Anaesthetic machine

The original concept of continuous-flow machines was popularised by Boyle's anaesthetic machine, invented by the British anaesthetist Henry Boyle at St Bartholomew's Hospital in London, United Kingdom, in 1917, although similar machines had been in use in France and the United States. Prior to this time, anaesthesiologists often carried all their equipment with them, but the development of heavy, bulky cylinder storage and increasingly elaborate airway equipment meant that this was no longer practical for most circumstances. Contemporary anaesthetic machines are sometimes still referred to metonymously as "Boyle's machine", and are usually mounted on anti-static wheels for convenient transportation. , made in the UK, 1947. This device was designed for self-administration by the patient. Many of the early innovations in anaesthetic equipment in the United States, including the closed circuit carbon-dioxide absorber (a.k.a. the Guedel-Foregger Midget) and diffusion of such equipment to anaesthesiologists within the United States can be attributed to Richard von Foregger and The Foregger Company. ==Flow rate==

In anaesthesia, fresh gas flow is the mixture of medical gases and volatile anaesthetic agents which is produced by an anaesthetic machine and has not been recirculated. The flow rate and composition of the fresh gas flow is determined by the anaesthetist. Typically the fresh gas flow emerges from the common gas outlet, a specific outlet on the anaesthetic machine to which the breathing attachment is connected. • High flow anesthesia supplies fresh gas flow which approximates the patient’s minute ventilation, which is usually about 3 to 6 litres per minute in a normal adult. • Low flow anesthesia supplies fresh gas flow of less than half the patient's minute ventilation of the patient, which is usually less than 3.0 litres per minute in a normal adult. • Minimal flow anesthesia supplies fresh gas flow of about 0.5 litres per minute. • Closed system anesthesia supplies fresh gas flow as needed to make up the recirculated gas volume to compensate for the patient’s need for oxygen and anesthetic agents. ==Anaesthetic vapouriser==

(yellow) and isoflurane (purple) vaporizers on the right An anesthetic vaporizer (American English) or anaesthetic vapouriser (British English) is a device generally attached to an anesthetic machine which delivers a given concentration of a volatile anesthetic agent. It works by controlling the vaporization of anesthetic agents from liquid, and then accurately controlling the concentration in which these are added to the fresh gas flow. The design of these devices takes account of varying: ambient temperature, fresh gas flow, and agent vapor pressure. There are generally two types of vaporizers: plenum and drawover. Both have distinct advantages and disadvantages. The dual-circuit gas-vapor blender is a third type of vaporizer used exclusively for the agent desflurane. Plenum vaporizers The plenum vaporizer is driven by positive pressure from the anesthetic machine, and is usually mounted on the machine. The performance of the vaporizer does not change regardless of whether the patient is breathing spontaneously or is mechanically ventilated. The internal resistance of the vaporizer is usually high, but because the supply pressure is constant the vaporizer can be accurately calibrated to deliver a precise concentration of volatile anesthetic vapor over a wide range of fresh gas flows. It was designed by Ivan Houghton for military use in 1981. Original design included trichloroethylene for its analgesic properties and halothane for main general anaesthesia; it was later used with isoflurane. Dual-circuit gas–vapor blender The third category of vaporizer (the dual-circuit gas–vapor blender) was created specifically for the agent desflurane. It is mounted on the anesthetic machine in the same way as a plenum vaporizer, but its function is quite different. It evaporates a chamber containing desflurane using heat, and injects small amounts of pure desflurane vapor into the fresh gas flow. A transducer senses the fresh gas flow. A warm-up period is required after switching on. The desflurane vaporizer will fail if mains power is lost. Alarms sound if the vaporizer is nearly empty. An electronic display indicates the level of desflurane in the vaporizer. The expense and complexity of the desflurane vaporizer have contributed to the relative lack of popularity of desflurane, although in recent years it is gaining in popularity. Historical vaporizers Historically, ether (the first volatile agent) was first used by John Snow's inhaler (1847) but was superseded by the use of chloroform (1848). Ether then slowly made a revival (1862–1872) with regular use via Curt Schimmelbusch's "mask", a narcosis mask for dripping liquid ether. Now obsolete, it was a mask constructed of wire, and covered with cloth. Pressure and demand from dental surgeons for a more reliable method of administering ether helped modernize its delivery. In 1877, Clover invented an ether inhaler with a water jacket, and by the late 1899 alternatives to ether came to the fore, mainly due to the introduction of spinal anesthesia. Subsequently, this resulted in the decline of ether (1930–1956) use due to the introduction of cyclopropane, trichloroethylene, and halothane. By the 1980s, the anesthetic vaporizer had evolved considerably; subsequent modifications lead to a raft of additional safety features such as temperature compensation, a bimetallic strip, temperature-adjusted splitting ratio and anti-spill measures. ==Components of a typical machine==

The breathing circuit is the ducting through which the breathing gases flow from the machine to the patient and back, and includes components for mixing, adjusting, and monitoring the composition of the breathing gas, and for removing carbon dioxide. A modern anaesthetic machine includes at minimum the following components: • Flowmeters such as rotameters for oxygen, air, and nitrous oxide • Vaporisers to provide accurate dosage control when using volatile anaesthetics • A high-flow oxygen flush, which bypasses the flowmeters and vaporisers to provide pure oxygen at 30-75 litres/minute • Systems for monitoring the gases being administered to, and exhaled by, the patient, including an oxygen failure warning device Systems for monitoring the patient's heart rate, ECG, blood pressure and oxygen saturation may be incorporated, in some cases with additional options for monitoring end-tidal carbon dioxide and temperature. Breathing systems are also typically incorporated, including a manual reservoir bag for ventilation in combination with an adjustable pressure-limiting valve, as well as an integrated mechanical ventilator, to accurately ventilate the patient during anaesthesia. ==Safety features of modern machines==

Based on experience gained from analysis of mishaps, the modern anaesthetic machine incorporates several safety devices, including: • an oxygen failure alarm (a.k.a. 'Oxygen Failure Warning Device' or OFWD). In older machines this was a pneumatic device called a Ritchie whistle which sounds when oxygen pressure is 38 psi descending. Newer machines have an electronic sensor. • Nitrous cut-off or oxygen failure protection device, OFPD: the flow of medical nitrous-oxide is dependent on oxygen pressure. This is done at the regulator level. In essence, the nitrous-oxide regulator is a 'slave' of the oxygen regulator. i.e., if oxygen pressure is lost then the other gases can not flow past their regulator. • hypoxic-mixture alarms (hypoxy guards or ratio controllers) to prevent gas mixtures which contain less than 21–25% oxygen being delivered to the patient. In modern machines it is impossible to deliver 100% nitrous oxide (or any hypoxic mixture) to the patient to breathe. Oxygen is automatically added to the fresh gas flow even if the anaesthesiologist should attempt to deliver 100% nitrous oxide. Ratio controllers usually operate on the pneumatic principle or are chain linked (link 25 system). Both are located on the rotameter assembly, unless electronically controlled. • ventilator alarms, which warn of low or high airway pressures. • interlocks between the vaporizers preventing inadvertent administration of more than one volatile agent concurrently • alarms on all the above physiological monitors • the Pin Index Safety System prevents cylinders being accidentally connected to the wrong yoke • the NIST (Non-Interchangeable Screw Thread) or Diameter Index Safety System, DISS system for pipeline gases, which prevents piped gases from the wall being accidentally connected to the wrong inlet on the machine • pipeline gas hoses have non-interchangeable Schrader valve connectors, which prevents hoses being accidentally plugged into the wrong wall socket The functions of the machine should be checked at the beginning of every operating list in a "cockpit-drill". Machines and associated equipment must be maintained and serviced regularly. Older machines may lack some of the safety features and refinements present on newer machines. However, they were designed to be operated without mains electricity, using compressed gas power for the ventilator and suction apparatus. Modern machines often have battery backup, but may fail when this becomes depleted. The modern anaesthetic machine still retains all the key working principles of the Boyle's machine (a British Oxygen Company trade name) in honour of the British anaesthetist Henry Boyle. In India, however, the trade name 'Boyle' is registered with Boyle HealthCare Pvt. Ltd., Indore MP. Various regulatory and professional bodies have formulated checklists for different countries. Machines should be cleaned between cases as they are at considerable risk of contamination with pathogens. ==See also==