Section 1 Spirometry and Measurement

What is spirometry?

Spirometry is a test of lung function that measures how much and how quickly air can be moved into and out of the lungs. The measurements are made using a spirometer.

What is a Spirometer?

A spirometer is an instrument used to measure respired volumes and flows (i.e. spirometry). Many spirometers are able to measure both inspiratory and expiratory airflow. 

Why Measure Spirometry?

Correctly performed spirometry, using an accurate spirometer provides:

• Rapid and objective assessment of airflow obstruction and restrictive conditions.
• Differentiation between asthma and COPD.
• Early detection and monitoring of disease progression (eg COPD).
• Quantitative assessment of the severity of airflow obstruction.
• Incorporate guideline recommendations for therapy based on COPD and asthma severity
  (COPD www.lungnet.org.au/ ;
  Asthma www.nationalasthma.org.au/publications/amh/step1.htm ).
• Quantitative assessment of the response to therapy.
• Population screening and case finding to detect airflow obstruction - especially smokers and ex-smokers (with and without symptoms), and all patients with respiratory symptoms.
• Encouragement and motivation for smoking cessation, especially if abnormal spirometry is obtained (provides a ‘teachable moment’).
• Feedback to the patient about their disease and effect of medication.
• More accurate and comprehensive assessment than peak flow.

Why Measure Spirometry in General Practice?

Primary care physicians are in a unique position to monitor the respiratory health of the community. The inclusion of spirometry as a routine test, especially in patients at risk of respiratory disease (eg smokers), will lead to earlier detection of respiratory disease and more effective intervention and treatment.

Ninety percent of non-asthmatic patients with airflow obstruction, have COPD. In addition, COPD is characterised by an accelerated decline in spirometric values.  The disease progresses slowly and the early signs (eg cough and sputum) are often ignored or are not significant enough to prompt the patient to seek treatment. Consequently, a diagnosis is often not made until about half of the lungs’ large reserve capacity is already lost. Causing significant symptoms.  Because there is a close relationship between the risk of COPD and the intensity and duration of smoking, spirometry is a very important test for the early detection of COPD in smokers and ex-smokers.  When provided with evidence of airflow limitation, patients are more likely to cease smoking and this will reduce the rate of FEV1 decline and thus modifies the natural history of the disease.

Although there is the possibility that a finding of normal spirometry in a smoker may reinforce their smoking habit, such findings can be used as ‘teachable moments’ when the patient has increased awareness of the risks. 

Medicare Rebate and 3+ Asthma Visit Plan (SIP)

Doctors who perform spirometry before and after the administration of a bronchodilator can claim under Medicare item 11506. The 3+ Asthma Visit Plan provides a SIP for practices who complete 3 scheduled visits for asthma management in patients with moderate to severe asthma.

Involvement of the Practice Nurse

Some doctors prefer to have their practice nurse perform the spirometry measurement which is the most time consuming part of the spirometric assessment. The doctor then interprets the spirometry result.

Definitions of Common Spirometric Indices

• FVC (Forced Vital Capacity) is the maximum volume of air that can be expired (or inspired) during a manoeuvre using maximal effort.
• SVC (Slow Vital Capacity) is the maximum volume of air that can be exhaled “slowly” following a full inspiration (or inhaled after a complete expiration). The SVC is similar to the FVC in subjects without airflow obstruction, but is often larger in subjects with airflow obstruction.
• FEV₁ (Forced Expired Volume in one second) is the volume of air that can be forcefully expired in the first second of the maximal expiration. It is a measure of how quickly full lungs can be emptied.
• FEV₁/FVC ratio is the FEV₁ expressed as a percentage of the FVC and gives a clinically useful indicator of airflow obstruction.
• FEF_25-75% (Forced Expiratory Flow between 25 and 75 percent of the FVC) is the average expired flow over the middle half of the FVC manoeuvre. It is regarded as a more sensitive but more variable measure of narrowing of the smaller airways than provided by FEV₁.