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From a practical point of view it is
necessary to perform calibration checks on spirometers: a
calibration syringe is generally needed. The frequency of
performing checks will vary with the clinical setting and
the type of instrument being used, and the need to adjust
the calibration will depend on whether it is outside control
limits. Flow-type spirometers generally require daily
calibration checks. An important determinant is the
stability of the calibration over time and this can only be
established with hindsight, having performed many
calibration checks on the instrument. All spirometers must
be recalibrated after cleaning or disinfection, or if an
unusual or unexpected result indicates a problem.
Typically, spirometers should be accurate (volume to within
± 0.05 L or ±3%, whichever is greater; flow to within ±0.2
L/sec or ±5%, whichever is greater) and calibrated
periodically with an accurate (certified) 3 L syringe.
When a spirometer is moved into a cooler or hotter
environment, it is important to allow time for it to reach
the new temperature and to measure it, otherwise the BTPS
correction factor will be incorrect. Similarly, the
calibration syringe needs to be at the same temperature as
the spirometer and for this reason it is usually stored near
the spirometer. In order to detect changes in overall
spirometer performance, the ventilatory function of one or
more subjects with stable respiratory function should be
measured and recorded regularly as part of an ongoing
quality control programme.
Records of calibration checks, quality control and service
history should be kept with the equipment. In the surgery,
testing yourself (if you have stable function) on your
spirometer every week or two is a practical way of ensuring
quality control. A variation of >5% in FEV1 or
FVC should alert you to a problem and the need to have your
instrument properly checked and serviced.
Flow measurement devices (e.g. pneumotachographs,
turbinometers) should be checked regularly for linearity
over the physiological range of flows (0-14 L per
second). A good test of linearity is to deliver a given
volume (e.g. with a 3 L syringe) at a wide variety of
flows, ensuring that the volume recorded by the instrument
is close to 3.00 L over the whole range of flows. When
3 L is passed into the spirometer it should record a volume
to within ±3.5%; that is, a spirometer is accurate if the
recorded volume is between 2.895 L and 3.105 L.
Peak flow meters can generally be expected to wear out after
about 12 to 24 months of heavy use, although there is little
published data to support this, whereas a
volume-displacement spirometer will usually last years if
properly maintained and serviced.
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