Diagnosis of Asthma
There are several methods of diagnosing asthma. The main ways are history of illness of the patient and signs and symptoms experienced by the patient. After review of the above and a physical exam, a diagnosis may be made. If the physician is unsure or the diagnosis needs to be supported by more objective data, there are three different types of tests that can be done. These include pulmonary function tests (PFTs), exercise testing, and bronchial provocation testing.
The main PFT used with asthma is the forced expiratory flow test. With this test the patient uses a peak flow meter to test how fast air can be expelled from the lungs. This is important in diagnosis as asthmatics suffer from narrowing of their airways which decrease the speed that air can be moved into and especially out of the lungs. Peak flow meters used with this test can come in many different styles but each measures air speed. A forced expiratory test is accomplished by asking the patient to take in a deep breath, placing the flow meter into their mouth and forming a tight seal, and then exhaling as fast as possible until all air possible is expelled. The average normal peak expiratory airflow speed is around 450 to 650 LPM, but this average changes based on age, height, and sex. Asthmatics show a reduced airflow compared to the normal expected airflow. Many newer flow meters can produce a readout which shows the flow waveform of the patient's exhalation. Other values can also be obtained with the newer meters that can also be helpful with the diagnosis of asthma. The degree of severity is determined by the forced expiratory volume in the first second of exhalation (FEV1), the forced vital capacity of the lungs (FVC), the forced expiratory flow during the middle portion of the FVC (FEF 25-75%), as well as other values. Any values pertaining to capacities or lung volumes are usually normal with asthmatics or can be larger than normal with severe attacks when air is being trapped in the lungs due to constriction of the airways. Any values pertaining to flow and flow rates will be decreased in the asthmatic and the degree of reduction reflects the degree of severity of the attack.
Bronchial provocation testing is used to diagnose asthma or to confirm diagnosis for a patient. These tests use agents which are aerosolized and breathed into the lungs by the patient. These agents will cause bronchial constriction in people with hypersensitive airways. The two major agents used are Methacholine and Histamine which are diluted by a pharmacist and prepared as an aerosol mixture. Due to the potential problems that may be incurred through this test, a strict dosing and delivery technique must be used. Each agent has its own dosage tables which are given in small increments by a nebulizer. There must be a waiting period after each dose to determine the amount of response to that specific dose. The amount of response is determined by sequential peak flow measurements. If little or no response is noted, the next dosage is given until a 20% decrease in peak flow is noted. The amount of agent needed to provoke constriction can show how much sensitivity the patient has and the severity of the patient's asthma.
Another form of testing is the exercise test. This is used to diagnose exercise induced asthma in patients thought to have this problem. This test is very extensive and must be done in the presence of well trained personnel as well has having a physician present. These tests use either a treadmill, bicycle, or stepper to exercise the patient. Special equipment is attached to the patient to record heart rate, blood saturations, air flow rates as well as the mix of gases being inhaled and exhaled (oxygen and carbon dioxide). Through the use of these devices it can be determined if the patient has asthma brought on by exercise or if their problem has other origins.