Spirometry

Spirometry is a quick and simple way to help document the severity of asthma. Unfortunately the physical exam in respiratory medicine has not proved to be reliable. In fact, studies have consistently shown that the patient is a much better judge of whether they have improved on objective tests than the physician who bases his/her opinion on a physical exam.

The test should be performed properly with three tries. The results on two of the tests should be within 5% of each other. Before looking at the FEV₁ and the FVC, one should note whether the curve is smooth and showing good effort. If the flow is suddenly shut off, this is due to closing of the larynx and suggests laryngospasm. This is best seen if flow volume loops are generated. A curve that is not smooth, may suggest the patient the patient is not delivering his or her best effort. Coaching and cheerleading will improve the results.

The FEV ₁, the FVC, and the FEV ₁/FVC ratio should be looked at and compared to normal values. If necessary, the results should be race corrected with Africans having lung values 80% of Caucasians. An FEV₁ less than 80% predicted and a FEV₁/FVC ratio less than 70% are the best tests to confirm obstruction. A 12% improvement in FEV₁ with bronchodilators suggest reversibility. To perform a steroid trial, give 30-40 mg of prednisone for 10-14 days. A 20% improvement that is greater than 300 ml suggests a response to oral steroids. A low FVC can be seen in the many causes of restrictive disease.

Spirometry is useful to have at least once a year so you can make sure the patient is not deteriorating and developing chronic bronchitis. It allows you to document what the best control is and to see whether a patient does respond to a trial of prednisone. It is helpful in documenting that it is safe for a patient to leave the emergency room after treatment.

Full PFTs are important if you suspect interstitial disease or significant emphysema. ABGs are necessary only if the patient seems in severe distress. It is important to realize that a PCO₂ that is normal is worrisome in asthmatics as they usually hyperventilate.

- Scott Rappard

Thanks to Dr. Dennis Bowie, Consultant respirologist at the Queen Elizabeth II Health Sciences Center in Halifax, Nova Scotia, for reviewing the draft copy of this article.