Diagnosis

A chest radiograph is usually part of the medical workup of suspected respiratory disease. However, a negative chest x-ray does not exclude significant lung damage. For example, immediately after toxic inhalational injuries, the chest x-ray is frequently normal. Furthermore, abnormalities on chest x-ray do not necessarily correlate with the degree of pulmonary impairment or disability, which are better evaluated by pulmonary function testing (PFT) and arterial blood gas evaluation (119).

Chest Computed Axial Tomography

A chest CT is better able to detect abnormalities of the pleura and medi-astinal structures than a plain chest x-ray, in large part because it is more sensitive to differences in density. A chest CT may also be performed after administration of intravenous contrast material to gain better visualization of the pulmonary hila. A high-resolution CT (HRCT) is a more detailed exam than a conventional chest CT and obtains sharp interfaces between adjacent structures. A high-resolution CT appears to be more sensitive for a number of diffuse lung processes, such as emphysema and interstitial lung disease (76).

Pulmonary Function Testing

Pulmonary function testing is used to detect and quantify abnormal lung function. This exam consists of spirometry, measurement of lung volumes and diffusing capacity, gas exchange analysis, and exercise testing. Although a pulmonary function laboratory is needed to do most of this evaluation, spirometry can be done at most regional evaluation centers. Often, the most useful of all pulmonary function measures are those obtained from spirometry: FEV1, forced vital capacity (FVC), and the FEV1/FVC ratio. The forced expiratory flow (FEF) from 25% to 75% of the vital capacity (FEF 25 to 75) and the shape of the expiratory flow-volume curve are more sensitive findings for mild airway obstruction. Results of spirometry can be compared to predicted values from reference populations (adjusted for age, height, and sex) and expressed as a percentage of predicted value. The determination of obstructive, restrictive, or mixed venti-latory defects can be ascertained from the comparison of observed with predicted values. However, as the commonly used reference population consists entirely of Caucasians, there can be problems using predicted values when evaluating non-Caucasian patients. Generally, the predicted value is lowered by approximately 10% to 15% to adjust for the smaller lungs of non-Caucasians (97).

Several other factors may affect the accuracy of spirometric findings including patient cooperation, poor testing methods, and unreliable equipment.

Peak Expiratory Flow Rate

Measurement of peak flow is a commonly used single-breath test that reflects the degree of airway obstruction. Many portable units exist. Peak flow measurements are helpful in the diagnosis of occupational asthma to document delayed responses after the work shift has ended. However, a major limitation of this method is that patient self-recording is needed and results can be inaccurate or manipulated (97).

Bronchoprovocation Testing

This evaluation is helpful in making the diagnosis of occupational asthma. Pulmonary function responses to inhaled histamine or methacholine are measured and give an indication of the presence and degree of nonspecific airway hyperresponsiveness. A dose-response curve is constructed for repeated measurements of FEV1 after progressively increasing the exposure doses of histamine or methacholine. The test is typically terminated after a 20% drop in FEV1. In asthmatic subjects, a relatively low cumulative dose of methacholine induces the 20% in FEV1 compared to normal subjects (97).

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