Breathing circuits

This is almost complete - needs tidying up, simplifying and summarising.

Based on Update in Anaesthesia Issue 7 (1997) Article 4 here, an ODP site here, a veterinary site Anesthesia Service and Equipment.

You need to have read and understood the basic respiratory physiology page first.

An ideal breathing system:

  1. Simple, safe and inexpensive
  2. Delivers intended inspired gas mixture
  3. Permits spontaneous, manual and controlled ventilation in all age groups
  4. Efficient, allows low fresh gas flow
  5. Protects patient from barotrauma
  6. Sturdy, compact, lightweight
  7. Permits easy removal of waste gases

Classification systems

  1. Afferent or efferent reservoir system. Whether the reservoir bag comes off the limb that the FGF goes into or out of. A, B and C from the Mapleson are afferent and D is efferent.
  2. Open / Semi closed (example of open: Hudson mask, Nasal Cannulae.)
  3. Rebreathing/non rebreathing
  4. With CO2 absorption or not
  5. The Mapleson classification (all semi open circuits, all can be rebreathing if FGF is low)

1954 by Professor W W Mapleson is commonest. Doesn't include the circle system.

  • Mapleson A - the Magill and Lack circuits
  • Mapleson B and C - Rebreathing of exhaled gases occurs even when very high fresh gas flow rates are used, since inspiration is taken from the same space into which the previous breath was expired. These are unsatisfactory for anaesthesia, but may be used in emergency for resuscitation.
  • Mapleson D - the modified Bain circuit.
  • Mapleson E - Ayre's T piece and the Bain circuit.
  • "Mapleson F" - not originally classified by Mapleson, but is used to refer to Jackson-Rees' modification of Ayre's T-piece.
  • The Humphrey ADE is a circuit that provides the ability to switch between the Mapleson A, D and E arrangements.

Some definitions


Mapleson A

Also known as the Magill circuit after Sir Ivan Whiteside Magill 1888-1986, Consultant Anaesthetist at Westminster Hospital, London - the inventor of endotracheal intubation.

Magill Circuit Magill Circuit Function

During Spontaneous Ventilation:

Maximally efficient. The best.

Inspiration -The valve closes and the patient inspires fresh gas from the reservoir tube.
Expiration - The patient expires into the reservoir tube. Toward the end of expiration, the bag fills and positive pressure opens the valve, allowing expired gas to escape.
Expiratory pause - FGF flushes Dead space Gas (DSG) out of valve. If flow sufficient then no DSG remains in circuit at beginning of next inspiration.

Theoretically if FGF=Alveolar ventilation then no rebreathing. This assumes that there is a regular respiratory pattern, the tidal volume is not too great, there is a sharp division between Vd and Vt = which obviously there is not.

Therefore usually FGF should be equal to or greater than MV. ~ 80ml/kg x 70kg = ~6l/min

The efficiency of the system is independent of waveform, this is not true of any other circuit.

During Controlled ventilation (IPPV)

Completely useless. The Worst

Inspiration - Need valve to be closed during inspiration to stop reservoir gases leaving circuit

Expiration - The patient expires, these expired gases mix with the FGF and partially fill the reservoir bag, some are vented.

Expiratory pause - FGF mixes, mixed gases leave

Next Inspiration - rebreathing

Needs FGF = 2.5 X MV to avoid i.e. 12-15l/min, highly inefficient

Modifications

1. The Lack circuit. Lack is a contemporary anaesthetist at Salisbury Hospital, UK.

lack.jpg (13571 bytes)

It is a Mapleson A system in which the exhaled gases travel down a central tube located within an outer corrugated tube towards the expiratory valve. Functionally identical but the expiration valve is moved further from the patient by having an expiratory limb running coaxially inside the inspiratory limb. The two tubes can run parallel. Allows scavenging of anaesthetic gases

2. Modified valve that closes during inspiration in controlled ventilation

To avoid the major failing of the Magill circuit (FGF is vented during IPPV) there are modifications of the Mapleson A system in which the expiratory valve is held shut during lung inflation. Such modifications include the Carden A system, the enclosed Magill system, and the afferent reservoir systems (EAR) - all of which may be called enclosed Mapleson A systems.

Controlled ventilation is effected by increasing the pressure inside the container by means of a self-inflating bag or a ventilator. The increased pressure in the  container squeezes the reservoir bag, while at the same time it holds the expiratory valve tightly shut. In this way, all the fresh gas and any dead space gas stored in the inspiratory tube is delivered to the patient. The valve opens again at end-expiration allowing alveolar gas to be vented

 During expiration, the expiratory valve opens when the enclosed reservoir bag is full and expired gas is vented via the ventilator or a valve attached to the inflation bag

3. Adding a non rebreathing valve. Resuscitation.

Artificial Ventilation This shows an "Ambu-bag" which is a self-reinflating bag with nonrebreathing valves to provide positive pressure ventilation during resuscitation with oxygen or air

The Ruebens non rebreathing valve is the simplest to understand. Picture...

Which ones are good for SV and IPPV? Some cannot allow the patients to spontanously breath through as the resistance to open the valve without IPPV is too high

Also known as a Bag Valve mask system or BVM.

Used with portable ventilator units.


Mapleson B and C

Rationale was that putting the FGF nearer the patient would increase the amount of FGF the patient breathes. This is untrue.

They are similar in construction, with the fresh gas flow entry and the expiratory valves located at the patient end of the circuit. They are not commonly used in anaesthetic practice, although the C system is used on intensive care units (?Why). High flows of gases are needed to prevent rebreathing of CO2 and this system was at one time combined with a canister of sodalime to absorb CO2 (Waters' "To and Fro" Circuit). However the canister proved too bulky for practical use and there was a risk of the patient inhaling soda lime dust.

During SV the inspiratory gases contain a large proportion of the alveolar gas that has just been exhaled into the reservoir. Therefore rebreathing occurs unless very high FGF are used.

The situation is not changed much (?) in controlled ventilation. The circuit is not widely used.

Mapleson C is used in resuscitation

And to provide PEEP. Commonly found in recovery areas - if your patient has fallen asleep after their anaesthetic (!) and is hypoventilating you might give them a hand breathing off the volatile by sticking a mask on and a Mapleson C (Occasionally known as a Waters circuit, not the same as the Water's to and fro circuit) by acheiving a good FM seal and twiddling the valve you can apply PEEP and hand ventilate with the bag.

An alternative is the the BVM system but it's not so easy to put PEEP on.


Mapleson D

Figure 4

Spontaenous

As can be seen from the above figure - during SV there is a degree of rebreathing

Controlled

During controlled ventilation the Mapleson D system functions more efficiently.

During expiration the corrugated tubing and reservoir bag fill with a mixture of fresh and exhaled gas. Fresh gas fills the distal part of the corrugated tube during the expiratory pause prior to inspiration. When the bag is compressed this fresh gas enters the lungs and when the expiratory valve opens a mixture of fresh and exhaled gas is vented. The degree of rebreathing that occurs depends on the FGF.

The Bain modification

bain.jpg (13999 bytes)

Advantages Problems
  • For the patient:
    • Low resistance,
    • Easy to sterilise,
  • For the anaesthetist:
    • Light weight, minimal apparatus at ETT,
    • Easy to scavenge,
    • Easy to connect to ventilator for IPPV,
    • No Valves,
    • SV or PPV equally simple,
    • Inexpensive materials,
    • Useful for all ages.
  • Many connections therefore disconnection risk increased
  • Uneconomical on fresh gas utilisation

Mapleson E and F

Most important design features for paediatric anaesthetic use include:

(Additionally desirable to conserve heat and moisture).

Original T-piece (Phillip Ayre 1937 - paediatric neurosurgical/cleft-lip and palate repair) was modified by Jackson Rees in 1950 & 1960, adding exp. limb to prevent air dilution and an open-ended 500 ml bag to allow respiratory monitoring and/or assistance.

Deadspace

Specific VD/VT ratio in children similar to adults, ie ~ 0.3 - therefore 3 kg infant with VT=21ml has a VD = 7ml.

VD/VT increases in deep anaesthesia with spontaneous ventilation, and can increase further if XS apparatus dead space is not limited. In ATP and Bain, dead space volume is determined by:

  1. FGF, and
  2. apparatus VD between FG outlet and patient.

In SV, PaCO2 increases unless FGF > 2.5-3x Vmin.
In CV (IPPV), ATP results in relatively more rebreathing 2 to higher PIFR.

Provided the FGF > Vmin, efficiency of CO2 washout is entirely dependant on FGF. Mean level of PaCO2 depends on VCO2 minus CO2 removal. (ie since CO2 removal is dependent in a non-absorbent system on FGF, the ETCO2 and PaCO2 depend on VCO2 and FGF).

The leak around ETTs in children is of little importance, as is the loss of dead space when cutting ETT length.

ADVANTAGES:

  1. Simple and lightweight;
  2. VD minimal;
  3. Resistance is low - a slight increases in exp. resistance may act like a low level of PEEP and help to offset the loss of FRC in general anaesthesia;

DISADVANTAGES:

  1. High FGF necessary;
  2. Dry gases inhaled unless humidified;
  3. Atmospheric pollution unless scavenging in place;
  4. Expense prohibitive if N2O not available - ie O2 plus volatile at high flows

 

 

Humphrey's ADE

The Mapleson A circuit is inefficient for controlled ventilation as is the Mapleson D circuit for spontaneous ventilation. David Humphrey has designed a single circuit (Figure 5) that can be changed from a Mapleson A system to a Mapleson D by moving a lever on the metallic block which connects the circuit to the fresh gas outlet on the anaesthetic machine. The reservoir bag is situated at the fresh gas inlet end of the circuit, and gas is conducted to and from the patient down the inspiratory and expiratory limbs of the circuit

Can confuse anaesthetists so they don't use it.

 


Circle system

Figure 6

An alternative to using high flow circuits is to absorb CO2 from the expired gases which are then recirculated to the patient. These circuits are known as circle systems, were first devised by Brian Sword in 1926 and require smaller amounts of fresh gas each minute.

Carbon dioxide is removed from the expired gas by passage through soda lime, a mixture of 94% calcium hydroxide and 5% sodium hydroxide, and 1% potassium hydroxide which reacts with CO2 to form calcium carbonate. Soda lime also contains small amounts of silica to make the granules less likely to disintegrate into powder and a chemical dye which changes colour with pH. As more carbon dioxide is absorbed the pH decreases and the colour of the dye changes from pink to yellow/white. When around 75% of the soda lime has changed colour it should be replaced . The soda lime canister should be mounted vertically on the anaesthetic machine to prevent the gases passing only through a part of the soda lime (streaming).

Fresh soda lime contains 35% water by weight which is necessary for the reaction between carbon dioxide and soda lime to take place. This generates considerable heat. The soda lime may rise in temperature to 40 centigrade. There are therefore additional advantages of using circle systems in that the gases within the circle are warmed and humidified prior to inspiration. (Baralyme is a commercially available CO2 absorber which contains 5% barium hydroxide instead of sodium hydroxide

Vaporiser Position. The vaporiser may be placed either outside the circle (VOC) on the anaesthetic machine in its conventional position, or rarely within the circle itself (VIC). Normal plenum vaporisers, with high internal resistance, cannot be used within the circle and a low internal resistance type vaporiser (such as the Goldman) is required. Drawover vaporisers such as the OMV are not recommended for use within the circle because of the risk of over-dosage. Since the gases are recirculated, if the vaporiser is placed in the circle, gas already containing volatile anaesthetic agent will re-enter the vaporiser and the resulting output will exceed the vaporiser setting. This is a particular danger during controlled ventilation when dangerously high concentrations can build up. Vaporisers should only be placed inside the circle (VIC) when inspired volatile anaesthetic agent monitoring is available. It is safer to use conventional plenum vaporisers mounted on the anaesthetic machine outside the circle. In this case the maximum volatile anaesthetic agent concentration achievable within the circle cannot exceed that set on the vaporiser.

Practical Use of Circle Systems. During the first 5 - 10 minutes of an inhalational anaesthetic using a volatile anaesthetic agent in oxygen and nitrous oxide, large amounts of the anaesthetic agent and nitrous oxide will be taken up by the patient, and the nitrogen contained in the patient's lungs and dissolved in their body will be washed out. If low fresh gas flows are used immediately the patient is connected to the circuit the nitrogen will not be flushed out of the circle system and will dilute the anaesthetic agent concentration. This may be prevented by using conventional fresh gas flows of 6litres/min for the first 5-10 minutes of each anaesthetic before reducing the flow rates.

Reducing the fresh gas flow rates. Inspired anaesthetic gases should contain no carbon dioxide and a minimum of 30% oxygen. Exhaled alveolar gas contains a lower concentration of oxygen and around 5% carbon dioxide which is removed from the exhaled gas on passage through the soda lime. A small amount of fresh gas is added before the next breath. At low fresh gas flow rates (<1000mls /min) unless 40-50% oxygen is supplied to the circle, the oxygen concentration within the circle can fall to unacceptably low levels due to the greater uptake of oxygen compared with nitrous oxide. Circle systems should preferably not be used at low flow rates without an oxygen analyser in the inspiratory limb. The lowest fresh gas flow rate of oxygen and nitrous oxide which can be used to ensure that the inspired oxygen concentration remains at a safe level is 1500mls/min (nitrous oxide 900mls/min and oxygen 600mls/min). Conventional flow meters and vaporisers become unreliable if flows are set lower than these levels.

These comments are less important if only oxygen and a volatile agent is being used in the circle. Under these circumstances there is no risk of oxygen dilution and the flows may be reduced to 1000mls/min.

With flows of >1500mls/min the inspired concentration of volatile agent will be similar to that set on the vaporisers. With flows <1500mls/min the volatile agent concentration may fall within the circuit and the setting on the vaporiser may need to be increased.

Halothane, isoflurane and enflurane are all safe to use in circle systems with soda lime, however trichloroethylene (no longer used in the USA or UK) produces a toxic metabolite and must not be used. When the circle system is not in use all fresh gas flows should be turned off to avoid wastage and to prevent the soda lime from drying out.

Several paediatric circle systems have been developed using smaller bore tubing and a one litre reservoir bag. The work involved in breathing through these systems is no greater than with a conventional Mapleson F system

Advantages of the circle circuit

Can use low FGF

Recirculation of volatiles saves money and the environment

Low functional deadspace due to the Y connector and the CO2 removal

 

Breathing circuits and ventilators

Some incorporate circuits - the Manley series.

Other ventilators have been designed to operate with existing breathing systems e.g. the Penlon Nuffield 200 and the Bain circuit