Asthma pulmonary function test

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Editor(s)-in-Chief: C. Michael Gibson, M.S., M.D. [1] Phone:617-632-7753; Philip Marcus, M.D., M.P.H. [2]; Associate Editor(s)-In-Chief: Lakshmi Gopalakrishnan, M.B.B.S. [3]

Overview

Asthma is defined as reversible airway obstruction that occurs spontaneously or with treatment. Measurement peak flow rates and spirometry are two valuable methods to assess pulmonary function. While measurement of airway function is possible in adults, most new cases that are diagnosed constitute the pediatric age group, who are unable to perform such tests. Thereby, diagnosis in children is based on a careful compilation and analysis of the individual's medical history and demonstration of symptomatic improvement with the administration of inhaled bronchodilator. In adults, diagnosis can be made with a peak flow meter that assess any airway restriction, diurnal variation and any reversibility following inhaled bronchodilator. Young asthmatics may experience only exercise-induced asthma; hence, testing peak flow at rest and after exercise may be beneficial. If in doubt, spirometry may be conducted to ascertain the diagnosis. Once the diagnosis is established, peak flow meter testing may be conducted to monitor the severity and progression of the disease. Capnography may be used in the emergency situations, to measure the amount of exhaled carbon dioxide and if used in conjunction with pulse oximetry may be possible to estimate the amount of oxygen dissolved in the blood, in order to determine the severity of an asthma attack as well as the predict the response to therapy.

Spirometry

• The preliminary investigation of choice to establish diagnosis of asthma is the assessment of lung function using spirometry that includes measurement of forced vital capacity (FVC) and forced expiratory volume in one second (FEV1). Baseline spirometry provides a fairly accurate assessment of the disease severity, degree of airway obstruction and also helps to differentiate between obstructive or restrictive patterns of airway disease.

• The classic spirometry findings consistent with airway obstruction include a reduced FEV1 and a subsequent reduction in the FEV1:FVC ratio. However, a normal to increased FEV1:FVC ratio would rule out the presence of an airway obstruction.

• The hallmark of asthma is the reversibility of airway obstruction that can be demonstrated with the initial spirometry by measuring the lung volumes before and after the administration of a short-acting bronchodilator. A 12% or greater increase in forced expiratory volume in 1 second (FEV1) is strongly suggestive of asthma.

• Single-breath counting is a measurement of how far an individual can count in a normal speaking voice after a maximal effort inhalation and serves as a convenient method to measure pulmonary function in children.

Peak Expiratory Flow Rate (PEFR)

• Peak expiratory flow rate is a measurement of an individual's maximum ability to expel air from the lungs.

• A significant reduction in the peak expiratory flow rate from normal predicted value, is suggestive but not diagnostic of asthma. However, the reversibility of peak flow rate following the administration of a short-acting bronchodilator may favor the diagnosis of asthma.

• A serial monitoring of peak expiratory flow rate helps to monitor the patient's response to therapy, evaluate the severity of symptoms and estimate the frequency of exacerbations.

• The PEFR is an effort dependent measurement that is recorded using a peak flow meter which is a small hand-held device and helps to define the patient's normal peak flow rate.

• Based on the American Lung Association, peak flow readings are classified into 3 zones of measurement namely the green, yellow, and red.

• The green zone means 80 to 100 percent of the usual or normal peak flow readings are clear. A peak flow reading in the green zone indicates that the asthma is under good control.

• The yellow zone means 50 to 80 percent of the usual or normal peak flow readings indicate caution. It may indicate narrowing of respiratory airways and hence an additional medication may be required.

• The red zone means less than 50 percent of the usual or normal peak flow readings indicate a medical emergency. It signifies severe airway obstruction that may warrant immediate medical attention.

• Limitations of PEFR:

• Reduced PEFR may be observed in both obstructive and restrictive airway disease and hence is not specific for asthma.
• PEFR does not differentiate asthmatic wheeze from other causes of wheeze. A predominant inspiratory monophonic wheeze is suggestive of vocal cord dysfunction, while a polyphonic wheeze is more consistent with asthma.
• PEFR is effort dependent and hence the validity of the measurement solely depends on the individual's effort and technique employed.
• PEFR readings is not consistent and varies with different instruments.

Diagnostic Criteria Based On The British Thoracic Society

• ≥20% difference on at least three days in a week for at least two weeks;
• ≥20% improvement of peak flow following treatment. For example:

• 10-minutes of inhaled beta-agonist such as salbutamol;
• 6-week of inhaled corticosteroid such as beclometasone;
• 14-days of 30mg prednisolone.

• ≥20% decrease in peak flow following exposure to a trigger such as exercise.