anaesthetic breathing systems and IVFT

what is the aim of general anaesthesia

ensure patient is in a state of controlled unconsiousness with associated analgesia and muscle relaxation

important stages of anaesthesia

pre-operative assessment

premedication

induction of anaesthesia

maintenance of anaesthesia

recovery from anaesthesia

what should pre-op assessment incl

history and clinical exam which may provide info on health status of patient and highlight any potential factors which may impact the risk of anaesthesia to your patient and so your choice of anaesthetic drugs 

ensure you have venous access to patient 

aim of premedication 

reduce anxiety ]ease handling patient 

smooth induction 

smooth maintenance

smooth recovery 

reduce dose of anaesthetic drugs required

provide pre-emptive analgesia 

following premedication what agents are used 

injectable agents - propofol

inhalation agents - isoflurane 

intravenous agents

used to induce anasthesia

easy to administer 

provide rapid, smooth induction 

generally short acting, allow enough time for vet to intubate patient and connect them to anaesthetic circuit for maintanence of anaesthetia 

then anaesthesia commonly maitained by saturating oxygen with appropriate level of inhalation agent 

require metabolism to deactivate and allow patient to fully recover from general anaesthesia

maintanence agents

volatile liquids which vapourise and saturate oxygen to controlled level

concentration of vapour within oxygen devlivered to patient can then be increased or decreased to deepen or lighten the level of anaesthesia 

benefit of using inhalation agent = delivery of oxygen and rapid change depth of anaesthasia by adjusting conc of gas delivered

caesing of inhalation agent allows patient to regain consciousness

recovery period

associated with greatest risk of morbitdity and mortality in anaesthesia

2 major types of breathing systems

rebreathing system

non-rebreathing system 

rebreathing systems

allow patient to rebreath expired gas

contains higher levels of CO2 whcih must be extracted before patient rebreaths economical but high resistance so only suitable for patients over 15kg 

non-rebreathing systems

should not allow patient to rebreath expired gas

expired gas must be flushed out circuit before patient takes nect inhale so utilise a higher fresh gas flow rate 

less economical but have low resistance so suitable for patients <25kg 

adapter to connect ET tube

designed so can only be connected to ET tube so cant incorrectly connect to anaesthesia machine 

expiratory valve

also called adjustable pressure limiting or APL valve

resevoir bag 

allows us to undertake intermittent positive pressure ventilation should patient stop breathing 

should have capacity of at least twice the normal tidal volume of patient 

fresh gas tubing 

delivers gas from anaesthetic machine to patient 

commonly green 

expired gas tubing 

takes expired gas away from patient and is white in image 

soda lime canister 

removes CO2 from expired gas rendering it safe to be rebreathed 

soda lime changes colour when reacts with CO2 allowing us to detect when canister is saturated and needs soda lime to be changed

will have gaps of air between granules = inter-granular volume - usually half canisters capacity , should be at least twice the maximium tidal volume of animal 

max tidal volume = 2-3 times normal resting tidal volume

bodily fluid compartments

semipermeable membranes that separate body compartments

capillary membrane - between intravascular space - freely permeable to water and electrolytes but not macromolecules

cell membrane - between interstitial space and cell - freely permeable to water 

fluid movement accross them is goverend by hydrostatic pressure - move low ti high 

hydrostatic pressure affected by osmotic and oncotic pressure 

osmotic created by solutes such as electrolytes, glucose and urea

oncotic created by proteins 

types of fluid

crystalloids - contain water, NaCl and other solutes - available in multiple concentrations, reffered to as tonicity of solution= describes the effective concentrations of solution when compared to plasma 

colloids - contain water, starch, NaCl and other solutes - contain macromolecules which tend to be retained in intravascular spce and exert osmotic effect = retention of fluid in intravascular space 

protein-based solutions incl non-oxygen carriers - plasma and oxygen carriers such as whole blood 

movement of fluid between intravascular and interstitial space depends on tonicity of fluid administered 

isotonic  - no net movement 

hypotonic = net movement from intravascular space → interstitial 

hypertonic = net movement from interstitial space → intravascular 

isotonic solutions commonly administerd to dehydrated patients e.g diarrhoea or vommiting or in shock

hypertonic administered to patient in shock to rapidly increase circulating blood volume but never to dehydrated patient 

dehydration

associated with loss of fluid from intravascular and interstitial space

to correct it need to calculate volume of isotonic fluid required to restore body fluid to normal 

consider - maintanence requirements, current deficit, ongoing losses

shock

circulatory shock caused by hypovolaemia, a loss of fluid from intravascular space = emergency situation 

immediate treatment required to increase intravascular volume so CO can be maintained 

shock can be with or without dehydration e.g 

• road accident = shock without dehydration 

• chronic diarrhoea = shock with dehydration 

1. Connection to ET tube

2. Expired gas tubing

3. Fresh gas tubing

4. (T-connector)

5. Reservoir bag

6. Expiratory valve

mapleson A 

used in patients >10kg 

• Fresh gas enters the system from the anaesthetic machine and fills the reservoir bag.

• At the start of inspiration, the lungs are empty, the reservoir bag is full and the breathing system is full of fresh gas.

• At the end of inspiration, the patient has breathed in. There is fresh gas in the breathing system and the lungs and the reservoir bag has decreased in size

• During expiration the lungs reduce in size forcing expired gas back into the tubing.

• By the end of expiration, the reservoir bag is full of gas displaced from the tubing by expired gas.

• During the expiratory pause, fresh gas continues to enter the breathing system but cannot re-enter the lungs.

• Pressure within the system is now high enough to open the expiratory valve. Fresh gas continues to flow into the system and therefore expels expired and dead space gas through the expiratory valve

very efficient for spontaneously breathing patients as resevoir bag is filled with fresh gas 

presence of expiratory valve at patient end of system can provide challenges for ops around head 

can make manual ventilation challenging 

considered inefficient for IPPV

mapleson D

used in patients <10kg

• Fresh gas enters the breathing system from the anaesthetic machine

• At the start of inspiration, the lungs are empty, the reservoir bag is full and the breathing system is full of fresh gas.

• At the end of inspiration, the patient has breathed in. There is fresh gas in the breathing system and the lungs and the reservoir bag has decreased in size

• During expiration the lungs reduce in size forcing expired gas back into the expired gas tubing

• By the end of expiration, the reservoir bag is full of gas displaced from the expiratory tubing by the expired gas

• During the expiratory pause, fresh gas continues to enter the breathing system but cannot re-enter the lungs.

• Pressure within the system is now high enough to open the expiratory valve. Fresh gas continues to flow into the system and therefore expels expired and dead space gas through the expiratory valve, so at the

next breath the animal will only breathe in fresh gas

less efficient than magill as require higher gas flow rate in order to ensure expired gas is cleared from resevoir bag prior to next inspiration

more suitable for performing IPPV than magill

Mapleson E

used in patients <10kg

• Fresh gas enters the breathing system from the anaesthetic machine

• At the start of inspiration, the lungs are empty, and the breathing system is full of fresh gas.

• At the end of inspiration, the patient has breathed in. There is fresh gas in the breathing system and the lungs.

• During expiration the lungs reduce in size and the flow of fresh gas in the fresh gas tubing means expired and dead space gas is forced into the expiratory tubing.

• By the end of expiration, the expiratory tubing is full of expired and dead space gas.

• The continued flow of fresh gas during the expiratory pause increases the pressure in the system and forces expired gas out of the tubing. Remember there is no expiratory valve in this system

has very low resistance = suitable for small patients

least efficient class of breathing system so high fresh gas flows required to clear expired gas from tubing

doesnt has resevoir bag so IPPV couldnt be performed if patient stops breathing

scavenging waste gases is hard

Mapleson F

connector on resevoir bag is not expiratory valve 

used in patients <10kg 

Fresh gas enters the breathing system from the anaesthetic machine

• At the start of inspiration, the lungs are empty, the reservoir bag is full and the breathing system is full of fresh gas.

• At the end of inspiration, the patient has breathed in. There is fresh gas in the breathing system and the lungs and the reservoir bag has decreased in size

• During expiration the lungs reduce in size and the flow of fresh gas in the fresh gas tubing means expired and dead space gas is forced into the expiratory tubing.

• By the end of expiration, the expiratory tubing contains expired and dead space gas and the reservoir bag is full.

• The continued flow of fresh gas during the expiratory pause increases the pressure in the system and forces expired gas out of the system via the reservoir bag. So that the patient will only breathe fresh gas during the next breath. Remember there is no expiratory valve in this system

low level resistance = suitable for small patients

resevoir bag can be used for IPPV

least efficient = high gas flows required to flush expired gas from system via resevoir bag

appropriate scavenging from system hard due to open bag set up

parallel lack

used in patients >7kg

• Fresh gas enters the breathing system from the anaesthetic machine

• At the start of inspiration, the lungs are empty, the reservoir bag is full and the breathing system is full of fresh gas.

• At the end of inspiration, the patient has breathed in. There is fresh gas in the breathing system and the lungs and the reservoir bag has decreased in size

• During expiration the lungs reduce in size forcing gas back into the breathing and the flow of fresh gas in the fresh gas tubing means expired and dead space gas is forced into the expiratory tubing.

• By the end of expiration, the expiratory tubing contains expired and dead space gas.

• During the expiratory pause, fresh gas continues to enter the breathing system but cannot re-enter the lungs.

• Pressure within the system is now high enough to open the expiratory valve. Fresh gas continues to flow into the system and therefore expels expired and dead space gas through the expiratory valve.

Efficient for spontaneous breathing as the reservoir bag is filled with fresh gas. The expiratory valve has been moved away from the patient, compared to the Magill, meaning that surgery around the

head would be less challenging and it is easier to adjust the valve if the patient requires ventilation. 

not suited to prolonged IPPV, as it allows for build-up of CO2 in the system in the same way as the Magill.

Co-axial bain 

used in patients >10kg

• fresh gas enters breathing system from anaesthetic machine 

• at start of inspiration lungs are empty, resevoir bag full and breathing system full of fresh gas

• end of inspiration patients has breathed in, fresh gas in breathing system and lungs and the resevoir bag has decreased in size

• during expiration lungs reduce in size forcing expired gas back into expired gas tubing 

• by end of expiration the resevoir bag is full of gas displaced from expiratory tubing by expired gas 

• during expiratory pause fresh gas continues to enter breathing system but can’t reenter lungs

• pressure within system is high enough to open expiratory valve. fresh gas continues to flow into system and expel expired and dead space has through expiratory valve, so next breath animal will only breath fresh gas

can be used for IPPV

modification of tubing allows for extension of fresh and expired gas tubing without increased resistance in system so allows for good anaesthetic machine positioning

due to co-axial of tubing theres potential for inner tube disconnection = lead to extensive rebreathing of expired gas

check tubing before each use and additional leak testing to test integrity of inner tube required on top of standard leak testing

circle system- rebreathing system 

used for patients >15kg

• Fresh gas flows into the system from the
  anaesthetic machine

• During inspiration the one-way expiratory valve closes and the inspiratory valve opens.

• Gas flows from the reservoir bag to the patient via the inspiratory tubing

• During expiration the lungs reduce in size forcing expired gas back into the circuit.

• The inspiratory valve closes and gas flows into the breathing bag via the expired gas tubing.

• The CO2 is absorbed by the soda lime.

• During the expiratory pause, the pressure is at its highest in the circuit and excess gas will exit the system via the expiratory valve

Only a single turn of the expiratory valve is required when setting up the circuit in order that excessive amounts of gas are not lost from
the system.
The advantages of rebreathing systems, such as the circle, include conservation of heat and moisture in respired gases; the efficiency of the
system due to low fresh gas flow rates required and the ability to perform IPPV easily.
However, the resistance in the system is high so it cannot be used for small patients, the need to replace soda lime and the slower
change in anaesthetic depth when the anaesthetic gas concentration is changed are potential disadvantages of this system.

what to consider when calculating the flow rate required for non-rebreathing systems

patients minute volume

breathing system being used 

minute volume

volume of gas patient is expering per minute

approx 200ml/kg/min

based on tidal volume 10ml/kg and respiratory rate of 20 breaths/min

if respiratory rate is higher it will increase minute volume 

flow rate required for rebreathing systems

10-50ml/kg/min

tend to start with initial flow rate of 2-4l/min for first 10 mins to clear circuit of air then reduce to 0.5-1l/min for maintenance 

if flow rate too high 

not an issue for patient but wasteful as fresh gas comprising O2 and inhalational agent expelled unecessarily 

if flow rate too low

expired alveolar gas wont be completely removed from system dueing expiratory pause and gas with increased levels CO2 will be rebreathed by patient

treatment of dehydration fluid therapy calculations

administered intravenously over 24hrs via continuous rate infusion using gravity or infusion pump, patient should be monitored and reevaluated over period incl HR, RR and mucous membranes and pulse quality

1. maintanence requirement = 2ml/kg/hr or 50ml/kg/24hr

2. deficit = volume in ml = % dehydration(0.05-0.12) x bodyweight(kg) x 1000

3. ongoing losses - estimate volume lost each time - 4ml/kg/episode x weight(kg), estimate frequency of loss over 24hr period e.g vomit 4 times a day, volume in ml = volume of loss per episode(ml) x frequency of loss per 24hrs

from this calculate volume per min and drip rate

treatment of shock fluid therapy calculations

initial treatment incl bolus administration of isotonic crystalloid fluids

fluids should be administered as quick as possible but often restricted by size of catheter placed to increase rate -

• apply 2 IV catheters

• apply pressure to fluid bag to force fluid into intravascular space rather than relying on gravity

bolus of 10-20ml/kg should be administered over 15-30mins

following administration re-evaluate patient - checl HR,RR, mucous membranes

if still in shock repeat bolus

monitor throughout - too much = pulmonary oedema

once normal circulatory volume restored switch to continuous rate infusion of fluids at appropriate rate