Anaesthesia for Thoracic Surgery


Anaesthesia for Thoracic Surgery


Thoracic anaesthesia offers a number of anaesthetic challenges:



In common with major surgery at other sites, thoracic patients frequently:



Diagnosis, staging and resection of intrathoracic malignant disease occupy a large part of thoracic surgical practice. There is also a need for drainage and obliteration of an expanded pleural space to remove infection, or to prevent lung collapse and re-accumulation of air or liquid in the pleural space. Resection of bullous lung disease may improve the respiratory mechanics of the chest if there is parenchymal lung disease elsewhere.



PREOPERATIVE ASSESSMENT



History and Examination


Thoracic patients often exhibit respiratory symptoms of cough, sputum, haemoptysis, breathlessness, wheeze and chest pain, or oesophageal symptoms of dysphagia, pain and weight loss. Other common chest features include hoarseness, obstruction of the superior vena cava, pain in the chest wall or arm, Horner’s syndrome, cyanosis and pleural effusion. Lung tumours may cause extrathoracic features by metastatic spread, principally to brain, bone, liver, adrenals and kidneys, or by endocrine effects such as finger clubbing, hypertrophic pulmonary osteoarthropathy, Cushing’s syndrome, hypercalcaemia, myopathies (e.g. Eaton–Lambert syndrome), scleroderma, acanthosis and thrombophlebitis. Anaemia, cardiac disease and lung disease may cause breathlessness.


To distinguish between loss of lung tissue and reversible airways disease, the patient’s own history of daily activity may reveal diurnal variation in breathlessness and associated symptoms of sputum, stridor and wheeze. Symptoms may conflict with the results of pulmonary function tests, which require voluntary effort, if the tests have been performed ineffectively. Wheeze during expiration and stridor during inspiration are likely to result from airway obstruction below and above the thoracic inlet, respectively.


Production of sputum is the most common stimulus to cough, which is therefore almost universal in cigarette smokers. A dry cough may result from tumour or external compression of the upper airways. Oesophageal tumours are associated with dysphagia. The restriction on ingestion of food exacerbates the cachexia of malignant disease. At induction of anaesthesia, patients are at risk of regurgitation of food and secretions from above the oesophageal obstruction.


Preoperative features of weight loss, protein-calorie malnutrition and hypoalbuminaemia make postoperative pulmonary infection, multi-organ failure and delayed wound healing more likely. Patients with pre-existing chronic lung disease are more likely to suffer postoperative pulmonary complications. Cyanosis may result centrally from intrapulmonary shunting caused directly by diseased tissue or because of lung collapse consequent to proximal airway obstruction. Peripheral cyanosis is possible in the face and arms if the superior vena cava becomes obstructed by mediastinal spread.


Many major thoracic surgical procedures are preceded by rigid bronchoscopy which requires clinical assessment of upper airway patency at the preoperative visit. Forced ventilatory effort by the patient may elicit stridor or wheeze, and palpation of the neck and inspection of the airway demonstrated on chest radiograph may reveal tracheal abnormality.




Investigations


A variety of preoperative investigations are performed to determine resectability. The chest radiograph may reveal changes months before symptoms are manifest. Of symptomatic patients, 98% have chest radiograph abnormalities. Lung tumours are central in 70% of patients and may show collapse or cavitation more peripherally in the lung. Tumours are commonly 3–4 cm in size by the time of presentation. Other features include tracheal deviation, obstruction of the superior vena cava, pleural effusions and air-filled cavities.


Tumour diagnosis and staging involve sputum cytology, bronchoscopy, needle biopsy, mediastinoscopy and mediastinotomy. Computed tomography (CT) and magnetic resonance imaging (MRI) of the chest may reveal the spread of disease. Biochemistry, bone scans and ultrasound scans of the abdomen may detect metastatic disease. Barium studies and oesophageal ultrasound are similarly able to diagnose and stage carcinoma of the oesophagus.


Investigations are used to quantify physiological reserve by measuring mechanical and parenchymal function, and cardiopulmonary interaction. Assessment should be on physiological reserve rather than chronological age. Rather than to deny surgery to patients, this is done to allocate resources to borderline patients to minimize their postoperative complications. Investigations to diagnose lung collapse, oedema, infection and bronchospasm allow patients to be presented in the best possible state on the day of surgery.



Whole-Lung Testing


Spirometry using voluntary effort, pulse oximetry and arterial blood gas tensions breathing air are influenced by the function of the whole lung.


Mechanical testing. Spirometry tests only the mechanical bellows function of the lung. Testing relies on voluntary effort and effective technique by the patient. From total lung capacity, the patient exhales to residual volume to measure the forced vital capacity (FVC). This is reduced in restrictive lung disease, such as cryptogenic alveolar fibrosis. The volume of gas exhaled forcibly in the first second gives the forced expiratory volume in 1 s (FEV1). By definition, FEV1 measures function when the lung is expanded well. In health, patients can exhale 70–80% of their vital capacity in 1 s, the FEV1%. The remainder of the vital capacity may take another 2 s to exhale. With obstructive lung disease, the FEV1% is reduced below 70% and the time taken to exhale the vital capacity is prolonged. The FEV1% of patients with restrictive lung disease is preserved, although the absolute value of FVC, and therefore the volume exhaled in 1 s, is reduced. Values of 2 L for FVC and 1.5 L for FEV1 offer a lower limit when screening for pneumonectomy. An FEV1 of 1.0 L is cautionary for single lobectomy because a whole-lung FEV1 > 800 mL after surgery is required to avoid dependence on mechanical ventilation.


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.



Parenchymal lung function. Arterial oxygen tension < 8 kPa or carbon dioxide tension > 6 kPa indicate increased risk for lung resection. The diffusing capacity of carbon monoxide (DLCO) correlates with the total functioning area of the alveolar capillary membrane. Predictive postoperative values may be calculated in the same way as for mechanical function. PPO DLCO values < 40% of predicted in health – normal range 100–150 mL min–1 mmHg–1 – correlate with increased postoperative respiratory complications.


Cardiopulmonary interaction. Cardiopulmonary exercise testing requires a patient to be able to pedal a bicycle ergometer and breathe through a mouthpiece. Increasing exercise to a peak allows calculation of maximum oxygen uptake expressed in mL kg–1 min–1. Patients with VO2max > 10 mL kg–1 min–1 have been able to withstand lobectomy, whereas a PPO VO2max > 15 mL kg–1 min–1 is required to contemplate pneumonectomy.




Invasive Assessment


In patients with borderline lung function for whom surgical resection offers great prognostic advantage, the risks and discomfort of invasive assessment of regional lung function may be worth the information obtained about likely residual function after surgery. Balloon occlusion of a main pulmonary artery before surgery or clamping a pulmonary artery during surgery allows some assessment of pulmonary artery pressures and oxygenation after resection. Inadequate blood oxygenation, arterial carbon dioxide tension greater than 6.0 kPa and mean pulmonary artery pressure greater than 25 mmHg at rest, or greater than 35 mmHg with exercise, indicate inadequate function and increased operative risk.


Any intrathoracic operative procedure places an immediate burden on the right ventricle. Using echocardiography, magnetic resonance scanning or direct pressure measurement, diagnosis of a failing right ventricle or coexisting pulmonary artery hypertension may render a patient’s chest pathology inoperable.



Treatment


Before surgery, patients should be motivated to stop smoking and lose excess weight. Reversible airway narrowing should be treated with bronchodilators such as salbutamol, terbutaline, theophylline, inhaled steroids or sodium cromoglycate. By giving antibiotics to treat chest infection, and loosening and removing bronchial secretions with inhaled nebulized water aerosols, chest physiotherapy and postural drainage, the incidence of pulmonary complications is reduced. Collapse in lung segments not intended for resection should be expanded, and pulmonary oedema treated by improving heart failure. Pulmonary hypertension should be treated where feasible. Overnight, stopping smoking improves bronchial reactivity and reduces carboxyhaemoglobin concentration. Eight weeks after cessation of smoking, the excessive production of mucus is reduced. This makes tracheobronchial clearance easier and improves small airway function.



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.



The right main bronchus is larger and less deviated from the midline than the left. The origin of the right upper lobe bronchus arises laterally 2.5 cm from the carina, whereas the origin of the left upper lobe arises laterally after 5 cm. These dimensions determine the relative ease of isolating and ventilating each lung independently using double-lumen bronchial tubes.


The oesophagus is a continuation of the pharynx at the level of the lower border of the cricoid cartilage (C6) 15 cm from the incisor teeth. It passes immediately anterior to the thoracic spine and aorta and descends through the oesophageal hiatus of the diaphragm at T10, to the left of the midline at the level of the seventh rib. There are four slight constrictions, at its origin, as it is crossed by the aorta and left main bronchus, and at the diaphragm, at 15, 25, 27 and 38 cm from the incisors.



Radiographic Surface Markings


The apices of the lungs extend 2.5 cm above the point at which the middle and inner thirds of the clavicle meet. Lung borders descend behind the medial end of the clavicle to the middle of the manubrium. The lung border is behind the body of the sternum and xiphisternum before sweeping inferiorly and laterally down to the level of the eleventh thoracic vertebra. On the left, at the level of the horizontal fissure at the fourth costal cartilage (T7), the medial border of the lung is displaced to the left of the sternal edge in the cardiac notch. The oblique fissure descends from 3 cm lateral to the midline at T4, inferiorly and anteriorly to the sixth costal cartilage 7 cm from the midline. The diaphragmatic reflection of the pleura extrudes below the lung to the lower border of T12.



INDUCTION AND MAINTENANCE OF ANAESTHESIA


All currently available anaesthetic agents have been used for thoracic surgery. They are used to maintain anaesthesia and haemodynamic stability during the procedure, while allowing the patient to breathe spontaneously in reasonable comfort immediately after surgery is complete. Large doses of i.v. opioid drugs are unlikely to achieve all these aims. Thoracic surgery patients may demonstrate sensitivity to drugs used in anaesthesia because of debility from the extent of their disease or from systemic effects, such as prolonged effects of neuromuscular blocking drugs in patients with Eaton–Lambert syndrome associated with carcinoma of the bronchus.



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.




The Lateral Position


In health, with the chest erect, the right lung takes 55% of the pulmonary blood flow and the left lung 45%. In the right lateral position, the right lung takes 65% and the left lung 35% because of the influence of gravity. In the left lateral position, differences are reversed; the right lung takes 45% and left lung 55%. These changes persist under anaesthesia. However, anaesthesia does affect the changes to ventilation of the two lungs in the lateral position. Awake, there is more ventilation to the dependent lung; similarly, the bases receive proportionately more ventilation than the apices when the chest is erect. The dependent lung has a less negative intrapleural pressure than the upper lung and is on a more favourable part of the pressure–volume curve. A change in pressure produces a greater change in volume in the dependent lung than in the upper lung. Under anaesthesia, conditions for ventilation between the two lungs are reversed. Functional residual capacity is reduced. With paralysis of the diaphragm, the mechanical advantage of the greater curve of the lower diaphragm is lost and the lower lung is compressed by the mediastinum and abdominal contents. Awkward positioning on the operating table may further impede the lower lung. The lower lung is now in a less favourable position on the pressure–volume curve and any change in pressure produces greater change in the volume of the upper lung than the dependent lung. Anaesthesia therefore produces much worse ventilation/perfusion mismatch in the lateral position, with more blood going to the dependent lung and more ventilation going to the upper lung. Application of positive end-expiratory pressure up to 10 cmH2O improves the changes in ventilation and tends to restore ventilation to the dependent lung.



One-Lung Anaesthesia


The principal indications for one-lung anaesthesia are:



Isolation of a diseased lung with sepsis or haemorrhage may be necessary to protect the healthy lung. When there is inadequate ventilation of both lungs because of a large bronchopleural or bronchocutaneous fistula, a large unilateral bulla or because the compliance of two lungs is so different that they require independent ventilation, satisfactory oxygenation may be obtained by ventilation of one lung alone. Pulmonary alveolar proteinosis may be treated by bronchoalveolar lavage. This requires that only one lung be lavaged with liquid at a time, whilst the other is protected. Video-assisted pulmonary and pleural surgery, and intrathoracic, non-pulmonary surgery such as oesophageal, aortic and spinal surgery, may require the lung to be collapsed to allow access to the operative structures.


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.

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May 31, 2016 | Posted by in ANESTHESIA | Comments Off on Anaesthesia for Thoracic Surgery
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