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Spirometry — What are they going on about?

[Kevin Baker - Respiratory Resource Nurse, Wanganui Hospital] With all this talk about COPD [Chronic Obstructive Pulmonary Disease] and the current emphasis on spirometry testing, I thought I would explain how this test is performed and what it can tell you about your lungs. Spirometry testing, in various forms, has been around for over 150 years. The original equipment depended upon the amount and speed of water displaced by the breath to measure the volume and flow of air in the lungs. As you can imagine, the more water displaced by the breath – the greater the lungs voluntary capacity. If the water is pushed out quickly then the airways are wide open – similar to having a good peak flow. Conversely a low lung volume implies small or restricted lungs and low airflow suggests narrow or obstructed airways. With the coming of the computer chip, engineers have replaced the old water spirometers with spirometers that use carefully-calibrated wind vanes or temperature differences to measure airflow, and determine if the results are normal. They are now accurate, portable, reliable and cheap ($2000 -$5000). Spirometry is useful in diagnosing, measuring and evaluating therapy in diseases such as asthma and COPD. A single test can be very useful in detecting early smoking-related changes in the lungs. One of the insidious consequences of smoking cigarettes is that the lung changes associated with COPD often occur long before there are symptoms of the disease. Spirometry will reliably detect these early changes. The size of the airways in an asthmatic are variable depending on trigger exposure, therapy and age, height and sex. Spirometry is useful in determining if therapy is returning the airway to a normal diameter. The most useful measures are the FEV1 (Forced expiratory volume in 1 second ) and the FVC (forced vital capacity). The FEV1 has proven to be a sensitive measure of airway caliber and is defined as the total volume of air, in liters, expired in the first second of forced expiration. A series of tests, done a few weeks or months apart are useful in fine tuning therapy. The test procedure is simple, but requires an experienced operator and a cooperative patient. After setting up the equipment and feeding in information on age, height and sex, the patient takes a full breath and exhales as forcefully as possible through the mouthpiece until their lungs feel completely empty. This can be very hard work and often causes lightheadedness, coughing, breathlessness and uncontrolled closing of the throat – nothing permanent I need to add. At least three “good blows” are required to ensure that an acceptable result is obtained. The results are then either printed out or displayed for review. It is important for the patient to work hard, and for the tester to be precise in the administration of the test. Most inaccurate tests are the result of operator error, rather than equipment failure. Spirometers are regularly tested to ensure they are accurate, and the heads should be disinfected frequently to reduce the chance of spreading bugs. Disposable one-way mouthpieces are recommended. We do not try and treat high blood pressure without blood pressure readings taken regularly; neither can we treat diabetes without measuring the blood sugar levels. With the introduction of relatively cheap and reliable spirometers, smokers, asthmatics and other folk with breathing problems will find spirometry a part of every routine check-up.

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