A comparison of linear and logarithmic auditory tones in pulse oximeters

• Pulse oximeters are used throughout medicine but pitch-to-value mappings are not consistent.
• Logarithmic mappings led to both more accurate estimations of saturation, and more accurate estimations of saturation rate change.
• Use of logarithmic tone scales, for example a semitone scale, could be used more extensively in practice.

This study compared the ability of forty anaesthetists to judge absolute levels of oxygen saturation, direction of change, and size of change in saturation using auditory pitch and pitch difference in two laboratory-based studies that compared a linear pitch scale with a logarithmic scale. In the former the differences in saturation become perceptually closer as the oxygenation level becomes higher whereas in the latter the pitch differences are perceptually equivalent across the whole range of values. The results show that anaesthetist participants produce significantly more accurate judgements of both absolute oxygenation values and size of oxygenation level difference when a logarithmic, rather than a linear, scale is used. The line of best fit for the logarithmic function was also closer to x = y than for the linear function. The results of these studies can inform the development and standardisation of pulse oximetry tones in order to improve patient safety.