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Pulse Oximetry: Perception, Pitch, Psychoacoustics, and Pedagogy.

May 2016 • Volume 122 • Number 5 www.anesthesia-analgesia.org 1253 Copyright © 2016 International Anesthesia Research Society DOI: 10.1213/ANE.0000000000001203 Pulse oximeter pitch presentation schemata are not created equal, and this can cause a problem for anesthesiologists who depend on the auditory information presented to give information about oxygen saturation (Spo2), heart rate, distal perfusion, and regularity of the heart rate. While attending to multiple patients in the operating room or intensive care unit, our attention is divided among manifold tasks and stimuli of varying importance. Increased task demands have been shown to degrade performance to detecting pitch differences of the pulse oximeter.1,2 Although control of the acoustic environment in our workplace is increasingly out of our control,3 it is exceedingly important to invoke concepts from the fields of music perception and cognition into standardization and refinement of the pulse oximeter. Loeb et al.4 reviewed the sounds of desaturation, surveying the pulse oximeter sonifications (continuous nonspeech audio to convey information or perceptualize data). Given that acoustic properties are not standardized, the research group examined 21 pulse oximeters and found that an Spo2 value of 100% varied from 479 to 925 Hz with grouping around 670 Hz, with a time of presentation varying between 55 and 150 milliseconds, and a Spo2 value of 1% varied between 41 and 404 Hz with grouping around 225 Hz. Thus, the low extreme of 100% Spo2 of 1 pulse oximeter (479 Hz) is not terribly different from the high extreme of 1% Spo2 of another pulse oximeter (404 Hz). Although an anesthesiologist may habituate to the sonification of a particular type of pulse oximeter, varying practice locations could introduce different products and present opportunity for error. Pitch presentation aside, pulse oximeters manifested linear or logarithmic step differences, which will be discussed in more detail herein. Loeb et al.4 did an excellent job in reviewing the previous work with pulse oximetry, and I encourage the reader to read the background presented in their work.5–12 Major suggestions for improvement from Loeb/Sanderson’s group include, but are not limited to: (1a) increasing the frequency range of presentation (the range of human speech is from 300 to 3000 Hz),13,14 (1b) using logarithmic pitch mapping,9 (2) pitch changing at every 1% Spo2 from 100% to 0%, and (3) increasing the perceived loudness as Spo2 decreases to avoid tones that are quieter than the 100% tone.4 These suggestions are excellent based on the previous, current, and ongoing work, and they deserve further discussion and exploration. Brown et al.9 conducted a study to assess the ability of anesthesiologists to judge absolute levels of Spo2, direction of change, and size of change in saturation using auditory pitch and pitch difference in a laboratory study using a linear pitch scale compared with a logarithmic pitch scale. A logarithmic scale is the scale used in Western music. For example, 3 octaves apart would have the frequencies of 200, 400, and 800, respectively (ratios of 1:2:4 [logarithmic]), not 200, 400, and 600 Hz (ratios of 1:2:3 [linear]). Further relating the logarithmic scale to music, the smallest unit of pitch difference typically used in music is found in the chromatic scale. The chromatic scale proceeds in semitone differences, the difference between 2 notes on a piano (e.g., C to C#). The frequency change from one tone to a semitone higher is approximately equivalent to [(first frequency) × (twelfth root of 2 = 1.0595)].15 The just-noticeable difference of pitch discrimination (in a quiet environment) is 1/12th of a semitone,16 but pitch perception performance is hampered by increased attentional demands.1,2 Therefore, it is imperative to ensure that the pitch scale used by pulse oximeters increases the chance of the anesthesiologist to recognize and anticipate the clinical trend of his or her patients in the operating room and intensive care unit. Comparing linear and logarithmic scales, anesthesiologists could produce significantly more accurate judgments of both absolute oxygenation values and size of oxygenation level difference when a logarithmic, rather than a linear, scale was used. Frequencies corresponding to 70% to 100% Spo2 for the linear and logarithmic scale were 620 to 920 Hz and 390 to 930 Hz, respectively. The line of best fit for the logarithmic function was also close to x = y (actual saturation = estimated saturation) and could lead the practitioner from just recognizing when reactively intervening when “beep” turns to “boop” to proactively intervening before patient decompensation. Schulte and Block further discuss the idea of a logarithmic scale where the pitch difference between semitones is a 6% change (e.g., [(665 Hz × 1.0595) − 665 Hz)/665 Hz]), whereas the pitch difference on a linear scale between 98% and 97% Spo2 is 0.75% (e.g., 5 /665 Hz). Pulse Oximetry: Perception, Pitch, Psychoacoustics, and Pedagogy