4. The anaesthetic machine

Question 1

Why do we need anaesthetic machines?

Answer 1

To allow for accurate and continuous delivery of gas and vapour (Oxygen/nitrous oxide) mixtures of a desired composition.

Question 2

What is an anaesthetic machine made up of?

Answer 2

• A gas supply
• Pressure gauges
• Pressure reducing valves
• Flow metres
• vaporisers
• A common gas outlet
• A breathing system
• A ventilator (on modern machines)

Question 3

What is the pin index system?

Answer 3

A system to prevent fitting the wrong cylinder onto the wrong gas yoke on the anaesthetic machine.

Question 4

Special requirements for gas cylinders?

Answer 4

• Identification on what a cylinder contains
• Correct storage protocol
• Cylinder valves must be sealed and secured

Question 5

What methods are used for piped gas?

Answer 5

• Cylinder manifold (large vet hospitals)
• Liquid oxygen storage
• Oxygen concentrators

Question 6

What different types of alarms are there?

Answer 6

Main alarm- provides an indication of the plant status and advance warning

Local alarm- indication that a problem has occurred as the point of use.

Question 7

What are vaporisers and how do they work?

Answer 7

Devices containing liquids which deliver clinically safe and effective concentrations of anaesthetic vapour.

They dilute the saturated vapour of volatile anaesthetics and yield a safe range. It splits the flow of gas into 2 chambers:

• One passes through the vaporiser chamber
• The other bypasses the chamber

Delivers anaesthetic and gas at the same time.

Question 8

What is required when filling the vaporiser?

Answer 8

Made for specific agents so the right one must be used- they only allow for a specific agent to be used.

Vaporisers must be checked to ensure that they contain enough liquid, the filling port is tightly closed and the dial turns smoothly.

Usually filled with sevoflurane, isoflurane or desflurane.

Question 9

What is a consequence of using halothane?

Answer 9

Malignant hypothermia .

Slow recovery- preferred to use in horses.

Question 10

What is the function of breathing systems?

Answer 10

• Deliver oxygen and anaesthetic agents from the machine to the patient
• Remove exhaled CO2
• Provides a mean of ventilating the lungs, by applying pressure on a reservoir bag
• Delivery of waste anaesthetic to the scavenging system.
• Gas sampling.
• Volume measurements
• Measure airway pressure

Question 11

What is Capnography?

Answer 11

How we measure CO2- tells us how well we are anesthetising our patient

Question 12

Define ‘Rebreathing’

Answer 12

to inhale previous respired gases, from which CO2 may have been removed; the amount of rebreathing will depend on gas flow, apparatus dead space and the design of the breathing system

Question 13

Define ‘Apparatus Deadspace’

Answer 13

volume of the breathing system occupied by gases that are rebreathed without any change in composition.

Question 14

Define ‘Minute volume’

Answer 14

The sum of all gas volumes exhaled in 1 minute

Question 15

Define ‘Tidal volume’

Answer 15

the volume of gas exhaled in one breath

Question 16

What are the components of a breathing system?

Answer 16

• Breathing tube
• Reservoir bag
• CO2 absorbent
• One-way valve
• Pressure Relief valve
• Fresh gas inlet
• pop off valve

Question 17

What is important to monitor whilst using a breathing system?

Answer 17

• Reservoir bag fatigue

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Question 18

How are breathing systems classified?

Answer 18

• according to the function (open/closed/semi-closed)
• whether they allow rebreathing
• whether the contain absorbent to remove CO2

Question 19

Which breathing systems do not all rebreathing?

Answer 19

• T-piece (Cats and small dogs)
• Bain (large dogs)
• Magill
• Lack (small/med dogs and cats)
• Mini-lack (dogs)
• Humphrey ADE

All require high fresh gas flow, none of theses systems cannot be used for horses.

Question 20

Which breathing systems allow rebreathing?

Answer 20

• Circle system
• To-and-fro system

Allow rebreathing of gases from which CO2 has been removed by an absorbent. Theses systems just go around in a circle, or back and forth.

Question 21

How do you decide on which breathing system to use?

Answer 21

• patient size
• convenience
• cost- rebreathing (closed) systems are more economical than non-rebreathing (open) systems

Question 22

Which substances are used to absorb CO2?

Answer 22

Sodaline canister (Calcium hydroxide or sodium hydroxide) white to begin with however turns pink when needs replacing.

Advantages include- decreased FGF requirements, expense of volatile agent, environmental pollution and loss of heat and moisture from patium.

Question 23

What is the role of the scavenging system?

Answer 23

Removes the anaesthetic vapours vented from the anaesthetic machine. It is necessary to protect personnel from exposure.

Question 24

What is assisted ventilation?

Answer 24

Intermittent positive pressure ventilation. Can be used during respiratory failure or if a patient hold their breath to provide repeated controllable breathing.

Can do more harm than good if improperly used.

Question 25

What is hypercapnia?

Answer 25

Elevation of the partial pressure of CO2, mild hypercapnia is normal during anaesthesia.

The normal range for arterial CO2 tension is considered 35-50mmHg

Question 26

How can we manage ventilation in anaesthesia?

Answer 26

Ventilation may be needed if hypoventilation occurs. Can be done by using the rebreathing bag (reservoir bag), however be careful not to overinflate the bag. Not easy to do in horses.

Question 27

What are the main considerations of mechanical ventilators?

Answer 27

must be:

• compact
• robust
• easy to operate
• economical to purchase, use and maintain

Capnography is important to monitor. In horses arterial blood gas needs to be used.

Question 28

What is important when weaning a patient off a ventilator?

Answer 28

• Ventilatory drive may become suppressed
• Ensures they are spontaneously breathing
• Performed at the end of surgery
• O2 saturation must be closely monitored