Download MBBS Neuroanaesthesia PPT 1 Anaesthesia Machine And Breathing Circuit Lecture Notes

Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Neuroanaesthesia PPT 1 Anaesthesia Machine And Breathing Circuit Lecture Notes

Anaesthesia machine and

breathing circuit

Introduction

? The Anesthesia gas machine is a device which delivers a

precisely known but variable gas mixture, including

anesthetizing and life sustaining gases.
Objective

?Become familiar with the basic design of an anesthetic machine
? Become familiar with the design and functioning of anesthetic

vaporizers.

?Become familiar with the design and functioning of the more

commonly used breathing circuits

History

before the invention of anaesthesia...

surgery was an agony...a nightmare....



Boyle's Apparatus- 2000s

Modern anaesthesia workstation
Types of Anaesthesia Machine

? INTERMITTENT - Gas flow only during inspiration
- Entonox appartus, Mackessons
? CONTINUOUS - Gas flows both during inspiration and expiration
- Boyle machine , forregar , dragger

Components of anaesthesia machine

? Comprise of three different pressure systems-
? High pressure system: from cylinder to pressure reducing valves
? Intermediate pressure system: from pressure reducing valves to

flowmeters

? Low pressure system: from flow meters to the common gas outlet on

machine

High pressure system

n Receives gasses from the high pressure E cylinders

attached to the back of the anesthesia machine

(2200 psig for O2, 745 psig for N2O)

n Consists of:

? Hanger Yolk (reserve gas cylinder holder)
? Check valve (prevent reverse flow of gas)
? Cylinder Pressure Indicator (Gauge)
? Pressure Reducing Device (Regulator)

n Usually not used, unless pipeline gas supply is off
Components of high pressure system
? Hanger yoke: orients and supports cylinder, provides a gas tight seal, ensures

unidirectional gas flow. Parts of hanger yoke assembly are:

? Body: principal framework of yoke, attached to body of machine
? Retaining screw: its threaded into distal end of yoke, fits into conical depression of

cylinder valve and yoke

? Nipple: gas enters machine through nipple
? Index pin: are below the nipple
? Washer/Bodok seal: Placed around nipple to produce seal between cylinder valve

and yoke.

? Filter: filters 100m particulates
? Check valve assembly: allows gas from cylinder to enter machine but not vice versa

A. Cylinder yokes. The empty right-hand yoke shows a Bodok seal and the pins of the

pin index system.

B. Diagram of cylinder yoke assembly
Pin-Index safety system-1952
? Safety mechanism so that one cylinder can not be fitted at the other's

position

? It consists of holes on the cylinder valve and two pins on the yoke

positioned to fit into the holes

? Pins are 4 mm in diameter and 6 mm long (except pin-7)
? The 7 hole positions are on the circumference of a circle of 9/16 inch

radius centred on the port.

? Six pin positions are located at an interval of 12 degree, with pin 7

located between 3 and 4.

Pin-index system

Bourdon Pressure Gauge
? Must be clearly marked with: Name or

chemical symbol of gas color assigned to

gas

? Each hanger yoke or group of

interconnected yokes should be supplied

with pressure indicator to display pressure

of cylinder supplied gas.

? If indicator is circular , the lowest pressure

indication should be between 6 o' clock and

9 o' clock position on clock face.

? Scale must be 33% more than maximum

filling pressures.
Pressure reducing devices

? Pressure in cylinder varies to maintain constant flow, with changing

supply pressure regulators are provided

? Reduces high and variable pressure found in cylinder to a lower and

more constant pressure found in the anaesthesia machine (40-45

psig)

Intermediate Pressure System

q Receives gasses from the regulator or the
hospital pipeline at pressures of 40-55 psig
q Consists of:
?Pipeline inlet connections
?Pipeline pressure indicators
?Piping
?Gas power outlet
?Master switch
?Oxygen pressure failure devices
?Oxygen flush
?Additional reducing devices
?Flow control valves
Pipeline Inlet Connections

? Mandatory N2O and O2, usually

have air and suction too

? Inlets are non-interchangeable

due to specific threading as per

the Diameter Index Safety

System (DISS)

? Each inlet must contain a check

valve to prevent reverse flow

(similar to the cylinder yoke)

DISS
Terminal outlets. Note the different diameter recesses (collar indexing

system) that match the collar on

the relevant probe

Flexible hose

probes. Note the

difference in size

of the indexing

collar

Colour-coded hoses.
Oxygen Pressure Failure Devices

? The oxygen concentration at the common gas

outlet does not fall below 19%.

? A Fail-Safe valve is present in the gas line

supplying each of the flowmeters (except O 2)

? This valve is controlled by the O2 supply

pressure and shuts off or proportionately

decreases the supply pressure of all other

gasses as the O2 supply pressure decreases

? Historically there are 2 kinds of fail-safe valves
? Pressure sensor shut-off valve (Ohmeda)
? Oxygen failure protection device (Drager)

Oxygen Supply Failure Alarm

? The machine standard specifies that whenever the oxygen supply pressure

falls below a manufacturer-specified threshold (usually 30 psig) a medium

priority alarm shall blow within 5 seconds.

? Electronic alarms: A pressure operated electric switch operates this alarm\
? Ohmeda: 28 psig
? Drager: 30-37 psig

? Pneumatic alarms (Bowman's Whistle): Uses a pressurized canister that is

filled with oxygen when the anesthesia machine is turned on. When the

oxygen pressure falls below a certain value, the alarm directs a stream of

oxygen through a whistle
Oxygen Flush Valve (O2+)

? Receives O2 from pipeline inlet or cylinder reducing device and

directs high, unmetered flow directly to the common gas outlet

(downstream of the vaporizer)

? Machine standard requires that the flow be between 35 and 75 L/min
? The ability to provide jet ventilation via the O2 flush valve is presence

of a check valve between the vaporizer and the O2 flush valve

(otherwise some flow would be wasted retrograde)

qHazards:
?May cause barotrauma
?Dilution of inhaled anesthetic

? Flush valves for gases other than oxygen are not permitted.

? Oxygen flush can be activated regardless of whether machine is

turned ON or OFF.

? Protective rim is present which prevents unintentional

activation
Master switch

? Turning the master switch to the `on' position
? activates both pneumatic and electrical
? functions of the machine as well as certain
? alarms and safety devices.

Second-Stage Reducing Device

? Located just upstream of the flow control valves
? Receives gas from the pipeline inlet or the cylinder reducing device

and reduces it further to 26 psig for N2O and 14 psig for O2

? Purpose is to eliminate fluctuations in pressure supplied to the flow

indicators caused by fluctuations in pipeline pressure
Flow/ Control valves (Needle Valves/ Pin Valves/

Fine Adjustment Valves/ Flow adjustment controls
? Controls rate of flow of gas through its associated flow indicator by

manual adjustment of a variable orifice.

? Current standard requires that there be only one flow control valve

for each gas. It must be adjusted or identifiable with it's flow indicator

Components

? Body.
-Screwed to the base of flow indicator

? Stem and Seat.

? Control knob.

Low Pressure System

? Extends from the flow control valves to the common gas outlet
? Pressure is only slightly above atmospheric pressure.
? Consists of:
? Flow meters
? Vaporizer mounting device
? Check valve
? Back pressure safety devices (Pressure relief device)
? Common gas outlet

Flowmeter tubes

? Measures and indicates RATE OF FLOW of gas .
? TYPES:
? Constant-pressure variable-orifice type
? Electronic flowmeter
Constant-pressure variable orifice

flowmeter
? Measures drop in pressure that occurs when gas passes

through resistance

? Correlates this pressure ?drop to flow

Flowmeter assembly

? When the flow control valve is

opened the gas enters at the bottom

and flows up the tube elevating the

indicator

? The indicator floats freely at a point

where the downward force on it

(gravity) equals the upward force

caused by gas molecules hitting the

bottom of the float

? Because the tube is tapered the

annular opening around the indicator

increases with height and more gas

flows around the float
Parts of flowmeter assembly

? Tube
? Float/Bobbin
? Stop at top of tube
? Scale
? Plastic shields
? Lights
? Each flowmeter assembly should be permanently identified with:
? Color of gas
? Chemical symbol / Name of gas

Tube

? Made of glass
? Gas passes between bobbin and inner wall of tube
? Tapered
? Flow increases from below upward
? Also known as `Thorpe' tube
Electronic flowmeter

Prefer digital system

? Solenoid valves

? Control flow on or

of valves

? Computer

controlled

Auxiliary oxygen flowmeter
Anti-hypoxia devices

? Mechanical devices- "Link-25 system"
? Pneumatic device- "ratio-mixer valve"
? Electronically controlled device- "penlon ltd"
Vaporizers
? A vaporizer is an instrument designed to change a liquid anesthetic

agent into its vapor and add a controlled amount of this vapor to the

fresh gas flow
Common gas outlet

? Receives all gases and vapors from machine.
? Most machine outlets have 15mm female connection with coaxial

22mm male connection.

? Miscellaneous : Antistatic wheels

? This list is by no means exhaustive and newer anaesthesia machines

have incorporated a lots of new features to enhance the safety.

Breathing system

? 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
? 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

? 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.
? Requirements of a Breathing System
? Essential:
? The breathing system must deliver the gases from the machine to the

alveoli in the same concentration as set and in the shortest possible

time;

? Effectively eliminate carbon-dioxide;
? Have minimal apparatus dead space; and
? Have low resistance.

vRequirements of a Breathing System
? 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
Table 1. Classification of breathing systems

Classification Of Breathing Systems

BREATHING SYSTEMS WITHOUT CO2 ABSORPTION.

BREATHING SYSTEMS WITH CO2 ABSORPTION.

Unidirectional flow:

Unidirectional flow

a) Non rebreathing systems.

Circle system with absorber.

B) Circle systems.







Bi-directional flow:

Bi-directional flow

a) Afferent reservoir systems.

To and Fro system.

Mapleson A



Mapleson B



Mapleson C

Lack's system.

B) Enclosed afferent reservoir systems

Miller's (1988)

c) Efferent reservoir systems

Mapleson D

Mapleson E

Mapleson F

Bain's system

d) Combined systems

Humphrey ADE



Mapleson D

? 3 phases of respiration
1) Inspiration
2) Expiration
3) End-expiratory pause
Bain's system
? 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 V E )
-70ml/kg/min with controlled

ventilation

Circle system

v Breathing Systems with CO2 Absorption
- Sodalime canister,
- Two unidirectional valves,
- Fresh gas entry, Y-piece to connect to the patient,
- Reservoir bag a relief valve and
- Low resistance interconnecting tubing.
Circle system

Circle system

v 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
Circle system

vOptimization of Circle Design
q Unidirectional Valves
?Placed in close proximity to patient to prevent backflow into

inspiratory limb if circuit leak develops.

qFresh Gas Inlet
?Placed between absorber & inspiratory valve. If placed downstream

from inspiratory valve, it would allow FG to bypass patient during

exhalation and be wasted. If FG were placed between expiration valve

and absorber, FG would be diluted by recirculating gas

Circle system

vOptimization of Circle Design
qAPL valve
? Placed immediately before absorber to conserve absorption capacity

and to minimize venting of FG

qBreathing Bag
? Placed in expiratory limb to decrease resistance to exhalation. Bag

compression during controlled ventilation will vent alveolar gas thru

APL valve, conserving absorbent
Circle system

? Circle system can be:
# closed (FGF= patient uptake, complete rebreathing after CO2

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)


Circle system

? Most commonly used
? Adult and child appropriate sizes
? Can be semi open, semi closed, 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
Circle system

Scavenging system

? The collection and the subsequent removal of vented gases from the

operating room

? Components
(1) the gas-collecting assembly
(2) the transfer means
(3) the scavenging interface
(4) the gas-disposal assembly tubing
(5) an active or passive gas-disposal assembly
Components of a scavenging system- APL valve, adjustable

pressure limiting valve

Our institute...
Summary...

? The gases that are required are Oxygen, Nitrous Oxide and Air.
? The gases enter the machine at high pressures which are then

reduced by pressure reducing valves.

? Controller knob on each rotameter causes gas to flow and lift the

bobbin

? The vaporizer is fitted with a dial so that the concentration of the

volatile agent can be varied

? The gas mixture is then delivered to the anaesthetic circuit.