Interpreting Pulmonary Function Tests

Educational Objectives

The goal of this program is to improve the interpretation of pulmonary function tests. After hearing and assimilating this program, the clinician will be better able to:

1. Interpret the results of pulmonary function tests.

2. Distinguish between obstructive lung disease and restrictive lung disease.

3. Diagnose and manage chronic obstructive pulmonary disease.

Summary

Full pulmonary function test (PFT): spirometry, lung volume, and diffusion; spirometry — mechanics of how air moves in and out of lungs; provides no information about lung function; lung volume — volume of all gases in chest; lung volume useful diagnostic measurement, but not for monitoring over time; plethysmography used to measure changes in volume within chest (using Boyle’s law; ie, pressure is inversely proportional to volume); diffusion or diffusing capacity — same as gas transfer or transfer factor; measures transfer of gas from alveoli to bloodstream

Reasons for obtaining PFT: evaluate symptoms, signs, and abnormal laboratory tests; assess preoperative risk, especially in patients undergoing chest surgery; determine prognosis; measure effect of disease on lung function; monitor responses to therapy; monitor for development of gas transfer abnormalities (using diffusing capacity of lung for carbon monoxide [DLCO]); establish level of disability

Spirometry: cannot be used to pinpoint definitive diagnosis; cannot be used to determine whether degree of abnormality associated with patient’s symptoms; normal PFT does not exclude presence of disease; abnormal tests may not reflect loss of pulmonary function; abnormal results not necessarily due to pulmonary factors; patient asked to inhale as hard, fast, and deeply as possible to total lung capacity (TLC), then exhale as far as possible to functional residual capacity, then resume normal breathing; volume measured directly, whereas capacity is sum of volumes; vital capacity (VC) — maximum amount individual exhales after maximum inhalation; equals sum of tidal volume plus inspiratory reserve volume plus expiratory reserve volume; TLC — sum of VC plus residual volume (RV; amount of air that remains in lungs after maximum expiration); flow-volume loop — flow represented on y-axis (positive or negative; in liters per second) and volume on x-axis; time not included; VC is amount of air in chest; helps to determine speed at which air released; rounded pattern of inspiratory limb normal; volume-time curve — probably most important measurement in PFT; can help determine whether data real and reproducible; for data to be real, individual must exhale for ≥6 sec; volume represented on y-axis and time on x-axis; shows proportion of VC that individual able to expire in first second of forced expiration (forced expiratory volume [FEV1]) or during plateau of forced expiration lasting ≥1 sec; test reliable if individual exhales and plateaus for 1 sec or exhales for ≥6 sec

Lung mechanics: involves 4 measurements (O2 saturation, forced VC [FVC; actual, predicted, and percentage predicted values], FEV1, and maximum voluntary ventilation [MVV]); MVV amount of air that individual able to inhale and exhale as fast as possible in 15 sec (result then multiplied by 4 for estimate of amount of air individual able to inhale and exhale in 1 min); not particularly useful because it does not distinguish among neuromuscular weakness, obstructive lung disease (OLD), or restrictive lung disease (RLD); FEV1/FVC (FEV1%) — amount of air expired in first second expressed as percentage of amount of air expired overall (should be ≈80% in most individuals); number derived, not measured, and estimation may be incorrect; forced expiratory flow25%-75% — flow rate at 25% to 75% of FVC; provides information about small airways (likely compromised in OLD); if slope significantly decreased, small airway disease possible; high TLC — indicates more air in chest than normal or hyperinflation; specific airway conductance (ability of airways to conduct air) — inverse of airway resistance; DLCO maneuver — ask patient to breathe (regular tidal breathing), then ask patient to take deep breath to TLC, then hold breath for 10 sec; at end of 10 sec, ask patient to expire to maximum; at time patient inhales, known amount of CO introduced; CO has high affinity for hemoglobin (Hb; more than O2 or CO2); if diffusion normal, CO binds to Hb, and little to no CO seen during exhalation, which indicates adequate DLCO; DLCO can be adjusted for alveolar volume according to age, weight, height, race, and/or gender; poor DLCO after adjustment indicates decreased amount of effective alveolar volume; ideally, DLCO should be corrected for Hb level (eg, Hb level decreased in anemic individuals)

Obstructive lung disease: asthma, COPD, and bronchiectasis; COPD comprises emphysema and chronic bronchitis (purely clinical diagnosis and does not require spirometry; ≥3 mo of chronic cough with sputum production over 2 consecutive years); obliterative bronchiolitis rare; differs from bronchiolitis obliterans organizing pneumonia (BOOP); airways do not function, and air trapped; BOOP is RLD (inflammation in alveoli extends to bronchi); RLD and OLD may balance each other out, which results in normal FEV1%; FEV1% <70% in individuals with OLD; rule of 10s — applies to OLD and RLD; if 80% normal, 70% to 80% considered mild, 60% to 69% moderate, 50% to 59% moderately severe, 40% to 40% severe, <40% very severe, and <30% extremely severe; grade level dictates therapy; Global Initiative for Chronic Obstructive Lung Disease (GOLD) categorization — mild, moderate, severe, and very severe stages; FEV1% always low, but FEV1 may be normal; also applies to severity of asthma; flow-volume loop shows coving pattern (indicating that small airways closing with forceful exhalation); presence of large airway lesion — pressures in extrathoracic airway (above sternal notch) completely reversed compared with all other pressures in chest; airway prone to collapse with forceful exhalation, because pleural pressure significantly exceeds transpleural pressure; lesion in chest that narrows airway narrows airway even further; on exhalation, blunting of flow-volume loop seen due to limitation of flow; inhalation normal; inspiratory limb normal, but expiratory limb blunted (intrathoracic problem results in expiratory limb problem and vice versa); rheumatoid arthritis affecting vocal folds limits flow during inhalation and exhalation

Restrictive lung disease: pleural diseases or any disease that prevents gas from entering chest; no correlation between description of symptoms and lung disease present; any condition that prevents diaphragm from dropping during inhalation results in RLD; includes interstitial lung disease, neuromuscular disease, and thoracic cage abnormalities (eg, kyphoscoliosis, history of spinal fixation); diagnostic criteria — TLC <80%; cannot be diagnosed without full PFT; normal FEV1, low FVC, and high FEV1% suggestive of restriction, but TLC needed to determine and diagnose RLD; if FEV1% normal or high, either condition normal or RLD present (OLD present if FEV1% low); severity — FVC 80% to 100%, normal; 70% to 80%, mild; 60% to 69%, moderate; 50% to 59%, moderately severe; <50%, severe; treatment — few effective treatments for RLD; testing — flow-volume loop narrow and pointed; plethysmography gold standard; gas dilution with helium or nitrogen (easier to perform, but not accurate in OLD); in RLD, TLC low (RV low and VC low); in significant OLD, VC normal or high, but RV hyperinflated; hyperinflation diagnosed with chest x-ray (flattened hemidiaphragm and wide anterior-posterior diameter) and PFT

Diffusing capacity of lung for CO: in active smoker, with CO bound to Hb, less CO trapped, which results in low DLCO (also seen in severe congestive heart failure, polycythemia, and severe anemia); if <75% with obstructive pattern, almost always emphysema; if <75% with restrictive pattern, almost always interstitial lung disease; if restrictive or normal pattern present, always consider pulmonary vascular disease; if mechanics normal, but DLCO 50% of predicted value, early interstitial lung disease, chronic thromboembolic disease, or pulmonary arterial hypertension in 99.9% of cases

Muscle pressure: measured in patients with neuromuscular disease and reported as actual and predicted; no set amount for lower limit, but if lower than expected with standard deviations, probably true neuromuscular weakness

Bronchodilator challenge: used to diagnose asthma; detects reversibility; response considered positive if either FEV1 or FVC changes by 12% and absolute volume increases by ≥200 mL (volume-time curve moves up and left, which indicates increased airflow); bronchoprovocation testing — methacholine or histamine testing; determines dose at which FEV1 decreases by 20%; relative and absolute contraindications present; useful only to exclude asthma and cannot be used to diagnose asthma (positive in 30% of smokers and in individuals with allergic rhinitis); highly nonspecific; asthma excluded if negative

Readings

Antwi DA et al: Analysis of lung function tests at a teaching hospital. Ghana Med J, 2011 Dec;45(4):151-4; Ford ES et al: Trends in the prevalence of obstructive and restrictive lung function among adults in the United States: findings from the National Health and Nutrition Examination surveys from 1988-1994 to 2007-2010. Chest, 2013 May;143(5):1395-406; Godfrey MS, Jankowich MD: The vital capacity is vital: epidemiology and clinical significance of the restrictive spirometry pattern. Chest, 2016 Jan;149(1):238-51; Keddissi JI et al: Bronchial responsiveness in patients with restrictive spirometry. Biomed Res Int, 2013;2013:498205; Lindberg A et al: Co-morbidity in mild-to-moderate COPD: comparison to normal and restrictive lung function. COPD, 2011 Dec;8(6):421-8; Mannino DM, Diaz-Guzman E: Interpreting lung function data using 80% predicted and fixed thresholds identifies patients at increased risk of mortality. Chest, 2012 Jan;141(1):73-80; Melbye H et al: Asthma, chronic obstructive pulmonary disease, or both? Diagnostic labeling and spirometry in primary care patients aged 40 years or more. Int J Chron Obstruct Pulmon Dis, 2011;6:597-603; Price DB et al: Real-world characterization and differentiation of the Global Initiative for Chronic Obstructive Lung Disease strategy classification. Int J Chron Obstruct Pulmon Dis, 2014 May 28;9:551-61; Sumino K et al: Writing Committee for American Lung Association Asthma Clinical Research Centers. Variability of methacholine bronchoprovocation and the effect of inhaled corticosteroids in mild asthma. Ann Allergy Asthma Immunol, 2014 Apr;112(4):354-60.e1; Vaz Fragoso CA et al: Phenotype of normal spirometry in an aging population. Am J Respir Crit Care Med, 2015 Oct 1;192(7):817-25.

Disclosures

For this program, members of the faculty and planning committee reported nothing to disclose.

Acknowledgements

Dr. White was recorded at the 34th Annual Internal Medicine Update, held July 29-31, 2016, in Mackinac Island, MI, and presented by the University of Michigan Medical School Department of Internal Medicine. For information about upcoming CME activities from the University of Michigan Medical School, please visit med.umich.edu/intmed/cme. The Audio Digest Foundation thanks the speakers and the University of Michigan Medical School for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 0 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 0 CE contact hours.

Lecture ID:

IM634601

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.