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Making the most of pulmonary function testing in the diagnosis of asthma: Are new monitoring tools on the horizon?

Article Excerpt
ABSTRACT: Although the results of a thorough history and physical examination often suggest the diagnosis of asthma, confirmatory testing is required and may be helpful in more subtle cases. Spirometry before and after bronchodilator administration is the first step for the initial diagnosis; it also is an important component of the long-term assessment of asthma control. When the results of spirometry are normal in a patient in whom asthma is suspected, bronchoprovocation challenge testing with methacholine is generally considered the next diagnostic step. Numerous alternative methods of bronchoprovocation testing have been developed, such as the challenge with adenosine 5'-monophosphate. Novel methods such as the forced oscillation technique and the measurement of exhaled nitric oxide hold promise for more effective diagnosis and monitoring of asthma in the future.

KEY WORDS: Pulmonary function testing, Spirometry, Bronchoprovocation testing, Asthma

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Asthma is a chronic inflammatory airway disease characterized by waxing and waning respiratory symptoms of cough, wheezing, and shortness of breath. It is a common and increasing problem, responsible for 1.8 million emergency department visits, 497,000 hospitalizations, and 4055 deaths in the United States in 2004. (1) Asthma is the underlying cause in up to 40% of young adults being evaluated for dyspnea, and it is an underrecognized cause of respiratory symptoms in older persons. (2,3) Pulmonary function testing plays a major role in the initial diagnostic evaluation, assessment of severity, and monitoring of patients with asthma. The purpose of this article is to review the current pulmonary function techniques used in the diagnosis of asthma, summarize recent advances in testing, and identify novel methods that hold promise for more effective diagnosis and management of this disease in the future.

CLINICAL PRESENTATION

It is important to briefly review the epidemiology and clinical presentation of asthma because disease prevalence and pretest probability can significantly influence the predictive value of pulmonary function testing. The global prevalence of asthma ranges from 1% to 18%, with an estimated prevalence of 11% in the United States. (4)

Table 1 displays the common risk factors associated with asthma. A family history of asthma clearly increases the risk of symptomatic disease, although the relationship is complex with multiple genes implicated in the pathogenesis. While the prevalence of asthma is nearly twice as high in boys as in girls younger than 14 years, the overall prevalence is higher in women than in men.

Obesity has been shown to be an independent risk factor, perhaps because of the influence of leptins on airway function; obesity also may play a role in asthma severity and control. (5) A history of childhood wheezing, eczema, and allergies is frequently associated with a diagnosis of asthma in young adults, while the clinical presentation in older adults is less strongly associated with an allergic diathesis.

The classic clinical presentation of asthma includes waxing and waning symptoms of dyspnea, cough, wheezing, and chest tightness. An exacerbation of symptoms is usually gradual in onset and cessation and is frequently associated with triggers, such as exposure to common allergens, cold weather, and viral infections. Physical exercise can also cause asthma symptoms and may be the only trigger in some patients. Nocturnal symptoms are common in patients with poorly controlled asthma, and increased numbers of airway inflammatory cells have been demonstrated in the early morning hours. (6)

Physical examination findings are frequently normal in persons with asthma, but the examination may reveal stigmata of allergic rhinitis, eczema, or airflow obstruction. Although the presence of wheezing may suggest a diagnosis of asthma, its predictive value is quite poor. (7) Chest radiography and pulse oximetry are recommended, although the results are frequently normal. Blood work may demonstrate peripheral eosinophilia or an elevated IgE level, but it is often not necessary or cost-effective in the initial diagnostic evaluation.

THE STANDARD TESTS

A variety of methods have been developed to objectively measure lung function to aid in the diagnosis of asthma (Table 2). Understanding the strengths and limitations of these tests can be vital to correctly interpret results and guide subsequent clinical decisions in patients with suspected or confirmed asthma.

Spirometry

The recently updated National Asthma Education and Prevention Program (NAEPP) guidelines place an increased emphasis on pulmonary function testing, using spirometry both for initial diagnosis and for long-term follow-up of asthma. (8) Objective measurement of lung function is important to confirm the diagnostic impression, to provide additional information regarding disease severity and associated risk, and to exclude other diagnoses.

Clinical signs and symptoms should be assessed at every health care visit. In addition, a variety of disease-specific quality of life measurements, such as the Asthma Quality of Life Questionnaire, have been developed to quantify the effect of asthma on a patient's health and well-being. Although these assessment tools can provide significant insight into the functional status of patients...

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