Different types of spirometer

Spirometers can measure volume directly or can calculate volume from a flow measurement. Currently most spirometers are flow-measuring devices. The most common spirometers measure the flow of air by:

Air passes through the spirometer and directed by the swirl plate into the turbine, rotating the vane through the sensor either an optical or capacitance sensor which emits a series of pulses to the microprocessor receiver.

The spirometer consists of a fixed resistance known as a pneumotachograph, either a Fleich or a lilly type. As air passes through the spirometer across the resistive element there is a drop in pressure from the air hitting the resistance to the other side. The difference in pressure across the resistive element is measured by a sensitive transducer and is proportional to the flow of air. The pressure difference is directly proportional to flow.

There are two methods. One method works on the principle of a flow of air passing through a tube with obstructive object, when the air meets the obstruction the flow into turbulent flow and creating waves. Each wave passes through an ultrasonic wave beam producing a pulse proportional to a specific volume. The pulses are counted and calculated electronically to produce a volume measurement.

The second method measures the velocity of a linier air through an augmentation or retardation of an ultrasound signal. There is a pair of transducers positioned opposite one another as the air passes through the transmission time between a pair is altered, the difference between the upstream and downstream flow is converted into a flow signal.

Flow-measuring devices are relatively cheap, compact and portable, and are more widely available than volume measuring devices.

The nature of the flow-measuring technology also makes them more suitable for viewing and printing the graphical display.

Its electronic format also allows results to be stored electronically, which also means it can be possible to upload these results directly to the patients’ medical records.

Ultimately, an informed decision should be made with the medical team based on necessity, practicality and ease of use.

Whilst spirometers come in a varied assortment of sizes, it is important to choose one with an adequate sized screen to allow a thorough inspection of the trace. To correlate individual spirometric traces with the correct data, the traces should be easily identifiable – such as colour coding the different traces, for example.

  • capable of accumulating volume for a minimum of 15 seconds – this is necessary to measure the FVC of patients with airways obstruction
  • accomodate large vital capacities it should be able to measure at least 8 litres (BTPS) with an accuracy of at least +/-3% of the reading, or 0.05 litres, whichever is greater
  • if using a flow-measuring device, this should be able to measure flows in the range of 0 litres/second to 14 litres/second
  • the total resistance to airflow at 14 litres/second, when measured with all testing hardware in place, must be less than 15cmH2O / 0.15kPa
  • given that water vapour may build up in the system following repeated measurements, the resistance requirements must be met under BTPS conditions for up to eight successive expired FVC measurements. These must be performed within a ten minute period

If you measure a volume–time curve plotted as a hardcopy, the volume scale must be ≥10 mm·L−1 (BTPS). For a screen display, 5 mm·L−1 is satisfactory.

Generally, a hard copy should be stored in the patient notes with a photocopy for your own records

In primary care, spirometry reports should be filed in an electronic copy and correctly coded in the electronic patient record.

Accurate spirometry is an integral component of COPD and asthma management.

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