Diagnosis of Chronic Cough
A physician should evaluate any chronic cough. The initial step in the evaluation is a medical history and physical examination. This is almost always followed by a chest x-ray. Depending on the results of the initial evaluation, further diagnostic tests may be performed. The tests performed depend on the suspected cause of the cough. Some diagnostic tests include:
Bronchoprovocation to Diagnose Chronic Cough
During this test, a patient inhales increasing doses of methacholine. Between each inhalation, the patient performs a breathing test, spirometry. Patients with asthma have a reduction in lung function at low doses of methacholine. Some patients without asthma also test positive, but in a much lower proportion. Often patients are initiated on asthma therapy without a methacholine bronchial challenge test. This then is used to confirm the diagnosis.
Bronchoscopy to Diagnose Chronic Cough
Fiberoptic bronchoscopy is employed to evaluate the airways for the presence of an endobronchial cancer or a foreign body. This is a painless procedure that allows direct examination of the main airways. The gag reflex is suppressed by spraying topical lidocaine onto the back of the throat. The patient is then given light sedation. The fiberoptic bronchoscope is a tube that is approximately as big around as a pencil. It is inserted into either the nose or the mouth and passed down the back of the throat into the trachea and then into each lung. There are no pain fibers in the tracheobronchial tree, so the procedure does not hurt. The patient may experience some increased coughing during this procedure. Small doses of lidocaine are sprayed into the lungs during the procedure to inhibit the cough receptors. The procedure can take 5 minutes to an hour. Longer procedures usually occur when biopsies are taken.
Endobronchial Biopsy to Diagnose Chronic Cough
If an abnormality seen in the airways suggests a tumor, a biopsy is taken. A small wire with biopsy forceps (similar to a very small alligator clip) attached to its end is passed through the bronchoscope. Small pieces of tissue are then removed. Typically, several biopsies are taken. In most cases, these tissue samples are sufficient to identify the abnormality. This procedure is called an endobronchial biopsy, because it is done inside the bronchus. The fiberoptic bronchoscope, because of its size, can be passed only into the larger bronchial tubes, about the first 3 divisions out of approximately 23 divisions of the bronchi.
Transbronchial Biopsy to Diagnose Chronic Cough
To obtain sample tissue further away from the main divisions of the bronchi, a transbronchial biopsy is performed. This procedure is usually performed under x-ray guidance (fluoroscopy), because one cannot visualize this area of the lung. The bronchoscope is advanced as far as possible, biopsy forceps are passed to the area of lung seen on x-ray, and tissue samples are taken.
Complications of Tests Used to Diagnose Chronic Cough
Bronchoscopy is a safe procedure, but any procedure carries the risk of complications. Fever occurs postprocedure in up to 50% of patients and usually resolves within a few hours. The following complications occur infrequently: significant bleeding, in less than 4% of biopsies, and pneumothorax (collapsed lung) in 1% to 5% of patients who undergo transbronchial biopsies. Rarely, complications arise from fiberoptic bronchoscopy that include reaction to the medications, trauma due to tube insertion, laryngospasm, bronchospasm, pneumothorax (only with transbronchial biopsies), bleeding, cardiac arrhythmia, myocardial infarction, ruptured lung abscess with flooding of the airways, postbronchoscopy fever or infection, and respiratory compromise. These major complications occur at rates between 0.08% and 5%. Postbronchoscopy, the patient usually needs a couple of hours to recover and for the gag reflex to return before eating or drinking.
Pulmonary Function Tests (PFTs) to Diagnose Chronic Cough
These tests demonstrate characteristic abnormalities in lung function. There are four components to pulmonary function testing: spirometry, postbronchodilator spirometry, lung volumes, and diffusion capacity. In the initial evaluation, all four components are often performed.
Spirometry is a testing procedure that measures the amount of air entering and leaving the lungs. This simple test can be performed in most physicians' offices, with the patient sitting comfortably in front of the spirometry machine. The machine measures airflow that passes through the inhalation port attached to the machine. The inhalation device is usually a disposable cardboard tube or a reusable tube that can be sterilized after use.
The patient inhales as deeply as possible and then forms a seal around the tube with their mouth. Then the patient exhales, as forcefully and rapidly as they can, until they can exhale no more. To be an adequate test, the patient must exhale all the air they possibly can and continue exhaling for at least another 6 seconds. Usually, three separate attempts are made and the best result is used for evaluation.
Multiple measurements are obtained from this maneuver. Those most commonly used for interpretation are (1) forced expiratory volume after 1 second [FEV1], (2) forced vital capacity [FVC], and (3) forced expiratory flow at 25%-75% of maximal lung volume [FEF25-75]. They are expressed as percentages of what is predicted for normal lung function, depending on the variables of height, age, race, and sex.
- Peak expiratory flow rate (PEFR) also can be obtained. PEFR can be compared with readings the patient obtains at home with their own peak flow meter. A peak flow meter is a portable device that can be carried by the patient. It consists of a small tube with a gauge that measures the maximum force with which one can blow air through the tube.
In healthy lungs, 70% to 75% of all the air exhaled after maximal inhalation (FVC) is exhaled within the first second (FEV1), known as the FEV1/FVC ratio. In lungs with chronic obstructive pulmonary disease (COPD), the FEV1/FVC ratio falls below 70% to 75%.
The absolute value of the FEV1 is reduced in some conditions, such as chronic obstructive pulmonary disease and restrictive ventilatory defects. In restrictive ventilatory defects the FVC is reduced proportionally, preserving a normal FEV1/FVC ratio. The FEV1 is used to quantify the severity of obstruction. A FEV1 < 70% of what is predicted for age, height, weight and race is considered mild; < 50% to 69% moderate; < 35% to 49% severe; and < 35% very severe. Sometimes the only abnormality seen is a reduction in the FEF25-75. Isolated reduction in the FEF25-75 is considered an early detector of very mild obstruction. It can also be a normal variant.
- Postbronchodilator Spirometry If the FEV1 indicates a possible obstruction, spirometry is often repeated after the administration of a bronchodilator, such as an inhaled beta-agonist. An FEV1 (forced expiratory volume after 1 second) that improves more than 12% suggests that the obstruction may be reversible or partially reversible. This procedure provides some information on the potential responsiveness of the airways to medication. It is also useful for determining whether steroid treatment has been beneficial, a few weeks after initiating therapy.
- Lung Volumes
Lung volumes may be measured in two ways, gas dilution or body plethysmography. The gas dilution method is performed after the patient inhales a gas, such as nitrogen or helium. The amount of volume in which the gas is distributed is used to calculate the volume of air the lungs can hold. Lung volume measurement by body plethysmography requires the patient to sit in an airtight chamber, usually transparent to prevent claustrophobia, and inhales and exhales into a tube. The pressure changes in the plethysmograph are used to calculate volumes.
The most important measurements obtained during lung volume determinations are residual volume and total lung capacity (TLC). These measurements vary with age, height, weight, and race and are usually expressed as an absolute number and a percentage of what is predicted with normal lung function. A high TLC demonstrates hyperinflation, that is, there is more air in the lungs than there should be, which is consistent with emphysema. Increased residual volume signifies air trapping, which means that the patient cannot exhale as much air as they should. This demonstrates an obstruction to exhalation. A low TLC suggests a restrictive ventilatory defect and another pulmonary process may be involved.
- Diffusion Capacity Diffusion capacity is a measurement of gases that transfer from the alveoli to the capillary. The patient inhales a very small amount (very safe) of carbon monoxide. How much of it is taken up by the blood is measured. A reduced diffusion capacity is consistent with emphysema, but may be seen in a many other lung diseases as well.