How would you distinguish obstructive from restric-tive lung disease, and asthma from chronic obstruc-tive pulmonary disease (COPD), using preoperative pulmonary function tests?
Pulmonary function tests (PFT) are comprised of spirometry and flow-volume loops. An arterial blood gas and diffusion of carbon monoxide (DLCO) can also be included. Typical values in obstructive and restrictive lung disease are shown in Table 80.2.
Asthmatic patients usually have normal PFT values between attacks and periods of exacerbation. Narrowing limited to the small airways can yield a normal FEV1/FVC, but the FEF25-75 will be decreased. Bronchospasm may be triggered during PFT evaluation with methacholine or histamine to assess airway reactivity in patients suspected of asthma with normal baseline PFT values.
For patients with COPD, measurements are repeated after inhaled bronchodilators to evaluate the degree of reversibility or the spastic component, as opposed to the fixed component due to inflammation and airway destruction.
FEV1/FVC is effort-dependent and requires patient cooperation for accurate measurement, while FEF25-75 is effort-independent. It is obtained by dividing the volume expired between 75% and 25% of the FVC by the time elapsed between these two points.
Flow-volume curves may also be helpful. A normal curve as well as typical curves from patients with obstruc-tive and restrictive disease are shown in Figure 80.1. By convention, inspiration is below the baseline and expira-tion above. In restrictive disease, airway resistance is normal with no flow limitation, while lung volumes are reduced. In obstructive disease, the expiratory flow curve shows a characteristic flattening due to increased airway resistance. Curves from patients with fixed airway obstruction and variable extrathoracic and intrathoracic obstructions are shown in Figure 80.2. Fixed obstruction, such as is seen in tracheal stricture or compression by a tumor or a goiter, causes a decrease in inspiratory and expiratory flows. Variable extrathoracic obstruction (e.g., caused by vocal cord paralysis or marked pharyngeal muscle weakness) causes collapse of the airway during inspiration, as the transmural pressure gradient is negative. On the other hand, with variable intrathoracic obstruction (e.g., caused by tracheal or endobronchial tumor) the airway narrowing increases during forced expiration.
Typically, the arterial blood gas (ABG) is normal in asthmatic patients between attacks. Depending on the severity of the attack, they will first show hypocapnia with normoxemia or mild hypoxemia. As the attack becomes more severe, normocapnia then hypercapnia will be seen, with worsening hypoxemia. The onset of hypercapnia is an ominous sign of impending respiratory failure. In patients with COPD, the ABG shows a variable degree of hypercapnia and hypoxemia. Patients with restrictive disease can have a normal ABG at rest with hypercapnia and hypoxemia developing during exercise, or they can have baseline abnormalities.