Sophie Young 21st February 2007 Anaesthetic Machines Sophie Young 21st February 2007

Aims and Objectives Components of anaesthetic machines Safety features of anaesthetic machine

Why do we need an anaesthetic machine? Supply compressed gases Measure flow of gases Add vapours in known concentrations Deliver vapours and gases to patient via a breathing system Scavenge waste Monitor machine and patient

Anaesthetic Machines “Pressurised gases are supplied by cylinders or pipelines to the anaesthetic machine which controls the flow of gases before passing them through a vaporiser and delivering the resulting mixture to the patient via the breathing circuit”

History Original Continuous-flow Anaesthetic Machine designed by Boyle in 1917 Most current anaesthetic machines derived from Boyles Machine Simpler anaesthetic machines – ‘draw over’ used by Armed Forces (no need for fresh gas supply)

Boyles Anaesthetic Machine http://www.nda.ox.ac.uk/wfsa/html/u06/u06_015.htm

Components of the Anaesthetic Machine Gas supplies Pressure gauges Pressure reducing valves Rotameters Vapourisers Common gas outlet High flow oxygen flush Pressure relief valve Oxygen supply failure alarm Breathing system Ventilator Scavenging Suction Monitoring devices

Basic Components of Anaesthetic Machine Fundamentals of Anaesthesia

Gas Supply - Pipelines Central supply point O2, N2O, air, medical vacuum supplied Pipelines throughout hospital Outlets – colour and shape coded Flexible colour coded pipelines run to anaesthetic machine Pipelines permanently fixed to machine (Non-Interchangable Screw Thread – NIST) Pressure 400kPa (4 bar)

Gas Supply - Cylinders Molybdenum steel Size E Filling pressures Colour coded Pin Index System Bodok seal Valve (open + close prior to attaching) – key and keyless Essentials of Anaesthetic Equipment

Pressure Gauges Pressure gauges to measure pipeline and cylinder pressures O2, N2O and air gauges on front of machine Gas colour coded and calibrated Bourdon Gauge www.frca.co.uk/article.aspx?articleid=100325

Pressure Regulators Function: Reduce pressure to protect machine Maintain constant supply to compensate for changing cylinder pressures + demand Reduce pressure from 13 700kPa to ~400kPa Compression spring acting on diaphragm coupled to inlet valve Essentials of Anaesthetic Equipment

Rotameters Measure flow rate of gas passing through them Calibration (accuracy +/- 2.5%) Components: flow control (needle) valve, tapered tube, bobbin (slits + dot) Low flow rates: flow laminar + function of gas viscosity (Poiseuille’s Law) High flow rates: flow turbulent + function of gas density Essentials of Anaesthetic Equipment

Problems with Rotameters Non-vertical tubes Tube damage/leakage (O2 enters back bar downstream from other rotameter outlets) Back pressure from common gas outlet – drop in bobbin level Bobbin sticking: Static electricity Dirt

Back Bar Horizontal part of machine between rotameters and common gas outlet Vapourisers mounted here so volatiles can be added to fresh gases Pressure ranges 0-30 kPa (1 kPa at outlet end) Non-return pressure relief valve downstream of vapourisers set at 35 kPa – prevents barotrauma to rotameters, vapourisers and patient

Vapourisers Add controlled amount inhalational agent, after changing it from liquid to gas, to fresh gas flow Design depends on SVP, boiling point and MAC of volatile being used – variable bypass or measured flow vapouriser Accurate delivery of anaesthetic agent regardless of time, gas flow and temperature Antispill mechanism Colour coded keyed filling devices Safety interlock device

Variable Bypass Vapouriser Essentials of Anaesthetic Equipment

Measured Flow Vapouriser http://www.frca.co.uk/article.aspx?articleid=100151

Common Gas Outlet Fresh gas mixture supplied to patient or ventilator Often mounted on swivel 22mm male – outlet’s external diameter 15mm female – outlet’s internal diameter

Safety Features of Anaesthetic Machines Non interchangable pipeline outlets Colour coded pipelines and cylinders Pin index system Pressure gauges for gases Colour coded rotameters Oxygen rotameter on left, protrudes, ridged Oxygen/nitrous oxide interlink device Emergency oxygen flush Oxygen failure warning device Keyed vapouriser fillers Pressure relief valve Anaesthetist to check the machine

Oxygen Failure Warning Device Upstream of rotameters Specifications (British Safety Standard): Audible alarm >60db, 1m from machine, >7 seconds Activated when O2 supply falls to 200kPa Power supply from O2 supply pressure Alarm can’t be switched off/reset until O2 supply restored Coupled to gas cut off valve – opens machine circuitry to air

Ritchie Whistle (1960) Fundamentals of Anaesthesia

Emergency Oxygen Flush Non-locking button Bypass rotameter block and back bar Delivers flow > 30 l/min Pressure ~400 kPa Risks: Barotrauma Awareness

Summary Basic components of anaesthetic machine Safety features of anaesthetic machine Any Questions?

References Al-Shaikh et al (2002) Essentials of Anaesthetic Equipment. Churchill Livingstone Pinnock et al (2003) Fundamentals of Anaesthesia. Cambridge Aitkenhead et al (2001) Textbook of Anaesthesia. Churchill Livingstone Davis et al (2005) Basic Physics and Measurement in Anaesthesia. Butterworth Heinemann Milner (1996) Update in Anaesthesia http://www.nda.ox.ac.uk/wfsa/html/u06/u06_015.htm Anaesthesia UK (2005) Temperature Measurement www.frca.co.uk/article.aspx?articleid=100325 Anaesthesia UK (2004) Precision Vapourisers http://www.frca.co.uk/article.aspx?articleid=100151