Moments with Morgan and Mickhail - Chapter 4, Part 3

Continue your journey to mastering anaesthesia—one chapter at a time. In this episode, Dr. J.R. Decker reads and discusses Chapter 4 (Part 3) of Morgan & Mikhail’s Clinical Anesthesiology (7th Edition). Follow as you read along to strengthen your foundations in anaesthesia, one clear and engaging session at a time. Perfect for trainees, revision, or daily listening.

Episode Transcript

(00:02):
Morgan and Michael's Clinical Anesthesiology 7th Edition,
Chapter 4, Part 3 Ventilators All modern anaesthesia machines
are equipped with a ventilator. Historically, ventilators used

(00:24):
in the operating room were simpler and more compact than
their intensive care unit counterparts.
This distinction has become blurred due to advances in
technology and an increasing need for ICU type ventilators.
As more critically I'll patientscome to the operating room, the

(00:48):
ventilators on some modern machines have the same
capabilities as those in the ICU.
Indeed, during the COVID-19 pandemic, anaesthesia
workstations were employed to provide mechanical ventilation
when traditional ICU ventilatorswere on available.

(01:12):
A complete discussion of mechanical ventilation and
ventilator design is contained in Chapter 57.
Overview. Ventilators generate gas flow by
creating a pressure gradient between the proximal airway and

(01:34):
the alveoli. Ventilator function is best
described in relation to the four phases of the venture.
Ventilatory cycle. Inspiration.
The transition from inspiration to expiration.
Expiration and the transition from expiration to inspiration

(02:01):
Although several classification schemes exist, the most common
is based on inspiratory phase characteristics and the method
of cycling from inspiration to expiration.
A inspiratory phase During inspiration, ventilators

(02:23):
generate tidal volumes by producing gas flow along a
pressure gradient. The machine generates either a
constant pressure, that is constant pressure generators, or
a constant gas flow rate, that is constant flow generators

(02:46):
during inspiration regardless ofchanges in long mechanics.
Non constant generators produce pressures or gas flow rates that
vary during the cycle but remainconsistent from breath to
breath. For instance, A ventilator that

(03:12):
generates a flow pattern resembling 1/2 cycle of a sine
wave, EG Rotary piston ventilator, would be classified
as a non constant flow generator.
An increase in airway resistanceor a decrease in lung compliance

(03:34):
would increase peak inspiratory pressure but would not alter the
flow rate generated by this typeof ventilator.
B Transition phase from inspiration to expiration.

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Termination of the inspiratory phase can be triggered by a
preset limit of time that is fixed duration, a set
inspiratory pressure that must be reached, or a predetermined
tidal volume that must be delivered.
Time cycled ventilators allow tidal volume and peak

(04:23):
inspiratory pressure to vary depending on long compliance.
Tidal volume is adjusted by setting inspiratory duration and
inspiratory flow rates. Pressure cycled ventilators will

(04:44):
not cycle from the inspiratory phase to the expiratory phase
until a preset pressure is reached.
If a large circuit leak decreases peak pressure
significantly, a pressure cycledventilator may remain in the
inspiratory phase indefinitely. On the other hand, a small leak

(05:09):
may not markedly decrease tidal volume because cycling will be
delayed onto the pressure limit is met Volume Cycled ventilators
vary the inspiration, duration, and pressure to deliver a preset
volume. In reality, modern ventilators

(05:32):
overcome the many shortcomings of classic ventilator designs by
incorporating secondary cycling parameters or other limiting
mechanisms. C Expiratory phase.
The expiratory phase of ventilators normally reduces

(05:55):
airway pressure to atmospheric levels or some preset value of
positive and expiratory pressure, that is, P Exhalation
is therefore passive. Flow out of the lungs is
determined primarily by airway resistance and lung compliance.

(06:19):
Expired gases fill up the bellows.
Excess is relieved to the scavenging system.
D Transition phase from expiration to inspiration
Transition into the next inspiratory phase may be based

(06:42):
on the preset time interval or achange in pressure.
The behaviour of the ventilator during this phase, together with
the type of cycling from inspiration to expiration,
determines the ventilator mode. During controlled ventilation,

(07:03):
the most basic mode of all ventilators, the next breath
always occurs after a preset time interval.
Thus, tidal volume and rates arefixed in volume controlled
ventilation, whereas peak inspiratory pressure and rates

(07:26):
are fixed in pressure controlledventilation.
Ventilator Circuit design 7 Traditionally, ventilators on

(07:48):
anaesthesia machines have a double circuit system design and
a pneumatically powered and electronically controlled.
Newer machines also incorporate microprocessor controls and
sophisticated and precise pressure and flow sensors to

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achieve multiple venturatory modes.
PIP acute tidal volumes and enhanced safety features.
A double circuit system ventilators In a double circuit

(08:34):
system design, tidal volume is delivered from a bellows
assembly that consists of a bellows in a clear rigid plastic
enclosure. A standing that is ascending
bellows is preferred as it readily draws attention to a

(08:56):
circuit disconnection by collapsing hanging that is
descending bellows are rarely used and must not be weighted.
Other older ventilators with weighted hanging bellows
continue to feel by gravity despite a disconnection in the

(09:18):
breathing circuit. The bellows in a double circuit
design ventilator takes the place of the breathing bag in
the anaesthesia circuit. Pressurised oxygen or air from
the ventilator power outlet thatis 45 to 50 lbs per square inch

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gauge is routed to the space between the inside wall of the
plastic enclosure and the outside wall of the bellows.
Pressurisation of the plastic enclosure compresses depleted
bellows inside, forcing the gas inside into the breathing

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circuit and patient. In contrast, during exhalation,
the bellows ascend as the pressure inside the plastic
enclosure drops and the bellows fills up with the exhaled gas.
A ventilator flow control valve regulates Dr Gas flow into the

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pressurising chamber. This valve is controlled by
ventilator settings in the control box.
Ventilators with microprocessorsalso utilise feedback from flow
and pressure sensors. If oxygen is used for pneumatic

(10:52):
power, it will be consumed at a rate at least equal to minute
ventilation. Thus, if oxygen fresh gas flow
is 2 litres per minute and a ventilator is delivering 6
litres per minute to the circuit, a total of at least 8

(11:15):
litres per minute of oxygen is being consumed.
This should be kept in mind if the hospital's medical gas
system fails and cylinder oxygenis required.
Some anaesthesia machines reduceoxygen consumption by

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incorporating A venturi device that draws in room air to
provide air stroke oxygen pneumatic power.
Newer machines may offer the option of using compressed air
for pneumatic power. A leak in the ventilator bellows

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can transmit high gas pressure to the patient's airway,
potentially resulting in pulmonary barotrauma.
This may be indicated by a higher than expected rise in
inspired oxygen concentration, that is if oxygen is the sole

(12:22):
pressurising gas. So machine ventilators have a
built in drive gas regulator that reduces the drive pressure,
for example to £25 per square inch gauge.
For added safety, double circuitdesign ventilators also

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incorporate a free breathing valve that allows outside air to
enter the rigid Dr chamber and the bellows to collapse if the
patient generates negative pressure by taking spontaneous
breaths during mechanical ventilation.

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B Piston Ventilators 8 In a piston design, the ventilator
substitutes an electric and electrically driven piston for
the bellows, and the ventilator requires either minimal or no

(13:29):
pneumatic that is, oxygen power.The major advantage of a piston
ventilator is its ability to deliver accurate tidal volumes
to patients with very long with very poor lung compliance and to
very small patients. C Spiel valve Whenever a

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ventilator is used on an anaesthesia machine, the circle
system's APL valve must be functionally removed or isolated
from the circuit. A bag stroke ventilator switch
typically accomplishes this. When the switch is turned to a

(14:22):
bag, the ventilator is excluded and spontaneous stroke manual
that is bag ventilation is possible.
When it is turned to ventilator,the breeding bag and the APL are
excluded from the breeding circuit.

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The APL valve may be automatically excluded in some
newer anaesthesia machines when the ventilator is turned on.
The ventilator contains its own pressure relief that is pop up
valve called the Spiel valve, which is pneumatically closed

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during inspiration so that positive pressure can be
generated. During exhalation.
The pressurising gas is vented out and the ventilator spill
valve is no longer closed. The ventilator bellows or piston

(15:30):
refills during expiration. When the bellows is completely
filled, the increase in cycle system pressure causes the
excess gas to be directed to discouraging system through the
spiel valve. Sticking of this valve can

(15:52):
result in abnormally elevated airway pressure during
exhalation pressure and volume monitoring.
Peak inspiratory pressure is thehighest circuit pressure
generated during an inspiratory cycle and it provides an

(16:16):
indication of dynamic compliance.
Plateau pressure is the pressuremeasured during an inspiratory
pause, that is a time of no gas flow, and it mirrors static
compliance. During normal ventilation of a

(16:39):
patient without lung disease, peak inspiratory pressure is
equal to or only slightly greater than plateau pressure.
An increase in both peak inspiratory pressure and plateau
pressure implies an increase in tidal volume or a decrease in

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pulmonary compliance. An increase in peak inspiratory
pressure without any change in plateau pressure signals an
increase in airway resistance orinspiratory gas flow rate.

(17:23):
Thus, the shape of the breeding circuit pressure waveform can
provide important airway information.
Many anaesthesia machines graphically display breathing
circuit pressure. Airway secretions or kinking of

(17:45):
the trachea tube can be easily ruled out with the use of a
suction catheter. Flexible fibre optic
bronchoscopy usually provides A definitive diagnosis.
We get to Table 4-2 Causes of increased peak inspiratory

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pressure with or without an increased plateau pressure.
So we have it in two parts, increased peak inspiratory
pressure and plateau pressure. Second part is increased peak

(18:32):
inspiratory pressure and unchanged plateau pressure.
So let's let's take it 1 by 1. Causes of increased peak
inspiratory pressure and plateaupressure, increased tidal
volume, decreased pulmonary compliance under which the

(18:59):
following pulmonary edoema, Trendelenburg position, plural
effusion, asitis, abdominal parking, peritoneal gas
insufflation, tension pneumothorax and endobronchial

(19:28):
intubation. Causes of increased peak
inspiratory pressure and unchanged plateau pressure.
Increased inspiratory gas flow rates.
Increased airway resistance under which we find kink,

(19:54):
endotypical tube bronchospasm, secretions, foreign body
aspiration, airway compression and endotracheal tube cough
herniation. Ventilator Alarms 9.

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Alarms are an integral part of all modern anaesthesia
ventilators. Whenever a ventilator is used,
disconnect alarms must be passively activated.
Anaesthesia workstations should have at least three disconnect

(20:55):
alarms. Low peak inspiratory pressure.
Low exhale tidal volume and low exhaled carbon dioxide.
The first is always built into the ventilator, whereas the

(21:18):
later 2 may be in separate modules.
A small leak or partial breedingcircuit disconnection may be
detected by subtle decreases in peak inspiratory pressure,
exhaled volume, or end tidal carbon dioxide before alarm

(21:43):
thresholds are reached. Other built in ventilator alarms
include High Peak inspiratory pressure, High Peak sustained
high airway pressure, negative pressure, and low oxygen supply

(22:06):
pressure. Most modern anaesthesia
ventilators also have integratedspirometers and oxygen analyzers
that provide additional alarms. Problems associated with

(22:27):
anaesthesia Ventilators A ventilator fresh gas flow
coupling 10. From the previous discussion, it
is important to appreciate that because the ventilator spill

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valve is closed during inspiration, fresh gas flow from
the machines common gas outlets normally contributes to the
tidal volume delivered to the patient.
For example, if the fresh gas flow is 6 litres per minute, the

(23:13):
inspiratory expiratory that is IE ratio is 1 to 2 and the
respiratory rate is 10 Brits perminute.
Each tidal volume will include an extra 200 meals in addition
to the ventilators. Output that is 6000 meals per

(23:39):
minute times 33% / 10 breaths per minute is approximately 200
meals per breath. Thus, increasing fresh gas flow
increases tidal volume, minute ventilation, and peak

(24:03):
inspiratory pressure. To avoid problems with
ventilator fresh gas flow coupling, airway pressure and
exhaled tidal volume must be monitored closely and excessive
fresh gas flows must be avoided.Current ventilators

(24:27):
automatically compensate for fresh gas flow coupling.
Piston style ventilators redirect fresh gas flow to the
reservoir bag during inspiration, thus preventing
augmentation of the tidal volumesecondary to fresh gas flow.

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B Excessive positive pressure. 11 Intermittent or sustained
high inspiratory pressures, thatis, greater than 30 millimetres
of mercury during positive pressure ventilation increase

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the risk of pulmonary barotrauma, for example,
pneumotorax or hemodynamic compromise, or both.
During anaesthesia. Excessively high pressures may
arise from incorrect settings onthe ventilator.

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Ventilator malfunction, fresh gas flow coupling as discussed
above, or activation of the oxygen flush during the
inspiratory phase of the ventilator.
Use of the oxygen flush valve during the inspiratory cycle of

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a ventilator must be avoided because the ventilator spill
valve will be closed and the APLvalve is excluded.
The surge of oxygen that is 600 to 1200 miles per second and the

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circuit pressure will be transferred to the patient's
lungs. In addition to a high pressure
alarm, all ventilators have a built in automatic or APL valve.
The mechanism of pressure limiting may be as simple as a

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threshold valve that opens at a setting pressure or electronic
sensing that abruptly terminatesthe ventilator.
Inspiratory phase C Tidal volumediscrepancies 12 Large

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discrepancies between the set and actual tidal volume that the
patient receives are often observed in the operating room
during volume controlled ventilation.
Courses include breeding circuitcompliance, gas compensation,

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ventilator fresh gas flow coupling as described above and
leaks in the anaesthesia machine, the breathing circuits
or the patient's airway. The compliance for standard
adult breathing circuits is about 5 mils per centimetre of

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water. Thus, if peak in spiritual
pressure is 20 centimetres of water, about 100 mils of set
tidal volume is lost to expanding the circuit.

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For this reason, breathing circuits for paediatric patients
are designed to be much stiffer,with compliances as small as 1.5
to 2.5 mils per centimetre of water.
Compression losses, normally about 3%, are due to gas

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compression within the ventilator bellows and may be
dependent on breeding circuit volume.
Thus, if tidal volume is 500 mills, another 15 mills of the
set tidal gas may be lost. Gas sampling for carpinography

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and anaesthetic gas measurementsrepresent additional losses
unless the sampled gas is returned to the breathing
circuit. Accurate detection of tidal
volume discrepancies is dependent on where the
spirometer is placed. Sophisticated ventilators

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measure both inspiratory and expiratory tidal volumes.
It is important to note that unless the spirometer is placed
at the Y connector in the breeding circuit, compliance and
compression losses will not be apparent.

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Several mechanisms have been built into the newer anaesthesia
machines to reduce tidal volume discrepancies during the initial
electronic self checkout. Some machines measure total
system compliance and subsequently use this

(30:30):
measurement to adjust the excursion of the ventilator.
Bellows or piston leaks may alsobe measured but are usually not
compensated. The actual method of tidal
volume compensation or modulation varies according to

(30:54):
manufacturer and model. In one design, a flow sensor
measures the tidal volume delivered at the inspiratory
valve for the first few breaths and adjusts subsequent metre Dr
gas flow volumes to compensate for tidal volume losses, that

(31:20):
is, feedback adjustment. Another design continually
measures fresh gas and vaporizerflow and subtracts this amount
from the metered Dr gas flow that is pre emptive adjustment.

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Ultimately, machines that use electronic control of gas flow
can decouple fresh gas flow fromtidal volume by delivery of
fresh gas flow only during exhalation.
Lastly, the inspiratory phase ofthe ventilator fresh gas flow

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may be diverted through a decoupling valve into the
breeding bag, which is excluded from the circle system during
ventilation. During exhalation, the
decoupling valve opens, allowingthe fresh gas that was

(32:32):
temporarily stored in the bag toenter the breeding circuit.
Waste gas scavengers 13 Waste gas scavengers dispose of gases

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that have been vented from the breeding circuits by the APL
valve and ventilator spill valve.
Pollution of the operating room environment with anaesthetic
gases may pose a health hazard to surgical personnel.

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Although it is difficult to define safe levels of exposure,
the National Institute for Occupational Safety and Health,
NIOSH, recommends limiting the room concentration of nitrous
oxide to 25 parts per million and halogenated agents to two

(33:41):
parts per million, that is, 0.5 parts per million.
If nitrous oxide is also being used in time integrated samples,
reduction to these trace levels is possible only with properly

(34:03):
functioning waste gas scavengingsystems.
To avoid the buildup of pressure, the gas, the excess
gas volume is vented through theAPL valve in the breathing
circuit and the ventilator spillvalve.

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Both valves should be connected to hoses, that is transferred
tubing leading to the scavenginginterface, which may be inside
the machine or an external attachment.
The pressure immediately downstream to the interface

(34:52):
should be kept between 0.5 and plus 3.5 centimetres of water
during normal operating conditions.
Disconviging interface may be described as either open or
closed. An open interface is open to the

(35:19):
outside atmosphere and usually requires no pressure relief
valves. In contrast, a closed interface
is closed to the outside atmosphere and requires negative
and positive pressure relief valves that protect the patient

(35:41):
from the negative pressure of the vacuum system and positive
pressure from an obstruction in the disposal tubing,
respectively. The outlets of the scavenging
system may be a Direct Line to the outside via a ventilation

(36:05):
duct, beyond any point of recirculation, that is passive
scavenging or a connection to the hospital's vacuum system
that is active scavenging. A chamber or reservoir bag

(36:27):
accepts waste gas overflow when the capacity of the vacuum is
exceeded. The vacuum control valve on an
active system should be adjustedto allow the evacuation of 10 to
15 litres of waste gas per minute.

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This rate is adequate for periods of high fresh gas flow,
that is induction and emergence,yet minimises the risk of
transmitting negative pressure to the breeding circuits during
lower flow conditions, that is maintenance.

(37:16):
Unless an open interface is usedcorrectly, the risk of
occupational exposure for healthcare providers is greater
with an open interface. Some machines may come with both
active and passive scavenger systems.

(37:44):
Anaesthesia Machine Check out list 14 Misuse or malfunction of
anaesthesia gas delivery equipment can cause major
morbidity or mortality. A routine inspection of

(38:09):
anaesthesia equipment before each use increases operator
familiarity and confirms proper functioning. the US Food and
Drug Administration FDA has madeavailable a generic checkout
procedure for anaesthesia gas machines and breathing systems.

(38:34):
This procedure should be modified as necessary depending
on the specific equipment being used and the manufacturer's
recommendations. Note that although the entire
checkout does not need to be repeated between cases on the
same day, the consensus use of an abbreviated checkout list is

(38:58):
mandatory before each anaesthetic procedure.
A mandatory check off procedure increases the likelihood of
detecting anaesthesia machine faults.
Some anaesthesia machines provide an automated system

(39:20):
check that requires A variable amount of human intervention.
These system cheques may includenitrous oxide delivery that is
hypoxic mixture prevention, agent delivery, mechanical and

(39:41):
manual ventilation, pipeline pressures, scavenging, breaking,
circuit compliance, and gas leakage.
Then we get to Table 4-3, talking about anaesthesia

(40:06):
apparatus checkout recommendations.
The checkout is or a reasonable equivalent should be conducted
before the administration of anaesthesia.
These recommendations are valid only for an anaesthesia system

(40:27):
that conforms to current and relevant standards, and it
includes an ascending bellows, ventilator and at least the
following monitors. Carpinograph pulse oximeter.
Oxygen analyzer, respiratory volume monitor that is
spirometer and breathing system.Pressure monitor with high and

(40:53):
low pressure alarms. Users are encouraged to modify
this guideline to accommodate differences in equipment design
and variations in local clinicalpractise.
Such local modifications should have appropriate peer review.

(41:16):
Users should refer to the appropriate operator manuals for
specific procedures and precautions.
So let's go through the checkoutrecommendations.

(41:37):
Emergency ventilation equipment One Verify that backup
ventilation equipment is available and functioning.
High pressure system Two check the oxygen cylinder supply.

(42:02):
A Open the oxygen cylinder and verify it is at least half full,
about £1000 per square inch gauge.
B Close the cylinder three. Check central pipeline supplies.

(42:23):
Check that hoses are connected and pipeline gauges.
Read about £50 per square inch gauge.
Low pressure system 4 Check the initial status of the low
pressure system. A Close flow control valves and

(42:50):
turn the vaporizers off B Check the fill level and tighten the
vaporizer's filler caps. Five Perform a leak check of the
machines low pressure system A. Verify that the machine master

(43:15):
switch and flow control valves are off B.
Attach the suction bulb to the common fresh gas outlet C.
Squeeze the bulb repeatedly until fully collapsed.

(43:35):
D Verify the bulb stays fully collapsed for at least 10
seconds E Open one vaporizer at a time and repeat steps.
C and DF. Remove the suction bulb and

(43:56):
reconnect the fresh gas hose. Six.
Turn on the machine master switch and all other necessary
electrical equipment. Seven.
Test the flow metres a. Adjust the flow of all gases

(44:22):
through their full range, checking for smooth operation of
floats and undamaged flow tubes B.
Attempt to create a hypoxic oxygen stroke nitrous oxide
mixture and verify the correct changes in the flow or alarm

(44:47):
scavenging system 8. Adjust and check the scavenging
system a. Ensure proper connections
between the scavenging system and both the APL that is purple
valve and the ventilator relief valve B.

(45:13):
Adjust the waste gas vacuum if possible C.
Fully open the APL valve and occlude the Y piece D with
minimum oxygen flow. Allow the scavenger reservoir

(45:33):
bag to collapse completely and verify that the absorber
pressure gauge reads about 0 E With the oxygen flush activated,
allow the scavenger reservoir bag to distend fully and then

(45:54):
verify that the absorber pressure gauge reads less than
10 centimetres of water. Monitors 13.
Sorry, that's should go to 9 now.
Breathing system calibrate 9. Calibrate the oxygen monitor A.

(46:24):
Ensure the monitor reads 21% in room air B.
Verify that the low oxygen alarmis enabled and functioning C.
Reinstall the sensor in the circuit and flush the breathing

(46:44):
system with oxygen D Verify thatthe monitor now reads greater
than 90% 10. Check the initial status
breathing system A. Set the selector switch to bag

(47:08):
mode B. Check that the breeding circuit
is complete, undamaged and unobstructed C Verify that
carbon dioxide absorbent is adequate D Install the breeding

(47:30):
circuit accessory equipment, forexample, humidifier PIPP valve
to be used during decades 11. Perform a leak check of the
reading of the breeding system a.

(47:51):
Set all gas flows to 0 or minimum B.
Close the APL that is pop up valve and occlude the Y piece C.
Pressurise the breathing system to about 30 centimetres of water

(48:12):
with oxygen flush D. Ensure that the pressure remains
fixed for at least 10 seconds E.Open the APL's to purple valve
and ensure that pressure decreases.

(48:37):
Manual and automatic ventilationsystems 12 Test ventilation
systems and unidirectional valves A Place a second breeding
bag on the Y piece. B Set appropriate ventilator

(49:01):
parameters for the next patient.C Switch to automatic
ventilation, that is ventilator mode.
D Turn the ventilator on and fill the bellows and breeding
bag with oxygen flush. E Set oxygen flow to minimum and

(49:30):
other gas flows to 0. F Verify that during inspiration
the bellows delivers the appropriate tidal volume and
that during expiration the bellows feels completely.

(49:50):
G Set the fresh gas flow to about 5 litres per minute.
H Verify that the ventilator bellows and simulated lungs feel
and empty appropriately without sustained pressure at end

(50:11):
expiration. I check for proper action of
unidirectional valves. J Exercise breathing circuit
accessories to ensure proper function.
K Turn the ventilator off and switch to manual ventilation,

(50:36):
that is bag stroke, EPL mode. L Ventilate manually and ensure
inflation and deflation of artificial lungs, an appropriate
field of system resistance and compliance, and remove the
second breeding bag from the white piece. 13 OK, talking

(51:04):
about monitors now. 13 Check, calibrate or set alarm limits
for all of all monitors. Carpinograph, pulse oximeter,
oxygen analyzer, respiratory volume monitor that is

(51:25):
spirometer and pressure monitor with high and low airway
pressure alarms. Final position 14 Check the
final status of the machine. A Vaporizers of BAPL valve open.

(51:53):
C selector switch to bag mode. D All flow metres to 0 or
minimum. E Patient suction level
adequate. F breathing system ready to use.

(52:16):
Case discussion Detection of a leak after induction of general
anaesthesia and intubation of a 70 KG patient for elective
surgery. A standing bellows ventilator is
set to deliver a tidal volume of500 mils at a rate of 10 breaths

(52:41):
per minute. Within a few minutes, the
anesthesiologist notices that the bellows fails to rise to the
top of its clear plastic enclosure during expiration.
Shortly thereafter, the disconnect alarm is triggered.

(53:02):
Question Why has the ventilator bellows fallen and the
disconnect alarm sounded? Answer.
Fresh gas flow into the breathing circuit is inadequate
to maintain the circuit volume required for positive pressure
ventilation. In a situation in which there is

(53:26):
no fresh gas flow, the volume inthe breathing circuit would
slowly fall because of the constant reoptake of oxygen by
the patient, that is, metabolic oxygen consumption and
absorption of expired carbon dioxide.
An absence of fresh gas flow could be due to exhaustion of

(53:50):
the hospital's oxygen supply. Remember the function of
Remember the function of the fail safe valve or failure to
turn on the anaesthesia machinesflow control valves.
These possibilities can be ruledout by examining the oxygen
pressure gauge and the flow metres.

(54:14):
A more likely explanation is a gas leak that exceeds the rate
of fresh gas. Flow leaks are particularly
important in closed circuits. Anaesthesia.
Question How can the size of theleak be estimated?

(54:39):
Answer. When the fresh gas inflow equals
the rate of gas outflow, the circuit volume will be
maintained. Therefore, the size of the leak
can be estimated by increasing the fresh gas flows until there
is no change in the height of the bellows from 1 expiration to

(55:02):
the next. If the bellows collapse despite
a high rate of fresh gas inflow,a complete circuit disconnection
should be considered. The site of the disconnection
must be determined immediately and repaired to prevent hypoxia

(55:22):
and hypercapnia. A resuscitation bag must be
immediately available and can beused to ventilate the patient if
there is a delay in correcting the situation.
Question Where are the most likely locations of a breathing

(55:44):
circuit disconnection or leak? Answer Frank Disconnections
occur most frequently between the right angle connector and
the trachea tube, whereas leaks are most commonly traced to the

(56:05):
base plate of the carbon dioxideabsorber.
In the intubated patient, leaks often occur in the trachea
around an uncuffed trachea tube or an inadequately filled cough.

(56:25):
There are numerous potential sites of disconnection or leak
within the anaesthesia machine and the breeding circuits.
However, every addition to the breeding circuits such as
humidifier provides another potential location for a leak.

(56:48):
Question How can these leaks be detected?
Answer Leaks may occur because the fresh gas outlets that is
within the anaesthesia machine or after the fresh gas inlet
that is within the breeding circuit.

(57:11):
Large leaks within the anaesthesia machine are less
common and can be ruled out by asimple test.
Pinching the tubing that connects the machines fresh gas
outlets to the circuits fresh gas inlets creates a back
pressure that obstructs the forward flow of fresh gas from

(57:36):
the anaesthesia machine. This is indicated by a drop in
the height of the flow metre floats.
When the fresh gas tubing is released, the floats should
briskly rebound and settle at the original height.

(58:00):
If there's a substantial leak within the machine, obstructing
the fresh gas tubing will not result in any back pressure and
the floats will not drop. A more sensitive test for
detecting small leaks that occurbefore the fresh gas outlets

(58:21):
involves attaching a suction bulb at the outlets as described
in Step 5 of Table 4-3. Correcting a Leak within the
Machine will usually require removing it from service.

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Leaks within a breathing circuitnot connected to a patient are
readily detected by closing the APL valve, occluding the Y piece
and activating the oxygen flush until the circuit reaches a
pressure of 20 to 30 centimetresof water.

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A gradual decline in circuit pressure indicates a leak within
the breathing circuits. That is Table 4-3.
Step 11. How are leaks?
That is a question. How are leaks in the breathing

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circuits located? Answer Any connection within the
breathing circuits is a potential site of a gas leak.
A quick survey of the circuits may reveal A loosely attached
breathing tube or a cracked oxygen analyzer adapter.

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Less obvious causes include detachment of the tubing used by
the disconnect alarm to monitor circuit pressures.
An open APL valve or an improperly adjusted scavenging
unit. Leaks can usually be identified

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audibly or by applying a SOAP solution to to suspect
connections and looking for bubble formation.
Leaks within the anaesthesia machine and breeding circuits
are usually detectable if the machine and circuits have

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undergone an established checkout procedure.
For example, steps 5 and 11 of the FDA recommendations in Table
4-3 will reveal most leaks. We've come to the end of Chapter

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4.