Breathing Systems

Dr. P. Narasimha ReddyProfessor

Dept. of Anaesthesiology

6 March 2011 1

Definition

A breathing system is defined as an

assembly of components which

connects the patient’s airway to the

anaesthetic machine creating an

artificial atmosphere, from and into

which the patient breathes.

6 March 2011 2

Purpose To deliver anesthetic gases and oxygen Offer a means to deliver anesthesia without

significant increase in airway resistance To offer a convenient and safe method of

delivering inhaled anesthetic agents To annoy you with yet one more thing to

memorize

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Components A fresh gas entry port / delivery tube A port to connect it to the patient's airway; A reservoir for gas, in the form of a bag An expiratory port / valve A carbon dioxide absorber if total rebreathing

is to be allowed Corrugated tubes for connecting these

components. Flow directing valves may or may not be used.6 March 2011 4

Requirements of a Breathing System

Essential:

The breathing system must

1. Deliver the gases from the machine to the alveoli in the same concentration as set and in the shortest possible time;

2. Effectively eliminate carbon-dioxide;

3. Have minimal apparatus dead space; and

4. Have low resistance.6 March 2011 5

Requirements of a Breathing System

Desirable: economy of fresh gas; conservation of heat; adequate

humidification of inspired gas;

Light weight; convenience during

use;

efficiency during spontaneous as well as controlled ventilation

adaptability for adults, children and mechanical ventilators;

provision to reduce theatre pollution

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Anesthesia Breathing Systems Resistance to flow can be minimized by:

Reducing the circuit’s length Increasing the diameter (who’s law is that??)

• Hagen-Poiseuille P = (L)(v)(V) r4

– P is pressure gradient. L is length. v is viscosity. V is flow rate– RESISTANCE IS INDIRECTLY PROPORTIONAL TO FLOW RATE WITH LAMINAR FLOW

» Flow = P1-P1/R where P1 is pressure at one end of a tube and P2 is pressure at the other end of the tube

– FOR TURBULENT FLOW, GAS DENSITY IS MORE IMPORTANT THAN VISCOSITY– RESISTANCE IS PROPORTIONAL TO THE “SQUARE” OF FLOW RATE (TURBULENT FLOW)– IN CLINICAL PRACTICE, FLOW IS USUALLY A MIXTURE OF LAMINAR & TURBULENT

FLOW

Avoiding the use of sharp bends (turbulent flow) Eliminating unnecessary valves Maintaining laminar flow6 March 2011 7

Another look at Poiseuille’s Law

Laminar flow: orderly movement of gas inside a “hose” (gas in the center of the tube moves faster than gas closer to walls)

Turbulent flow: resistance is increased (seen with sudden narrowing or branching of tube)

Laminar flow becomes Turbulent when Reynold’s number is >2000

Poiseuille’s Law follows Laminar flow R = 8 n l (R: resistance, n: viscosity, l: length, r: radius)

r4

Example: doubling the radius of the tube will decrease the resistance 16 times (2)4=16

6 March 2011 8

Classification Of Breathing Systems

McMohan in 1951 Open no rebreathing Semiclosed partial rebreathing Closed total rebreathing

Dripps et al Insufflation, Open, Semiopen, Semiclosed and

Closed taking into account the presence or absence of

Reservoir, Rebreathing, CO2 absorption and Directional valves

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Conway Breathing systems with CO2 absorber Breathing systems without CO2

absorber.

Sub-classified by Miller in 1988

Classification Of Breathing Systems

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Unidirectional flow: Non rebreathing systems. Circle systems.

Breathing Systems Without CO2 Absorption

Bi-directional flow:1. Afferent reservoir systems.

Mapleson A Mapleson B Mapleson C Lack’s system.

2. Enclosed afferent reservoir systems Miller’s (1988)

3. Efferent reservoir systems Mapleson D Mapleson E Mapleson F Bain’s system

4. Combined systems Humphrey ADE

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Breathing Systems With CO2 Absorption.

Unidirectional flow

Circle system with absorber.

Bi-directional flow

To and Fro system.

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Non-rebreathing systemUnidire

ctional

Flow

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Disadvantages of Nonrebreathing system

1. Fresh gas flow has to be constantly adjusted

2. There is no humidification of inspired gas.

3. There is no conservation of heat.

4. They are not convenient as the bulk of the valve has to be positioned near the patient.

5. The valves can malfunction due to condensation of moisture and lead to complications.

6 March 2011 14

Bi-Directional Flow

These systems depend on the FGF for effective elimination of CO2.

functioning can be manipulated by changing parameters like Fresh gas flow, alveolar ventilation, apparatus dead space

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Fresh Gas Supply

If there is no FGF into the system, the patient will get suffocated.

If the FGF is low, most systems do not eliminate carbon-dioxide effectively, and

If there is an excess flow there is wastage of gas. It is imperative to specify optimum FGF for a

breathing system for efficient functioning. FGF should be delivered as near the patient's

airway as possible.6 March 2011 16

Elimination Of Carbon-Dioxide

Normal production of carbon-dioxide in a 70 kg adult is 200 ml per minute

Normal end-tidal concentration of carbon-dioxide is 5%.

For eliminating 200 ml of carbon-dioxide as a 5% gas mixture, the alveolar ventilation has to be

200 x 100 / 5 = 4,000 ml.

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alveolar ventilation

< 4 L/min Hypercarbia

> 4 L/min Hypocarbia alveolar ventilation

5 L/min with 1 % CO2

8 L/min with 2.5 % CO2 Normocarbia

Elimination Of Carbon-Dioxide

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Apparatus Dead Space

It is the volume of the breathing system from the patient-end to the point up to which, to and fro movement of expired gas takes place

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The dynamic dead space will depend on the FGF and the alveolar ventilation

The dead space is minimal with optimal FGF.

If the FGF is reduced below the optimal level, the dead space increases and

The whole system will act as dead space if there is no FGF

Apparatus Dead Space

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Sub-Classification Of Bi-Directional Flow Systems

1. Afferent reservoir system (ARS).

2. Enclosed afferent reservoir systems (EARS).

3. Efferent reservoir systems (ERS).

4. Combined systems.

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Enclosed afferent reservoir system has been described by Miller and Miler.

If the reservoir is placed in this limb as in Mapleson D, E, F and Bain systems, they are called efferent reservoir systems (ERS).

The efferent limb is that part of the breathing system which carries expired gas from the patient and vents it to the atmosphere through the expiratory valve/port.

Bi-Directional Flow Systems

The afferent limb is that part of the breathing system which delivers the fresh gas from the machine to the patient.

If the reservoir is placed in this limb as in Mapleson A, B, C and Lack's systems, they are called afferent reservoir systems (ARS).

Afferent Limb

Efferent Limb6 March 2011 22

Afferent Reservoir Systems The Mapleson A, B and C systems

have the reservoir in the afferent limb, and do not have an efferent limb

Lack system has an afferent limb reservoir and an efferent limb through which the expired gas traverses before being vented into the atmosphere. This limb is coaxially placed inside the afferent limb.

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Afferent Reservoir Systems

These AR systems work efficiently during spontaneous breathing provided the expiratory valve is separated from the reservoir bag and FGF by at least one tidal volume of the patient and apparatus dead space is minimal

6 March 2011 24

If the FGF is close to the expiratory valve as in Mapleson B & C, the system is inefficient both during spontaneous and controlled ventilation.

Afferent Reservoir Systems

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Mapleson’s analysis 1. Gases move enbloc. They maintain their

identity as fresh gas, dead space gas and alveolar gas. There is no mixing of these gases.

2. The reservoir bag continues to fill up, without offering any resistance till it is full.

3. The expiratory valve opens as soon as the reservoir bag is full and the pressure inside the system goes above atmospheric pressure.

4. The valve remains open throughout the expiratory phase without offering any resistance to gas flow and closes at the start of the next inspiration.6 March 2011 26

AR Systems – Functional analysis

Spontaneous Breathing

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Magill System – Controlled Ventilation

First Breath

Expiration

Next Inspiration

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Lack's system

This system functions like a Mapleson A system both during spontaneous and controlled ventilation

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Anesthesia Breathing Systems

Mapleson Advantages

• Used during transport of children• Minimal dead space, low resistance to breathing• Scavenging (variable ability, depending on FGF used)

Disadvantages• Scavenging (variable ability, depending on FGF used)• High flows required (cools children, more costly)• Lack of humidification/heat (except Bain)• Possibility of high airway pressures and barotrauma• Unrecognized kink of inner hose in Bain• Pollution and higher cost• Difficult to assemble6 March 2011 30

Enclosed Afferent Reservoir (Ear) Systems

Coaxial version

Non coaxial version

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Efferent Reservoir Systems 6 mm tube as the afferent

limb that supplies the FG from the machine.

The efferent limb is a wide-bore corrugated tube to which the reservoir bag is attached

The expiratory valve is positioned near the bag

All these ER systems are modifications of Ayre's T-piece

6 March 2011 32

Ayre's T-piece Light metal tube 1 cm

in diameter, 5 cm in length with a side arm

Used as such, it functions as a non-rebreathing system

FGF equal to peak inspiratory flow rate of the patient

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Efferent Reservoir Systems

In an attempt to reduce FGF requirements ER systems are constructed

The functioning of all these systems are similar

These systems work efficiently and economically for controlled ventilation as long as the FG entry and the expiratory valve are separated by a volume equivalent to atleast one tidal volume of the patient6 March 2011 34

ER systems - Functional Analysis

Factors affecting CO2 elimination

FGF, respiratory rate, expiratory pause, tidal volume and CO2 production

Spontaneous Breathing

Factors other than FGF cannot be manipulated in a spontaneously breathing patient

6 March 2011 35

Factors affecting CO2 elimination

FGF, respiratory rate, tidal volume and pattern of

ventilation

ER systems - Functional Analysis

Controlled Ventilation

These parameters can be totally controlled by the anaesthesiologist6 March 2011 36

Anesthesia Breathing Systems

Bain system (http://www.capnography.com/Circuits/bainsystem.htm)

Coaxial (tube within a tube) version of Mapleson D

Fresh gas enters through narrow inner tube Exhaled gas exits through corrugated outer

tube FGF required to prevent rebreathing:

• 200-300ml/kg/min with spontaneous breathing (2 times VE)

• 70ml/kg/min with controlled ventilation6 March 2011 37

Bain at work (spontaneous) Spontaneous: The breathing system should be filled

with FG before connecting to pt. During inspiration, the FG from the machine, the reservoir bag and the corrugated tube flow to the pt.

During expiration, there is a continuous FGF into the system at the pt’s end. The expired gas gets continuously mixed with the FG as it flows back into the corrugated tube and the reservoir bag. Once the system is full, the excess gas is vented to the scavenger.

During the expiratory pause the FG continues to flow and fill the proximal portion of the corrugated tube while the mixed gas is vented through the valve. 6 March 2011 38

Bain at work (spontaneous) During the next inspiration, the pt breathes in FG

as well as the mixed gas from the corrugated tube. Many factors influence the composition of the inspired mixture (FGF, resp rate, expiratory pause, TV and CO2 production in the body). Factors other than FGF cannot be manipulated in a spontaneously breathing pt.

It has been mathematically calculated and clinically proved that the FGF should be at least 1.5 to 2 times the patient’s minute ventilation in order to minimize rebreathing to acceptable levels.

6 March 2011 39

Bain at work (controlled) Controlled: To facilitate intermittent positive pressure

ventilation, the APL has to be partly closed so that it opens only after sufficient pressure has developed in the system. When the system is filled with fresh gas, the patient gets ventilated with the FGF from the machine, the corrugated tube and the reservoir bag.

During expiration, the expired gas continuously gets mixed with the fresh gas that is flowing into the system at the patient’s end.

During the expiratory pause the FG continues to enter the system and pushes the mixed gas towards the reservoir.  

6 March 2011 40

Bain at work (controlled)

When the next inspiration is initiated, the patient gets ventilated with the gas in the corrugated tube (a mixture of FG, alveolar gas and dead space gas).

As the pressure in the system increases, the APL valve opens and the contents of the reservoir bag are discharged into the scavenger (gas follows the path of least resistance)

6 March 2011 41

Anesthesia Breathing Systems

Bain Advantages

• Warming of fresh gas inflow by surrounding exhaled gases (countercurrent exchange)

• Improved humidification with partial rebreathing

• Ease of scavenging waste gases

• Overflow/pressure valve (APL valve)

• Disposable/sterile

6 March 2011 42

Anesthesia Breathing Systems

Bain Disadvantages

• Unrecognized disconnection• Kinking of inner fresh gas flow tubing• Requires high flows• Not easily converted to portable when commercially used

anesthesia machine adapter Bain circuit used Look at the Bain and identify what makes it

modified from the standard Mapleson D

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Bain is a Modified Mapleson D

(APL)

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Pethick’s Test for the Bain Circuit

A unique hazard of the use of the Bain circuit is occult disconnection or kinking of the inner hose (fresh gas delivery hose). To perform the Pethick’s test, use the following steps: Occlude the patient's end of the circuit (at the elbow). Close the APL valve. Fill the circuit, using the oxygen flush valve (like

pressurizing the circuit when you are doing a leak test) Release the occlusion at the elbow and flush. A Venturi

effect flattens the reservoir bag if the inner tube is patent.

6 March 2011 45

PaCO2 Isopliths

1. When FGF is very high the PaCO2 becomes ventilation dependent (as during spontaneous respiration).

2. When the minute volume exceeds the FGF substantially, the PaCO2 is dependent on the FGF

40 mmHg

35 mmHg

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Combined System Humphry ADE:with two reservoirs one in afferent and

onein efferent limb System can be changed from ARS to ERS by changing the

position of the lever.

Can be used for adults and children.

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Breathing Systems with CO2 Absorption

1. sodalime canister, 2. Two unidirectional

valves, 3. Fresh gas entry, 4. Y-piece to connect to

the patient, 5. Reservoir bag 6. a relief valve and 7. low resistance

interconnecting tubing.

Circle System

Functional Analysis

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3 Essential Factors

There should be two unidirectional valves on either side of the reservoir bag and the canister,

Relief valve should be positioned in the expiratory limb only,

The FGF should enter the system proximal to the inspiratory unidirectional valve.6 March 2011 49

Optimization of Circle Design

Unidirectional Valves Placed in close proximity to pt to prevent

backflow into inspiratory limb if circuit leak develops.

Fresh Gas Inlet Placed b/w absorber & inspiratory valve. If

placed downstream from insp valve, it would allow FG to bypass pt during exhalation and be wasted. If FG were placed b/w expiration valve and absorber, FG would be diluted by recirculating gas6 March 2011 50

Optimization of Circle Design APL valve

Placed immediately before absorber to conserve absorption capacity and to minimize venting of FG

Breathing Bag Placed in expiratory limb to decrease

resistance to exhalation. Bag compression during controlled ventilation will vent alveolar gas thru APL valve, conserving absorbent

6 March 2011 51

Circle system can be:

closed (fresh gas inflow exactly equal to patient uptake, complete rebreathing after carbon dioxide absorbed, and pop-off closed)

semi-closed (some rebreathing occurs, FGF and pop-off settings at intermediate values), or

semi-open (no rebreathing, high fresh gas flow)

6 March 2011 52

Anesthesia Breathing Systems

Circle systems Most commonly used Adult and child appropriate sizes Can be semiopen, semiclosed, or closed

dependent solely on fresh gas flow (FGF) Uses chemical neutralization of CO2 Conservation of moisture and body heat Low FGF’s saves money

6 March 2011 53

Functional analysis

During inspiration fresh gas flow goes through inspiratory limb passing through U1.

No flow in expiratory limb. During expiration no flow in the inspiratory limb,

gases pass through U2 goes to canister, CO2 is absorbed then mixes with fresh gas flow in reservoir bag.

When bag is full expiratory valve opens.

6 March 2011 54

Maintanence of closed systems Condensed water in the circuit must be

removed. Breathing hoses should be changed in

between cases Reusable tubes washed and hanged Valves opened and cleaned with alcohol Sodalime should be changed regularly

Color change in majority of granules If etco2 shows base line elevation

When it was not used for longer time6 March 2011 55

Advantages

1. Economy: The FGF could be reduced to as low as

250 - 500 ml of oxygen. The consumption of Halothane / Isoflurane

has been found to be around 3.5 ml/hour

2. Humidification

3. Reduction of heat loss

4. Reduction in atmospheric pollution6 March 2011 56

Disadvantages1. A greater knowledge of uptake and

distribution is required to master closed circuit anaesthesia.

2. Inability to alter any concentration quickly.

3. Real danger of hypercapnia exists with a) an inactive absorber,

b) incompetent unidirectional valves and

c) incorrect use of absorber bypass.6 March 2011 57

Disadvantages of Circle System

Greater size, less portability Increased complexity

Higher risk of disconnection or malfunction Increased resistance (of valves during

spontaneous ventilation) Dissuading use in Pediatrics (unless a circle pedi

system used) Difficult prediction of inspired gas

concentration during low fresh gas flow6 March 2011 58

Bacterial Contamination

Slight risk of microorganism retention in Circle system that could (theoretically) lead to respiratory infections in subsequent pts

Bacterial filters are incorporated into EXPIRATORY LIMB of the circuit

6 March 2011 59

Gases and vapour concentration in circle

system Internal volume of apparatus with 2 kg

absorbent Inter granular space 1 litre Breathing hoses 1 litre Pathways with in the absorber 1 litre Patient FRC 1.25 litres Total 4.25 litres

Into this anesthetic gas is diluted.6 March 2011 60

Checking breathing systems – WHY????

Fatalities have occurred when the user User was unfamiliar with equipment Included a non standard item Not noticied improper assembly Not noticed the development of a fault

6 March 2011 61

protocols The components of the system must

conform to the international quality Assembly should be proper All tapered connections are push and

twist technique APL valve if closed air tight fitting must

be there If valve opened gases must go freely Coaxial breathing systems integrity

should be tested 6 March 2011 62

Summary

Defined a Breathing System

Classified the Breathing Systems

With CO2 Absorber Unidirectional

Without CO2 Absorber Bidirectional

Analyzed the functional analysis

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6 March 2011 64