1 What are pulmonary function tests and how are they used?

The term pulmonary function test (PFT) refers to a standardized measurement of a patient’s airflow (spirometry), lung volumes, and diffusing capacity for inspired carbon monoxide (DLCO). These values are always reported as a percentage of a predicted normal value, which is calculated on the basis of the age and height of the patient. Used in combination with the history, physical examination, blood gas analysis, and chest radiograph, PFTs facilitate the classification of respiratory disease into obstructive, restrictive, or mixed disorders.

2 What is the benefit of obtaining pulmonary function tests?

The primary goal of preoperative pulmonary function testing, also called spirometry, is to recognize patients at high or prohibitive risk for postoperative pulmonary complications. Abnormal PFTs identify patients who will benefit from aggressive perioperative pulmonary therapy and in whom surgery should be avoided entirely. However, no single test or combination of tests will definitely predict which patients will develop postoperative pulmonary complications.

3 Besides abnormal pulmonary function tests, what are recognized risk factors for postoperative pulmonary complications?

Risk factors for pulmonary complications include:

Age >70 years

Obesity

Upper abdominal or thoracic surgery

History of lung disease

Greater than 20-pack/year history of smoking

Resection of an anterior mediastinal mass

4 What factors should be taken into consideration when interpreting pulmonary function tests?

The predicted PFT values are based on test results from healthy individuals and vary according to age, height, gender, and ethnicity. For example, vital capacity and total lung capacity are 13% to 15% lower in blacks than in whites. Reliable testing requires patient cooperation and a skilled technician.

5 Describe standard lung volumes

The tidal volume (TV) is the volume of air inhaled and exhaled with each normal breath. Inspiratory reserve volume (IRV) is the volume of air that can be maximally inhaled beyond a normal TV. Expiratory reserve volume (ERV) is the maximal volume of air that can be exhaled beyond a normal TV. Residual volume (RV) is the volume of air that remains in the lung after maximal expiration (Figure 9-1).

Figure 9-1 Subdivisions of lung volumes and capacities. ERV, Expiratory reserve volume; FRC, functional residual capacity; IC, inspiratory capacity; IRV, inspiratory reserve volume; RV, residual volume; TLC, total lung capacity; TV, tidal volume; VC, vital capacity.

6 What are the lung capacities?

Lung capacities are composed of two or more lung volumes. Total lung capacity (TLC) is the sum of IRV, tidal volume (TV), ERV, and RV. Vital capacity (VC) is the sum of IRV, TV, and ERV. Inspiratory capacity (IC) is the sum of IRV and TV. Functional residual capacity (FRC) is the volume of air in the lung at the end of a normal expiration and is the sum of RV and ERV (see Figure 9-1).

7 What techniques are used to determine functional residual capacity?

Measuring FRC is the cornerstone for determining the remainder of the lung volumes. The FRC can be measured by three different techniques:

Helium equilibration/dilution

Nitrogen washout

Body plethysmography

Plethysmography is most accurate for determining FRC in patients with obstructive airway disease and applies Boyle’s law, which states that the volume of gas in a closed space varies inversely with the pressure to which it is subjected. All measurements of FRC, if performed correctly, are independent of patient effort.

8 What information is obtained from spirometry?

Spirometry is the foundation of pulmonary function testing and provides timed measurements of expired lung volumes (Figure 9-2). With automated equipment it is possible to interpret more than 15 different measurements from spirometry alone. Forced vital capacity (FVC), forced expiratory volume in 1 second (FEV₁), FEV₁/FVC ratio, and flow between 25% and 75% of the FVC (mean maximal flow [MMF]_25-75) are the most clinically helpful indices obtained from spirometry. Although spirometry demonstrates airflow limitations, it does not determine the cause (e.g., airway obstruction vs. decreased alveolar elastic recoil vs. decreased muscle strength). It is also effort dependent and requires a motivated patient.

Figure 9-2 Spirogram. FEV₁, Forced expiratory volume in 1 second; FRC, functional residual capacity; FVC, forced vital capacity; MMF, mean maximal flow; RV, residual volume; TLC, total lung capacity.

9 What is the diffusing capacity for the single-breath diffusion capacity (DLCO)?

The DLCO measures the rate of uptake of the nonphysiologic gas carbon monoxide (CO). CO is used because of its affinity for hemoglobin and because it reflects the diffusing capacity of the physiologic gases oxygen and carbon dioxide. DLCO is dependent on membrane-diffusing capacity and the pulmonary vasculature and thus is a measure of functioning alveolar capillary units. This test has been used as an indicator of suitability for pulmonary resection and a predictor of postoperative pulmonary morbidity.

10 What disease states cause a decrease in DLCO?

Any disease process that compromises the alveolar capillary unit may cause a decrease in DLCO. Three major types of pulmonary disorders cause a decrease in DLCO:

Obstructive airway disease

Interstitial lung disease

Pulmonary vascular disease

11 What disease states cause an increase in DLCO?

In general, conditions that cause a relative increase in the hemoglobin concentration result in an increased DLCO. Congestive heart failure, asthma, and diffuse pulmonary hemorrhage are the most common causes of an increased DLCO. A perforated tympanic membrane may cause an artifactually high DLCO by permitting an escape of CO by a nonpulmonary route.

12 Review obstructive airway diseases and their pulmonary function test abnormalities

Obstructive airway diseases, including asthma, chronic bronchitis, emphysema, cystic fibrosis, and bronchiolitis, exhibit diminished expiratory airflow and involve airways distal to the carina. The FEV₁, FEV₁/FVC ratio, and the forced expiratory flow at 25% to 75% of FVC (FEF_25-75) are below predicted values. A decreased FEF_25-75 reflects collapse of the small airways and is a sensitive indicator of early airway obstruction. The FVC may be normal or decreased as a result of respiratory muscle weakness or dynamic airway collapse with subsequent air trapping. Table 9-1 compares the alterations in measures of lung function in various obstructive lung diseases. Table 9-2 grades the severity of obstruction based on the FEV₁/FVC ratio.

13 Review restrictive lung disorders and their associated pulmonary function test abnormalities

Disorders that result in decreased lung volumes include abnormal chest cage configuration, respiratory muscle weakness, loss of alveolar air space (e.g., pulmonary fibrosis, pneumonia), and encroachment of the lung space by disorders of the pleural cavity (e.g., effusion, tumor). The characteristic restrictive pattern is a reduction in lung volumes, particularly TLC and VC. Airflow rates can be normal or increased.

14 What is a flow-volume loop and what information does it provide?

Using routine spirometric values, flow-volume loops assist in identifying the anatomic location of airway obstruction. Forced expiratory and inspiratory flow at 50% of FVC (FEF₅₀ and FIF₅₀) are shown in Figure 9-3. Note that expiratory flow is represented above the x-axis, whereas inspiratory flow is represented below the axis. In a normal flow-volume loop the FEF₅₀/FIF₅₀ ratio is 1.

Figure 9-3 Idealized flow-volume loop. EXP, expiratory; FEF₅₀, expiratory flow at 50% of forced vital capacity; FIF₅₀, inspiratory flow at 50% of forced vital capacity; FVC, forced vital capacity; RV, residual volume; TLC, total lung capacity. (Flow in L/sec is abbreviated V.)

(From Harrison RA: Respiratory function and anesthesia. In Barash PG, Cullen BF, Stoelting RK, editors: Clinical anesthesia, Philadelphia, 1989, Lippincott, pp 877-994, with permission.)

15 What are the characteristic patterns of the flow-volume loop in a fixed airway obstruction, variable extrathoracic obstruction, and intrathoracic obstruction?

Upper airway lesions (e.g., tracheal stenosis) are fixed when there is a plateau during both inspiration and expiration. The FEF₅₀/FIF₅₀ ratio remains unchanged. An extrathoracic obstruction occurs when the lesion (e.g., tumor) is located above the sternal notch and is characterized by a flattening of the flow-volume loop during inspiration. The flattening of the loop represents no further increase in airflow because the mass causes airway collapse. The FEF₅₀/FIF₅₀ ratio is >1. An intrathoracic obstruction is characterized by a flattening of the expiratory loop of a flow-volume loop, and the FEF₅₀/FIF₅₀ ratio is <1. The lesion causes airway collapse during expiration (Figure 9-4).

Figure 9-4 Flow-volume loops in a fixed, extrathoracic, and intrathoracic airway obstruction. The hashmarks represent flow at 50% of vital capacity. Exp, Expiratory; Insp, inspiratory; RV, residual volume; TLC, total lung capacity.

(From Kryger M et al: Diagnosis of obstruction of the upper and central airways, Am J Med 61:85–93, 1976.)

16 What is the value of measuring flow-volume loops in a patient with an anterior mediastinal mass?

Injudicious anesthetic induction and paralysis in patients with an anterior mediastinal mass (e.g., lymphoma, thymoma, thyroid mass) may result in an inability to ventilate the patient and cardiovascular collapse caused by compression of the tracheobronchial tree, vena cava, pulmonary vessels, or heart. The change from spontaneous negative-pressure respirations to assisted positive-pressure ventilation is a significant factor in this collapse. The preoperative evaluation of flow-volume loops in sitting and supine positions helps to assess potentially obstructive lesions of the airway and identify patients in whom alternative management may be indicated.

17 What are the effects of surgery and anesthesia on pulmonary function?

All patients undergoing general anesthesia and surgical procedures (particularly in the thorax and upper abdomen) exhibit changes in pulmonary function that promote postoperative pulmonary complications. For instance, VC is reduced to approximately 40% of preoperative values and remains depressed for at least 10 to 14 days after open cholecystectomy. Upper abdominal procedures result in a decrease in FRC within 10 to 16 hours; FRC gradually returns to normal by 7 to 10 days. The normal pattern of ventilation is also altered, with decreased sigh breaths and decreased clearance of secretions.

18 What pulmonary function test values predict increased perioperative pulmonary complications after abdominal or thoracic surgery?

See Table 9-3.

TABLE 9-3 Pulmonary Function Criteria Suggesting Increased Risk for Abdominal and Thoracic Surgery

DLCO, Diffusing capacity for inspired carbon monoxide; FEV₁, forced expiratory volume in 1 second; FVC, forced vital capacity; MVV, maximal voluntary ventilation; RV, residual volume; VO₂, oxygen consumption.

* Pneumonectomy.

† Lobectomy.

‡ Segmentectomy.

Data from Gass GD, Olsen GN: Preoperative pulmonary function testing to predict postoperative morbidity and mortality, Chest 89:127–135, 1986.

19 Are there absolute values of specific pulmonary function tests below which the risk of surgery is prohibitive?

No single PFT result absolutely contraindicates surgery. Factors such as physical examination, arterial blood gases, and coexisting medical problems also must be considered in determining suitability for surgery. Ultimately the decision to operate weighs all risks and benefits and is a collaborative decision between the surgeon, the anesthesiologist, and the patient.