There are many types of spirometer currently in use, and in general they perform measurements accurately. Spirometers broadly fall into two categories — those that measure volume directly, such as bellows-type spirometers; and those that measure flow and derive volume. The flow-based spirometers use a pneumotachograph or turbine, which records pressure change with time and integrates the flow-time signal to obtain volume. Computer enhancement has greatly improved the accuracy of these instruments.
Spirometers have an assortment of graphical displays, from paper printouts to digital real-time visual display of either flow-volume or volume-time curves. Hard copy can be obtained from the latter, or the signal can be passed through a computer for storage or printing. More expensive and accurate equipment tends to be used in lung function laboratories in hospitals, and simpler (less expensive) portable electronic units in primary care. The cheapest hand-held spirometers for primary care tend to provide only numerical values for FEV1 and FVC, but no printout. This is undesirable, as it offers no way of assessing the accuracy and quality of the blows.
All spirometers require calibrating, preferably with a 3L syringe. Calibration is carried out daily in lung function laboratories, but far less so in primary care. Some manufacturers of spirometers claim their equipment's calibration remains stable over 2-3 years without regular calibration.
It is important for reliable consecutive readings on a given patient that the same spirometer should be used. Some research from the UK [3,4] has compared pneumotachograph, turbine and wedge bellow spirometers. The FEV1 volumes were similar, but measurements of FVC and vital capacity (VC) with the turbine machine were 400-500 mL less than with the other types of spirometer. The authors suggest this could be due to inadequate volume measurement at low flow rates, which is frequently found in patients with moderate to severe COPD. Since these studies, manufacturers of turbine spirometers have made modifications to correct the tendency to low readings of FVC.
The essential requirements for a spirometer are thus:
• The need for calibration.
• A hard copy or visual display of blows in real time, to assess accuracy and reproducibility.
• Ideally, the ability to superimpose traces with repeated blows.
• Some electronic spirometers calculate the percentage variation between blows, or give a bleep if blows are performed inadequately.
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