ECG:Chest Pain =Spirometry:Dyspnea

Q:What is the best and most efficient method ofevaluating pulmonary function in primary careoffice practice?

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

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

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

Normally the FEV₁ accounts for 70% or more of the FVC; the ratio istherefore important. In obstructive diseases, the FEV₁:FVC ratio is less than70%. The obstructive diseases include asthma, COPD, cystic fibrosis, and anumber of other disorders in which airflow is reduced. In restrictive diseases,such as sarcoidosis and congestive heart failure, the FEV₁:FVC ratio increasesbecause the FVC is low. Thus, ratios of 85% or more strongly suggest a restrictivedisease, but the actual measurement of the residual volume may be neededfor confirmation.

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

Flow-volume loops. These provide the same information as the time-volumecurves of simple spirometry, but expressed in a different way. I prefer thevolume over time curves, because you can directly see the FEV₁, the FVC, andthe expiratory time. In the flow-volume curves you see the peak flow, whichis an indicator of good effort, but you do not see the FEV₁ or the expiratorytime, 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 curvealso shows the inspiratory curve, which is abnormal in states of upper airwayobstruction. These conditions are relatively rare, however, and are best managedby a pulmonologist.

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

An acceptable alternative to FVC. Because healthylungs empty in 6 seconds, this brief amount of time is allthat is required in office spirometry. The forced expiratoryvolume in 6 seconds (FEV₆) is a good surrogate for theFVC, and the FEV₁:FEV₆ provides as reliable a ratio as theFEV₁:FVC. It has the added advantage of being less physicallydemanding for the patient.

Spirometric abnormalities predict all causes of mortality.That is why the inventor of the spirometer, JohnHutchinson, coined the term "vital capacity," or capacity tolive. No primary care practitioner can effectively managepatients with obstructive or restrictive diseases withoutoffice spirometry.

Getting the message out. "Test your lungs, knowyour numbers" is the motto of the national health care initiativeknown as the National Lung Health Education Program.Most patients know their blood type, blood pressurerange, and cholesterol levels. They also need toknow how their lung function measures up--that is, theirlung age. Lung age is the age when a person's measuredlung function is normal. If a person's lung age is mucholder than the chronologic age, lung disease is present.

For example, an asymptomatic 40-year-old man whois 6 ft 1 in and whose FEV₁ is 3.76 L has the lungs of a70-year-old man. Normal FEV₁ for this man would be4.49 L (83% of predicted).

FOR MORE INFORMATION:

• Ferguson GT, Enright PL, Buist AS, Higgins MW. Office spirometry for lunghealth assessment in adults: a consensus statement from the National LungHealth 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 surrogatefor FVC in the spirometric diagnosis of airflow obstruction and restriction. Am JResp Crit Care Med. 2000;162:917-919.