Anesthetic Equipment

Gas SuppliesBulk Supply of Anesthetic Gases

In the majority of modern hospitals, piped medical gases and vacuum (PMGV) systems have been installed

only a few cylinders are kept in reserve, attached usually to the anesthetic machine

The PMGV services comprise five sections:– bulk store– distribution pipelines in the hospital– terminal outlets, situated usually on the walls or ceilings of the

operating theatre suite and other sites– flexible hoses connecting the terminal outlets to the anesthetic

machine– connections between flexible hoses and anesthetic machinesResponsibility for the first three items lies with the engineering and pharmacy departments. Within the operating theatre, it is partly the anesthetist’s responsibility to check the correct functioning of the last two items

Bulk Store: Oxygen

In small hospitals:Oxygen cylinder manifolds consist of two groups of large cylinders (size J)

Bulk Store: Oxygen

In large hospitals: Liquid oxygen store Liquid oxygen is stored

at a temperature of approximately −165 °C at 10.5

If the pressure increases above 17 bar (1700 kPa), a safety valve opens and oxygen runs to waste

Liquid oxygen plants are housed some distance away from hospital buildings because of the risk of fire

Even when a hospital possesses a liquid oxygen plant, it is still necessary to hold reserve banks of oxygen cylinders in case of supply failure

Other Gases Nitrous Oxide

– Nitrous oxide and Entonox may be supplied from banks of cylinders connected to manifolds similar to those used for oxygen

Medical Compressed Air – Compressed air is supplied from a bank of cylinders into the

PMGV system– Air of medical quality is required, as industrial compressed

air may contain fine particles of oil Piped Medical Vacuum

– Piped medical vacuum is provided by large vacuum pumps

Terminal Outlets Six types of terminal outlet are found commonly in

the operating theatre The terminals are color-coded and also have non-

interchangeable connections specific to each gas: – Vacuum (colored yellow) – a vacuum of at least 53 kPa (400

mmHg) should be maintained at the outlet, which should be able to take a free flow of air of at least 40 L min–1

– Compressed air (colored white/black) at 4 bar – this is used for anesthetic breathing systems and ventilators

– Air (colored white/black) at 7 bar – this is to be used only for powering compressed air tools and is confined usually to the orthopedic operating theatre

– Nitrous oxide (colored blue) at 4 bar– Oxygen (colored white) at 4 bar– Scavenging designed to accept a standard 30-mm connection

Outlet of central oxygen supply system

Gas Cylinders Modern cylinders are constructed from

molybdenum steel They are checked at intervals by the manufacturer

to ensure that they can withstand hydraulic pressures considerably in excess of those to which they are subjected in normal use

The cylinders are provided in a variety of sizes (A to J), and color-coded according to the gas supplied

Cylinders attached to the anesthetic machine are usually size E

The cylinders comprise a body and a shoulder

Gas Cylinders Cylinder valves should be opened slowly to prevent sudden

surges of pressure The color codes used for medical gas cylinders in the United

Kingdom are shown in Table 15.2. Different colors are used for some gases in other countries

Cylinder sizes and capacities are shown in Table 15.3. Oxygen, air and helium are stored as gases in cylinders and

the cylinder contents can be estimated from the cylinder pressure

Nitrous oxide and carbon dioxide cylinders contain liquid and vapor

The cylinder pressure cannot be used to estimate its contents because the pressure remains relatively constant until after all the liquid has evaporated and the cylinder is almost empty

The contents of nitrous oxide and carbon dioxide cylinders can be estimated from the weight of the cylinder

The anesthetic machine comprises: A means of supplying gases either from attached

cylinders or from piped medical supplies via appropriate unions on the machine

Methods of measuring flow rate of gases Apparatus for vaporizing volatile anesthetic agents Breathing systems and a ventilator for delivery of

gases and vapors from the machine to the patient Apparatus for scavenging anesthetic gases in

order to minimize environmental pollution.

vaporizerbellow

Corrugated tube

Soda lime

Flow meter

ventilator

APL valve

Scavenging system

The Anesthesia Machine

High Intermediate Low Pressure Circuit

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

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

Basic Anesthetic Monitoring

ASA Standards for Basic Anesthesia Monitoring

Prepared byDr. Mahmoud Abdel-Khalek

Basic Anesthetic Monitoring

The primary goal of anesthesia is to keep the patient as safe as possible in the perioperative period

Anesthesia and surgery are serious invasions on the physiologic stability of the human body

Careful monitoring of the patient during and after surgery allows the anesthesiologist to identify problems early, when they can still be corrected

Proper monitoring of the patient can reduce the risks involved in anesthesia and surgery

Some of the physiologic disturbances that occur in the perioperative period include: apnea, respiratory depression, airway obstruction, cardiac depression, hypertension, hypotension, hypervolemia, hypovolemia, arrhythmias, blood loss, fluid shifts, weakness, bradycardia, tachycardia, hyperthermia, and hypothermia

Standards of Care

Proper monitoring standards are well-defined

The most widely accepted current anesthesia

monitoring standards are those that have been

published by the by the American Society of

Anesthesiologists (ASA)

The ASA standards were initially published in

1986, and were updated afterwards

The ASA Standards for Basic Anesthetic Monitoring Standard I states that a qualified anesthesia

provider will be present with the patient throughout the anesthetic

Standard II states that the patient's oxygenation, ventilation, circulation, and temperature will be continually monitored

Assessment of oxygenation involves two parts: measurement of inspired gas with an oxygen analyzer and assessment of hemoglobin saturation with a pulse oximeter and observation of skin color

Assessment of ventilation is by clinical assessment and preferably capnography

The ASA Standards for Basic Anesthetic Monitoring

Tracheal intubation must be verified clinically and by detection of exhaled CO2

Mechanical ventilation must be monitored with an audible disconnect monitor

Assessment of circulation involves continuous ECG monitoring, blood pressure measurement at least every five minutes, and continuous monitoring of peripheral circulation by such means as palpation, ausculation, plethysmography, or arterial pressure monitoring

The patient's temperature must be measured if changes are anticipated, intended, or suspected

Oxygen analyzers Oxygen Analyzers Oxygen analyzers are an

integral part of the newer anesthesia machines The purpose of the oxygen analyzer is to

confirm that oxygen is being delivered to the patient and that concentration of oxygen in the gas mixture is adequate

The oxygen analyzer provides one last check before the gas mixture is delivered to the patient

For the analyzer to be useful, it must be calibrated and the low-limit alarm must be working

The two main types of oxygen analyzers are galvanic (fuel cell), and the polarographic

Pulse Oximeters

Pulse oximetry have been a major advance in improving the safety of anesthesia

The pulse oximeter provides continuous monitoring of hemoglobin saturation using a two-wavelength light absorption technique

The monitor filters out the effects of ambient light, tissue, skin pigment, tissue, and venous blood It focuses on the pulsatile absorption which due to pulsatile arterial blood

Pulse oximeters were developed in the early 1980's and rapidly proved their value in anesthesia

Pulse oximetry allows rapid, beat-by-beat, noninvasive monitoring of blood oxygenation

Disadvantages of pulse oximetry are that it is motion sensitive, and that substances like carbon monoxide, methemoglobin, and some dyes like nail polish affect the readings

Capnography

The most common method of exhaled CO2 measurement is sidestream infrared (IR) capnography

Gas from the circuit is drawn into an infrared measurement chamber CO2, N2O, H20, and inhaled anesthetic agents all absorb infrared light, but at slightly different frequencies

Newer monitors have precise light sources and filters that specifically measure the individual gases These monitors provide breath-by-breath gas analysis

Problems with IR capnographs are that moisture can cause blockage of the gas path, and that they can't measure oxygen or nitrogen

Abnormalities:– Complete absence of waveform

Circuit disconnection Cardiac arrest Esophageal intubation Complete respiratory obstruction

Automatic blood pressure monitors Current automatic

noninvasive blood pressure monitors work on the oscillometric technique

The cuff inflates well above the systolic pressure and then deflates slowly The monitor first senses oscillations as the cuff drops to systolic pressure

The point at which the oscillations are the strongest is read as the mean pressure

Most of these devices calculate the diastolic pressure after they measure the systolic and mean pressures The system is normally very reliable and accurate, but motion (especially

shivering) on the part of the patient or the surgeon leaning against the cuff will cause false readings or failure to get a reading

Patient injury is possible if the tubing becomes kinked Values may be in error if the cuff is not the proper size

ECG Monitor

Lead placement is important in ischemia detection The most sensitive lead is lead V5, detecting about 75% of ischemic

episodes Lead II plus lead V5 raise the detection rate to 80%, whereas leads II, V4, and V5 together detect 98% of ischemic events

Current top-of-the line monitors do automated ST analysis which is more reliable than individual practitioner assessment as long as the measurement points are correct

The ECG monitor can provide a lot of information to the anesthesiologist

Arrhythmia detection and identification of tachycardia and bradycardia are important uses

The ECG monitor may also provide the first indication of myocardial ischemia However the absence of ST depression does not guarantee that ischemia is not present

Ventilation Monitors

Continuous measurement of exhaled tidal volume can detect circuit leaks and hypoventilation

The spirometers on the anesthesia machines may give false readings if moisture blocks the inner workings

Current anesthesia machines also have overpressure alarms and overpressure "pop-off" valves

Current anesthesia machines have ventilator disconnect alarms and built-in spirometers

The spirometers have high and low limit alarm settings

Temperature Monitors

Monitoring of skin temperature is nearly useless

Upper esophageal and nasopharyngeal temperature are affected by airway temperature

Lower esophageal temperature is normally a good reflection of core or blood temperature

Tympanic membrane temperature is also a good indication of core temperature but it is not practical in the operating room environment

Peripheral Nerve Stimulators

Peripheral nerve stimulation (PNS) monitoring is not required by the ASA standards However, it is an important safety monitor in patients who a receiving neuromuscular blocking drugs

Train-of-four monitoring assesses the level of nondepolarizer blockade and double-burst stimulation assesses return of strength at the end of the case

Clinical monitoring of neuro- muscular blockade during an anesthetic is difficult without a PNS monitor

Clinical assessment of strength is important, however, at the conclusion of an anesthetic before a final decision is made to extubate the patient

Thank You