Anaesthesia for Thoracic Surgery
Thoracic anaesthesia offers a number of anaesthetic challenges:
Control of the airway during bronchoscopy.
Protection of the airway in patients with oesophageal disease, lung abscess, bronchopleural fistula or haemoptysis.
Positioning a double-lumen tracheal tube to maintain safe anaesthesia in the lateral position with the chest opened and one lung collapsed.
Postoperative care of a patient after lung tissue resection.
In common with major surgery at other sites, thoracic patients frequently:
have parenchymal lung disease in addition to their presenting complaint
experience severe pain after surgery
are at risk of substantial haemorrhage
PREOPERATIVE ASSESSMENT
Investigations
Whole-Lung Testing
Predicted postoperative (PPO) lung function may be calculated using lung segments. From a total of 19 segments (three in the upper lobes, two in both the middle lobe and lingual and four in the left and five in the right lower lobes; Fig. 33.1), the fraction of lung remaining is multiplied by the preoperative spirometry measurement to give the predicted postoperative measurement of spirometry (PPO FEV1 = preoperative FEV1 × (1 – (resected segments/19))). Using whole-lung spirometry to predict postoperative lung function may be invalidated if the regional function of the lung is not known. For example, a patient with an FEV1 of 1.5 L may have the same or better FEV1 after lobectomy if the main bronchus of the affected lobe was occluded completely at the time of testing before surgery. The oxygenation of blood of such a patient may be improved by the removal of a non-functioning lung or lobe through which considerable right-to-left shunt existed.
ANATOMY
The bronchial tree and the views obtained when facing the patient are illustrated in Figure 33.2 and the bronchopulmonary segments are shown in Figure 33.1. The trachea leads from the cricoid cartilage below the larynx at the level of the sixth cervical vertebra (C6) and passes 10–12 cm in the superior mediastinum to its bifurcation at the carina into left and right main bronchi at the sternal angle, T4/5. During inspiration, the lower border of the trachea moves inferiorly and anteriorly. The trachea lies principally in the midline, but is deviated to the right inferiorly by the arch of the aorta. The oesophagus is immediately posterior to the trachea and behind it is the vertebral column. The wall of the trachea is held patent by 15–20 cartilaginous rings deficient posteriorly where the trachealis membrane, a collection of fibroelastic fibres and smooth muscle, lies. It is wider in transverse diameter (20 mm) than anteroposteriorly (15 mm). The trachea passes from neck to thorax via the thoracic inlet at T2.
INDUCTION AND MAINTENANCE OF ANAESTHESIA
Lateral Thoracotomy
Many thoracic procedures are performed through a posterolateral thoracotomy incision between fifth and eighth ribs. Patients have to be positioned on their side with the neck flexed, dependent shoulder brought forward and the arm raised under the pillow to protect the shoulder and brachial plexus. The upper shoulder is flexed to 90° and the arm supported. Hips and knees are flexed together with a pillow between the legs. Padding, strapping, lower leg compression devices and diathermy pad complete the preparation for surgery (Fig. 33.3). Positioning with the chest flexed laterally away from the operative side on a beanbag which is then aspirated of air, or breaking the operating table, may improve surgical access. The upper wrist has a tendency to flex, so radial artery cannulae cause less trouble when on the dependent side. Peripheral and jugular veins are more accessible on the operative side.
One-Lung Anaesthesia
The principal indications for one-lung anaesthesia are:
Ventilation of one lung alone may require a double-lumen tracheal tube (Fig. 33.4), a bronchial blocker (Fig. 33.5) or a bronchial tube. The double-lumen tube has greatest flexibility to allow changes from ventilation of two lungs to one lung then back to two lungs during or at the end of surgery. It allows aspiration of the main bronchi independently, and insufflation of oxygen to the non-ventilated lung. It has a larger external diameter than the bronchial blocker or bronchial tube and may be difficult to position correctly if tracheal and bronchial anatomy is distorted. The two separate lumens are narrow and present a high resistance to spontaneous ventilation. This is overcome by positive pressure ventilation, but a single-lumen tube may have to be substituted at the end of surgery if resumption of spontaneous ventilation is not immediate. A bronchial blocker with a hollow lumen allows insufflation of oxygen, some suctioning and may be used with a jet ventilator, which overcomes some of the disadvantages of the technique. The Rüsch bronchial blocker illustrated in Figure 33.5 has a 170 cm long, 2 mm stem (outside diameter) with a central lumen to a 2.75 mm diameter balloon, which accepts 5 ml of air. It may be passed down a bronchoscope with a lumen greater than 2.8 mm diameter or through the 15 mm tapered connector with a 1.8 mm seal shown.