Volume-Displacement Spirometers

Conventional spirometers provide a direct measure of respired volume from the:

• displacement of a bell (water sealed);
• piston (rolling seal); or
• bellows (e.g. wedge bellows).

The results are normally presented as a graphic display of expired volume against time (a spirogram). The indices FEV₁, FVC and VC are generally manually calculated (including correction to BTPS) from the spirogram by the operator and for this reason volume-type spirometers are considered time consuming and less convenient for routine use in the doctor's surgery.

Generally, volume spirometers are simple to use, accurate, reliable, easy to maintain and provide a clear and permanent record of the test. They are, however, less portable than flow spirometers, and more difficult to clean and disinfect.

Flow-Sensing Spirometers

Over recent years advances in electronics and microprocessor technology have led to the development of a new range of portable spirometers. Flow spirometers generally utilise a sensor that measures flow as the primary signal and calculate volume by electronic (analog) or numerical (digital) integration of the flow signal. The most commonly used flow sensors detect and measure flow from:

• the pressure drop across a resistance (e.g. pneumotach or orifice);

• the cooling of a heated wire (anemometer);

• electronically counting the rotation of a turbine blade; or

• the time of flight of an ultrasonic sound pulse directed across the expired gas flow (ultrasonic sensor).

For the general practitioner these devices have largely replaced the volume spirometer because they are usually portable and they automatically calculate a large range of ventilatory indices, provide immediate feedback on the quality of each blow, select the best result, store patient results, calculate reference values for the patient being tested and provide a print-out of the results including the spirogram and flow-volume loop. These features, together with their portability, ease of use and maintenance (e.g. cleaning and disinfection) have resulted in the increasing popularity of flow-based spirometers.

Some flow spirometers have single patient use disposable sensors, effectively eliminating the need for cleaning and disinfection. However, the accuracy of each new sensor may need to be established. Accuracy and reproducibility depend on the stability and calibration of the electronic circuitry and appropriate correction of flow and volume to BTPS conditions.

Spirometers need to be calibrated (or their accuracy validated) regularly (see Appendix A).

Monitoring Devices

Mechanical devices for personal use by patients, such as the peak flow meter, have been available for several decades for serial monitoring of lung function and have proven useful in the management of asthma. Most peak flow meters are robust and provide reproducible results essential for serial monitoring. However, they often have limited accuracy and, because they provide only a single effort-dependent index of ventilatory function, they have limited application in the initial assessment of respiratory diseases.

Measurements of PEF are reduced in diseases causing airways obstruction. Peak flow monitoring is particularly useful for following trends in lung function, quantifying response to treatment and identifying trigger factors in asthma.

Portable peak flow meters are a reasonably reliable tool for patients to monitor their own airway function.

Recently several small, inexpensive yet accurate battery-powered devices for measuring ventilatory function (including FEV₁) have been developed, some of which can store the test data which can be downloaded onto a computer for review and statistical analysis.

Factors to Consider when Choosing a Spirometer

• Ease of use
• Provision of easy to read real-time graphic display of the manoeuvre
• Provision of immediate quality feedback concerning the acceptability of blows, including reproducibility
• Provision to interface with clinical software packages
• Provision of customisable final spirometry report
• Provision to print the final report
• Price and running costs
• Reliability and ease of maintenance
• Training, servicing and repair of the spirometer provided by supplier
• Ability to trial the spirometer in your setting before purchase
• Provision of a disposable sensor or a breathing circuit that can be easily cleaned and disinfected
• Provision of appropriate normal reference values with lower limits of normal
• Robustness
• Provision of a comprehensive manual describing the spirometer’s operation, maintenance and calibration
• Calibration requirements
• Conformance with accepted spirometry performance standards
• Compliance with electrical safety standards

A summary of the specifications and features of spirometers currently available in Australia and New Zealand is provided in the Spirometer Users’ and Buyers’ Guide, which is published on the National Asthma Council Australia website.

Faced with such a large variety of spirometers, general practitioners have to choose an instrument suitable for use in their own surgery. Readers are advised to contact their State Asthma Foundation for further information and advice on peak flow meters, and local respiratory laboratories regarding spirometers.

Content Updated 28 March, 2008