UNDERSTANDING THE GA MACHINE

MODULE FOR HOUSE OFFICER ROTATION IN ANAESTHESIA

What is a GA machine?

• An electronic mechanical device for:1. Administration of inhalation anesthetic gases and

volatile anesthetic agents.

2. Controlling ventilation

A typical modern anaesthetic machine

• a Physiological monitor,

• b ventilator controls, • c dual flowmeter

tubes, • d vaporizer locks, • e multiple vaporizers, • f master gas on/off

switch, • g Bourdon gauges, • h ventilator bellows, • i APL valve,• j emergency oxygen, • k patient monitor

connection, • l CO2 absorber,• m reservoir bag.

1. A delivery system for medical gases, 2. A flowmeter to measure gas flow, 3. Vaporizers for volatilizing and combining the

anesthetic agents with oxygen and carrier gases,

4. A circuit for delivering the gas to the patient, 5. Monitoring equipment or gauges, 6. A ventilator, and 7. A scavenging system to collect and discharge

excess gas

Components of a GA machine

Delivery system for medical gases

1. Hospital pipe lines. • From the hospital medical gas system (wall outlet).• Pressure should be around 400 kPa (60 psi; 4

atmospheres).• Colour coded fitted with a Schrader connector or

Noninterchangeable screw-threaded (NIST).2. Reserve gas cylinders 

• Oxygen, air, and nitrous oxide • attached via a specific yoke with a Bodok seal. • The regulators for the cylinders are set at 300 kPa (45 psi;

3 atmospheres).

Note: If the cylinders are left on and the machine is plugged into the wall outlet, gas from the wall supply will be used preferentially, since it is at a higher pressure.

Medical gases colour coding

Noninterchangeable screw threaded (NIST)

• Noninterchangeable screw-threaded (NIST) connectors showing– a pipeline end and– b corresponding machine connection.

Medical gas terminal outlets

Schrader probes

Gas cylinders• The cylinders are fixed to the anaesthetic machine using a gas specific pin

indexed yoke. • The pins on the yoke match holes drilled on the face of the cylinder valve to

prevent connection with a cylinder of the wrong gas. • All valve yokes are indelibly marked with the gas they have been designed

to carry. • Between the face of the cylinder and the face of the yoke lies a thin

aluminium and neoprene washer, the Bodok seal. • Once the gas has entered the yoke, it passes through a filter, and then a

one-way check valve. • The function of the check valve is to prevent egress of gas, in the event that

there is more than one cylinder yoke for a given gas and a cylinder is missing.

• When new, the check valve is highly efficient, allowing a maximum leak rate of 15 ml/min.

• However, with use, the check valve becomes inefficient, and if a cylinder is not present a blanking plug should be fitted.

• From the cylinder yoke, the gas flows through the reducing valves into the machine.

Cylinder Yoke

• A nitrous oxide cylinder yoke. – a The two pins of the

pin index safety system,

– b the metal-rimmed Bodok seal,

– c on the right of the yoke is the noninterchangeable screw thread (NIST) pipeline attachment.

A nitrous oxide cylinder yoke with a blanking plug installed

Pin Index Safety System (PISS)

• A safety system to ensure that pneumatic connections between a gas cylinder and a machine that uses pressurized gases are not connected to the wrong gas yoke

– O2: 2-5

– N2O: 3-5

– Air: 1-5

– CO2: 1-6

– Heliox: 2-4

Pin Index Safety System (PISS)

Flowmeters• Made of tempered glass and are individually

calibrated for the gas. • Gas flow rate should be read at the top of the

plumb bob and skirted floats, or at the widest point of the ball.

• A safety feature of the flow meter block is that the oxygen flow control is always on the left-hand side of the block, and uses a different type of knob so that it can instantly be distinguished from the other gases.

• O2 is the last gas to enter the manifold to minimizes the possibility of O2 leakage from cracks in the other flow meter tubes causing inadvertent delivery of a hypoxic mixture.

• Most manufacturers now fit anti-hypoxia devices to their machines so that it is impossible to deliver less than 25–30% oxygen when nitrous oxide is used.

Anti-hypoxic device

• A mechanical interface that physically tethers the oxygen flow control valve itself to the nitrous oxide flow control valve, producing a certain minimal ratio of O2 to NO2.

• Both the ratio of sprocket teeth

and the individual gas supply pressures to the flow meters determine the final 3:1 minimal ratio of NO2 to O2 flow so that a minimum 25% of O2 will be delivered if attempts are made to increase the NO2 flow greater than the 3:1.

Back bar (Selectatec system)

• Part of the anaesthetic machine to which the vaporizers are attached.

• Many manufacturers use the Selectatec system, which allows the user the freedom to add or remove specific anaesthetic agent vaporizers.

Diagram of a generic two-gas anesthesia machine

Common gas outlet• Medical gases and vapours exit the anaesthetic machine via a 22

mm male/15 mm female conically tapered connector. • This connector is sometimes fixed, other manufacturers supply a

swivel connector. • Historically an emergency oxygen flush has been located close to

the common gas outlet, though with integrated machines this is less common.

• It is required to provide a minimum of 30 litre/min oxygen, though some machines produce flow rates of up to 70 litre/min.

• In addition to the common gas outlet, many manufacturers provide one or more 400 kPa auxiliary gas drive points.

• These points are fitted with mini-Schrader sockets and supply either air or oxygen.

• A mechanical devices attached to the GA machine back bar to produce a controlled and predictable concentration of anesthetic vapor in the carrier gas passing through the vaporizer.

• Vaporizer design has been influenced by the physical properties of the different agents and techniques to ensure their safe and accurate delivery.

Vaporizers

Vaporizers

• Colour coded:– Isoflurane: purple– Sevoflurane: Yellow– Desflurane: Blue– Halotane: Red

• Common vaporizers:– Tec (Temperature compensated) 4,5

(Isoflurane, sevoflurane)– Tec 6 (desflurane)

• Sevoflurane and halotane can be used for inhalational induction

• Desflurane and Isofurane can not be used for inhalational induction

Vaporizers

Vaporizers

• Important safety features include:– Keyed fillers– Low filling port– Secured vaporizers (less

ability to move them about minimizes tipping)

– Interlocks (selectatec)– Concentration dial

increases output in all when rotated counterclockwise

– Agents level

Vaporizers (TEC 5)

Plenum vaporizers

• Designed for use with gas flowing under positive pressure through the vaporization chamber.

Plenum vaporizers

• Specific only for desflurane• The volatile agent’s sum electrically

heated to 390C, thus takes time to warm up ~ 5 minutes

• Can be refill while in used• Knob to be dialed up gradually to the

desired concentration as can cause tachycardia if suddenly turn up to a high concentration.

• Can cause bradycardia.• Can cause increase in secretions• Can not be used for inhalational

induction agent• Suitable for day surgery (ambulatory)

anaesthesia, fast onset and off set.• Volatile agent level by LED displayed

Vaporizers (TEC 6)

• The TEC 6 is electrically powered to heat the desflurane to 39°C, producing a stable SVP of about 1500 mm Hg.

• The desflurane concentration is set by a dial controlling a variable restrictor downstream from the vaporizing chamber.

• A differential pressure transducer continually senses the pressure in the vapour upstream of the variable restrictor as well as ‘backpressure’ from the fresh gas flow.

• An additional stop point on the control dial at 12% before accessing higher concentrations (the airway irritability of desflurane can provoke coughing in the spontaneously breathing patient at high inspired concentrations).

• Two further heating elements in the upper structure prevent vapour condensation and a reduction in delivered concentration.

• A tilt detector shuts off the vaporizer if it senses a tilt greater than 15° from the vertical.

• A filler port that accepts only the desflurane bottle nozzle to prevent accidental filling with other volatile agents.

Vaporizers (TEC 6)

Inhalational agents

• The ideal characteristics of an inhalational agent are:– quick onset and offset– low airway irritability– cardiovascular stability– low incidence of side-effects.

Inhalational agents

Anesthetic Breathing Circuit

Breathing circuit

1. Most commonly a circle attachment, or a Bain's breathing system, which are breathing hoses connected to a anaesthesia face mask

2. A heat and moisture exchanger (HME) with or without bacteria-viral filter (HMEF).

Circle breathing systemSe7en components: 1. a fresh gas source; 2. Inspiratory and

expiratory unidirectional valves;

3. inspiratory and expiratory limbs of the breathing circuit;

4. a Y-type connector; 5. a pop-off valve; 6. a reservoir bag; and7. a carbon dioxide

absorbent canister

Other breathing systems

Ventilators• Usually Volume and

Pressure control mode.• In some ventilator:

SIMV, PSVpro mode• Volume control: set VT~

7-10mls/kg• Pressure control: set IP~

10-15mmHg• Respiratory rate• I:E ratio• PEEP• Alarms setting

Monitors

• Physiological monitors to monitor the patient's: 

1. Heart rate, 2. ECG, 3. Non-invasive blood pressure and 4. Plethysmography with Oxygen saturation 5. End-tidal CO2, 6. Temperature, 7. Arterial blood pressure 8. Central venous pressure9. The gases compositions delivered to the patient

(and breathed out) is monitored continuously.

Scavenging system• To remove expired anaesthetic gases from the operating room.

Scavenged gases are usually vented to the atmosphere.

• Common sources for waste gas contamination of the operating room: 1. anesthetics given with uncuffed endotracheal tubes or poorly fitting

face masks, 2. spilled agent from refilling vaporizers, 3. excessive use of the oxygen flush valve, 4. failure to turn off the flowmeters at the end of the case, and 5. poorly adjusted closed scavenging interfaces. 6. Other sources of contamination may come from leaks at connection

sites of the pipeline supplies to the anesthesia machine, 7. from internal leaks in the anesthesia machine, and 8. from patient gas sampled from the breathing circuit for airway gas

analysis (capnography and anesthetic agent detection).

Scavenging system

Safety features

• The most important safety feature is the machine check. • The operator’s manual for a specific machine should

always be consulted when using a new machine for the first time.

• The second most important safety feature is the oxygen supply failure alarm. Once sounding, the only way to silence the alarm is to restore the oxygen supply.

• The anaesthetic machine is fitted with a pressure relief valve.

• It is set to open at 30–42 kPa and is designed to protect the machine not the patient.

• Subjecting the lungs to this pressure would cause severe barotrauma.