Nick Robson

ANAESTHESIA BREATHING SYSTEMS

References:

CLASSIFICATION

Several different nomenclatures in existence, eg

Further subdivided:

Confusing and cumbersome, because the same systems are frequently referred to by different people using a variety of names.

Makes sense to classify in terms of their geometry (Conway - 1985) into 2 broad classes , ie, systems:

The majority of systems allocated to the first group are the "Mapleson" systems. These are rebreathing systems, since the potential exists for expired CO2 rebreathing.

The second group contains CO2 absorption circuits, of which the CIRCLE is most commonly in use. (Watters "to-and-fro" circuit is the other configuration). These have a means of CO2 absorption and utilising one-way valves mechanically separating inspired and expired gases, can virtually eliminate expired CO2 rebreathing.

Non-rebreathing circuits exist also with a NRB valve close to the patient's airway to vent expired gases eg Laerdal resuscitation bags.

McIntyre (1986) classifies into 2 groups also:

Three main functions of anaesthesia circuits are:

Open circuits are of historic interest only apart from paediatric induction supplementing ambient air with high flow O2/N2O delivered by a hose into the anaesthetist's cupped hands.

A. CARBON DIOXIDE WASHOUT CIRCUITS

1. MAPLESON A - (Magill) CIRCUIT

FGF enters at the end remote from the patient. Behaviour varies markedly between spontaneous and controlled ventilation:

(a) Spontaneous ventilation

(b) Controlled ventilation

2. MAPLESON B & C SYSTEMS

3. MAPLESON D, E and F SYSTEMS

4. BAIN MODIFICATION OF THE MAPLESON D SYSTEM

Numerous formulae exist to predict FG requirements in spont. breathing:

Capnography has removed the "hit and miss" nature of these calculations. FGF > 6-8 l.min-1 in an adult should be adequate, ie > 70-100 ml.kg.min-1.

Rebreathing does occur but increased Vmin in spontaneous respiration will prevent CO2 retention, (unless respiratory depressants used in large doses).

During controlled ventilation, FGF of 70 ml.kg.min-1 and Vmin =120-150ml.kg.min-1 adequate to prevent significant rebreathing, although this nearly always occurs to a degree (unless FGF > 3x minute ventilation ie ~ 200-250 ml.kg.min-1).

Advantages of Bain circuit:

Problems with Bain circuits:

5. LACK COAXIAL SYSTEM - (Modification of Magill)

Coaxial with FGF down large diameter outer corrugated tube, functionally resembling the Magill. Inner narrow tube is connected to the expiratory valve.

FGF = VE avoids significant rebreathing in spont. ventilation (ie FGF > 70 ml.kg-1).

During spontaneous ventilation gas taken from the outer tube and the reservoir bag, which have been filled (depending upon FGF rate) during the inspiratory pause.

In CV same flow considerations apply as with the Magill.

During expiration, the bag fills and XS gas is vented along the inner tube to the relief valve. At end insp'n the inner tube contains only exp. gas, and the outer tube contains exp. gas near the patient, and some FG near the machine and within the reservoir bag. During the exp. pause, the FG drives the exp. gas out of the inner tube to the vent valve. In CV FGF > 150 ml.kg.min-1prevents significant rebreathing, unless Vmin 200ml.kg.min-1, when FGF becomes the CO2 controller and may be reduced to 100 ml.kg.min-1.

6. AYRE'S T-PIECE

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

B. CO2 ABSORPTION CIRCUITS

Most common in use today is the circle system.

Components -

  1. absorber and absorbent;
  2. FG inlet;
  3. unidirectional valves;
  4. pressure gauge;
  5. breathing tubes;
  6. reservoir bag;
  7. "Y" piece; and
  8. O2 sensor.

Optional extras include - bacterial filters, VIC vaporisers, humidifiers (HMEs) etc.

1. ABSORBER AND ABSORBENT

Canister containing CO2 absorbent - soda lime - may be single or dual, of metal, glass or (typically) plastic construction. Intergranular space > VT, volume of air in a filled canister is approx. 50% of total. Larger cross-sectional diameter allow less turbulence with reduced resistance and less dust.

Correctly packed canisters allow absorption throughout, rather than in a columnar fashion. Baffles in canister reduces gas tracking down the walls, where spaces relatively larger.

Bypass mechanism can isolate absorber from circuit and allow PaCO2 to occur without decreased minute ventilation.

Advantages of CO2 absorption

  1. lower FGF, improved economy;
  2. less pollution;
  3. heat and moisture are conserved;
  4. flammable gases and vapours (historically) contained.
  5. inhaled mixture composition is more constant.

Soda lime

2. FRESH GAS INLET


Connection between CGO and (usually) the inspiratory side of the absorber top mount.

3. UNIDIRECTIONAL VALVES


2 in each circle system - inspiratory, and expiratory limbs. Attached to the absorber usually, but can be located at the "Y" piece. 2 main designs:

Valve requirements - low resistance essential, therefore large SA,

Problems:

4. EXHALE VALVES - (pop-off, or adjustable pressure limiting valves)


Designed to vent or spill XS gas from the circle. Generally combined with scavenging collection assembly. Necessary in all circuits where gases are vented by pressure from the circuit - eg circle, Bain, Magill.

Minimum spill pressure sl.> that required to fill the reservoir bag (< 2.5 cmH2O usually).

(i) Heidebrink type


Disc valve seated on annular knife edge. Light spring connected to a screw cap allows the opening pressure to be adjusted from 1-40 cmH2O, beyond which valve is totally closed. Creates very little outflow resistance. Initially mica, changed to metal, now mica again.

(ii) CIG - Medishield exhale valve


Similar principle, but differs in that it has a safety relief mechanism activated at pressures > 50 cmH2O, with manual override by depressing the tightening screw. Relieves at 2-50 cmH2O in normal flow range, but relief pressure increases to 50-100 cmH2O at flows > 30 l.min-1.

(iii) Berner valve


Three-position valve - behaves like Heidebrink at normal pressures, ie opening at various spill pressures, fully closed and thirdly, closes completely on sharp rise in pressure. Useful during hand ventilation of a patient. Gas spill still occurs in expiration. Avoids the need for adjustment if FGF is altered.

5. PRESSURE GAUGE - manometer


Attached to absorber, calibrated in cmH2O and mmHg with positive and negative scales.

Anaeroid type.

Reflects changing pressures within the circuit, DOES NOT accurately display Paw, because of resistance and compliance characteristics of the components of the circuit, proximal to patient airway.

6. BREATHING HOSES


Function - gas conduit to patient (reservoir in some circuits). Length does not influence VD within the circuit.

Black antistatic:

Disposable polypropylene hoses

7. RESERVOIR BAG


Stores gas between respiratory cycles. Provides information to anaesthetist in manual ventilation regarding pulmonary compliance and airway patency. Rubber or neoprene construction. Antistatic - resistance not less than 106 ohms.

Sizes from 500 ml - 5 litres.

> 2 litre bags have standard 22 mm connections.

2 l bags have 15 mm connections.

Compliance tested - inflated to 2.5 cmH2O and then filling continued to 4x nominal volume. After deflation to P = 2.5 cmH2O again, should not have increased volume by more than 10% and inflation pressure should have been 30-50 cmH2O.

Can usually hold 10x nominal volume before bursting. Offers no protection against pressure extremes in IPPV - 2 l bag has peak pressure of > 75 cmH2O at 4x nominal volume.

8. "Y" PIECE


Plastic or metal. May house non-return valves. All have two standard 22 mm male connections to fit the breathing hoses. Patient connection port is a 15 mm female fitting, OR a 15 mm female port coaxial within a 22 mm male fitting.

Metal "Y" pieces require care with positioning around the face - risk of nerve damage.

9. SENSOR FROM O2 ANALYSER


Fitted to a right angled portion of a "T" fitting located at the inspiratory limb of the circuit - polarographic analyser commonly.

10 FITTINGS