Spirometry Made Simple

By: Thomas L. Petty, M.D.

Published in Advance for Managers of Respiratory Care

January, 1999, Volume 8, pp 37, 38, 41.

Simple, in-office spirometry is the key to diagnosing patients with both common and less common lung diseases. Spirometry is also extremely valuable in assessing the risk of other common diseases primary care physicians (PCPs) encounter, including lung cancer, heart attack and stroke.

In fact, abnormal spirometry can predict all mortality causes. Physicians need spirometry results to judge responses to therapy, as well as to plot the course and prognosis of all lung disease B most notably asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis and interstitial lung disease.

Despite spirometry's value, its routine use is something PCPs have been slow to accept. This is because they are unsure what spirometry measures and how to use those measurements to reach a diagnosis.

THE BASICS

Simply stated, spirometry measures airflow from fully inflated lungs. The lungs within the thorax create balanced opposing elastic forces. (See Figure 1) These elastic forces are equal, yet pull in opposite directions, which leaves a small amount of air in the lungs. This air is called the residual volume.

 

Similarly, the amount of air forced, or blown, out of the lungs is known as the forced vital capacity (FVC). The forced expiratory volume (FEV1) is the amount of air blown out in the first second. Normally, the lung empties 70 percent to 75 percent of its total air volume within one second.

The factors associated with expiratory airflow are related to pressure and resistance. In patients with ventilatory abnormalities, such as emphysema, airflow is limited by loss of elastic recoil, and in those with conditions like bronchospasm, mucosal edema, mucus retention and inflammation, the airway narrows. (See Figure 2)

Physicians should consider spirometry a simple expression of a complex process. Such is the case with blood pressure, which represents cardiac output, peripheral vascular resistance, blood volume, blood viscosity and the complex neurogenic and humeral factors that control peripheral resistance. Lung function measures elastic recoil, large and small airway resistance, interdependence between airways and alveoli, and muscular effort and coordination. (See Table 1)

Table 1:

 

Blood Pressure

(sphygmomanometry)

120/80

Lung Function

(spirometry)

3.0 FEV1/4.0 FVC

Cardiac output

Peripheral resistance

Blood volume

Blood viscosity

Renin-angiotension axis

Elastic recoil

Airways resistance

Large airways

Small airways

Interdependence, etc.

Muscular effort and coordination, etc.

INTERPRETATION TIPS

Normal values for lung function in spirometry are based on patient age, gender and height. For instance, younger, taller, men have a larger predicted FVC and FEV1 than older, shorter, women. Because of arm and leg span differences, black non-smokers tend to have normal values of about 15 percent less than white non-smokers. Conveniently, modern spirometer software has nomograms that compare the patients= values with predicted norms.

Physicians use two conventions in the display of the spirometric curves B volume over time and the flow volume loop. I like the volume over time method, the most preferred historically, because it allows physicians to directly visualize the vital capacity, FEV1 and the expiratory time. Neither the FEV1 nor the expiratory time can be directly visualized on the expiratory flow volume loop, but they are displayed on the printout.

However; the second method, the flow volume loop, has the advantage of displaying the peak flow and some expiratory flow volume curve (FVC) features, such as convexity in states of loss and elastic recoil, both of which have clinical meaning to pulmonologists. A visual display of FEV1, FVC, FEV1/FVC actual values and percent of predicted normal values by simple spirometers is often more suitable for PCPs. Flow volume pattern recognition is actually much simpler than the pattern recognition applied daily to EKG interpretation. However, modern spirometers also give an interpretation of the normal or various degrees of airflow obstruction or restriction.

The algorithm depicted in Figure 3 can summarize spirometric interpretation. Since airflow disorders lower the FEV1 before the FVC, the ratio between the two is the starting point in interpretation. If the patient=s FEV1/FVC ratio is low, for instance less than 70 percent, then the patient has an obstructive defect. Patients with complete or nearly complete airway obstructions that resolve with inhaled bronchodilator use have asthma. Those with irreversible airflow obstructions have COPD. However, considerable overlaps between asthma and COPD are common.

Marked hyperinflation, which leads to air trapping, lowers vital capacity. Patients with low FVC and a low FEV1 may have a combined ventilatory defect. Physicians can assess the patient for such as defect by measuring lung compartments via a chest x-ray or simply observing whether the patient has a hyperinflated chest. If the FEV1/FVC ratio is normal or high, the patient could either be normal or, in the case of a low FVC, have a restrictive defect. The simple algorithm in Figure 3 is a gross oversimplification, but a reasonable first step in evaluation.

Spirometric abnormalities are also predictors of heart attack, lung cancer and stroke. A low vital capacity in heart disease may be due to pulmonary congestion, pleural effusion, cardiomegaly or muscular weakness, which impairs the chest bellows function. We don=t know how COPD is linked to lung cancer. But, with any degree of airflow obstruction, the risk of lung cancer is three to five times higher, depending on the patient=s age and smoking history. Spirometric abnormalities are actually predictive of all causes of premature mortality.

EVALUATION TOOL

In addition to being a valuable diagnostic tool, a spirometer is also a helpful monitoring evaluation tool to judge responses to therapy. Yet many physicians do not use it that way.

While no physician would give insulin to a diabetic without measuring blood sugar or an antihypertensive to a patient without measuring blood pressure, these same physicians often prescribe powerful beta agonists, anticholinergics and even corticosteroids without performing spirometry. This failure to document the presence of airflow in response to therapy has led to many lawsuits involving steroid complications.

Given these facts, it is certainly time for all primary care physicians to equip themselves with a simple spirometer; they are easy to use, can result in the discovery of important prognostic information for patients, and can guide beneficial therapy. Simple, economical handheld devices are already available, and, as manufacturers and physicians alike begin to appreciate the value of spirometry in the primary care setting, models that cost $500 or less will also become available.

We must abandon the notion that spirometry is a mystical thing done in a pulmonary function laboratory. Times have changed. We are entering a new era.

SPIROMETRY IN SMOKERS

The National Lung Health Education Program (NLHEP) now recommends simple two-parameter spirometry for all smokers over 20 pack years, and for anyone with cough, dyspnea, excess mucus or wheeze.

If a patient has normal spirometry in middle age, it's highly unlikely that any abnormality will occur in his lifetime. So, if tests on a patient over 40 yield normal results, the patient does not need repeat spirometry unless symptoms of dyspnea, cough, wheeze or excess mucus occur.

By contrast, if a patient has abnormal test results, he faces the strong likelihood of accelerated lung function losses over time, unless bronchoactive drugs can reverse the process. Patients with airflow obstruction and 30 or more pack years of smoking have a 2 percent to 3 percent chance of having occult carcinoma at the time of testing. Subsequent testing will reveal more lung cancer, i.e., up to 5 percent in heavy smokers with airflow obstruction. These patients are candidates for sputum cytology in a qualified laboratory, followed by chest x-rays and bronchoscopy if malignant or pre-malignant cellular abnormalities are found.

 

Dr. Petty is a professor of medicine at the University of Colorado Health Sciences Center in Denver, and chairman of the National Lung Health Education Program. He is also an ADVANCE editorial advisory board member.