Interpreting Pulmonary Function Tests
STEPS IN INTERPRETING PULMONARY FUNCTION TESTS
1. Notice the patient’s age, height, weight, sex, to have a mental picture of body habitus. Age, height and sex determine normal lung volumes and are used to calculate the predicted values for that person. Normal predicted values are between 80 to 120%.
2. Look at Total Lung Capacity (Body Plethysmograph). If it’s below 80% of predicted, the patient has restrictive disease. Restriction is due to: 1) intrinsic lung disease (interstitial lung disease or pulmonary edema) or an extrinsic pulmonary process such as 2) a chest wall disorder (obesity, kyphoscoliosis, etc.) or 3) neuromuscular weakness.
3. Look at the flow volume loop. Was it a good effort? Does it rise to a peak and then come down to the x-axis in a nearly linear triangular shape? That’s normal. Concavity suggests obstruction. Sometimes loops from restricted patients bulge outwards.
4. Look at the FEV1 (FEV1/FVC) and FVC individually. Their value in liters can be reduced below 80% either by restriction or obstruction. If the TLC is low, this is likely due to restriction. If the TLC is normal or increased the FEV1 and FVC may be reduced due to obstruction (see steps 3 and 5 to confirm). These values help you to know how bad the problem is.
5. Look at the FEV1% (FEV1/FVC). In health, the value should be within 5% above or below the value predicted. In restriction from an intrinsic lung disease (interstitial lung disease), the FEV1% is typically high because these processes cause stiff lungs with decreased compliance. In obstruction, the FEV1% is usually low but can be normal in some cases. Why? Because some patients with obstructive lung disease have proportional reduction in both their FEV1 and FVC.
6. Did the patient receive bronchodilators? If so, a response to the drug is an increase of both 200 ml AND 12% in the FEV1 OR FVC.
7. Look at the airways resistance, Raw. If it is elevated, the patient probably has narrowing of the large central airways (either asthma, upper airway obstruction, chronic bronchitis).
8. Look at the diffusing capacity, DLco. A high DLco means that the lungs have too much blood in them: think of asthma, obesity or intrapulmonary hemorrhage. If the DLco is low, this may be due to 1) anemia or 2) the lungs don’t have their full share of pulmonary capillaries: the patient may have emphysema or intrinsic lung disease such as scleroderma or sarcoidosis, or 3) pulmonary vascular disease (pulmonary hypertension or embolism). Cirrhosis can also cause a low DLco.
9. Did the patient perform testing of respiratory muscle strength? These include negative inspiratory force, NIF and positive expiratory force, PEF. Normal values are greater than -60 or +60 respectively.
10. Check to see if there is a large amount of poorly-ventilated lung. This is done by comparing the two measurements of total lung capacity. The TLCBB – TLCsingle breath gives a value in liters that should not exceed 0.5 L. The body box (BB) method measures ALL the air in the lungs while the gas dilution (single breath) maneuver cannot measure air in poorly ventilated lung units thus will underestimate lung volume in obstructive lung diseases such as asthma and emphysema.