Anesthetic Equipment

The commonest type of anesthetic machine in use is the continuous-flow anesthetic machine, which is designed to provide an accurate and continuous supply of medical gases ( such as oxygen and nitrous oxide ), mixed with an accurate concentration of anesthetic vapor (such as isoflurane), and deliver this to the patient at a safe pressure and flowModern machines incorporate a ventilator, suction unit, and patient-monitoring devices

vaporizerbellow

Corrugated tube

Soda lime

Flow meter

ventilator

APL valve

Scavenging system

The Anesthesia Machine

High Intermediate Low Pressure Circuit

High Pressure System Receives gasses from the high

pressure E cylinders attached to the back of the anesthesia machine (2200 psig for O2, 745 psig for N2O)

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

Usually not used, unless pipeline gas supply is off

E Size Compressed Gas Cylinders

Cylinder Characteristics

Oxygen

Nitrous Oxide

Carbon Dioxide

Air

Color White (green)

Blue Gray Black/White (yellow)

State Gas Liquid and gas

Liquid and gas

Gas

Contents (L) 625 1590 1590 625

Empty Weight (kg)

590 590 590 590

Full Weight (kg)

676 880 890

Pressure Full (psig)

2000 750 838 1800

Hanger Yolk

Hanger Yolk: orients and supports the cylinder, providing a gas-tight seal and ensuring a unidirectional gas flow into the machine

Index pins: Pin Index Safety System (PISS) is gas specificprevents accidental rearrangement of cylinders (eg switching O2 and N2O)

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 yolk)

Outlet of central oxygen supply system

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

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 Hazards

– May cause barotrauma– Dilution of inhaled anesthetic

Low Pressure System

Consists of:– Flow meters– Vaporizer mounting device– Check valve– Common gas outlet

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

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

Classification of Vaporizers

Methods of regulating output concentrationConcentration calibrated (eg variable bypass)Measured flow

Method of vaporization

Flow-overBubble throughInjection

Temperature compensation

ThermocompensationSupplied heat

Generic Bypass Vaporizer

Flow from the flowmeters enters the inlet of the vaporizer

The function of the concentration control valve is to regulate the amount of flow through the bypass and vaporizing chambersSplitting Ratio = flow though

vaporizing chamber/flow through bypass chamber

Factors That Influence Vaporizer Output

Flow Rate: The output of the vaporizer is generally less than the dial setting at very low (< 200 ml/min) or very high (> 15 L/min) flows

Temperature: Automatic temperature compensating mechanisms in bypass chambers maintain a constant vaporizer output with varying temperatures

Back Pressure: Intermittent back pressure (eg positive pressure ventilation causes a higher vaporizer output than the dial setting)

Factors That Influence Vaporizer Output

Atmospheric Pressure: Changes in atmospheric pressure affect variable bypass vaporizer output as measured by volume % concentration, but not (or very little) as measured by partial pressure (lowering atmospheric pressure increases volume % concentration and vice versa)

Carrier Gas: Vaporizers are calibrated for 100% oxygen Carrier gases other than this result in decreased vaporizer output

The Circuit: Circle System

Arrangement is variable, but to prevent re-breathing of CO2, the following rules must be followed:– Unidirectional valves

between the patient and the reservoir bag

– Fresh-gas-flow cannot enter the circuit between the expiratory valve and the patient

– Adjustable pressure-limiting valve (APL) cannot be located between the patient and the inspiratory valve

The carbon dioxide absorberSodalime (CaOH2 + NaOH + KOH + silica) or

Baralyme (Ba[OH] 2 + Ca[OH]2) contained in the

absorber combines with carbon dioxide, forming

CaCO2 and liberating heat and moisture (H2O)

A pH-sensitive dye changes to a blue-violet

color, indicating exhaustion of the absorbing

capacity

The canister should be changed when 25% to

50% of the contents has changed color, although

it should continue to absorb satisfactorily until at

least the contents of the top canister have

changed color

Circle System

Advantages:– Relative stability of inspired concentration– Conservation of respiratory moisture and

heat– Prevention of operating room pollution– PaCO2 depends only on ventilation, not

fresh gas flow– Low fresh gas flows can be used

Disadvantages:– Complex design = potential for

malfunction– High resistance (multiple one-way valves)

= higher work of breathing

The reservoir bag

The reservoir bag is located on the expiratory limb

The reservoir bag accumulates gas between inspirations

It is used to visualize spontaneous ventilation and to assist ventilation manually

Adults require a 3-L bag Children a 2-L bag Most new machines have a valve used to

switch between the reservoir bag and the ventilator

Older machines may require that the bag be removed and a hose to the ventilator be connected

The Adjustable Pressure Limiting (APL) Valve

User adjustable valve that releases gases to the scavenging system and is intended to provide control of the pressure in the breathing system

Bag-mask Ventilation: Valve is usually left partially open During inspiration the bag is squeezed pushing gas into the inspiratory limb until the pressure relief is reached, opening the APL valve

Mechanical Ventilation: The APL valve is excluded from the circuit when the selector switch is changed from manual to automatic ventilation

Scavenging Systems

A scavenging system channels waste gases away from the operating room to a location outside the hospital building

The ambient concentration of anesthetic gases in the operating room should not exceed 25 ppm for nitrous oxide and 2 ppm for halogenated agents

Specific anesthetic gas-scavenging systems should be used routinely These systems consist of a collecting system, a transfer system, a receiving system, and a disposal system

Scavenging Systems

The disposal system may be passive or active, although passive systems are inadequate for modern hospitals

A passive system consists of wide-bore tubing that carries gases directly to the exterior or into the exhaust ventilation ducts

Active systems can be powered by vacuum systems, fans, pumps, or Venturi systems

Scavenging Systems The disposal system may be passive or

active, although passive systems are inadequate for modern hospitals

A passive system consists of wide-bore tubing that carries gases directly to the exterior or into the exhaust ventilation ducts

Active systems can be powered by vacuum systems, fans, pumps, or Venturi systems

Gas Analysis Several methods are used to monitor concentrations

of oxygen, carbon dioxide, and anesthetic gases in the breathing system.

The oxygen analyzer is the single most important monitor for detection of a hypoxic gas mixture.

Capnometry, the measurement of carbon dioxide, has many uses, including monitoring the adequacy of ventilation and detection of breathing system faults.

Breath-to-breath monitoring of anesthetic concentrations provides tracking of anesthetic uptake and distribution.

Most gas analyzers incorporate alarms. Among the techniques for measurement are the following: Mass spectrometry, infrared analysis and oxygen concentrations analysis

Anesthesia Ventilators Most modern anesthesia machines are fitted

with a mechanical ventilator that uses a collapsible bellows within a closed chamber.

The bellows is compressed intermittently when oxygen or air is directed into the chamber, thereby pressurizing it.

The ventilators are time cycled flow (as opposed to pressure) generators, controlled both mechanically and electronically, and pneumatically driven (requiring 10 to 20 L of driving gas per minute).

Ventilator controls vary among makes and models.

Some ventilators require setting of minute ventilation, rate, and inspiratory-expiratory (I:E) ratio to produce the desired tidal volume; other ventilators allow direct adjustment of tidal volume, with I : E ratio being dependent on the inspiratory flow rate, which is set independently.

Anesthesia Ventilators

Ventilator controls vary among makes and models. Some ventilators require setting of minute ventilation, rate,

and inspiratory-expiratory (I:E) ratio to produce the desired tidal volume; other ventilators allow direct adjustment of tidal volume, with I : E ratio being dependent on the inspiratory flow rate, which is set independently.

Although gas-driven ventilators can be safely driven with either oxygen or air, most often oxygen is chosen and is supplied by pipeline. Whether or not cylinder gases are used to drive the ventilator in the event of pipeline failure is usually determined by the user. If the machine is set up to drive the ventilator using cylinder oxygen, mechanical ventilation should be discontinued in the event of pipeline failure to conserve oxygen supplies.

Flow Generator Ventilators

Flow generators deliver a set tidal volume regardless of changes in patients' compliance ( unlike pressure generators ) but will not compensate for system leaks and may produce barotrauma because high pressures can be generated.

They reliably deliver the preset tidal volume (even in the presence of a small leak).

The risk o f barotrauma is minimal because most patients presenting to the operating room have healthy normally compliant lungs.

Pressure Generator Ventilators

For infants and patients with diseased lungs, the maintenance of preset tidal volumes may produce unacceptably high airway pressures and increased risk of barotrauma.

Pressure generators are more appropriate in these situations, because airway pressure is controlled and barotrauma risk minimized.

Checking Anesthesia Machines

Check: Emergency ventilation

equipment High-Pressure system Low-Pressure system Scavenging system Breathing system Manual and automatic

ventilation system Monitors

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