Accuracy and linearity of positive airway pressure devices: a technical bench testing study

To analyze the accuracy and linearity of different CPAP devices outside of the manufacturers' own quality control environment.

Accuracy (how well readings agree with the gold standard) and linearity were evaluated by comparing programmed pressure to measured CPAP pressure using an instrument established as the gold standard. Comparisons were made centimeter-by-centimeter (linearity) throughout the entire programming spectrum of each device (from 4 to 20 cm H2O).

A total of 108 CPAP devices were tested (1836 measurements); mean use of the devices was 956 hours. Twenty-two of them were new. The intra-class correlation coefficient (ICC) decreased from 0.97 at pressures programmed between 4 and 10 cm H2O, to 0.84 at pressures of 16 to 20 cm H2O. Despite this high ICC, the 95% agreement limit oscillated between -1 and 1 cm H2O. This same behavior was observed in relation to hours of use: the ICC for readings taken on devices with < 2,000 hours of use was 0.99, while that of the 50 measurements made on devices with > 6,000 hours was 0.97 (the agreement limit oscillated between -1.3 and 2.5 cm H2O). "Adequate adjustments" were documented in 97% of measurements when the definition was +/- 1 cm H2O of the programmed pressure, but this index of adequate adjustment readings decreased to 85% when the +/- 0.5 cm H2O criterion was applied.

In general, the CPAP devices were accurate and linear throughout the spectrum of programmable pressures; however, strategies to assure short- and long-term equipment reliability are required in conditions of routine use.