Certified Respiratory Therapist Practice Exam

What measurement confirms a diagnosis of obstructive lung disease when accompanied by elevated RV and TLC?

1. Decreased FEF25-75%

2. Decreased DLCO

3. Normal DLCO

4. Increased FEV1

The correct answer is: Decreased DLCO

To confirm a diagnosis of obstructive lung disease, a decrease in the forced expiratory flow (FEF25-75%) is significant. In patients with obstructive lung diseases, the airflow limitation leads to a reduction in this measurement, indicating that there is a problem with airflow on forced expiration, particularly in the mid to late phases of the exhalation. Elevation of residual volume (RV) and total lung capacity (TLC) often accompanies obstructive lung disease due to air trapping, which further emphasizes the impairment in airflow. While DLCO (diffusing capacity of the lungs for carbon monoxide) can be evaluated in various lung diseases, a decreased DLCO does not specifically confirm obstructive lung disease. It is more indicative of parenchymal lung disease or disorders affecting the alveolar-capillary membrane. In contrast, a normal DLCO could suggest an isolated obstructive process, while increased FEV1 (forced expiratory volume in one second) contradicts the characteristics of obstructive disease since it typically shows a disproportionate reduction compared to other volumes, especially FVC (forced vital capacity). Thus, interpreting these various measures can clarify the presence of obstruction and confirm the underlying disease process.