The National Heart, Lung and Blood Institute Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma (http://www.nhlbi.nih.gov/guidelines/asthma/) recommends the use of spirometry in caring for all patients with asthma—including children—both to establish the diagnosis and also for regular monitoring of the disease. This objective measure of lung function is recommended because history and physical examination do not reliably correlate with lung function, and peak flow measurements are not an adequate substitute. One of the primary goals of asthma treatment is to maintain normal lung function, and thus it must be measured regularly to assure that this goal is being met. The guidelines indicate that all children 5 years of age and older can perform spirometry, and in fact many children as young as 3 years of age can produce interpretable spirometry.
One of the barriers to the incorporation of this essential management tool into the routine office practice of pediatricians is the mistaken notion that the test results are difficult to interpret. In fact, if a methodical approach is followed, and if particular attention is paid to the flow-volume curve, physicians can easily tell whether the child generated interpretable results, whether the results are normal or abnormal, whether the pattern is obstructive or restrictive (if the result is abnormal), and whether there is a response to bronchodilator. The numerical values in terms of percent of predicted are then used only to confirm the impression of the flow-volume curve.
When interpreting a spirometry report, attention should be focused on the expiratory flow-volume curve, which plots flow (in liters per second on the vertical axis) and volume (in liters) on the horizontal axis. Patients are asked to inhale completely, put the spirometer mouthpiece in their mouth, and blast the air out as hard and as fast as they can. They continue to blow in one continuous breath until they can not expel any more air. This maneuver is similar to blowing out candles on a birthday cake—a description that is helpful for many patients.
At the beginning of this maneuver on the flow-volume curve, even though the patient’s lungs are full before they start blowing, there is no flow and no volume (0 on both axes). When the patient blasts the air out, there is a sudden increase in flow (on the vertical axis), which then steadily declines back to 0 when the end of the expiratory effort is reached. During this effort, the absolute volume of air exhaled is recorded on the horizontal axis. Connecting these 3 points, before starting the expiration (0 flow, 0 volume), the peak of the blast (high flow), and the end of the expiratory effort (0 flow, expired volume) creates a triangle, which is the shape of a normal expiratory flow-volume curve as below.
The dashed line in the figure indicates the predicted flow-volume curve for a given patient. The spirometry software generates this predicted curve based on information provided about the patient—specifically his or her age, height, sex, and ethnicity.
Error #1. The first of these errors is a weak effort, or failing to blast the air out initially. This generates a flow-volume curve that does not have a peak, but rather is rounded across the top as below.
Error #2. The second common error is a short effort, or quitting too soon. When this occurs, the flow-volume curve may have an initial peak, but suddenly drops off back to the baseline as depicted below.
Error #3. The third and final common error is a lack of reproducibility. It is important for patients to perform at least 2 expiratory efforts that generate nearly identical or superimposable flow-volume curves. Such reproducibility can only be achieved with maximal expiratory efforts. An example of efforts that are not reproducible, and therefore not acceptable, are shown below.
By noting these 3 common errors, we now have the first 3 of our criteria for interpreting spirometry. All 3 criteria depend on whether the patient performed adequate efforts.
Criteria for Acceptable Efforts
1. Peak: does each effort (flow-volume curve) have a sharp initial peak?
2. Finish: does each effort extend all the way down to the baseline at the end?
3. Reproducible: are at least 2 of the efforts superimposable?
If the answer to any one of these is no, the unacceptable efforts can be deleted and the patient can be coached to perform acceptable maneuvers, each of which has a peak and finishes, and at least 2 identical recordings. Note that these criteria for acceptable efforts are different than the criteria for normal efforts discussed below.
The ultimate interpretation of the flow-volume curves may be that they show a normal, obstructive, or restrictive pattern, but they all must meet the 3 criteria above for acceptable efforts.
After the patient has performed acceptable maneuvers as above, evaluation of the flow-volume curves can turn to whether they are normal. Abnormal spirometry patterns fall into 2 categories: obstructive and restrictive.
An Obstructive Air Flow Pattern
Patients with obstructive lung disease, such as asthma, have difficulty getting the air out. Given enough time, the total amount of air they exhale may be normal. Also, because the initial blast of air reflects large airways function, and because asthma is a disease of small airways, the peak on the flow-volume curve may also be normal in patients with asthma. (This is one reason why peak flow measurements are not an adequate substitute for spirometry.)
However, by definition, in obstructive lung disease, the rate at which the air is coming out is reduced (ie, the flow is reduced). This generates a "scoop" on the downward limb of the flow-volume curve, which is shown on the graph below.
A Restrictive Airflow Pattern
The other pattern of abnormality on spirometry is restrictive. Patients with restrictive lung disease, such as pulmonary fibrosis or a chest wall abnormality, have difficulty getting air in. Because they are able to inhale less air, they have less air coming out on their forced expiratory maneuver. However, there is no obstruction to this airflow. This generates a flow-volume curve that has the normal triangular shape (ie, it is not "scooped") but the triangle is smaller than would be predicted, as seen below.
We now have the remaining criteria for interpreting spirometry, having to do with whether the curve is normal or abnormal—and whether the pattern is obstructive or restrictive if the curve is abnormal:
1. Is the flow-volume curve shaped like a triangle? (if scooped = obstructive)
2. Is the triangle as big as the predicted curve? (if too small = restrictive)
If the spirometric maneuver has been adequately performed by the first 3 criteria, yet it is abnormal by 1 of the last 2 criteria, administer a bronchodilator to assess for reversibility of the abnormality. This would most often apply to an obstructive pattern. The graph below depicts such bronchodilator reversibility.
The Bottom Line
In summary, your evaluation of a spirometry report should focus on the flow-volume curves. These are assessed initially to determine whether the patient performed adequate maneuvers (peak? finish? reproducible?). Once the efforts performed have been deemed to be adequate, the flow-volume curves can be evaluated as normal (triangular in shape and close in size to the predicted curve), obstructive (not triangular, but instead "scooped"), or restrictive (triangular in shape, but much smaller than the predicted curve). Finally, any abnormality present can be assessed for reversibility with a bronchodilator.