Forced Vital Capacity. Forced Expiratory Volume in One Second. Obstructive Vs. Restrictive lung diseases. Pulmonary Function Test PFT

Terminology and Definitions Forced vital capacity (FVC): The volume of air which can be forcibly and maximally exhaled out of the lungs after the patient has taken in the deepest possible breath. Forced expiratory volume in one second (FEV1): the volume of air which can be forcibly exhaled from the lungs in the first second of a forced expiratory maneuver. FEV1/FVC-FEV1 percent (FEV1%): this number is the ratio of FEV1 to FVC and it indicates what percentage of the total FVC was expelled from the lungs during the first second of forced exhalation.

What can a PFT be used for? Pulmonary function abnormalities can be grouped into two main categories: Obstructive defects. Restrictive defects. This grouping of defects is based on the fact that the routine spirogram measures two basic components – air flow and volume. Generally the idea is that if the flow is impeded, the defect is obstructive, and if volume is reduced, a restrictive defect may be the reason for the pulmonary disorder.

Obstructed airflow Flow through the tubular passageways of the lung can be reduced for a number of reasons: 1. Narrowing of airways due to bronchial smooth muscle contraction as is the case in asthma. 2. Narrowing of airways due to inflammation and swelling of bronchial mucosa and the hypertrophy and hyperplasia of the bronchial glands as is the case in bronchitis. 3. Material inside the bronchial passageways physically obstructing the flow of air as is the case in excessive mucus plugging, inhalation of foreign objects or the presence pushing and invasive tumors.

Cont… 4. Destruction of lung tissue with the loss of elasticity and hence the loss of external support of the airways as is the case in emphysema. 5. External compression of the airways by tumors and trauma.

Restricted airflow “restriction” in lung disorders always means a decrease in lung volumes. This term can be applied with confidence in patients whose total lung capacity has been measured and found to be significantly reduced. There are a variety of restrictive disorders which are the following: 1. Intrinsic restrictive lung disorders: - sarcoidosis. - pneumonectomy (loss of lung). - pneumonia.

Cont… 2. Extrinsic restrictive lung disorders: - Scoliosis, khyphosis. - pregnancy. - gross obesity. - ascites. 3. Neuromuscular restrictive lung disorders: - generalized weakness. - paralysis of diaphragm. - myasthenia gravis. - poliomyelitis.

Criterion for obstructive and restrictive diseases Forced vital capacity FVC: In obstructive diseases  lung’s air volume will be more slowly expelled and will be a smaller volume than would be expected in a normal, healthy individuals. Explanation: because of physical obstruction and airway collapse during exhalation.

Cont… In restrictive diseases  FVC will also be smaller. Explanation: - due to mechanical limitations. - the thoracic cage does not have the ability to expand very much. - so, the amount of air that can be forcefully inhaled or exhaled from the lungs is smaller to start with because of disease.

Note : Since FVC will be smaller in both obstructive and restrictive disorders, FVC alone cannot be used to diagnose obstructive and restrictive disorders all by itself. Usually no one worries about FVC unless it’s 80%- 85% of predicted volumes.

Forced Expiratory volume in one second FEV1: In general, it’s common in healthy individuals to be able to expel 75%-80% of their vital capacity in the first second of the FVC test (i.e., FEV1/FVC ratio = % ). In obstructive diseases  have low FEV1 and low FEV1/FVC ratio. Restrictive lung disease  also have low FEV1 and low FEV1/FVC ratio.

restrictiveobstructive FVC FEV1 FEV1/FVC

Systematic way of interpratation There are a systematic way to read the pulmonary function test and be able to evaluate it for the presence of obstructive or restrictive disease. The following steps will be helpful: Step 1. look at the forced vital capacity (FVC) to see if it’s within normal limits. Step2. look at the forced expiratory volume in one second (FEV1) and determine if it’s within the normal limits. Step3. If both are normal, then you do not have to go any farther- the patient has a normal PFT.

Step4. if FVC and/or FEV1 are low, then the presence of disease is highly likely. Step5. if step 4 indicates that there is a disease, then you need to go to % predicted for FEV1/FVC. If % predicted for FEV1/FVC is : patient has obstructed lung disease < 70 % Patient have restricted lung disease > 85 %

Case # 1. % predictedMeasured valuesPredicted values 65 %4.00 liters6.00 litersFVC 40 %2.00 liters5.00 litersFEV1 60 %50 %83 %FEV1/FVC

Step1. FVC is low (i.e., must be % of predicted values) Step2. FEV1 is low. Step3. % predicted of FEV1/FVC is 60 % (i.e., < 70 %). Decision : this person have obstructive lung disease.

Case # 2. predictedMeasured valuesPredicted values 78%4.43 liters5.68 litersFVC 72%3.52 liters4.90 litersFEV1 94%79 %84 %FEV1/FVC

Step1. FVC is low. Step2. FEV1 is low. Step3. % predicted of FEV1/FVC is 94 % (i.e., >85 %). Decision : this person have restricted lung disease.