Continuous patient monitoring is one of the anesthesiologist's most important intra-operative functions. It is an ASA monitoring standard that anesthesiologists continually evaluate their patient's oxygenation, ventilation, circulation and temperature during any anesthetic procedure. Increasingly, physiologic monitors are used to automatically and quantitatively measure these processes. For the most part, these measurements are transmitted from the monitors to the anesthesiologist through visual displays. Unfortunately, anesthesiologists cannot reliably detect visual signals in the operating room. One reason for this is that anesthesiologists have other responsibilities besides watching the physiologic monitors. Anesthesiologists spend less than one-third of their overall time looking at monitors and, therefore, have only a limited percentage of time when they can receive information from visual displays. Even when they are looking at the displays, anesthesiologists may not perceive important information unless they are consciously seeking it.

Auditory displays overcome these inherent limitations. Transmission can occur from the display to the anesthesiologist at any time, since the auditory system is omnidirectional. The auditory system also tends to highlight changing and relevant sounds, and suppress monotonous ones. This aids the anesthesiologist in detecting important changes.

The pulse oximeter tone is an example of an auditory display that has been widely accepted and enormously successful in the practice of anesthesia. The tone conveys two variables of importance to the anesthesiologist: heart rate and saturation. Most anesthesiologists feel that this helps them to continuously monitor these variables. Indeed, there is evidence from videotaped simulator exercises that anesthesiologists quickly detect changes in saturation by perceiving changes in the pitch of the pulse oximeter tone. Many anesthesiologists are familiar with the feeling that they are most conscious of the pulse oximeter tone when the heart rate or saturation changes; this is an example of the powerful pre-attentive processing that occurs in the auditory system.

We have developed and tested an auditory display of six physiologic variables (click here for a demonstration). The display consists of two independent auditory streams: the cardiovascular signal and the respiratory signal. These signals are "self-labeling" in the sense that they are designed to mimic real physiologic sounds: the cardiovascular signal is a low-pitched, repetitive, thudding sound reminiscent of a heartbeat, and the respiratory signal is a higher-pitched amplitude-modulated white noise that simulates the sound of breathing. A cardiovascular signal is generated with each heartbeat and a respiratory signal is generated with every breath. The two sounds are modulated by six physiologic variables, as described in the table, below (click here for a detailed description of the auditory display algorithms).

We evaluated our auditory display against a standard visual display using 14 anesthesia residents as subjects. The subjects monitored simulated vital signs on a personal computer to detect and identify pre-specified events. They detected events as quickly and accurately with the auditory display as with the visual display, and detected events faster when the auditory display was used to augment the visual display. They correctly identified 60% of the events with the auditory display versus 88% with the visual display, and they were quicker to identify the events when the auditory display was used to augment the visual display. We feel that auditory displays have the potential to improve patient monitoring in the operating room.

Listen to the demonstration and judge for yourself!

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