INTERMITTENT FLOW APPARATUS

Typical Number in Hospital: 10

Cost Bands: 4

References: 2

An intermittent flow or demand flow apparatus is a type of anaesthetic/analgesic machine in which gases only flow in response to the patient's respiratory effort. These are often portable and are used for dental anaesthesia, emergency surgery, and for analgesia during labour. All the common types are designed to deliver a mixture of nitrous oxide and oxygen, although some can be used with vaporizers, but these must be low- resistance types (draw-over vaporizers). The basic principle of demand flow apparatus is that cylinders of high-pressure gas fill one or more reservoirs (bags or bellows) at low pressure via pressure reducing valves. The reservoir (or mixing chamber in the case of variable mixture machines) is connected to the breathing circuit via a light valve which opens under the slight negative pressure created when the patient attempts to breath in. During expiration the reservoirs refill.

The simplest form of intermittent flow apparatus uses premixed gases such as in the Entonox apparatus; most others include a mechanism for mixing the gases. Examples of mixing valve types are the Mckesson, Lucy Baldwyn, Walton 5, AE, and Quantiflex RA. Some of these can also deliver a continuous flow of the mixed gases and include features found on other anaesthetic machines, such as oxygen flush and rotameter flowmeters.