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

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