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.

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.