Investigation of Asthma

CLINICAL FEATURES AND DIAGNOSIS
The characteristic symptoms of asthma are wheezing, dyspnea, and coughing which are variable, both spontaneously and with therapy. Symptoms may be worse at night, and patients typically awake in the early morning hours. Patients may report difficulty in filling their lungs with air. There is increased mucus production in some patients, with typically tenacious mucus that is difficult to expectorate. There may be increased ventilation and use of accessory muscles of ventilation. Prodromal symptoms may precede an attack, with itching under the chin, discomfort between the scapulae, or inexplicable fear (impending doom).

Typical physical signs are inspiratory, and to a great extent expiratory, rhonchi throughout the chest, and there may be hyperinflation. Some patients, particularly children, may present with a predominant nonproductive cough (cough-variant asthma). There may be no abnormal physical findings when asthma is under control.

Diagnosis
The diagnosis of asthma is usually apparent from the symptoms of variable and intermittent airways obstruction, but is usually confirmed by objective measurements of lung function.

LUNG FUNCTION TESTS
Simple spirometry confirms airflow limitation with a reduced FEV1, FEV1/FVC ratio, and PEF. Reversibility is demonstrated by a >12% and 200 mL increase in FEV1 15 min after an inhaled short-acting 2-agonist or, in some patients, by a 2- to 4-week trial of oral glucocorticoids (prednisone or prednisolone 30–40 mg daily). Measurements of PEF twice daily may confirm the diurnal variations in airflow obstruction. Flow-volume loops show reduced peak flow and reduced maximum expiratory flow. Further lung function tests are rarely necessary, but whole body plethysmography shows increased airway resistance and may show increased total lung capacity and residual volume. Gas diffusion is usually normal but there may be a small increase in gas transfer in some patients.

AIRWAY RESPONSIVENESS
The increased AHR is normally measured by methacholine or histamine challenge with calculation of the provocative concentration that reduces FEV1 by 20% (PC20). This is rarely useful in clinical practice, but can be used in the differential diagnosis of chronic cough and when the diagnosis is in doubt in the setting of normal pulmonary function tests. Occasionally exercise testing is done to demonstrate the post-exercise bronchoconstriction if there is a predominant history of EIA. Allergen challenge is rarely necessary, and should only be undertaken by a specialist if specific occupational agents are to be identified.

The patient performs the peak flow meter test twice a day for about 2 weeks and records the results for review in a follow up appointment. The first test should be performed after waking in the morning, before taking bronchodilator medications. The patient should perform the peak expiratory flow maneuver 3 times and record the highest measurement. The second test should be done in the afternoon or early evening after taking a bronchodilator. Peak flows vary during the day and the early morning peak is lower than the evening peak. A variability greater than 20% indicates a reversible airway obstruction.

HEMATOLOGIC TESTS
Blood tests are not usually helpful. Total serum IgE and specific IgE to inhaled allergens (RAST) may be measured in some patients.

IMAGING
Chest roentgenography is usually normal but may show hyperinflated lungs in more severe patients. In exacerbations, there may be evidence of a pneumothorax. Lung shadowing usually indicates pneumonia or eosinophilic infiltrates in patients with bronchopulmonary aspergillosis. High-resolution CT may show areas of bronchiectasis in patients with severe asthma, and there may be thickening of the bronchial walls, but these changes are not diagnostic of asthma.

SKIN TESTS
Skin prick tests to common inhalant allergens are positive in allergic asthma and negative in intrinsic asthma, but are not helpful in diagnosis. Positive skin responses may be useful in persuading patients to undertake allergen avoidance measures.

Normal results for spirometry

The results of spirometry tests are compared to predicted values based on the patient's age, gender, and height. For example, a young adult in good health is expected to have the following FEV values:

FEV-0.5—50-60% of FVC
FEV-1—75-85% of FVC
FEV-2—95% of FVC
FEV-3—97% of FVC
In general, a normal result is 80–100% of the predicted value. Abnormal values are:

mild lung dysfunction—60–79%
moderate lung dysfunction—40–59%
severe lung dysfunction—below 40%


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