atropine 0.6 mg and morphine 7.5 mg intramuscularly 45 minutes prior to surgery. Anaesthesia was induced with a sleep dose of thiopentone followed by 100 mg suxamethonium intravenously to facilitate intubation. There was no difficulty in laryngoscopic visualisation of vocal cords but a Magill cuffed tracheal tube of adult size, 8.5 mm internal diameter (ID), could not be passed beyond the vocal cords. With subsequent attempts using additional doses of suxamethonium, intubation could be achieved only by gentle screwing movements of a cuffed tube of size 5 mm ID. Prior attempts to pass the tracheal tube of size 7, 6 and 5.5 mm ID had been futile. Following intubation, anaesthesia was maintained with conventional balanced anaesthesia technique using Mapleson D breathing circuit initially and FIO2 of 0.4. The patient's respiration was controlled using pancuronium. During operation pulse rate and BP were monitored. Arterial oxygen saturation (SaC2) and end tidal CO2 (ETCO2) were also monitored continuously using a pulse oximeter (Biox 3700) and capnograph (Ohmeda 5200). Though SaO2 remained within the normal range of 97–100%, the ETCO2 remained persistently elevated between 55–60 mm Hg indicating hypercarbia. So the fresh gas flow in D circuit was stepped up, in increments, from 5 to 8 and ultimately 10 L/min followed by attempted hyperventilation. But due to narrow size of the tube it was not possible to hyperventilate and reduce ETCO2. Subsequently the breathing circuit was changed to Mapleson A with a Mitochell's valve using slow rates of ventilation (10–12 breaths/min). With this non-rebreathing system ETCO2 could be maintained between 40–45 mm Hg throughout the operation.
During surgery the growth was found to be infiltrating in and around the trachea and into the oesophagus resulting in marked compression of the trachea. Palliative resection of the growth was carried out and tracheostomy was performed below the level of compression of trachea. Subsequently anaesthesia was maintained through an adult size tracheostomy tube with a fresh gas flow of 5 L/min. During the remaining part of operation ETCO2 fell to normal (35–37 mm Hg).
At the end of surgery neuromuscular block was reversed with atropine and neostigmine. Recovery and postoperative period were uneventful.
Whenever a patient of thyroid goitre especially a malignant one presents for surgery, the anaesthesiologist must be alert to the possibility of airway obstruction. Careful clinical examination along with radiography of thoracic inlet and CT scan usually help in preoperative diagnosis of tracheal compression or displacement. In this case a important radiological finding has been missed preoperatively. It was only at the time of intubation when difficulty was experienced in negotiating the larger endotracheal tube beyond vocal cords, that we realised the possibility of a narrowed and deviated trachea. A re-examination of the neck radiogram showed the narrowing of tracheal lumen and its displacement to the left in the region of the 7th cervical vertebra to 2nd thoracic vertebra (Fig. 1). In such cases, even if the airway appears normal preoperatively, the anaesthetist must keep the possibility of a difficult airway in mind and take all precautions in anaesthetic management. He must pre-oxygenate these patients to avoid hypoxia during difficult intubation. He must also keep endotracheal tubes of smaller sizes at hand. It is also preferable to use armoured tubes to prevent obstruction of the tube during surgical manipulation of the tumour . Tracheomalacia and its attending complications also should be kept in mind.
In this case oxygenation of the patient was well maintained due to high percentage of oxygen in the inspired mixture but inadequate expiration through narrow tracheal tube led to hypercarbia in the initial stages of anaesthesia. This was corrected by changing over to a non-rebreathing circuit and a low respiratory rate and allowing sufficient time for complete expiration. Immediately after tracheostomy ETCO2 came to normal. Jet ventilation would have been an alternative means of ventilation through the narrow tracheal tube.
This case highlights the importance of careful preoperative assessment of the airway in patients with thyroid goitre.