APNOEA ALARM/MONITOR

Typical Number in Hospital: 20

Cost Bands: 2

References: 2,3

This device monitors breathing and sounds an alarm if no breathing is detected for more than a preset time limit (e.g. 15- 30 s). The two main applications for such devices are monitoring the breathing of premature babies and monitoring the correct action of ventilators in the operating theatre and intensive care unit.

An apnoea alarm for use on babies may operate in one of several ways:

1. Impedance plethysmography in which the electrical impedance of the chest is measured via skin electrodes. The impedance falls as the chest expands and so a signal roughly corresponding to the breathing pattern is obtained. This signal is applied to a circuit to identify when chest movement is insufficient to indicate breathing. In its simplest form this might be a rectifier, integrator and level detector which alarms if the voltage falls below a preset level. The whole device may be incorporated into an ECG monitor using the same electrodes.

2. Chest wall movement detectors may be used which detect the expansion in the chest circumference during breathing. These may be simple strain gauges included in a thread or tape around the chest, or mercury in elastic tubes which varies in electrical resistance as the tube is stretched. The term 'pneumograph' is often used to describe a wider bore air-filled tube around the chest in which the pressure will vary with the breathing movement.

3. General movement of the infant may be a suitable substitute for actual chest wall movement since the apnoeic episodes experienced by young babies are normally associated with cessation of all movement. This may be detected with a crude form of radar device which directs a 10 GHz electromagnetic wave at the baby and detects changes in the phase of the returning echo and converts this into an activity signal to trigger the apnoea detecting circuit.

4. Movement may also be detected by a variety of sensing systems such as a pressure mattress using thermocouples to detect the flow of air which occurs between segments of an air mattress during movement, or a small air-filled bulb may be taped to the abdomen of the baby and the variations of pressure in the bulb be detected.

ventilator alarms may monitor the source of power (gas or electricity) to the ventilator and will monitor pressure or flow in the breathing circuit. Typically they are set to alarm if the pressure exceeds a given value and if the cycling of pressure ceases for more than a preset interval. This type, which works in the breathing circuit, is preferable since it will also alarm if there is a disconnection. Examples of ventilator alarms are the Blease, and the East Ventilarm.