Ventilation Measurements: These dynamic pulmonary function tests measure two things. One is the resistance that inspired and expired air meet in your airways. (Remember that airway obstruction increases resistance.) The other is airflowthe amount of air entering or leaving your lungs in a particular time periodwhich is partially determined by resistance.

Resistance must be measured in a plethysmograph (which can also be used for the indirect determination of residual volume, functional residual capacity, and total lung capacity). The plethysmograph is a five-foot-high box, two and one-half feet in both width and depth, with one or more clear sides. You sit inside this box, wearing a nose clip and breathing through a mouthpiece. The basic maneuver is repeated several times. First you breathe quietly, then pant gently about once per second. While you are panting, a shutter will close off the mouthpiece for just a few seconds. As soon as it reopens, you breathe quietly again. The information gained from this test is the speed with which air flows out of your lungs and the effort your respiratory system is making (its "driving pressure") to push air out at this speed. Then airway resistance and various lung volumes are calculated from this initial data.

Airflow is measured with a technique that is probably one of the most fruitful technological developments for assessing airway obstruction. You breathe through a mouthpiece connected to specialized monitoring and recording equipment that gives an instant-by-instant comparison (on a graph) of airflow speed with the volume of air already in the lungs at that instant. This is recorded on the graph during one maximum expiration (from total lung capacity to residual volume) followed by one maximum inspiration (from residual volume back to total lung capacity). The recording's shape on the graph is curved, and is called a flow-volume curve. Since airway obstruction is magnified during expiration, the expiratory portion of this flow-volume curve produces a very characteristic picture when airway obstruction is present.

The flow-volume curve contains particularly useful information for determining the degree of airway obstruction: (1) the amount of air that can be pushed out of the lungs in one second (called FEVl for "forced expiratory volume in one second"); (2) the highest airflow rate achieved during this one second (called PEFR for "peak expiratory flow rate"); and either (3) the average airflow for the middle 50% of the expiratory curve (called FEF25-75% for "forced expiratory flow for the mid-50%"); or (4) the airflow occurring when exactly one-half of the vital capacity has been expired (called V50 or "expiratory airflow at 50% of vital capacity.") When the airways are obstructed, these airflow measurements usually decrease.

Assessing the Reversibility of Obstruction: Traditionally, the airway obstruction produced in COPD was considered irreversible. Happily, current wisdom finds that it is actually partially reversible in many COPD patients. To see if you fall into this category and, if you do, to determine the degree of reversibility, two sets of pulmonary function tests will be done, one right after the other. Between the two sets, you will be given a spray bronchodilator medication (and instructed how to use it). If your second set of tests shows significantly improved airflow, your airway obstruction can be improved by this medication.

Failure to respond to the bronchodilator on the first try does not automatically mean that this medication cannot improve your airflow. A retest should be done, because there are several possible reasons for an initial failure to respond. What are these reasons? Unfamiliarity with the proper technique for taking a bronchodilator often prevents enough of the drug from reaching the airways where it is needed. Or, if the primary area of obstruction is in the smaller airways, it can't be reached by an inhaled bronchodilator. Finally, a bronchodilator cannot counteract obstruction caused by mucus plugging, inflammation, or structural changes in the airways.

The only problem with the pulmonary function retest after using the bronchodilator concerns the lack of firm guidelines for establishing whether or not adequate airflow improvement has indeed occurred. Various tentative recommendations are all that currently exist, with most physicians using their own personal criteria. The following recommended criteria for true bronchodilation seem logical to us: post-bronchodilator measurements should be greater than the initial measurements by: 15% in FEV1, 20% in FEF25-75%, and/or 10% in vital capacity even without any improvement in airflow rates.

Gas Exchange Measurements: The ventilation tests tell your doctor to what degree your airways are obstructed and roughly how much of your lung tissue is lost or nonfunctioning. But they do not tell him how well your remaining functional lung tissue is doing at getting oxygen into the blood and carbon dioxide out of it.

To evaluate your lungs' ability to achieve adequate gas exchange, one thing your doctor needs to know is how much oxygen passes into your blood in a specific amount of time. The doctor learns this from a diffusion test. He may also want to know exactly how much oxygen and carbon dioxide are in your arterial blood (the blood flowing from your lungs into your heart). This information comes from a blood gas analysis.

Measuring the actual transfer of oxygen from your lungs to your blood is difficult. This diffusion test would require sampling blood directly from your heart. Happily, there is a much easier way of gaining this information. They dilute a tiny amount of carbon monoxide in the air you breathe. You hold one large breath of this air for ten seconds before breathing out. Comparing the amounts of carbon monoxide in the air you breathed in and the air you breathe out after ten seconds tells us the degree of pressure in your air sacs that is pushing oxygen into your bloodstream.