PULMOIIARY FUNCTION EVALUATION



In the pulmonary function laboratory, routine studies can be grouped into four categories: lung volumes, air flow, diffusing capacity, and maxi­mal pressures. Additional studies such as mea­surements of lung compliance rarely provide ad­ditional information to that obtained by more easily performed measurements.

The lung is conveniently divided into four vol­umes and three capacities, as shown in Figure 18-1. The components of the vital capacity can be obtained with routine spirometry. The residual volume (RV), however, must be measured indi­rectly, since it represents the air left in the lungs at completion of a full expiration. In fact, we ac­tually measure functional residual capacity (FRC) rather than RV, since the former, i.e., the volume at the end of a normal expiration, is a more re­producible point. The expiratory reserve volume (ERV) is then subtracted from FRC to obtain the residual volume.

Three techniques are commonly used to meas­ure FRC: nitrogen washout, helium dilution, and body plethysmography. The first two techniques are limited by the ability of the test gas to either wash out or equilibrate completely with all por­tions of the lung. In the presence of significant airways obstruction this will not occur and the FRC will be significantly underestimated. Body plethysmography eliminates this problem and measures the total thoracic gas volume, whether it is located in a bulla or in direct communication with the airway, and thus provides a more ac­curate reflection of the FRC.

The dynamics of airflow can be evaluated dur­ing a forced expiratory maneuver by recording the change in volume against time to calculate flow rate or by directly measuring volume and flow (Figs. 18-2 and 18-3). The flow-volume loop is particularly useful in demonstrating the presence of upper airway obstruction, which by affecting primarily the peak inspiratory and expiratory flows gives a characteristic loop (Fig. 18-3). An estimate of total airway resistance can be deter­mined by the body plethysmograph.
The measurement of the diffusing capacity for carbon monoxide (DLco) is an indicator of the ad­equacy of the alveolar-capillary membrane and so is reduced when the latter is decreased, as in pul­monary fibrosis, emphysema, and pulmonary vas­cular disease. In patients with a restrictive phys­iological defect, diffusing capacity helps to differentiate chest bellows (DLco normal) from parenchymal disease (DLco decreased). While it pro­vides little insight into the mechanism of abnor­mal lung function, it is useful as a marker of improvement or deterioration of the existing dis­ease process.

Measurement of maximal static respiratory pressures is probably the most sensitive and spe­cific method of diagnosing respiratory dysfunc­tion in patients with neuromuscular disease. Max­imal inspiratory pressure is obtained by recording mouth pressure during a maximal inspiratory ef­fort from residual volume, and maximal expira­tory pressure is recorded during a maximum ex­piratory effort from total lung capacity.

Interpretation of pulmonary function studies requires consideration of the technical quality of the tracings and a knowledge of the degree of var­iation for a particular index. Small deviations in vital capacity may be abnormal, whereas larger deviations in DLco are required to confidently di­agnose abnormality. Fixed percentages of a nor­mal value should not be considered to indicate disease.
Measurement of lung volumes and flow rates after certain challenges such as methacholine, ex­ercise, cold air, or exposure to organic or inor­ganic substances helps in the diagnosis of bronchospasm. Acute reversibility is determined by their repetition after bronchodilator adminis­tration. However, failure of flow to improve fol­lowing a single dose of a bronchodilator does not necessarily indicate irreversible disease and does not exclude the possibility of a clinical response to bronchodilator treatment.

More complex testing is occasionally required to answer specific questions. Exercise studies are valuable in judging the degree of disability as well as elucidating the cause of dyspnea on exertion. Expired gas, minute ventilation, heart rate, and arterial oxygenation are measured during in­creased workloads. The degree of limitation and the relative contribution of ventilatory and car­diovascular factors can be assessed. Polysomnog­raphy is an essential tool in the diagnosis of sleep apnea.