Although a variety of esoteric tests can be performed in a fully equipped pulmonary function laboratory, a respiratory evaluation almost always begins with the forced vital capacity (FVC) maneuver, which is routinely, though somewhat imprecisely referred to as spirometry. When properly performed, spirometry can provide information about restrictive and obstructive diseases; a normal FVC essentially rules out restriction, while a reduced 1 second forced expiratory volume (FEV₁) expressed as a fraction of FVC (FEV₁/FVC) establishes the presence of obstruction. Obstruction may have a variety of causes, including the normal effects of aging, but repetition of the test may provide further information. A significant change in FEV₁ after administration of a bronchodilator, or after an interval of time, establishes reversible obstruction, which, in the absence of another identifiable cause, strongly suggests asthma.

A low FVC may be caused by a restrictive or obstructive process (due to air trapping, in the latter instance), and should be evaluated by lung volumes. Gas dilution methods are commonly available, but as with spirometry, air trapping can give rise to falsely reduced measurements of volume. Whole-body plethysmography is a far more accurate method for measuring lung volumes. The evaluation of restrictive disease often includes diffusing capacity (or transfer factor), which is complementary to arterial blood gas (ABG) analysis in evaluating [V with dot above]/[Q with dot above] mismatching. However, diffusing capacity is sensitive to a number of confounding variables, and reproducibility is often problematic.

In an aviator, identification of chronic lung disease involving the lung parenchyma should prompt consideration of ABG measurement. ABG should also be measured in any individual with COPD. (Blood gas analysis is rarely indicated in an asthmatic because, except during a serious exacerbation, arterial saturation will usually be normal in all but the most severe asthmatics.) Aircraft cabin altitude is typically restricted to a maximum of 8,000 ft (2,438 m) for civilian aircraft, although certain military aircraft routinely maintain a cabin altitude of 10,000 ft (3,048 m). At those altitudes, an adult with normal cardiorespiratory status will have a resting arterial partial pressure of oxygen of approximately 60 mm Hg. With preexisting lung disease, the arterial partial pressure of oxygen may be further decreased. Measuring peripheral saturation by oximetry is not a satisfactory substitute for assessing oxygenation, because saturation does not appreciably fall until arterial partial pressure drops below 60 mm Hg. Affected aircrew with mild or moderate disease severity will usually be on the relatively flat portion of the oxyhemoglobin dissociation curve (see Figure 2.4).

The list of restrictive diagnoses outnumbers obstructive diseases, but obstructive disease is more commonly encountered in practice, and one disease in particular accounts for most pulmonary problems in aviation. The identification of obstruction on spirometry that shows a significant (>15%) improvement after a bronchodilator challenge is strong evidence for asthma, but asthma is a variable disease, and may not be evident at the time spirometry is performed. Where asthma is suspected despite normal spirometry, bronchial provocation testing (BPT) can be employed. A variety of provocative agents have been utilized in the laboratory, of which nebulized histamine and particularly methacholine are the best standardized. With methacholine challenge, the usual parameter measured is the PC20, the concentration of methacholine required to provoke a 20% reduction in FEV₁. A PC20 of less than 8 mg/mL has typically been considered positive. Alternatively, current American Thoracic Society guidelines recognize a borderline category (2). Under this classification, individuals with a PC20 equal to or greater than 16 mg/mL are considered normal, whereas the development of bronchospasm at concentrations lower than 4 mg/mL is considered hyperreactivity. Those who show a 20% drop in FEV₁ at concentrations between these limits are considered to be borderline. Although it is most often employed as a diagnostic maneuver in patients presenting in atypical manner, methacholine challenge testing can also be used to assess response to therapy. Because of the time and expense involved, the latter indication has not found wide acceptance in clinical practice, but demonstrating suppression of reactivity in response to treatment can be very useful in aircrew.

A few caveats are in order. BPT is not useful for screening; the prevalence of airway hyperreactivity considerably exceeds that of clinical asthma, and will fluctuate with other factors such as recent respiratory infections. The results of BPT must be integrated with the clinical picture. Also, methacholine challenge testing is not useful for ruling out subsequent relapse in remitted childhood asthmatics. In one recent study, approximately a third of those with negative bronchoprovocation tests subsequently relapsed (3). This is equivalent to the historical rate of relapse of remitted childhood asthma.

The chest x-ray is indispensable in the evaluation of respiratory disease. The reason is simple enough; for many diseases, the lungs provide their own contrast medium. At full inspiration, the chest cavity is predominantly filled with air, and disease processes such as edema, inflammation, neoplasia, and fibrosis are associated with water density. The standard chest film is much less useful in the evaluation of obstructive diseases. The disruption of alveolar membranes typical of emphysema and the narrowing of endobronchial lumina seen in bronchitis and asthma do not appreciably affect the radiodensity of the lung.

Computed tomography (CT) of the chest significantly increases resolution, and better delineates adenopathy, early fibrosis, small nodules, and numerous other abnormalities. It provides much clearer imaging of bullae than does routine chest x-ray, and the use of a high-resolution format also allows the identification of subpleural blebs, something not possible with a standard roentgenogram.

Accurate diagnosis of most clinical sleep disorders requires overnight polysomnography (PSG). Although a full night’s
sleep study in a sleep disorders laboratory is cumbersome, simpler methods sacrifice too much information. Typically, PSG will include electroencephalography (two to three leads), electromyography (genioglossus and anterior tibialis), electrooculography, electrocardiography (usually lead II), and oximetry. Airflow and respiratory effort are the two respiratory parameters most commonly measured.

Staging of sleep is reasonably standardized, because most laboratories use the Rechtschaffen and Kales scoring system. Unfortunately, there is considerable variation in the definition of respiratory events, especially hypopneas. Similarly, the definition of what constitutes an arousal is inconsistent from laboratory to laboratory. Scoring of sleep apnea is predominantly based on the apnea-hypopnea index (AHI), which measures the average number of sleep-disordered breathing events per hour. Up to 5 events per hour are considered to be normal, whereas more than 30 events per hour are generally classified as severe, though that classification is also variable.

Multiple sleep latency testing (MSLT) is the most commonly used objective test to quantify daytime sleepiness. Typically performed on the day following PSG, it consists of a total of four or five monitored 20-minute naps at 2-hour intervals, with sleep latency defined as the interval between “lights-out” and the first 30 seconds of scorable sleep, and rapid eye movement (REM) latency as the interval from the beginning of the first epoch scored as sleep to the first epoch of REM sleep. Normal sleep latency is greater than 10 minutes, whereas less than 5 minutes is considered pathologic sleepiness. Normally, REM sleep is not seen during any trial; two or more episodes of sleep-onset REM suggest the diagnosis of narcolepsy.

While MSLT measures the tendency to fall asleep, maintenance of wakefulness testing (MWT) measures the ability to stay awake. The two propensities tend to be inversely correlated, and yet they are distinct, with some subjects showing disparate results on the same day. MWT is performed with the individual instructed to try to stay awake while sitting in a darkened, quiet room. The test consists of four 20- or 40-minute trials performed at 2-hour intervals. Normal latency is 18 minutes, with a lower limit of 11 minutes. MWT is assumed to be more occupationally relevant than MSLT, because it more nearly addresses the issue of concern in the occupational setting. It should be noted, however, that the MWT was validated in subjects who did not have a particular interest in the outcome.