If you have asthma or have taken care of a patient with chronic lung disease, you may be familiar with pulmonary function tests (PFTs). As a critical care nurse, I didn’t have a great deal of experience with PFTs; I was familiar with ventilator weaning protocols and had knowledge of tidal volumes, vital capacity, and minute ventilation, but didn’t possess a deep understanding of PFTs and how these tests are helpful in diagnosing respiratory disorders and tailoring treatments.
What are PFTs and what do they tell us?
PFTs are used to diagnose pulmonary disorders, identify potential causes of dyspnea, determine if a functional abnormality is obstructive or restrictive, assess the advancement of lung disease, monitor the effectiveness of therapies (i.e., bronchodilators) on lung function, and assess pulmonary function before and after surgical procedures (Lippincott Advisor, 2019; Smith et al., 2021). The most common PFTs include spirometry, lung volumes, and diffusing capacity.
Spirometry measures the maximal volume of air moved in a single breath in relation to time (Smith et al., 2019). Spirometry measurements include the total volume of air during forced expiration following maximal inspiration (FVC); the volume of air exhaled forcefully in the first second of expiration (forced expiratory volume in one second, FEV1
); and the FEV1
/FVC ratio. The FEV1
/FVC ratio is important in identifying obstructive airway disease and restrictive disease. To ensure accuracy, spirometry is repeated at least three times (maximum of eight attempts), while the person administering the test observes for a sharp peak in the flow curve and an expiration longer than six seconds.
- A normal or decreased FVC and a decreased FEV1 results in a reduced FEV1/FVC ratio, an indication of obstructive airway diseases (i.e., COPD or asthma).
- A reduced FVC and a normal or increased FEV1/FVC ratio suggests restrictive disease, if associated with reduced lung volumes (Kaminsky, 2022).
A bronchodilator response test
is performed when airflow is limited on spirometry. In this test, spirometry is repeated after the patient inhales a bronchodilator (i.e., four puffs of albuterol). The patient is assessed for improvement, or reversibility, as evidenced by an increase in the FEV1
or FVC of more than 12 percent and greater than 200 m L. If the FEV1
/FVC results return to normal, this often indicates asthma (Smith et al, 2019).
Lung volumes are obtained to complement spirometry. Body plethysmography is used to assess lung volumes; common measurements include total lung capacity (TLC), the total volume of the lungs when maximally inflated; functional residual capacity (FRC), the amount of air remaining in the lungs after normal expiration; and residual volume (RV), the amount of air that remains in the lungs at the end of maximum forced expiration.
- Decreased lung volumes suggest restrictive disease if coupled with a normal or increased FEV1/FVC ratio.
- Increased lung volumes suggest hyperinflation due to obstructive airway disease when coupled with a decreased FEV1/FVC ratio.
- Coexisting restriction and obstruction can be identified when the TLC and FEV1/FVC ratio are reduced (Kaminsky, 2022).
Diffusing capacity of the lung for carbon monoxide (DLCO)
Diffusing capacity of the lung for carbon monoxide (DLCO
) assesses gas exchange by measuring the milliliters of carbon monoxide (CO) diffused per minute across the alveolocapillary membrane.
- Decreased DLCO with restrictive disease indicates intrinsic lung disease.
- Normal DLCO with restrictive disease indicates a non-pulmonary cause of restriction.
- Significantly decreased DLCO with obstructive airway disease indicates emphysema.
- Reduced DLCO with normal lung volumes indicates possible pulmonary vascular disease.
Adjuncts to PFTs (Kaminsiky, 2022)
Exercise tests are used to evaluate for dyspnea and exercise intolerance. Three commonly used tests are the six-minute walk test (6MWT), the incremental shuttle walk test (ISWT), and the endurance shuttle walk test (ESWT). The 6MWT provides information about the distance walked and exertional desaturation when combined with pulse oximetry. The ISWT is a 12-level test in which the patient walks at progressively increasing speed for 12 minutes over a 10-mteter course. The test is stopped when the patient becomes short of breath, is unable to maintain the required speed, or completes the 12 levels. In the ESWT, the patient walks between two markers spaced 10 meters apart. The speed is based on results from the ISWT and the patient walks until they can no longer maintain the pace or after 20 minutes.
Assessment of pulse oxygen saturation (SpO2
) at rest or with exertion can be used to identify a gas exchange problem and to titrate the amount of oxygen needed to maintain adequate oxygenation. Arterial blood gases (ABGs) can help confirm carbon dioxide retention, increased bicarbonate levels, and/or chronic hypoxemia. ABGs also help determine the extent of gas exchange impairment in patients with a low normal SpO2
(i.e., less than 92%).
PFTs may cause adverse effects to patients with conditions that are negatively impacted by increased intracranial, intra-abdominal, intrathoracic, or intraocular pressures (Smith et al., 2021). PFTs may be contraindicated in patients with acute coronary insufficiency, angina, or recent myocardial infarction. Assess for respiratory distress, changes in heart rate and blood pressure, coughing, bronchospasm, and physical exhaustion in these high-risk patients.
- Before PFTs (Lippincott Advisor, 2019)
- Avoid smoking and vaping for 12 hours before the test.
- Eat a light meal and avoid too much fluid prior to the test.
- Wear loose, comfortable clothing.
- Hold bronchodilators as described below (Kaminsky, 2022):
- Hold short-acting inhaled beta agonists (i.e., albuterol, salbutamol) for four to six hours prior to testing.
- Hold the short-acting muscarinic antagonist ipratropium for 12 hours prior to testing.
- Hold long-acting beta agonist bronchodilators (i.e., salmeterol, formoterol) for 24 hours prior to testing.
- Hold ultra-long-acting beta agonists (i.e., indacaterol, olodaterol, vilanterol) for 36 hours prior to testing.
- Hold long-acting muscarinic antagonists, also called anticholinergic agents, (i.e., glycopyrrolate [glycopyrronium], tiotropium, aclidinium, and umeclidinium) for 36 to 48 hours prior to testing.
- During some of the tests, a nose clip and will be applied and instructions will be given to breath with a deep inspiration and a forceful continuous expiration into the mouthpiece of the spirometer.