Researchers are still looking for accurate methods to predict a person's chances of developing airway obstruction. None of the current ways used to diagnose COPD detects the disease before irreversible lung damage occurs. While many measures of lung function have been developed, those most commonly used determine:
- air-containing volume of the lung (lung volume),
- the ability to move air into and out of the lung,
- the rate at which gases diffuse between the lung and blood, and
- blood levels of oxygen and carbon dioxide.
Lung volumes are measured by breathing into and out of a device called a spirometer. Some types of spirometers are very simple mechanical devices which record volume changes as air is added to or removed from them. Other kinds are more sophisticated and use various types of electronic equipment to determine and record the volume of air moved into and out of the lungs. The three volume measures most relevant to COPD are forced vital capacity (FVC), residual volume (RV), and total lung capacity (TLC). The forced vital capacity is the maximum volume of air which can be forcibly expelled after inhaling as deeply as possible. Not all of the air in the lungs is removed when measuring the vital capacity. The amount remaining is called the residual volume. The total lung capacity is the combination of the forced vital capacity and residual volume. While most of the measured lung volumes or capacities change to some degree with COPD, residual volume usually increases quite markedly. This increase is the result of the weakened airways collapsing before all the normally expired air can leave the lungs. The increased residual volume makes breathing even more difficult and labored.
It is necessary to compare the results of several different tests to make a correct diagnosis of COPD.
Because COPD results in narrowed air passages, a measure of the rate at which air can be expelled from the lungs can also be used to determine how severe the narrowing has become. In this test, the forced vital capacity maneuver, the patient is asked to inhale as deeply as possible, and on signal, exhale as completely and as rapidly as possible. The volume of air exhaled within 1 second is then measured. This value is referred to as the forced expiratory volume in 1 second (FEV 1 ). When FEV 1 is used as an indicator of lung function, the average rate of decline in patients with chronic obstructive lung disease is observed to be two to three times the normal rate of 20-30 milliliters per year. This volume may also be expressed in terms of the percent of the vital capacity which can be expelled in 1 second. As COPD progresses, less air can be expelled in 1 second. A greater than expected annual fall in FEV 1 is the most sensitive test for COPD and a fairly good predictor of disability and early death.
Another measure of lung function is called diffusing capacity. For this, a more complicated test determines the amount of gas which can move in a given period of time from the alveolar side of the lung into the blood. A number of conditions can cause the diffusing capacity to decrease. However, in COPD the decrease is the result of the destruction of alveolar walls which leads to a significant decrease in surface area for diffusion of oxygen into the blood.
Because the primary function of the lung is to remove carbon dioxide from the blood and add oxygen, another indicator of pulmonary function is the blood levels of oxygen and carbon dioxide. As chronic obstructive pulmonary disease progresses, the amount of oxygen in the blood decreases and carbon dioxide increases.
In most cases, it is necessary to compare the results of several different tests in order to make the correct diagnosis, and to repeat some tests at intervals to determine the rate of disease progression or improvement. Measurement of FEV 1 and FEV 1 /FVC ratio should be a routine part of the physical examination of every COPD patient. It is hoped that current research will result in more accurate and earlier measures for detecting lung destruction and diminished function.