pulmonary function test of the month The Detection of Collapsible Airways Contributing to Airflow Limitation* Edward D. Chan, MD; and Charles G. Irvin, PhD The detection of collapsible airways, which may be a component of asthma and emphysema, has important therapeutic implications. We describe a patient with significant airways collapse contributing to his airflow limitation and discuss how collapsible airways can be assessed by the volume difference between what exits the lung as determined by a spirometer and the volume compressed as measured by the plethysmograph. More simply, a large volume difference between the slow and forced vital capacity (SVC-FVC), easily obtained from spirometry, may be used as a surrogate index of airway collapse. (Chest 1995; 856-59) Herein we present a patient as an example of air- flow limitation due largely to airways collapse. The point of this case is that in patients who present with evidence of airflow limitation without hyperin- flation, airways collapse is a possibility. Further we will demonstrate how this cause of airflow limitation can be assessed with flow-volume relationships de- rived from either a body plethysmograph or more simply from spirometry. CASE REPORT A 66-year-old man was referred for a chronic nonproductive cough of about 1-year duration. The cough has been more frequent in the early evening hours and is often triggered by cold air, deep breathing, and perfume. There is no significant associ- ated nasal or sinus symptoms. His medical history is notable for childhood "asthma," hypertension, insulin-dependent diabetes mellitus, coronary artery bypass surgery, and a benign nasal polyp. He smoked a pack of cigarettes a day for 30 years but quit over 20 years ago. He is a restaurant owner with no current or past toxic fume or dust exposure although he has had significant sec- ond-hand cigarette smoke exposure. He appeared healthy but anxious on examination. He was afe- brile with a pulse 84/min, BP of 160/90 mm Hg, and a respira- tory rate of 16/min. There was a paroxysmal cough. The skin, nodes, thyroid, heart, abdomen, and extremities were unremark- able. There was a right nasal polyp. The chest wall was normal and the lung examination was significant for a prolonged expira- tory phase with diffuse expiratory wheezes. Chest radiograph showed the previous sternotomy with clear lung fields and a normal sized heart. Computed tomography of Key words: airway collapse; asthma; gas compression; ob- structive lung disease the sinuses was unremarkable except for the nasal polyp. The re- sults from a fiberoptic laryngoscopy were normal. His pulmonary function test was performed with care to avoid prior treatment with bronchodilator or steroids. Pulmonary Function The spirometry (Table 1) showed a reduced forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) (both are 60% of predicted) and yet the FEV,/FVC ratio is 70% or 104% of predicted and the specific conductance (SGaw) is 86% pre- dicted. There is evidence of some element of reversible airflow limitation based on a significant bronchodilator response to albuterol with improvement in SGaw of 58% and in FEV, of 20% from baseline. Moreover, his lung volume decreased (Fig 1) with bronchodilator therapy (isovolume shift) with a reduction in to- tal lung capacity (TLC) of 220 mL, functional residual capacity (FRC) of 340 mL, and residual volume (RV) of 100 mL; hence, flows at absolute volume are improved.' In assessing airway collapse, the flow volume loops in which the volume measured at the mouth and that which is measured in the plethysmograph were superimposed (Fig 2). When the flow dif- ference between the two curves at any particular volume are compared, it can be appreciated that there is compression of al- veolar gas that is greater than seen in normal subjects. This is also confirmed by the large discrepancy between the slow vital capacity (SVC) and FVC of 1,200 mL (Table 1). This later find- ing is not explained by poor patient effort as there were signifi- cant gas compression and good test reproducibility; in addition, the American Thoracic Society end-of-test criteria were met. Lastly, evidence for collapsible airways is noted from the flow volume loops since the flow during eupnea exceeds that at forced expiration (Fig 3). DISCUSSION Airflow limitation intrinsic to the lung may be due to reversible or irreversible airways disease or to a combination of both. One form of "irreversible" Detection of Collapsible Airways Contributing to Airflow Limitation (Chan, Irvin) FEVg=forced expiratory volume in the first second; FRC=functional residual capacity; FVC=forced vital ca- pacity; MEFV=maximal expiratory flow volume; Raw= airway resistance; RV=residual volume; SGaw= specific conductance; SVC=slow vital capacity; ATGV=thoracic gas volume measured by body plethysmograph; TLC=total lung capacity; AV=volume exiting the lung measured by spirometry; Vmax,50,m=flow rate at 50% of VC by spirom- etry; Vmax,50,p=flow rate at 50% of VC by plethysmo- graph *From the Division of Pulmonarv Sciences and Critical Care Medicine, University of Colorado Health Sciences Center, and the National Jewish Center for Immunology and Respiratory Medicine, Denver. 856 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21710/ on 06/02/2017