Moments with Morgan and Mikhail - Chapter 4, Part 2

Continue your journey to mastering anaesthesia, one chapter at a time.In this episode, Dr. J.R. Decker reads and discusses Part 2 of Chapter 4 of Morgan & Mikhail’s Clinical Anesthesiology (7th Edition). Follow by listening alone or reading along with your textbook to strengthen your foundations in anaesthesia, one clear and engaging session at a time.Perfect for trainees, revision, or daily listening.

Episode Transcript

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Morgan and Mikhail's Clinical Anesthesiology, 7th Edition,
Chapter 4, Part 2 Vaporizers, Volatile anaesthetics.
For example, halothine, isofluorine, desfluorine,

(00:23):
sevofluorine must be vaporised before being delivered to the
patient. Vaporizers have concentration
calibrated dials that precisely add volatile anaesthetic agents
to the combined gas flow from all flow metres.

(00:45):
They must be located between theflow metres and the common gas
outlet. Moreover, unless the machine
accepts only one vaporizer at a time, all anaesthesia machines
should have an interlocking or exclusion device that prevents

(01:06):
the concurrent use of more than one vaporizer.
A physics of vaporisation at temperatures encountered in the
operating room. The molecules of a volatile
anaesthetic in a close containerare distributed between the

(01:29):
liquid and gaseous phases. The gas molecules bombard the
walls of the container, creatingthe saturated vapour pressure of
that agent. Vapour pressure depends on the
characteristics of the volatile agents and the temperature.

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The greater the temperature, thegreater the tendency for the
agent for the liquid molecules to escape into the gaseous
phase, and the greater the vapour pressure.
Vaporisation requires energy, that is the latent heat of

(02:13):
vaporisation, which results in aloss of heat from the liquid.
As vaporisation proceeds, the temperature of the remaining
liquid anaesthetic drops and thevapour pressure decreases,
unless heat is readily availableto enter the system.

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Vaporizers contain a chamber in which a carrier gas becomes
saturated with a volatile agent.A liquid's boiling point is the
temperature at which is vapour pressure is equal to the
atmospheric pressure. As the atmospheric pressure

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decreases, that is, as in higheraltitudes, the boiling point
also decreases. Anaesthetic agents with low
boiling points are more susceptible to variations in
barometric pressure than agents with higher boiling points.

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Among the commonly used agents, desflurane has the lowest
boiling point that is 22.8°C at 760 millimetres of mercury.
B Copper kettle The copper kettle vaporizer is no longer

(03:53):
used in clinical anaesthesia. However, understanding how it
works provides invaluable insight into the delivery of
volatile anaesthetics. It is classified as a measured
flow vaporizer or flow metre controlled vaporizer In a copper

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kettle, the amount of carrier gas bubbled through the volatile
anaesthetic is controlled by dedicated flow metre.
This valve is turned off when the vaporizer circuit is not in
use. Copper is used as a construction

(04:38):
metal because it's relatively high specific heat, that is the
quantity of heat required to raise the temperature of 1
gramme of substance by 1°C, and high thermal conductivity, that
is the speed of heat conductancethrough a substance, enhance the

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vaporizer's ability to maintain a constant temperature.
All the gas entering the vaporizer passes through the
anaesthetic liquid and becomes saturated with vapour. 1 mill of
liquid anaesthetic yields approximately 200 mills of

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anaesthetic vapour. Because the vapour pressure of
volatile anaesthetics is greaterthan the partial pressure
required for anaesthesia, the saturated gas leaving a copper
kettle has to be diluted before it reaches reaches the patient.

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Then we have the schematic of a copper kettle vaporizer, and it
should be noted that 550 mils per minute of halothane vapour
is added for each 100 mils per minute of oxygen flow that
passes through the vaporizer. For example, the vapour pressure

(06:10):
of halothane is 243 millimetres of mercury at 20°C.
So the concentration of halothane exiting a copper
kettle at one atmosphere will be243 / 760 or 32%.

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If 100 mils of oxygen enters thekettle, roughly 150 mils of gas
exits. That is the initial 100 mil of
oxygen plus 50 mil of saturated halothane vapour, 1/3 of which
would be saturated halothane vapour.

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For a 1% concentration of halothane that is minimum
alveolar concentration. That's mark of 0.5%.
To be delivered, A50 mill of Haloten vapour and 100 mill of
carrier gas that left the copperkettle have to be diluted within

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a total of 5000 mills of fresh gas flow.
Thus every 100 mils of oxygen passing through a Halothin
vaporizer translates into a 1% increase in concentration if the

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total gas flow into the breathing circuits is five
litres per minute. Therefore, when the total flow
is fixed, the flow through the vaporizer determines the
ultimate concentration of the anaesthetic.

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Isoflorane has an almost identical vapour pressure, so
the same relationship between copper kettle flow, total gas
flow and anaesthetic concentration exists.
However, if total gas flow decreases without an adjustment

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in copper kettle flow, that is exhaustion of nitrous oxide
cylinder, the delivered volatileand aesthetic concentration
rises rapidly to potentially dangerous levels.
See Modern conventional vaporizers 5.

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All modern vaporizers are agent specific and temperature
corrected, capable of deliveringa constant concentration of
agent regardless of temperature changes of flow through the
vaporizer. Turning a single calibrated
control knob counterclockwise tothe desired percentage diverts

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an appropriately small fraction of the total gas flow into the
carrier gas, which flows over the liquid and aesthetic in a
vaporising chamber, leaving the balance to exit the vaporizer
unchanged because some of the entering gas is never exposed to

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an aesthetic liquid. This type of agent specific
vaporizer is also known as a variable bypass vaporizer.
Next we get to Figure 4-8, explaining vaporizer technology.

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You would do well to take your time and go through it.
Temperature compensation is achieved by a strip composed of
two different metals wielded together.
The metal strips expand and contract differently in response

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to temperature changes. When the temperature decreases,
differential contraction causes the strip to bend, allowing more
gas to pass through the vaporizer.
Such bimetallic strips are also used in home thermostats.

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As the temperature rises, differential expansion causes
the strip to bend the other way,restricting gas flow into the
vaporizer. Altering total fresh gas flow
rates within a wide range does not significantly affect

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anaesthetic concentration because the same proportion of
gas is exposed to the liquid. Given that these vaporizers are
agent specific, filling them with the incorrect anaesthetic
must be avoided. For example, unintentionally

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filling a several fluorine specific vaporizer with
halothane could lead to an anaesthetic overdose.
First, halothens higher vaporizer pressure.
That is, 243 millimetres of mercury versus 157 millimetres

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of mercury will cause a 40% greater amount of anaesthetic
vapour to be released. 2nd halothane is more than twice as
potent as several fluorine mark of 0.75 versus 2.0.

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Conversely, filling a halothane vaporizer with several fluorine
will cause an anaesthetic under dosage.
Modern vaporizers offer agent specific keyed filling ports to
prevent filling with an incorrect agent variable.

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Bypass vaporizers compensate forchanges in ambient pressures,
that is, altitude changes maintaining relative anaesthetic
gas partial pressure. It is the partial pressure of
the anaesthetic agent that determines its concentration
dependent physiological effects.Thus, there is no need to

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increase the selected anaesthetic concentration when
using a variable bypass vaporizer at altitude.
At altitude, because the partialpressure of the anaesthetic
agent will be largely unchanged,although at lower ambient
pressures gas passing through the vaporizer is exposed to

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increase vaporizer output. Because of Dalton's law of
partial pressure, the partial pressure of the anaesthetic
vapour will remain largely unaffected compared with partial
pressures obtained at sea level.D Electronic vaporizers

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Electronically controlled vaporizers must be utilised for
DES fluorine and may be used forall volatile anaesthetics in
some anaesthesia machines. One DES fluorine vaporizer.

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DES fluorine's vapour pressure is so high that at sea level it
almost boils at room temperature.
This high volatility, coupled with a potency of only one fifth
that of other volatile agents, presents unique delivery

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problems. First, the vaporisation required
for general anaesthesia producesa cooling effect that would
overwhelm the ability of conventional vaporizers to
maintain a constant temperature.Second, because it vaporises so

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extensively, A tremendously highfresh gas flow will be necessary
to dilute the carrier gas to clinically relevant
concentrations. These problems have been
addressed by the development of specific DES fluorine

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vaporizers. A reservoir containing DES
fluorine, that is, DES fluorine sump, is electrically heated to
39°C. That is significantly higher
than its boiling point, creatinga vapour pressure of true

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atmospheres. Unlike A variable bypass
vaporizer, no fresh gas flows through this.
No fresh gas flows through this desflorine sump.
Rather, pure desflorine vapour joins the fresh gas mixture
before exiting the vaporizer. The amount of desflorine vapour

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released from the sump depends on the concentration selected by
turning the total, the control dial and the fresh gas flow
rate. Although the Tech 6 Plus
maintains a constant this fluorine concentration over a
wide range of fresh gas flow rates, it cannot automatically

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compensate for changes in elevation like the variable
bypass vaporizers can decrease ambient pressure, for example.
High elevation does not affect the concentration of the agents
delivered, but it decreases the partial pressure of the agent.

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Thus, at high elevations 1 was manually increase the
desfluorine concentration control 2 Aladdin GE cassette
vaporizer Gas flow from the flowcontrol is divided into bypass

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flow and liquid chamber flow. The latter is conducted into an
agent specific colour coded cassette, known as the Aladdin
cassette, in which the volatile anaesthetic is vaporised.
The machine accepts only one cassette at a time and

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recognises the cassette through magnetic labelling.
The cassette does not contain any bypass flow channels.
Therefore, unlike traditional vaporizers, liquid anaesthetic
cannot escape during handling and the cassettes can be carried

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in any position. After leaving the cassette, the
now an aesthetic saturated liquid chamber flow and the now
an aesthetic saturated liquid chamber flow reunites with the
bypass flow before exiting the fresh gas outlet.

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A flow restrictor valve near thebypass flow helps adjust the
amount of fresh gas that flows to the cassette.
Adjusting the ratio between the bypass flow and liquid chamber
flow changes the concentration of volatile anaesthetic agent

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delivered to the patient. Sensors in the cassettes measure
pressure and temperature, thus determining agent concentration
in the gas leaving the cassette.Correct liquid chamber flow is

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calculated based on desired fresh gas concentration and
determined cassette gas concentration.
Common fresh gas outlets in contrast to In contrast to the

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multiple gas inlets, the anaesthesia machine has only one
common gas outlet that supplies gas to the breathing circuit,
the term fresh gas outlet. Is also often used because of
its critical role in adding new gas of fixed unknown composition

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to the circle system. Unlike older models, some newer
anaesthesia machines measure andreport common outlet gas flows.
An anti disconnect retaining device is used to prevent
accidental detachment of the gasoutlet hose that connects the

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machine to the breathing circuit.
The oxygen flush valve provides a high flow that is 35 to 75
litres per minute of oxygen directly to the common gas
outlets. By passing the flow metres and

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vaporizers. It is used to rapidly refill or
flush the breeding circuits. But because the oxygen may be
supplied at a lying pressure of 45 to 55 PSIG, there is a real
potential for long barotrauma tooccur.

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For this reason, the flush valvemust be used cautiously when
whenever a patient is connected to the breathing circuit.
Moreover, inappropriate use of the flush valve or a situation
of stock valve may result in theback flow of gases into the low

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pressure circuits causing dilution of inhaled anaesthetic
concentration. Some machines use a second stage
regulator to drop the oxygen flush pressure to a lower lower
level. A protective ring around the

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flush bottom limits the possibility of unintentional
activation. The breathing circuits in
adults. The breeding system most

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commonly used with anaesthesia machines is the circle system,
though a being circuit is occasionally used.
The components and use of the circle system were previously
discussed in Chapter 3. It is important to note that gas

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composition at the common gas outlet can be controlled
precisely and rapidly by adjustments in flow metres and
vaporizers. In contrast, gas composition,
especially volatile and aesthetic concentration in the
breathing circuit, is significantly affected by other

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factors, including an aesthetic uptake in the patient's lungs,
minute ventilation, total fresh gas flow, the volume of the
breeding circuits, and the presence of gas leaks.
The use of high gas flow rates during induction and emergence

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decreases the effects of such variables and can diminish the
magnitude of discrepancies between the fresh gas outlets
and circle system and aesthetic concentrations.
Measurement of inspired and expired anaesthetic gas
concentration also greatly facilitates anaesthetic

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management. Oxygen analysis.
General anaesthesia must not be administered without an oxygen
analyzer in the breeding circuit.
Three types of oxygen analyzers are available, polarographic

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that is Clark electrode, galvanic, that is fuel cell, and
paramagnetic. The first two techniques use
electrochemical sensors that contain cathode and anode
electrodes embedded in an electrolyte gel separated from

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the sample gas by an oxygen permeable membrane, usually
Teflon. As oxygen reacts with the
electrodes, a current is generated that is proportional
to the oxygen partial pressure in the sample gas.

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The galvanic and polarographic sensors differ in the
composition of their electrodes and electrolyte gels.
The components of the galvanic cell are capable of providing
enough chemical energy so that the reaction does not require an

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external power source. Although the initial cost of
paramagnetic sensors is greater than that of electrochemical
sensors, paramagnetic devices are self calibrating and have no
consumable parts. In addition, their response time

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is fast enough to differentiate between inspired and expired
oxygen concentrations. All oxygen analyzers should have
a low level alarm that is automatically activated by
turning on the anaesthesia machine.

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The sensor should be placed intothe inspiratory or expiratory
limb of the circle system's breeding circuit, but not into
the fresh gas line. As a result of the patient's
oxygen consumption, the expiratory limb has a slightly

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lower oxygen partial pressure than the inspiratory limb,
particularly at low fresh gas flows.
The increased humidity of expired gas does not
significantly affect most modernsensors.

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Spirometers. Spirometers, also called
respirometers, are used to measure exhaled tidal volume in
the breeding circuits on all anaesthesia machines, typically
near the exhaled exhalation valve.

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Some anaesthesia machines also measure the inspiratory tidal
volume just past the inspiratoryvalve or the actual delivered
and exhaled tidal volumes at theY connector that attaches to the
patient's airway. A common method employs A

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rotating vein of low mass in theexpiratory limb in front of the
expiratory valve of the circle system, that is, vein anemometer
or right respirometer. The flow of gas across veins

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within the respirometer causes their rotation, which is
measured electronically, photoelectrically, or
mechanically during positive pressure ventilation.
Changes in exhaled tidal volumesusually represent changes in

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ventilator settings, but they can also be due to circuit
leaks, disconnections of ventilator malfunction.
These spirometers are prone to errors caused by inertia,
friction, and water condensation.

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For example, right respirometersunder read at low flow rates and
over read at high flow rates. Moreover, measurements of
exhaled tidal volumes at this location in the expiratory limb

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include gas that expanded the corrugated tubing in the circuit
and was not delivered to the patient.
The difference between the volume of gas delivered to the
circuit and the volume of gas actually reaching the patient
becomes very significant with long, compliant breathing tubes,

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rapid respiratory rates, and increased airway pressures.
Circuit pressure 6. Breathing circuit pressure is
always measured somewhere between the expiratory and

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inspiratory unidirectional valves.
The exact location depends on the model of anaesthesia
machine. Breathing circuit pressure
usually reflects airway pressureif it is measured as close to
the patient's airway as possible.

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The most accurate measurements of both inspiratory and
expiratory pressures can be obtained from the Y connection.
A rise in airway pressure may signal worsening pulmonary
compliance, an increase in tidalvolume, or an obstruction in the

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breeding circuit. In the trachea tube or the
patient's airway, a drop in pressure may indicate an
improvement in pulmonary compliance, a decrease in tidal
volume, or a leak in the circuit.

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If circuit pressure is being measured at carbon dioxide at
the carbon dioxide absorber, however, it will not always
mirror the pressure in the patient's airway.
For example, clamping the expiratory limb of the breathing
tubes during exhalation will prevent the patient's breath

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from exceeding the lungs. Despite this buildup in airway
pressure, a pressure gauge at the absorber will read 0.
Because of the intervening one way valve, some machines have
incorporated auditory feedback for the pressure changes during

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ventilator use. Adjustable pressure limiting
valve. The adjustable pressure limiting
that is APL valve, sometimes referred to as the pressure
relief or pop up valve, is usually fully open during

(32:43):
spontaneous ventilation but mustbe partially closed during
manual or assisted bag ventilation.
The APL valve often requires fine adjustments if it is not
closed sufficiently. Excessive loss of circuit volume

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due to leaks prevents manual ventilation.
At the same time, if it is closed too much or is fully
closed, a progressive rise in pressure could result in
pulmonary paro trauma, for example, pneumotorax or

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hemodynamic compromise, or both.As an added safety feature, the
APL valves on modern machines act as true pressure limiting
devices that can never be completely closed.
The upper limit is usually 70 to80 centimetres of water.

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Humidifiers. Absolute humidity is defined as
the weight of water vapour in one litre of gas, that is
milligrammes per litre. Relative humidity is the ratio
of the actual mass of water present in a volume of gas to

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the maximum amount of water possible at a particular
temperature. At 37°C and 100% relative
humidity, absolute humidity is 44 milligrammes per litres,

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whereas at room temperature thatis 21°C and 100% humidity, it is
18 milligrammes per litre. Inhaled gas Gases in the
operating room are normally administered at room temperature

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with little or no humidification.
Gases must therefore be warmed to body temperature and
saturated with water by the upper respiratory tract.
Tracheal intubation and high fresh gas flow bypass this

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normal humidification system andexpose the lower Airways to dry,
that is less than 10 millimetresof water per litre room
temperature gases. Prolonged humidification of
gases by the lower respiratory tracts leads to dehydration of

(35:47):
mucosa, altered ciliary functionand if excessively prolonged,
could potentially lead to anticipation of secretions,
actoelectasis and even ventilation perfusion.
Mismatching body heat is also lost as gases are warmed and

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even more importantly, as water is vaporised to humidify the dry
gases. The heat of vaporisation for
water is 560 calories per grammeof water vaporised.
Fortunately, this heat loss accounts for only 5% to 10% of

(36:41):
total intraoperative heat loss, is not significant for a short
procedure that is less than one hour and usually can easily be
compensated for with a forced air warming blanket.
Humidification and heating of inspiratory gases may be most

(37:05):
important for small paediatric patients and older patients with
severe underlying lung pathology, for example cystic
fibrosis. A passive humidifiers.

(37:32):
Humidifiers added to the breeding circuits minimise water
and heat loss. The simplest designs are
condenser humidifiers or heat and moisture exchanger, that is,
HME units. These passive devices do not add

(37:55):
heat or vapour, but rather contain a hygroscopic material
that traps exhaled humidification and heat, which
is released upon subsequent inhalation.
Depending on the design, they may substantially increase

(38:17):
apparatus Dead Space that is more than 60 miles per cube. 60
mils per litre cube, sorry 60 mils litre cube, which can cause
significant rebreathing in paediatric patients.
They can also increase breathingcircuit resistance and the work

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of breathing during spontaneous respirations.
Excessive saturation of an HME with water or secretions can
obstruct the breeding circuits. Some condenser humidifiers also
act as effective philtres that may protect the breeding

(39:05):
circuits and anaesthesia machinefrom bacterial or viral cross
contamination. This may be particularly
important when ventilating patients with respiratory
infections or compromised immunesystems.

(39:30):
B Active humidifiers Active humidifiers are more effective
than passive ones in preventing in sorry in preserving moisture
and heat. Active humidifiers add water to

(39:50):
gas by passing the gas over a water chamber that is Passover
humidifier or through a saturated weak that is weak
humidifier, bubbling it through water that is bubble through
humidifier, or mixing it with vaporised water.

(40:15):
That is vapour phase humidifier.Because increasing temperature
increases the capacity of a gas to hold water vapour, heated
humidifiers with thermostatically controlled
elements are most effective. The hazards of heated

(40:40):
humidifiers include thermal lunginjury, that is, inhaled gas
temperature should be monitored and should not exceed 41°C,
nosocomial infection, increased airway resistance from excess

(41:02):
water condensation in the breeding circuit, interference
with flow metre function and an increased likelihood of circuit
disconnection. The use of these humidifiers is
particularly valuable in children as they help prevent

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both hypothermia and the plugging of small trachea tubes
by dried secretions. Of course, any design that
increases airway Dead Space should be avoided in paediatric
patients. Unlike passive humidifiers,

(41:46):
active humidifiers do not re philtre respiratory gases.