Anesthetic Delivery Systems

Question 1

On an anesthesia machine, what components are

found in the low pressure system?

Answer 1

The low pressure system on an anesthesia machine contains the
flowmeters, hypoxia prevention devices, unidirectional valves,
pressure relief devices, and the common gas outlet.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 67-77.

Question 2

On an anesthesia machine, what components are

found in the intermediate pressure system?

Answer 2

The intermediate pressure system of an anesthesia machine
contains the pneumatic part of the master switch, pipeline inlet
connections and pressure indicators, the gas power outlet, oxygen
flush valve, oxygen pressure failure devices, and flow control
valves.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 59-66.

Question 3

Where does the oxygen flush receive its gas supply?

Answer 3

The oxygen flush receives an unmetered oxygen supply from the
cylinder pressure regulator or the pipeline inlet at a flow rate
between 35 and 75 liters/minute.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 62.

Question 4

When is use of the oxygen flush control valve

contraindicated in a mechanically ventilated patient?

Answer 4

The oxygen flush control valve delivers oxygen at a rate of 35-75
L/min and a pressure of 50 psig. To avoid barotrauma, it should
never be used when there is no outlet for excess gas pressure to
escape. This situation exists when the patient is in the inspiratory
phase of mechanical ventilation when the inspiratory valve is open
to the patient but there is no vent for gases to escape.
Longnecker DE, Newman MF, Brown DL, Zapol WM.
Anesthesiology. 2nd ed. New York: McGraw-Hill; 2012: 621.

Question 5

What are the three functions of the hanger-yoke

assembly on the anesthesia machine?

Answer 5

The hanger yoke assembly orients the gas cylinder, supports the
weight of the cylinder, provides for uni-directional gas flow, and
provides an air-tight seal.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 53.

Question 6

How does a variable-bypass vaporizer work?

Answer 6

A variable-bypass vaporizer splits the fresh gas flow into two
parts. One portion enters the vaporizing chamber where it
becomes saturated with anesthetic agent. The other portion
enters the bypass chamber and does not come into contact with
the anesthetic agent. By controlling the amount of gas that enters
the vaporizing chamber, the amount of anesthetic agent delivered
to the patient can be controlled.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 202.

Question 7

Is the Tex-6 vaporizer a variable-bypass vaporizer?

Why or why not?

Answer 7

The Tec 6 vaporizer is heated to 39 degrees Celsius and
pressurized to 2 atmospheres. It is not a variable bypass
vaporizer and unlike these devices, no fresh gas flows through the
sump.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 63.

Question 8

Why should you avoid using a dessicated CO2

absorbent with desflurane?

Answer 8

Prolonged exposure of desflurane or isoflurane to dessicated CO2
absorbent can result in the degradation of the anesthetic with the
resulting production of carbon monoxide. Increased temperature
and higher doses of anesthetic agent both increase the amount of
carbon monoxide produced. The inhaled anesthetic that produces
the greatest amount of carbon monoxide is desflurane.
Longnecker DE, Newman MF, Brown DL, Zapol WM.
Anesthesiology. 2nd ed. New York: McGraw-Hill; 2012: 641.

Question 9

Which vaporizer is electrically heated?

Answer 9

The desflurane vaporizer is electrically heated. The vaporizers for
other contemporary gases are temperature compensated and
have a variable-bypass, flow-over design.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 202.

Question 10

What is the ‘pumping effect’ and when might it occur?

Answer 10

The ‘pumping effect’, an intermittent back pressure caused by
positive pressure ventilation or use of the oxygen flush valve
results in elevated vaporizer output. It is more prominent with low
flows, low vaporizer dial settings, low levels of anesthetic in the
vaporizer chamber, high respiratory rates, and high peak
inspiratory pressures.
Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC,
Ortega R. Clinical Anesthesia. 7th ed. Philadelphia, PA:
Lippincott Williams and Wilkins; 2013: 664-665.

Question 11

What is the function of the bimetallic strip in a

contemporary vaporizer?

Answer 11

The bimetallic strip is temperature sensitive and expands and
contracts in response to ambient temperature changes to alter the
total gas flow between the vaporizing and bypass chambers. If the
temperature of the liquid anesthetic decreases, the bimetallic strip
allow more gas flow into the vaporizing chamber to compensate
for the decreased vapor pressure of the cooler liquid.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 203.

Question 12

Which gas flowmeter should be positioned last in the

anesthesia manifold and why?

Answer 12

The oxygen flowmeter should be positioned last in the sequence
of flowmeters in the manifold to reduce the likelihood that a leak in
one of the flowmeters would allow oxygen to escape and lower the
inspired oxygen concentration.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 201.

Question 13

Pressing the oxygen flush valve allows oxygen to

bypass what components?

Answer 13

Pressing the oxygen flush valve creates a direct communication
between the low and high pressure circuits and allows oxygen to
bypass the flowmeters and manifold on its way to the outlet.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 202.

Question 14

How many liters of oxygen will an E-cylinder hold?

Nitrous oxide? Carbon dioxide? Air?

Answer 14

An E-cylinder will hold about 660 liters of oxygen, 1590 liters or
either nitrous oxide or carbon dioxide, or 625 liters of air.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 8.

Question 15

When an anesthesia machine is connected to a
pipeline gas supply, should the backup cylinder valves
be open or closed? Why?

Answer 15

When connected to a pipeline gas source, the cylinders should be
kept closed so that a pipeline pressure failure would be detected
earlier and to prevent pressure variations in the pipeline from
draining oxygen from the cylinders.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 60.

Question 16

What safety mechanisms are built into the pipeline

inlet connections on the anesthesia machine?

Answer 16

The pipeline inlet connections on an anesthesia machine are fitted
with diameter index safety system fittings to prevent connection of
a gas line to the wrong connector on the machine. They also have
100 micron (or less) filters and a unidirectional check valve.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 24.

Question 17

What is the heat of vaporization?

Answer 17

The heat of vaporization of a liquid is the number of calories
required at a certain temperature to convert one gram of the liquid
into a vapor. (It is the amount of energy required for a liquid to
convert into a vapor.) As a liquid such as a volatile anesthetic or
nitrous oxide converts into a gas, it loses heat and cools.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 202.

Question 18

Running a high flow of nitrous oxide can cause frost to
develop on the cylinder and valves. Why does this
occur?

Answer 18

The vaporization of a compressed or liquified gas absorbs heat.
This heat is taken from the components of the cylinder and the
surrounding air resulting in the accumulation of frost. Frost does
not form on the inside of the cylinder because it contains no water
vapor.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 201.

Question 19

A leak in the anesthesia machine due to the vaporizer

is most commonly caused by what?

Answer 19

A loose filler cap.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 203.

Question 20

How does the oxygen supply failure alarm work?

Answer 20

The oxygen supply failure alarm sets off an alarm if the oxygen
pressure falls below a standard set by the manufacturer (which is
typically 30 psig). It must engage within 5 seconds of the
disconnect and cannot be disabled by the anesthetist. It is
designed to help prevent hypoxia from lack of oxygen flow, but
does not prevent the flow of anesthesia gases (and therefore
could still possibly allow a hypoxic mixture of gases to be
delivered).
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 61-62.

Question 21

In the event your oxygen supply fails and you must
switch indefinitely to your backup oxygen tanks, what
are appropriate measures to reduce the amount of
oxygen consumed and prolong the duration of your
backup oxygen supply?

Answer 21

Most anesthesia machines utilize oxygen as the driving gas to
power the ventilator. By reducing fresh oxygen flow rates and
eliminating the use of the ventilator by allowing the patient to
breathe spontaneously or ventilating via the reservoir bag, you will
prolong the backup oxygen supply.
Longnecker DE, Newman MF, Brown DL, Zapol WM.
Anesthesiology. 2nd ed. New York: McGraw-Hill; 2012: 621.

Question 22

What is the function of the diameter index safety

system?

Answer 22

The diameter index safety system connects the wall outlet gas
supply to the anesthesia machine and has a line pressure of 45-
55 psi. It is designed for medical gas lines at pressures of 1380
kPa (200 psi) or less
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 49-51.

Question 23

What is the function of the nitrous oxide shutoff valve

on the anesthesia machine?

Answer 23

The nitrous oxide shutoff device (formerly known as the fail safe
device) triggers an alarm and closes the supply of nitrous oxide
and other gases when the oxygen pressure falls below 25 psi.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 52-54.

Question 24

Why is the flowmeter tube (also called a Thorpe tube)

tapered?

Answer 24

The diameter of the flowtube is tapered such that it is smaller at
the bottom than at the top. At lower gas flows, it takes less
pressure to support the bobbin and the diameter is smaller. At
higher gas flows, the diameter is larger and more gas is allowed to
pass around the bobbin.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 54-55.

Question 25

Which gas should be positioned last in the flowmeter

sequence on an anesthesia machine?

Answer 25

Oxygen should be placed last in the sequence of flowmeters to
reduce the risk of delivery of a hypoxic mixture of gases if a leak
develops in one of the upstream flowmeters.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 201.

Question 26

How should a flowmeter indicater be assessed?

Answer 26

If using a bobbin, the flow rate should be read at the top of the
indicator. When using a ball-shaped float, the flow rate should be
read at the middle of the ball.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 201.

Question 27

You are testing your anesthesia machine in the
morning before cases begin. You disconnect the
pipeline supply and turn on the backup oxygen cylinder
to make sure it functions correctly. The oxygen
cylinder is full and connectly correctly but the
anesthesia machine still does not pressurize. What
could be the cause?

Answer 27

If you have verified that the backup oxygen cylinder is full,
connected properly, and opened and the anesthesia machine still
does not pressurize, then there is a problem in the high or
intermediate pressure system in the anesthesia machine.Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment.
5th ed. Philadelphia, PA: Lippincott Williams and Wilkins, 2008:
407.

Question 28

How do you calculate the inspired oxygen
concentration if you are administering air and oxygen
simultaneously?

Answer 28

The formula to calculate the inspired oxygen concentration if you
are administering both air and oxygen together is: 100 X (1.0 X
O2 L/min) + (0.21 X Air L/min)/(O2 L/min + Air L/min)
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011: 25.

Question 29

What is the only machine safety device that detect

problems downstream from the flowmeters?

Answer 29

The oxygen monitor
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 216.

Question 30

What is the result of closing the APL valve off when

the patient is breathing spontaneously?

Answer 30

Closing the APL valve while a patient is breathing spontaneously
will result in an increase in the breathing circuit pressure. The
breathing bag can become distended and the patient can be
exposed to CPAP.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 118.

Question 31

A patient is being mechanically ventilated with a
respiratory rate of 8/minute, a tidal volume of 700 mL,
the oxygen flow rate is 1L/minute and nitrous oxide is
being delivered at 1L/minute. How much oxygen is
being consumed per minute?

Answer 31

The ventilator is driven by oxygen and requires an amount equal to
the minute ventilation, which in this case is 5.6 L/min. The oxygen
flow is 1 L/min and therefore, the total flow consumed is 6.6 L/min.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 76-77.

Question 32

Which mechanical ventilation modes allows for

spontaneous ventilation?

Answer 32

Ventilation modes that allow spontaneous ventilation include: IMV,
SIMV, MMV, PSV, and HFJV. CMV, AC, and PCV do not.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 1291.

Question 33

What change would you see if there was a leak in the

bellows in an oxygen-driven ventilator?

Answer 33

Oxygen driven ventilators use a 50 psi pressure source to drive
the ventilator. If a leak develops in the bellows then the patient is
exposed to that higher pressure and peak inspiratory pressure
would rise. The fraction of inspired oxygen would also increase.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 209.

Question 34

What are the regulations concerning the amount of
pressure a breathing bag can exert? Who sets these
regulations?

Answer 34

According to the standards set by the ASTM, 3-liter reservoir bags
(also called breathing bags) that are distended to four times their
normal size should not exert a pressure less than 35 cm H2O or
greater than 60 cm H2O. Bags that are 1.5 L in size or smaller
shall not exert a pressure less than 30 cm H2O or greater than 50
cm H2O when distended to this extent. The lowest pressure to be
exerted is to ensure that the bag is capable of exerting enough
minimum pressure to ventilate a patient and the highest pressure
is to limit the amount of barotrauma that a patient may be exposed
to when using the breathing bag.
Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment.
5th ed. Philadelphia, PA: Lippincott Williams and Wilkins, 2008:
196.

Question 35

When the bag/vent selector is set to Vent Mode, what

components are eliminated from the breathing circuit?

Answer 35

The reservoir bag and the APL valve.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 209.

Question 36

What is wasted ventilation? Name four factors that will
increase the amount of tidal volume lost during
inspiration due to wasted ventilation.

Answer 36

Wasted ventilation is a reduction in the tidal volume delivered to
the patient due to distention of the breathing system components.
The tidal volume lost increases with increased airway pressures,
increased tidal volume, increased compliance of the breathing
system components, and increased breathing system volume.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 113.

Question 37

What factors result in higher tidal volumes than are set
for the patient when the anesthesia machine ventilator
is in use?

Answer 37

Unless you are using an anesthesia machine that is designed to
compensate for it, when the ventilator is being used a portion of
the fresh gas flow will contribute to the tidal volume. This effect
increases if the fresh gas flow rates are high, if the I:E ratio is
high, and if the patient has a slower respiratory rate.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 171.

Question 38

What are the potential complications of overfilling a

vaporizer?

Answer 38

Possible complications from overfilling a vaporizer include: liquid
agent entering the fresh gas line and potentially delivering a lethal
dose of agent to the patient or complete failure of the vaporizer.
Some vaporizers have the filling port positioned so that overfilling
cannot occur, but if the vaporizer is tipped overfilling is still a
possibility.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 103.

Question 39

Describe the steps of the chemical reaction that occur

in the soda lime-based CO2 absorber cannister

Answer 39

The reaction that occurs within the CO2 absorber between soda
lime and carbon dioxide occurs in two steps. The first is CO2 +
H2O –> H2CO3. The second portion of the reaction is H2CO3 +
2NaOH –> Na2CO3 + 2H2O + heat. Therefore, H2CO3 is an
intermediate product of the reaction, but not a final product as it is
degraded by the secondary reaction. The final products are
Na2CO3, H2O, and heat.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 37.

Question 40

What is channeling in a CO2 absorber cannister?

Answer 40

Channeling occurs when gases flow through passages of lower
resistance in the CO2 cannister. When this occurs, most of the
CO2 absorbent granules are bypassed and the efficiency of the
absorber decreases. It can be minimized by gently shaking the
cannister prior to use.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 213.

Question 41

What are the advantages/disadvantages of a singlecannister
CO2 absorber system over a doublecannister
system?

Answer 41

A single canister CO2 absorber system is easier to change, but
double canister systems provide more complete CO2 absorption,
less frequent canister changes, and lower resistance to gas flow.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 38-39.

Question 42

What are the three classifications of a circle system
type of anesthetic breathing apparatus? How do they
differ?

Answer 42

Semi-open, semi-closed, and closed. Semi-open requires high
fresh gas flow rates to prevent rebreathing. Semiclosed involves
rebreathing of anesthetic and respiratory gases and is the most
commonly used system in the U.S. A closed system requires
matching the inflow of gas precisely to the amount of gas
consumed by the patient.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 207.

Question 43

Which preserves circuit humidity more effectively, a

single-limbed circuit or a two-limbed circuit?

Answer 43

A single-limbed circuit will improve humidification to a slightly
greater degree than a two-limbed circuit.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 212.

Question 44

In a circle anesthesia circuit, what determines the

amount of gas that is rebreathed by the patient?

Answer 44

In a circle anesthesia circuit, the fresh gas flow rate determines
the amount of gas that returns to the patient and is rebreathed.
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011: 26.

Question 45

Can you use both an HME and a heated humidifier at

the same time to preserve circuit humidity?

Answer 45

No. The combination of an HME and a heated humidifier can
produce a dangerous resistance to airflow.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 212.

Question 46

You are using a circle anesthetic circuit and realize
that the carbon dioxide absorbent has become
exhausted. You decide to convert the circuit to an
‘open circuit’. What is the easiest way to do this during
the case?

Answer 46

An ‘open’ circuit implies that the patient is not rebreathing any of
the exhaled gases. Although this is wasteful, it can also be useful
in the event the CO2 absorbent becomes exhausted during a case
and you need to prevent the patient from rebreathing exhaled
carbon dioxide. All that’s necessary to convert a circle circuit to
‘open’ is to increase the fresh gas flow rate to above the patient’s
minute ventilation.
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011: 26.

Question 47

How much carbon dioxide will a patient inhale using a

normal circle anesthesia circuit?

Answer 47

None. A circle circuit incorporates the carbon dioxide absorbers
which will absorb any exhaled CO2.
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011:

Question 48

How will a PEEP valve affect the patient’s work of

breathing?

Answer 48

A PEEP valve will increase the amount of work the patient has to
exert during the exhalation phase.
Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment.
5th ed. Philadelphia, PA: Lippincott Williams and Wilkins, 2008:
200-201

Question 49

What is the Bain circuit?

Answer 49

The Bain circuit is a modification of the Mapleson D circuit
utilizing a coaxial system in which the fresh gas hose lies within
the corrugated tubing through which exhaled gas flows. The
exhaled gas passes in the opposite direction of the fresh gas,
establishing a countercurrent, which helps warm the inspired gas.
It requires a fresh gas flow of 1-2 times that of the minute
ventilation to prevent rebreathing.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 36.

Question 50

The Bain circuit is a modification of what other

anesthetic breathing system?

Answer 50

The Bain circuit is a coaxial version of the Mapleson D system.
The fresh gas supply tube passes inside the corrugated expiratory
tubing. The Bain circuit may be used for controlled or
spontaneously breathing patients.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 206.

Question 51

Which anesthetic breathing circuit does not have a gas

reservoir bag?

Answer 51

The Mapleson E does not have a gas reservoir bag. The Bain
circuit and all of the Mapleson circuits except for the Mapleson E
have a gas reservoir bag.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia,
PA: Elsevier Saunders; 2011: 205.

Question 52

What are the two components of the circle system test
on an anesthesia machine? Between what two
machine components does the circle system test?

Answer 52

The circle system test evaluates the circle breathing system from
the common gas outlet to the y-piece and consists of two parts,
the leak test and the flow test.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 215-216.

Question 53

How is the leak test performed on an anesthesia

machine

Answer 53

The leak test is performed by closing the pop-off valve, occluding
the y-piece, and evaluating for a drop in pressure after
pressurizing the circuit to 30 cm H2O using the flush valve.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 215-216.

Question 54

What is the most common critical incident in

anesthesia?

Answer 54

The most common critical incident in anesthesia is due to
breathing circuit disconnections which most commonly occur at
the y-piece.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 43.

Question 55

How is the flow test performed on an anesthesia

machine?

Answer 55

The flow test checks the integrity of the unidirectional valves by
removing the y-piece and breathing through each individual hose
to observe the movement of each valve.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed. Philadelphia:
Elsevier Saunders; 2011: 215-216.

Question 56

How can pain affect oxygen saturation during low-flow

anesthesia?

Answer 56

During low-flow anesthesia, the amount of oxygen delivered must
be higher than the patient’s metabolic demand regardless of the
FiO2. For example, if your oxygen flow rate is 300 mL/min and the
patient’s metabolic demand is 300 mL/min, then the oxygen supply
meets the demand. If however, the patient experiences pain, it is
likely that his heart rate, blood pressure, and metabolic demand
for oxygen will increase. As his oxygen consumption increases
above 300 mL/min the supply no longer meets the demand and
the oxygen saturation will begin to fall.
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011: 29.

Question 57

What is a heat and moisture exchanger (HME)? What

are three disadvantages of using an HME?

Answer 57

Heat and moisture exchangers (HME) are inexpensive, silent,
easy-to-use devices that help retain heat and moisture within the
anesthesia circuit. Advantages of HMEs include: no need for
water or electrical power source and no risk of: hyperthermia,
overhydration, burns, or electrical shock. They all act as large
particle filters and many serve as effective bacterial and viral filters.
The primary disadvantages of HMEs are that they are not nearly
as effective at warming and humidifying the patient’s airway as
water-based, electrical devices. They also increase deadspace
and can increase the work of breathing.
Airway obstruction can occur if the HME becomes blocked with
fluid, blood, secretions, nebulized drugs, or if a mechanical defect
is present in the device. If increased airway resistance is
experienced during an anesthetic, the peak pressure should be
measured both with and without the HME in place.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 208-213.

Question 58

What are the two types of heat and moisture

exchangers (HMEs) and how are they different?

Answer 58

There are two primary types of HMEs: hydrophobic and
hygroscopic. Hydrophobic HMEs have a hydrophobic membrane
with small pores that is pleated to provide a greater surface area.
Hygroscopic HMEs contain a wool, foam, or paperlike material
that is coated with a chemical that helps it to retain moisture. High
ambient temperature may decrease the effectiveness of
hydrophobic HMEs compared to hygroscopic HMEs.
Hydrophobic HMEs may also be more effective at preventing the
transmission of the hepatitis C virus than hygroscopic HMEs.
Also, if hygroscopic HMEs become wet, they may lose their ability
to filter airborne pathogens and airway resistance may increase
substantially. The airway resistance of hydrophobic HMEs
increases only slightly if wet. Hygroscopic HMEs are considered
more effective at preserving heat and humidity than hydrophobic
HMEs.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 209-211

Question 59

What driving pressure is required to deliver adequate
oxygen flow through a 14 or 16 gauge catheter used
for an emergency cricothyrotomy?

Answer 59

When performing transtracheal jet ventilation through a 12 or 14
gauge catheter, a driving pressure of 50 psi is required to ensure
sufficient gas flow into the lungs.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 1237.

Question 60

In what cases should an HME be avoided?

Answer 60

An HME should be avoided when a patient exhibits thick, copious
secretions which could enter the circuit and occlude the HME.
They should also not be used when there is a leak that prevents
the exhalation of gas through the HME such as a
bronchopleuralcutaneous fistula or when an uncuffed
endotracheal tube is used.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 211.

Question 61

Can you use two HME’s in a series to improve

humidification in an anesthesia circuit?

Answer 61

Yes. Using two HME’s in a series can improve humidification, but
care must be taken to monitor for the effects of increased
deadspace or work of breathing.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 211.

Question 62

How does the color of a gas cylinder correspond with

its contents in the US?

Answer 62

Oxygen cylinders are green, nitrous oxide cylinders are blue,
carbon dioxide cylinders are gray, air cylinders are yellow, nitrogen
cylinders are black, and helium cylinders are brown. These colors
can vary outside of the United States.
Dorsch JA, Dorsch SE. A Practical Approach to Anesthesia
Equipment. Philadelphia, PA: Lippincott Williams and Wilkins,
2011: 9.

Question 63

An E-cylinder oxygen tank pressure gauge reads 1000

psig. How much oxygen is left in the tank?

Answer 63

A full E-cylinder oxygen tank holds 660 liters at a pressure of 2000
psig. Because an oxygen tank’s pressure is proportional to its
contents, if the pressure falls by half, then the number of liters
falls by half as well. Using this ratio, the tank would have 330
liters left in it.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 11-12.

Question 64

What is the critical temperature of nitrous oxide in

degrees Celsius? What is the significance of this?

Answer 64

The critical temperature of nitrous oxide is 36.5 degrees Celsius.
This means that because its critical temperature is above room
temperature, it will remain in liquid form without a refridgeration
system.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 12.

Question 65

When the pressure gauge drops below 745 psig on a
nitrous oxide E-cylinder at room temperature, how
much nitrous oxide is left in the tank?

Answer 65

Because E-cylinder nitrous oxide tanks contain nitrous oxide in
both the liquid and gas state, the only accurate way to determine
the amount of gas left in the tank is by weighing it. A full Ecylinder
tank will hold 1590 liters of nitrous oxide. When the liquid
form is consumed and the tank pressure drops below 745 psig,
however, the amount of nitrous oxide in the gas phase is about
400 liters.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 12-13.

Question 66

What is Entonox?

Answer 66

Entonox is a 50:50 mixture of oxygen and nitrous oxide. It is most
commonly used for dental procedures, dressing changes, and
obstetric anesthesia.
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011: 13-14.

Question 67

What is heliox?

Answer 67

Heliox is a mixture of helium and oxygen. Helium is less dense
than oxygen and improves respiratory mechanics by increasing
laminar flow (a reduction in Reynolds number) through the airways
and decreasing the work of breathing. Heliox can alleviate the
symptoms of increased airway resistance, but doesn’t dilate the
airways like a bronchodilator.
Sandberg W, Urman RD, and Ehrenfield JM. The MGH Textbook
of Anesthetic Equipment. Philadelphia, PA: Elsevier; 2011: 14.