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 (D_LCO) 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 D_LCO 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.

Regional Lung Function

Ventilation/perfusion lung scans offer an indication of regional lung function. The relative contribution of the two separate lungs may be determined horizontally but regions are indicated as upper, middle and lower zones vertically within a lung rather than individual lobes or lung units.

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.

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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