Inhaled Anesthetics: Delivery Systems

Question 1

The gas supply system of an anesthesia workstation can be divided into three sections: high-pressure, intermediate-pressure, and low-pressure.

The only high-pressure elements in the anesthesia machine are …

The intermediate pressure section starts from…

The low-pressure section begins at the …

Answer 1

the auxiliary gas tanks (E-cylinders) on the back of the anesthesia machine

the pipelines or from the stepped-down input from the E-cylinders, and extends up to the flowmeter control valves

flowmeter control valves, includes the flowmeters and anesthetic vaporizer, and ends at the fresh gas outlet

Question 2

System designed to prevent the misconnection of hospital gas supply lines to the anesthesia workstation

Answer 2

Diameter Index Safety System (DISS)

Question 3

System designed to prevent incorrect gas cylinder connections in the anesthesia workstation

Answer 3

Pin Index Safety System (PISS)

Question 4

Checking the E-cylinders is part of an automatic machine checkout

T or F

Answer 4

F

Checking the E-cylinders is not part of an automatic machine checkout. The practitioner must manually open each cylinder and check the pressure gauges on the front of the machine. In the case of a two-tank oxygen manifold, the tanks must be serially opened and checked

Question 5

Why is important keep the auxiliary E-cylinders closed during normal operation using pipeline gases?

Answer 5

It is imperative to keep the auxiliary E-cylinders closed during normal operation using pipeline gases because of the possibility of small leaks in the high-pressure system, or fluctuations in pipeline pressures allowing flow from the cylinder to be activated. An open oxygen cylinder may allow the anesthesiologist to be unaware of catastrophic pipeline failure. When the oxygen cylinder is closed, the immediate result of oxygen pipeline failure is a low oxygen pressure alarm. The auxiliary E-cylinder can then be opened, ensuring continued flow of oxygen to the patient while troubleshooting occurs

Question 6

The characteristics of a gas that influence its flow rate through a given constriction are … (laminar flow) and … (turbulent flow)

Answer 6

viscosity

density

Question 7

Which flowmeter sequence is better to reduce the chance of delivering a hypoxic mixture in case of a gas leak?

Answer 7

A safer configuration exists when oxygen is located in the downstream position

• It is important to remember that in the case of a leak in the oxygen flow tube, a hypoxic mixture may result even when oxygen is located in the downstream position

Question 8

The ideal gas law is

Answer 8

PV = nRT

R (the universal gas constant)= 8.314 L kPa/mol K or 62.364 L mm Hg/molK

n: number of molecules or moles

Question 9

Dalton’s Law of Partial Pressures

Answer 9

Ptotal =P1 +P2 +P3 + …

• combining this whit the ideal gas law: PA = (nA/ntotal) Ptotal = (v/v%) Ptotal

Question 10

Describe the saturated vapor pressure, or simply vapor pressure

Answer 10

Volatile liquids, such as inhaled anesthetic agents, are characterized by a high propensity to enter the gas phase, or vaporize. When a volatile liquid is exposed to air or other gases, molecules at the liquid surface with sufficient kinetic energy escape and enter the vapor phase. This process is known as evaporation, which is purely a surface phenomenon (in contrast to boiling, which occurs throughout the liquid). If liquid volatile anesthetic is placed within a contained space, such as a vaporizer, molecules will escape into the vapor phase until the rate of evaporation equals the rate of return to the liquid phase (a process known as condensation). When this equilibrium is reached, the gas above the liquid is said to be “saturated” with anesthetic. The anesthetic molecules in the gas phase create a partial pressure known as the saturated vapor pressure, or simply vapor pressure. Liquids with a greater tendency to evaporate and generate higher vapor pressures are described as “more volatile

Question 11

Vapor pressure is affected by changes in atmospheric pressure

T or F

Answer 11

F

Vapor pressure is NOT affected by changes in atmospheric pressure

Question 12

Describe the Avogadro Hypothesis

Answer 12

The volume that an ideal gas occupies at a given temperature and pressure is related to the number of molecules of gas present, but not the size or identity of the molecules

Question 13

Explain the latent heat of vaporization and how it affects the volatile anesthetics delivery

Answer 13

When a liquid such as a volatile anesthetic evaporates into the gas phase, energy is required to overcome the attractive intermolecular forces between molecules in the liquid phase (a property known as cohesion). The needed energy is absorbed from the surroundings in the form of heat, and is the reason why the human body is cooled by the evaporation of sweat. The amount of energy absorbed by a specific liquid during evaporation is referred to as the latent heat of vaporiza- tion. It is more precisely defined as the amount of energy in joules or calories (1 calorie = 4.184 joules) required to change 1 g of liquid into vapor at a constant temperature.

In a well-insulated container, the energy for vaporization must come from the liquid itself. In the absence of an outside heat source, the remaining liquid cools as vaporization progresses. This leads to significant reductions in vapor pressure and therefore the number of volatile anesthetic molecules in the gas phase. If vaporizer design does not mitigate and compensate for evaporative cooling, output will decrease.

Question 14

Explain the boiling point

Answer 14

The boiling point of a liquid is defined as the temperature at which vapor pressure equals atmospheric pressure and the liquid begins to undergo rapid vaporization. From the definition above, it is important to note that the boiling point changes depending on atmospheric pressure. Whereas evaporation is a surface phenomenon, boiling is a bulk phenomenon that occurs throughout the interior of the liquid

Question 15

Explain the specific heat

Answer 15

The specific heat is the amount of energy required to increase the temperature of 1 g of a substance by 1°C.

Question 16

Vaporizers are first designated as in-circuit or out-of-circuit, which describes their relationship to the patient’s breathing circuit. Virtually all modern vaporizers are …

Answer 16

out-of-circuit, and their controlled output is introduced into the breathing circuit through a fresh gas line

Question 17

The second designation of vaporizers involves the specific types of vaporizers, and these currently include the …

Answer 17

1) variable bypass vaporizer (e.g., GE/Datex-Ohmeda Tec 7)

2) the dual- circuit vaporizer (e.g., the classic GE/Datex-Ohmeda Tec 6 desflurane vaporizer)

3) the cassette vaporizer (e.g., GE/ Datex-Ohmeda Aladin cassette)

4) the injection vaporizer (e.g., the Maquet vaporizer)

5) and the now historical measured-flow vaporizer (e.g., Copper Kettle).

Question 18

How does a variable bypass vaporizer function?

Answer 18

Selecting a vaporizer output (turning the vaporizer “on”) diverts an agent-specific ratio of gas through the pressure-compensating labyrinth, into the vaporizing chamber where it becomes saturated with anesthetic vapor, and then past the concentration cone where it reunites with the fresh gas stream. The temperature compensation device further adjusts the ratio of bypass to vaporizing chamber flow, to compensate for changes in anesthetic vapor pressure resulting from temperature changes. As the liquid anesthetic cools by evaporation, more gas is diverted to the vaporizing chamber to compensate for the decrease in vapor pressure. The labyrinth compensates for pressure fluctuations within the vaporizer from the gas supply side and the breathing circuit side to stabilize vaporizer output; it is not present to compensate for changes in atmospheric pressure.

Question 19

Describe the impact of the fresh gas flow in the variable bypass vaporizer

Answer 19

This factor is notable only at the extremes of flow rates and at higher concentration control dial settings.

At low flow rates (<250 mL/min), the output tends to be slightly less than the dial setting due to the relatively high density of volatile anesthetic agents. Insufficient turbulence is generated in the vaporizing chamber to advance the vapor molecules upward.

At high flow rates (such as 15 L/min), the output of most variable bypass vaporizers is somewhat less than the dial setting. This discrepancy is due to: cooling during rapid evaporation, incomplete mixing, and failure to saturate the carrier gas in the vaporizing chamber. In addition, the resistance characteristics of the bypass chamber and the vaporizing chamber can vary as flow increases

Question 20

Describe the impact of carrier gas composition when a variable bypass vaporizer is used

Answer 20

Variable bypass vaporizer output can be influenced by fresh gas composition. This phenomenon is the result of differences in the solubility of carrier gases in volatile anesthetic liquids. This effect is most pronounced when nitrous oxide is introduced or removed as a carrier gas.

In a experimental example, a change in carrier gas from 100% oxygen to 100% nitrous oxide results in a sudden decrease in halothane output (expressed as volume percent) followed by a slow increase to a new, lower, steady-state.

Because nitrous oxide is more soluble than oxygen in the liquid anesthetic within the vaporizer sump, more of the carrier gas dissolves, and the volume output from the vaporizing chamber is transiently reduced.
Once the anesthetic liquid becomes saturated with nitrous oxide, the vaporizing chamber output increases and achieves a new steady state.

The explanation for the new steady-state output value is less well understood. Differences in density and viscosity between oxygen and nitrous oxide are likely responsible because these physical properties affect the relative amount of gas flow through the bypass and vaporizing channels

Question 21

How higher altitude affect the function of an variable bypass vaporizer

Answer 21

Vapor pressure is independent of barometric pressure. Therefore as altitude increases and barometric pressure declines, the partial pressure of anesthetic agent in the vaporizing chamber remains constant despite decreases in the partial pressures of other constituent breathing gases and the total ambient pressure. This situation results in significantly increased volume percent concentration of anesthetic agent within the vaporizing chamber and at the outlet of the vaporizer. However, because anesthetic depth is determined by the partial pressure of volatile agent in the brain, the clinical impact is minor

The minimal alveolar partial pressure (MAPP) at altitude is the same as at sea level because it is a partial pressure, whereas the MAC increases because it is a simple concentration. The partial pressure output of a variable bypass vaporizer changes proportionally less than the volume percent concentration as altitude increases. Because the partial pressure of volatile agent determines anesthetic depth, the operator does not need to adjust the dial to a higher setting to compensate for barometric pressure. This holds true for variable bypass vaporizers, but not for the desflurane Tec 6–style vaporizer

Question 22

How do hyperbaric condictions affect the variable bypass vaporizer delivery

Answer 22

Under hyperbaric conditions, the partial pressure of volatile anesthetic in the vaporizing chamber remains constant despite an increase in ambient pressure and the partial pressure of the other gases.
The net theoretical effects on variable bypass vaporizers are a significant decrease in anesthetic concentration (v/v%) and a mild decrease in partial pressure output.
However, the partial pressure of halothane was noted to increase slightly with increasing barometric pressure under experimental conditions. Possible explanations for this finding include the effect of increased atmospheric gas density on the flow of gas through the vaporizer and the increased thermal conductivity of air at higher pressure.
The clinical significance of these small changes in partial pressure output under hyperbaric conditions is unclear

Question 23

The Tec 6 desflurane vaporizer was the first clinically available
vaporizer to be electrically heated and pressurized. How does it work?

Answer 23

The vaporizer has two independent gas circuits arranged in parallel. Fresh gas from the flowmeters enters at the fresh gas inlet, passes through a fixed restrictor (R1), and exits at the vaporizer gas outlet. The vapor circuit originates at the desflurane sump, which is a reservoir of desflurane vapor. It is electrically heated to 39°C, a temperature much higher than desflurane’s boiling point. At 39°C, the vapor pressure in the sump is approximately 1300 mm Hg (∼2 atm). Just downstream from the sump is the shut-off valve. After the
vaporizer warms up, the shut-off valve fully opens when the
concentration control valve is turned to the “on” position.
A pressure-regulating valve located downstream from the shut-off valve down regulates the pressure to the pressure of the background gas.
The operator controls the output of desflurane by adjusting the concentration control valve (R2), which is a variable restrictor

Question 24

How does higher altitude affect the Tec 6 desflurane vaporizer
output?

Answer 24

The Tec 6 device is more accurately described as a dual-gas blender than a vaporizer.
Regardless of ambient pressure, the Tec 6 will maintain a constant volume percent output (v/v%), not a constant partial pressure.
This means that at high altitudes, the partial pressure of desflurane will decrease in proportion to the reduction in atmospheric pressure divided by the calibration pressure (normally 760 mm Hg) per the following formula:

Required dial setting (%) = Normal dial setting × (760 mm Hg) / [Ambient pressure (mm Hg)]

Question 25

How does the carrier gas composition affect the Tec 6 desflurane vaporizer output?

Answer 25

Vaporizer output most closely matches the dial setting when oxygen is the carrier gas because the Tec 6 vaporizer is calibrated with 100% oxygen.
When a carrier gas other than 100% oxygen is used at low flow rates, a clear trend toward reduction in vaporizer output emerges. This reduction correlates with the decrease in viscosity of the carrier gas. Nitrous oxide is less viscous than oxygen, and generates lower backpressure upstream of resistor R1. As a result, the working pressure is reduced. At low flow rates with nitrous oxide as the carrier gas, vaporizer output is approximately 20% less than the dial setting.

Question 26

How do the GE/Datex-Ohmeda Aladin and Aladin2 Cassette Vaporizers work?

Answer 26

The Aladin cassette vaporizing system is best described, during most circumstances, as a computer-controlled variable bypass vaporizer.

It consists of a bypass section and vaporizing chamber, the latter of which is contained within the interchangeable cassett.

A fixed restrictor located in the bypass chamber causes gas flow from the inlet to split into two streams. One stream passes through the bypass chamber, and the other is diverted to the vaporizing chamber where it passes through a one-way check valve. This valve prevents retrograde flow of anesthetic vapor into the bypass chamber, and its presence is unique to the Aladin system. The one-way check valve is essential for precise delivery of desflurane (see below). Failure of the check valve to close can result in anesthetic overdose due to retrograde flow into the bypass chamber

Within the cassette, anesthetic agent vaporizes freely to saturated vapor pressure. A flow control valve, modulated by a central processing unit (CPU), precisely meters the amount of gas flow through the vaporizing chamber, which then rejoins the bypass flow. The CPU receives input from multiple sources: the concentration control dial, pressure and temperature sensors inside the vaporizing chamber, and flow sensors in the bypass and vaporizing chambers.

The CPU also receives input from the flowmeters regarding the carrier gas composition, which can affect vaporizer output as described above. Using these data, the CPU precisely regulates fresh gas flow through the vaporizing chamber to obtain the desired vapor concentration output

Question 27

How do the GE/Datex-Ohmeda Aladin and Aladin2 Cassette Vaporizers function when desflurane is used?

Answer 27

When vaporizing chamber pressure exceeds that in the bypass chamber, the one-way check valve closes and prevents carrier gas from entering the cassette. Carrier gas passes straight through the bypass chamber and its flow sensor. Under these conditions, the CPU adjusts the flow control valve to meter in the appropriate flow of pure desflurane vapor needed to achieve the desired final concentration. The vaporizer then begins functioning as an injector,
as opposed to resembling a variable bypass unit.

Question 28

To prevent rebreathing of CO2 in a traditional circle system, three rules must be followed: …

Answer 28

(1) a unidirectional valve must be located between the patient and the reservoir bag on both the inspiratory and expiratory limbs;

(2) the fresh gas inflow cannot enter the circuit between the expiratory valve and the patient;

(3) the APL valve cannot be located between the patient and the inspiratory valve

Question 29

Why does calcium hydroxide [Ca(OH)2] can’t be the unique base used in an CO2 absorbent?

Answer 29

Because CO2 does not react quickly with Ca(OH)2, water and small amounts of stronger base catalysts are required to speed up the reaction

Question 30

Which are the main degradation products of volatile anesthetics formed in the CO2 absorbents?

Answer 30

Today, the main degradation products of concern are compound A, associated with the use of sevoflurane, and carbon monoxide (CO), mainly associated with the use of desflurane, enflurane, and isoflurane. Other degradation products include formaldehyde and methanol

Question 31

Several physical factors may predispose to higher concentrations of compound A in the breathing circuit, including …

Answer 31

□ Low-flow or closed-circuit anesthetic techniques
□ Higher concentrations of sevoflurane
□ Type of absorbent (KOH or NaOH-containing)
□ Higher absorbent temperatures
□ Fresh absorbent

Question 32

LiOH-based absorbents and newer Ca(OH)2-based absorbents that are free of KOH and NaOH generate … amounts of compound A

Answer 32

zero or negligible

Question 33

Several factors increase the production of CO and risk of carboxyhemoglobinemia, describe then

Answer 33

□ Inhaled anesthetic used (for a given MAC multiple, the magnitude of carbon monoxide production is desflurane ≥ enflurane > isoflurane > > halothane = sevoflurane)
□ Degree of desiccation of the absorbent
□ Type of absorbent (KOH or NaOH-containing)
□ Higher temperature
□ Higher concentrations of anesthetic
□ Low fresh gas flow rates
□ Smaller patient size

Question 34

LiOH absorbent produces essentially no CO and maintains excellent CO2 absorption

T or F

Answer 34

T

Question 35

One extremely rare but potentially life-threatening complication related to CO2 absorbent is the development of extreme exothermic reactions that lead to fires and explosions. Specifically, this seems to occur when …

Answer 35

desiccated strong base absorbents (particularly Baralyme) interact with sevoflurane. Under experimental conditions, desiccated Baralyme absorbers exceeded 200°C (392°F) and higher, and fire was noted in some of the breathing circuits. The buildup of very high temperatures, flammable degradation products (formaldehyde, methanol, and formic acid), and oxygen or nitrous oxide-rich gases within the absorber provide all the ingredients necessary for combustion

Question 36

Explain how the color indicator of conventional absorbers demonstrate that the absorptive capacity of the material has been depleted

Answer 36

Conventional absorbents contain an indicator dye, ethyl violet, that allows anesthesia personnel to visually assess the functional integrity of the absorbent. Ethyl violet is a substituted triphenylmethane dye that undergoes a color change around pH 10.3. When the absorbent is fresh, the pH exceeds 10.3 and the dye is colorless. As the absorbent becomes exhausted, the pH drops below 10.3 and the dye becomes purple. The color change indicates that the absorptive capacity of the material has been depleted

Question 37

Describe de Mapleson A

Answer 37

The Mapleson A, also known as the “Magill circuit,” has a spring-loaded pop-off valve located near the facemask. It is the only Mapleson circuit where fresh gas flow enters from the end of the circuit opposite the patient (in this case, near the reservoir bag).

Question 38

Describe the Mapleson B and C

Answer 38

In the B and C systems, both the pop-off valve and fresh gas inlet tubing are located near the patient. The Mapleson C is known as the “Waters to-and-fro” circuit and the only differs from B because it lacks a corrugated tube

Question 39

Describe the Mapleson D, E and F

Answer 39

in the Mapleson D, E, and F, or “T-piece” group, fresh gas enters near the patient, and excess gas is vented off at the opposite end of the circuit.

The Mapleson D have a APL valve near the reservoir bag.

The Mapleson E doesn’t have a bag and an APL valve

The. Apples on F doesn’t have an APL valve

The Mapleson F circuit is known as the “Jackson-Rees” modification of the Mapleson E (also known as “Ayre’s T-piece”)

Question 40

Ventilation considerations of the Mapleson A system

Answer 40

Of all the circuits, only the Mapleson A has markedly different performance when used for spontaneous versus controlled ventilation. During spontaneous ventilation, exhaled alveolar gas is vented through the pop-off during the expiratory phase. With the next inspiration, the patient primarily draws in fresh gas (and a small amount of dead space gas). Thus the Mapleson A has the best efficiency of the six systems for spontaneous breathing. A fresh gas inflow rate of greater than or equal to minute ventilation is sufficient to prevent rebreathing of CO2.

However, the Mapleson A has the worst efficiency during controlled ventilation. As the reservoir bag is squeezed to initiate inspiration, exhaled alveolar gas first flows into the patient. The pop-off valve then opens and vents significant amounts of the fresh gas stream away from the patient during the inspiratory phase. Significant rebreathing of CO2 occurs unless minute ventilation is very high (>20 L/min). The key factor determining Mapleson A performance is the timing when the pop-off valve opens: during expiration for spontaneous ventilation, and during inspiration for controlled ventilation

Question 41

The relative efficiency of different Mapleson systems with respect to prevention of rebreathing are: … during spontaneous ventilation, and … during controlled ventilation

Answer 41

A > DFE > CB

DFE > BC > A

Question 42

The “T-piece” systems DEF are slightly more efficient than systems BC. To prevent rebreathing CO2, the DEF systems require a fresh gas inflow rate of approximately … times minute ventilation, whereas the fresh gas inflow rates required for BC systems are some- what higher

Answer 42

2 to 2.5

Question 43

Explain the Bain system

Answer 43

The Bain circuit is a coaxial circuit and a modification of the Mapleson D system. Fresh gas flows through a narrow inner tube nested within the outer corrugated hose. The central fresh gas tubing enters the corrugated hose near the reservoir bag, but the fresh gas actually empties into the circuit at the patient’s end. Exhaled gases pass down the corrugated hose, around the central tubing, and are vented through the pop-off valve near the reservoir bag.

The main hazards related to use of the Bain circuit are an unrecognized disconnection or kinking of the inner fresh gas hose

Question 44

What is the difference between ascending bellows ventilators and descending bellows ventilators? Which one is safer?

Answer 44

Bellows-type ventilators can be classified according to the direction that they move during patient exhalation.

Ascending bellows rise with exhalation, and descending bellows fall with exhalation.

Older pneumatic ventilators and some newer anesthesia workstations use weighted descending bellows, but most contemporary bellows ventilators employ an ascending bellows design.
Of the two configurations, the ascending bellows is considered safer. An ascending bellows will not fill if total disconnection occurs, or it may only partially fill if a circuit leak exceeds the fresh gas flow rate, providing an important visual cue for a circuit disconnect or leak. The bellows of a descending bellows ventilator, on the other hand, will continue its regular upward and downward movement despite patient disconnection: the drive gas pushes the bellows upward during the inspiratory phase, and during the expiratory phase the bellows “fills” with entrained room air instead of the patient’s exhaled gas, because of the weighted bellows. The pressure monitor and the volume monitor may be fooled even if disconnection is complete

Question 45

On workstations without a fresh gas decoupling feature, inappropriate activation of the oxygen flush valve during
the inspiratory phase of mechanical ventilation can …

Answer 45

add a substantial amount of volume to the circuit and can result
in baroand/or volutrauma because excess pressure and volume may not be able to be vented from the breathing circuit