A. Evaluation of Pulmonary Function

1. Lung Volumes
   [Figure] "Lung Volumes"
2. Flow Rates: spirometry, flow volume loops
3. Diffusing Capacity
4. Blood pCO2 and pO2 levels

B. Spirometry
[Figure] "Forced Expiratory Spirometry"

1. Subject inhales to Vmax then forcefully exhales
2. Volume leaving lung is recorded as a function of time (spirometry)
3. Total volume which leaves lung appears as maximum deflection on spirometer
4. Parameters obtained from Spirometry
   1. Forced Vital Capacity (FVC)
   2. Forced Expiratory Volume in first second (FEV1)
   3. Ratio: FEV1/FVC >80% is normal
   4. Maximum midexpiratory flow rate (MMF)
5. Normal Values
   [Figure] "Expiratory Flow and Vital Capacity"
   1. Highly dependent on height, weight, age, sex
   2. Normal nomograms have been developed to classify results
   3. Normal FVC (young, healthy, average man) ~ 5 liters
   4. Normal FEV1 ~ 80% of FVC; that is, FEV1 ~4 liters
   5. Maximal expiratory flow rate: men >3.5 liters/second, women > 3.0 liters/second
6. PFTs for Screening
   1. PFTs should not be done (for COPD screening) in healthy adults without symptoms [1,2]
   2. PFTs to detect early COPD prior to symptoms even in smokers is not recommended [1,2]

C. Diffusion Capacity

1. Ability of gases to cross alveolar-capillary membrane
   1. The properties of this membrane are solely responsible for gas crossing
   2. Carbon Monoxide (CO gas) is usually used to quantitate the diffusion capacity
   3. This is because CO has rapid transport across membrane
2. The Diffusion Limit of CO = DLCO is usually reported in pulmonary function tests
   1. Measured by inspiration of CO + Helium
   2. Hold breath 10 seconds, then analyze exhaled air
3. Ficks Law: Gas Flux (J) = k • A • delta P / delta L
   1. A = area of membrane, major determinant of gas flux
   2. delta P = pressure gradient (note that PaCO ~ 0)
   3. delta L = thickness of membrane
4. DL is defined as DL = k • A / delta L
   1. Determined only by properties of membrane:
   2. Area of membrane (most important)
   3. Thickness of membrane (least important)
   4. Permeability constant k (important in pulmonary edema, others)
5. Increased DLCO
   1. Fever
   2. Polycythemia
   3. Exercise
   4. Lying Down
6. Reduced DLCO
   1. Hemorrhage, Anemia
   2. Standing up
   3. Emphysema (pure chronic bronchitis maintains relatively normal DLCO)
   4. Atelectasis
   5. Loss of lung parenchyma: pneumonia, pneumonectomy, fibrosis, others
   6. Pulmonary Vascular Disease such as pulmonary hypertension, vasculitis

D. Types of Disease

1. Obstructive
   1. FEV1/FVC < 75%
   2. FEV1 of 50-75% of predicted is mild obstruction
   3. FEV1 of 35-50% of predicted is moderate obstruction
   4. FEV1 of 20-35% of predicted is severe obstruction
   5. MMF usually decreases with FEV1
   6. MMF low but with a normal FEV1 implies small airways disease
2. Restrictive Disease
   1. FVC < 75% of predicted value
   2. FEV1 / FVC often in normal range
   3. Total lung capacity (TLC) and residual volume (RV) are reduced

E. Flow-Volume Curves
[Figure] "Flow-Volume Curves"

1. Normal Spirogram is V (volume) vs. time
2. Slope of this curve is Flow = dV/dt
3. Plot of Flow vs. Volume (V) is a flow-volume curve
4. Used to distinguish airflow obstruction and restrictive disease
5. Also useful in diagnosis and localization of upper airway obstruction

References

1. US Preventive Services Task Force. 2008. Ann Intern Med. 148(7):529
2. Lin K, Watkins B, Johnson T, et al. 2008. Ann Intern Med. 148(7):535