Both constant background and intermittent noise occur in the ICU, both of which indicate undisturbed functioning of devices. There are also a number of audible alarm signals indicating equipment malfunction and limit overload.
The constant background noise of ventilators, suction devices, and intra-aortic balloon pumps, the periodic beep of the ECG, etc. are calming for staff and the conscious patient as they confirm that all machines in use are functioning normally. Therefore it is unnecessary to develop silent ventilators, balloon pumps, etc. because medical personnel have become accustomed to their particular sounds which are part of the controlling environment and are at low decibel (dB(A)) levels.
Most equipment used in the ICU incorporates alarms which consist of a visual and an audible component. Unfortunately, there is no uniform classification of alarms. They can be divided into two main categories: the first type are warnings which indicate that a life-support machine is not working in the desired manner but that the patient's condition has not yet deteriorated; the second type indicate emergencies, i.e. they monitor physiological function and show that a deleterious change in the patient has already occurred (Kerrand Hayes 19.8.3).
As an illustration, Tab.!®.!. lists the 24 alarm variables in a mechanically ventilated patient on hemofiltration. When four patients are treated in one room, which is possible in an open ICU ward, up to 100 different alarms must be distinguished. An inexperienced nurse may easily become confused and unable to distinguish several concomitant alarms, which may have disastrous consequences (Croppefa/ 1994). The only way to cope with this problem is to switch off unimportant alarms
(e.g. a motor syringe with no life-supporting medication) and to set the alarm range for a large scale. This change in alarm strategy must be made with the agreement of all responsible staff (doctors and nurses). As all device defects and changes in the patient's condition are indicated only after serious deterioration, the time available for solving problems is very short.
Table 1 Variables monitored by alarms on a ventilated and hemofiltrated ICU patient
The design of new equipment demands the co-operation of industry, ICU personnel, and psychoacoustic experts in order to plan the noise profile of a device, i.e. the background noise and alarms. The increasing number of technical appliances changes the sound pressure level in the ICU; different categories of alarms could be used to distinguish life-threatening events from insignificant disturbances.
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