Breathing Systems & IVFT

Clinical Skills - Year 2

What are the aims of general anaesthesia?

to render the patient unconscious

to provide analgesia

to allow muscle relaxation

What are the stages of general anaesthesia?

1. preoperative assessment

2. premedication (pain relief and sedative typically)

3. induction

4. maintenance

5. recovery

What are the 2 main forms of anaesthetic agents?

injectable agents and inhalational agents

What are the features of injectable anaesthetic agents?

administered intravenously

easy to administer

allow a rapid, smooth induction

short-acting

can be used for maintenance but typically used for induction

What are the features of inhalational anaesthetic agents?

volatile liquids

liquids are vaporised and saturate oxygen

allows delivery of oxygen to the patient

allows a rapid change in depth of anaesthesia

typically used for maintenance

What are the 5 main components of an anaesthetic machine?

fresh gas supply

flowmeter with bypass valve

vaporiser

breathing system with pressure relief valve

scavenging system

What is the function of a bypass valve on an anaesthetic machine?

to bypass the anaesthetic and deliver only oxygen from the flowmeter to the patient (without anaesthetic)

Where on a flowmeter do you read the level of gas?

top of the bobbin or the centre of the ball

What does the scavenging system of an anaesthetic machine do?

collects and safely removes exhaled and excess waste anaesthetic gases from the patient’s breathing circuit and ventilator to prevent them from leaking into the operating room

What are the problems with using a face mask in an anaesthetic breathing system?

anaesthetic can leak out into the operating room

What is the function of breathing systems in an anaesthetic machine?

to delivery anaesthetic vapour

to deliver oxygen

to remove carbon dioxide

What are the 5 main components of breathing systems?

adapter that connects to an endotracheal tube

expiratory valve

reservoir bag to allow intermittent positive pressure ventilation

fresh gas tubing

expired gas tubing

What is the function of a soda lime canister in an anaesthetic machine?

to absorb carbon dioxide exhaled by the patient during anaesthesia

How do you know when a soda lime canister needs to be replaced?

it will change colour either from white to pink or pink to white when saturated with carbon dioxide

What are the features of a non-rebreathing system?

the patient does not rebreathe expired gas

expired gas is flushed from the system before the patient’s next breath

less economical

less resistance

At which weight must an animal use a rebreathing system instead of non-rebreathing?

above 25kg

What modifications were made to the Mapleson A breathing system to create the Parallel Lack system?

a second tube was added to allow the APL valve to be moved further away from the head during head surgery

this model can be used on patients over 7kg

What modifications were made to the Mapleson D system to create the Co-axial Bain system?

the fresh gas tubing was passed through the expiratory tubing

How do you ensure that the flow rate is correct in a non-rebreathing circuit?

ensure that the fresh gas flow rate exceeds the minute volume by a certain amount (depending on the circuit) in order to prevent rebreathing of expired gases

What is minute volume?

the volume of gas expired per minute

What is the approximate minute volume of a patient in a non-rebreathing system?

200 ml/ kg/ min

What are the features of a rebreathing system?

soda lime canister

expiratory valve and connector with one-way valves

patient rebreathes expired gas

economical

increased resistance

Which patients are suitable for rebreathing systems?

patients over 15kg

What is the approximate minute volume of a patient in a rebreathing system?

10-50 ml/ kg/ min

What is wrong with having a gas flow rate too high?

a lot of anaesthetic and gas will be wasted

What is wrong with having a gas flow rate too low?

expired gas will not be fully removed during the expiratory pause and the patient will rebreathe carbon dioxide

What are the different types of fluid used in IVFT?

crystalloids - water, NaCl and other solutes

colloids - water, NaCl, starch and other solutes

protein-based solutions

What is the tonicity of a fluid?

the osmolarity of the solution relative to plasma

What is dehydration?

the loss of fluid from the intravascular and interstitial space

What is required to calculate the volume of isotonic fluid required to restore the body fluid of a patient to normal?

maintenance requirement

percentage dehydration (deficit)

ongoing losses

What clinical signs would you look for to identify dehydration?

demeanour

skin turgor

mucous membranes

globe/eye position

What is hypovolaemia?

loss of fluid from the intravascular space

What equipment do you need to administer an IV catheter?

catheter

bung

needle/ stylet

connector

cap

How should you set up a fluid giving set?

1. carefully unpack sterile giving set

2. turn off flow on giving set

3. insert giving set into port on fluid bag

4. attach fluid bag to drip stand

5. note the volume of the drip (usually 20 drips/ml)

6. slowly run fluid through the giving set to eliminate air bubbles

7. stop fluid from running until ready to attach to the catheter

How do you calculate how much fluid to administer to a dehydrated patient?

1. calculate maintenance requirement: 2ml/kg/hr

2. calculate water deficit: % dehydration x bodyweight(kg) x 1000

3. factor in ongoing losses: volume per episode (ml) x frequency of loss per 24 hours

What equipment will you need for this skill?

anaesthetic machine

anaesthetic breathing systems

endotracheal tubes

scavenger

fluid bag

giving set

intravenous injection limb

calculator

intravenous catheter

catheter tape

Which stage of anaesthesia is associated with the greatest risk of mortality?

recovery

What are the 2 main types of breathing system?

rebreathing and non-rebreathing

Which component of a breathing system is shown here?

adapter to connect to an ET tube

Which component of a breathing system is shown here?

expiratory/APL valve

Which component of a breathing system is shown here?

reservoir bag

Which component of a breathing system is shown here?

gas tubing

Which component of a breathing system is shown here?

soda lime (only in rebreathing)

What does APL valve stand for?

adjustable pressure-limiting valve

What is the function of the expiratory/APL valve in a breathing system?

to allow expired gas to exit the breathing system once the pressure in the system exceeds a certain level to prevent the patient from rebreathing expired air

What is the function of the reservoir bag in a breathing system?

to allow you to begin intermittent positive pressure ventilation if the patient stops breathing

What capacity should a reservoir bag have?

atleast double the normal tidal volume of the patient

What is the function of the fresh gas tubing in a breathing system?

to deliver fresh gas from the anaesthetic machine to the patient

What is the function of the expired gas tubing in a breathing system?

to take expired gas away from the patient

What is a common way to differentiate fresh gas tubing from expired gas tubing in a breathing system?

expired tubing is typically white and fresh is typically green

How do you calculate the maximum tidal volume of a patient?

2-3 times the normal resting tidal volume

What is the inter-granular volume in a soda lime canister?

the volume of air found in gaps between the soda lime granules

What is the ideal inter-granular volume of a soda lime canister?

at least twice the maximum tidal volume of the patient

What is the function of the flowmeter in an anaesthetic machine?

to control the flow of gas to the patient

What unit is flow rate measured in?

L/min

What factors affect the flow rate of gas for a patient?

weight of patient/ patient’s minute volume

anaesthetic breathing system type being used

When might you use the bypass valve on a flowmeter?

if the patient stops breathing under anaesthesia and you need to quikcly flush the anaesthetic out of the breathing system

What is the function of the vaporiser in an anaesthetic machine?

to mix the appropriate controlled level of inhalational agent with oxygen

How does a vaporiser measure the amount of inhalational agent delivered?

as a percentage of the gas produced

How does a scavenging system work?

it contains activated charcoal which removes organic gases which have not been metabolised by the patient because anaesthetic is heavier than oxygen so sinks to the bottom of the canister while oxygen remains above

What components of a breathing system are used to identify the model?

connection to ET tube

expired gas tubing

fresh gas tubing

T-connector

reservoir bag

expiratory valve

Which breathing system model is shown here?

Mapleson A/ Magill

What type of breathing system is classified using the Mapleson classification system?

non-rebreathing

What is the pathway of gas flow in a Mapleson A breathing system?

1. fresh gas enters system into reservoir bag

2. patient inhales - fresh gas enters lungs from reservoir bag (so it decreases in size)

3. patient exhales - lungs reduce in size and expired gas is forced out into tubing

4. reservoir bag is now full of fresh gas

5. fresh gas continues entering the system but cannot get into lungs

6. pressure is high enough for expiratory valve to open

7. fresh gas coming in forces expired gas out through valve

When might you use/not use a Mapleson A breathing system?

on patients over 10kg who are spontaneously breathing

not for operations around the head as set up makes manual ventilation difficult and system is inefficient for IPPV

What does IPPV stand for?

intermittent positive pressure ventilation

Which breathing system model is shown here?

Mapleson D

Which Mapleson models are not used in veterinary medicine?

B and C

What are the features of a Mapleson A/Magill breathing system?

connector to an ET tube

expiratory valve

reservoir bag

single tubing

What are the features of a Mapleson D breathing system?

connector to an ET tube

expiratory valve

reservoir bag

fresh gas tubing

expired gas tubing

T connector

What is the pathway of gas flow in a Mapleson D breathing system?

1. fresh gas enters breathing system

2. patient inhales and reservoir bag decreases in size

3. patient exhales and forces expired gas through the expired tubing to the reservoir bag (mix of fresh and expired in bag)

4. fresh gas continues entering the system and increases the pressure, causing the expiratory valve to open

5. fresh air forces expired air out through the valve, leaving only fresh gas in the system for the next inhalation

When might you use/not use a Mapleson D breathing system?

on patients under 10kg and who might require IPPV

less efficient than the Mapleson A so would use this if possible

Which breathing system model is shown here?

Mapleson E/ Ayre’s T-piece

What are the features of the Mapleson E/ Ayre’s T-piece breathing system?

connector to an ET tube

green fresh gas tubing

white expired gas tubing

(no expiratory valve or reservoir bag)

What is the pathway of gas flow in the Mapleson E/ Ayre’s T-piece breathing system?

1. fresh gas enters the system

2. patient inhales and breathes in fresh air

3. fresh gas continues entering system

4. patient exhales and expired gas is forced into expired tubing as fresh tubing is full of fresh air

5. the continued flow of fresh gas increases pressure and forces expired gas out of the tubing

When might you use/not use a Mapleson E/ Ayre’s T-piece breathing system?

patients under 10kg (system has very low resistance)

least efficient system

no reservoir bag so cannot perform IPPV - do not use on high risk patients

Which breathing system model is shown here?

Mapleson F/ Jackson-Rees

What are the features of the Mapleson F/ Jackson-Rees breathing system?

connector to an ET tube

reservoir bag

green fresh gas tubing

white expired gas tubing

(no expiratory valve)

What is the pathway of gas flow in the Mapleson F/Jackson-Rees breathing system?

1. fresh gas enters the system

2. patient inhales and pulls air from and tubing which slightly deflates the reservoir bag

3. fresh gas continues flowing into system and filling fresh tubing

4. patient exhales and expired gas is forced into expired tubing and reservoir bag as fresh tubing is full

5. continued flow of fresh gas increases pressure and forces expired air out through the reservoir bag

6. patient inhales again and breathes in fresh air from fresh tubing

What are the 2 least efficient non-rebreathing system models?

Mapleson E and F

When might you use/not use a Mapleson F/ Jackson-Rees breathing system?

on patients under 10kg as very low resistance

on patients who might require IPPV as reservoir bag enables this

high fresh gas flows required so would avoid in larger patients

Which breathing system model is shown here?

Parallel Lack (modification of Mapleson A)

What are the features of a Parallel Lack breathing system model?

connector to an ET tube

expiratory valve

reservoir bag

fresh gas and expired gas tubing connected by a Y connector

What is the pathway of gas flow in the Parallel Lack breathing system?

1. fresh gas enters system and fills tubing and reservoir bag

2. patient inhales and pulls air from tubing and bag

3. fresh gas continues flowing into fresh tubing

4. patient exhales and forces gas into expiratory tubing as fresh tubing is full of gas

5. fresh gas keeps entering system and increasing the pressure, which forces expired air out of tubing through the expiratory valve

When might you use/not use a Parallel Lack breathing system?

for a patient over 7kg able to breath spontaneously

for surgery around the head, as the expiratory valve is further away from the patient so allows more space for surgeon to work

not suited to a patient who would require prolonged IPPV as it allows build-up of CO2 in system

Which breathing system model is shown here?

Co-axial Bain (modification of Mapleson D)

What are the features of a Co-axial Bain breathing system?

connector to an ET tube

expiratory valve

reservoir bag

green fresh gas tubing which sits inside white expired gas tubing

What is the pathway of gas flow in a Co-axial Bain breathing system?

1. fresh gas enters system and fills tubing and reservoir bag

2. patient inhales and pulls gas from fresh tubing and bag

3. fresh gas continues flowing into fresh tubing

4. patient exhales and forces expired gas into expired tubing, displacing gas into the bag

5. fresh gas continues entering which increases pressure, forcing expired gas out through the expiratory valve

When might you use/ not use a Co-axial Bain breathing system?

on patients who might require IPPV

on patients over 10kg

during radiography as tubing can be extended to allow for good positioning

potential for inner tube disconnection, so must check tubing before each tube

Which breathing system model is shown here?

Circle system

What are the features of a circle breathing system?

soda lime canister

fresh and expired gas tubing

expiratory valve

connection housing one-way valves

reservoir bag

What is the pathway of gas flow in a circle breathing system?

1. fresh gas flows into the system and fills fresh tubing and reservoir bag

2. patient inhales and expiratory valve closes and inspiratory valve opens

3. fresh gas flows from the reservoir bag to the patient through fresh tubing

4. patient exhales and forces gas out, so the inspiratory valve closes due to pressure in the fresh tubing from gas flowing in

5. expired gas flows into reservoir bag through expiratory tubing and into soda lime canister

6. CO2 in expired gas is absorbed by the soda lime

7. pressure is now at highest in circuit so excess gas exits system via expiratory valve (but not all)

What are the advantages of using rebreathing systems?

heat and moisture in expired gases are conserved

very efficient

can perform IPVV easily

What are the disadvantages of using rebreathing systems?

high resistance so cannot be used in patients below 15kg

must replace soda lime

slower change in anaesthetic depth when anaesthetic gas concentration is changed

How do you calculate the required flow rate for a patient using a non-rebreathing system?

calculate the minute volume of the patient and multiply this by a factor depending on the model being used:

Mapleson A = x1

Mapleson D = x2-2.5

Mapleson E/F = 2.5-3x

What flow rate would you typically start a rebreathing system with for the first 10 mins and why?

2-4 L/min (and then reduce to 0.5-1 L/min)

to clear the circuit of air

What percentage of a patient’s body weight is made up by water?

60-65%

What percentage of a patient’s body water is extracellular?

20%

What percentage of a patient’s body water is intracellular?

40%

What is the capillary membrane freely permeable to?

water and electrolytes