Diagnosis of Asthma Part 2-By Dr.Karim
The most common widely used laboratory investigation done in all patients suspected to have bronchial asthma is Spirometry.
Spirometry specifically measures the volume and speed of air that the patient can inhale (breathe in) and exhale (breathe out). It is done using a device called the spirometer.
To understand how spirometry works, we need to know some definitions first.
Forced vital capacity (FVC):
It is the volume of air that can be forcibly blown out After full inspiration
Forced expiratory volume in 1 second (FEV1):
It is the volume of air that can be forcibly blow out in 1 second After full inspiration . Normal values of FEV1 differ according to age, sex, height, weight and smoking. FEV1 is the best measure for assessing severity of obstruction.
FEV1 /FVC ratio (FEV1%):
This is the ratio of FEV1 to FVC .
In healthy adults, this should be 0.75 – 0.8
In asthmatics, there is an increased airways resistance due to bronchoconstriction, increased mucus secretion and inflammatory edema ( please refer to the article Pathophysiology of asthma)
When airway resistance increases, the expiratory flow (speed of air expired or exhaled) in 1 second decreases. Thus FEV1 value decreases
FVC value decreases as well but to a lesser extent than FEV1 value , this is due to the prevention of all the air to be emptied out during forced expiration.
FEV1/FVC ratio should be measured before and after the patient receives a bronchodilator drug and any difference should be noted as results will improve greatly if the patient is an asthmatic.
FEV1 and FVC are measure in Liters.
Examples for better understanding
A 65 year-old man undergoes pulmonary function testing as part of a routine health-screening test. He had no pulmonary complaints. He is a lifelong non-smoker. His pulmonary function test results are as follows :
FEV1 = 3.20 Liters
FVC = 4.2 Liters
so FEV1/FVC = 0.76 (76%)
Remember the normal value of FEV1/FVC is 0.75 to 0.8 , so the previous patient has normal pulmonary functions.
A 54 year-old man presents to his primary care provider with difficulty breathing and a cough. He is a non-smoker . His pulmonary function test results are as follows:
Before using bronchodilator After using bronchodilator
FEV1 = 2.18 Liters FEV1 = 2.83 Liters
FVC = 3.19 Liters FVC = 4 Liters
so FEV1/FVC = 0.68 (68%) thus this patient suffers from air flow obstruction. Notice the improvement of pulmonary functions after using bronchodilators, this is diagnostic of bronchoconstriction and thus asthma.
While the Spirometry is done in a lab, another indicator of pulmonary functions that can be done at home is a small hand held device called the Peak Flow meter. It measures the Peak expiratory flow rate (PEFR) This is the person’s maximum speed of expiration.
Peak Flow Meter
In PEFR, the patient breathes forcibly into a peak flow meter and notes the readings
80-100% is considered normal
50-79% indicates that respiratory airways are narrowing and additional medication may be required
Less than 50% indicates a medical emergency and immediate action should be taken at a hospital.