ECG:Chest Pain =Spirometry:Dyspnea

Q:What is the best and most efficient method of evaluating pulmonary function in primary care office practice?

A:Pulmonary function tests include spirometry, measurement of lung compartments and diffusion tests, as well as more sophisticated physiologic analyses. Spirometry, the most common and useful of these tests, is to dyspnea what the ECG is to acute chest pain. Unlike ECG, however, spirometry can identify occult disease in asymptomatic patients.

The importance of spirometry. Simple office spirometry is recommended for your patients aged 45 years or older who smoke cigarettes. It is also warranted for patients with respiratory symptoms, such as chronic cough, episodic wheezing, and exertional dyspnea, in whom it can detect airways obstruction resulting from asthma or chronic obstructive pulmonary disease (COPD).

Spirometry measures the volume of air expired with maximum force after a full inspiratory effort (forced vital capacity [FVC]), as well as expiratory flow rate. The residual volume (RV) is the volume of air remaining in the lungs at the end of a maximal expiration. The amount of air expired in the first second is the forced expiratory volume in 1 second (FEV₁). The total lung capacity is the sum of the FVC and the RV.

Normally the FEV₁ accounts for 70% or more of the FVC; the ratio is therefore important. In obstructive diseases, the FEV₁:FVC ratio is less than 70%. The obstructive diseases include asthma, COPD, cystic fibrosis, and a number of other disorders in which airflow is reduced. In restrictive diseases, such as sarcoidosis and congestive heart failure, the FEV₁:FVC ratio increases because the FVC is low. Thus, ratios of 85% or more strongly suggest a restrictive disease, but the actual measurement of the residual volume may be needed for confirmation.

If you think of the FEV₁ as a flow test and the FVC as a volume test, spirometric interpretation becomes simple. Spirometry alone does not make the diagnosis. The 2 fundamental spirometric measurements must be put in the context of the patient’s responses to challenge with such therapeutic agents as bronchodilators and corticosteroids.

Flow-volume loops. These provide the same information as the time-volume curves of simple spirometry, but expressed in a different way. I prefer the volume over time curves, because you can directly see the FEV₁, the FVC, and the expiratory time. In the flow-volume curves you see the peak flow, which is an indicator of good effort, but you do not see the FEV₁ or the expiratory time, although the computer does provide this information.

There may be some qualitative value in looking at the flow-volume patterns, but this is not really important for the clinician. The flow-volume curve also shows the inspiratory curve, which is abnormal in states of upper airway obstruction. These conditions are relatively rare, however, and are best managed by a pulmonologist.

Pet peeves and myths. Some spirometer manufacturers put a number of extraneous values on the printout. For example, the middle part of the expiratory flow curve reflects the forced expiratory flow between 25% and 75% of the expressed volume, or FEF_25%-75%. Some clinicians still believe, incorrectly, that the FEF_25%-75% is a measure of small airways disease. Remember that alveoli empty into small airways, small airways empty into large airways, and the whole breath empties into the spirometer. The FEV₁ and FVC are measured by a flow sensor or by instruments that use volume displacement.

An acceptable alternative to FVC. Because healthy lungs empty in 6 seconds, this brief amount of time is all that is required in office spirometry. The forced expiratory volume in 6 seconds (FEV₆) is a good surrogate for the FVC, and the FEV₁:FEV₆ provides as reliable a ratio as the FEV₁:FVC. It has the added advantage of being less physically demanding for the patient.

Spirometric abnormalities predict all causes of mortality. That is why the inventor of the spirometer, John Hutchinson, coined the term “vital capacity,” or capacity to live. No primary care practitioner can effectively manage patients with obstructive or restrictive diseases without office spirometry.

Getting the message out. “Test your lungs, know your numbers” is the motto of the national health care initiative known as the National Lung Health Education Program. Most patients know their blood type, blood pressure range, and cholesterol levels. They also need to know how their lung function measures up—that is, their lung age. Lung age is the age when a person’s measured lung function is normal. If a person’s lung age is much older than the chronologic age, lung disease is present.

For example, an asymptomatic 40-year-old man who is 6 ft 1 in and whose FEV₁ is 3.76 L has the lungs of a 70-year-old man. Normal FEV₁ for this man would be 4.49 L (83% of predicted).

FOR MORE INFORMATION:

• Ferguson GT, Enright PL, Buist AS, Higgins MW. Office spirometry for lung health assessment in adults: a consensus statement from the National Lung Health Education Program. Chest. 2000;117:1146-1161.
• Petty TL. Simple office spirometry. Clin Chest Med. 2001;22:845-859.
• Swanney MP, Jensen RL, Crichton DA, et al. FEV6 is an acceptable surrogate for FVC in the spirometric diagnosis of airflow obstruction and restriction. Am J Resp Crit Care Med. 2000;162:917-919.