Vaporizers

Question 1

Describe basic characteristics of Sevoflurane.

Answer 1

-Ultane or “Sevo”
-rapid induction and awakening
-rapid elimination results in quick awakening (2nd most
rapid)
**reacts with absorbent to form Compound A which is
nephrotoxic
*used for peds inhalation inductions

Question 2

Describe basic characteristics of Desflurane.

Answer 2

-Suprane or “Des”
*requires specially designed electrically heated vaporizer
*extreme airway irritant (no inhalation inductions, use
with IV agents like propofol and give after pt. asleep
*cause tachycardia (caution with CAD pts.) (if given too
quickly, HR can reach >150s)
-rapid elimination, very quick emergence (most rapid)

Question 3

Describe basic characteristics of Isoflurane.

Answer 3

-Forane
-once considered “gold standard”, esp. for neuro surgery
*can cause coronary artery steal syndrome
*significant peripheral vasodilation leads to hypotension
(more than other agents)

Question 4

Describe basic characteristics of Halothane.

Answer 4

-historically, was agent of choice for inhalation (rare now)
*potent bronchodilator (good for pts. with reactive airway
disease like asthma)
*may cause halothane hepatitis
**sensitizes the myocardium to dysrhythmias esp. when
combined with catecholamines and/or epi in local
anesthetics ( i.e. stimulated when placing ETT)
*emergence significantly slower compared to others

Question 5

Describe basic characteristics of nitrous oxide.

Answer 5

*** NOT A VOLATILE AGENT
-used to supplement inhalation agents (can replace air)
-only anesthetic gas with sympathomimetic activity
(like small dose of atropine; differs from des)
**should not exceed 70% (with 30% O2; with fire risk
cases when you want to use less O2, can increase air)
**diffuses into air filled spaces, increasing pressure
(colon, stomach, eye, ETT cuff, anywhere with air)
**100% O2 should be given during emergence to avoid
diffusion hypoxia

Question 6

what is a vaporizer?

Answer 6

device for converting a volatile liquid agent into a breathable vapor

Question 7

describe vaporizers?

Answer 7

• concentration calibrated dials
• placed b/w flow meters and CGO
• all current vaporizers are agent specific
• interlocking and exclusion safety mechanisms

Question 8

define vapor.

Answer 8

gaseous molecules from substance that is liquid at room temp at 1 atm (760 mmHg) pressure

Question 9

describe what affects liquid vaporization.

Answer 9

-different liquids vaporize at different rates
-increased temp, increases vaporization
-increased vaporization, decreases temp of the liquid
below due to latent heat of vaporization (if no heat
source)
-decrease liquid temp, decreases vaporization

Question 10

what is vapor pressure?

Answer 10

pressure of the vapor above the liquid or solid in a close container

Question 11

what is saturated vapor pressure?

Answer 11

equilibrium pressure of a vapor above its liquid or solid at that temperature
**what we want

Question 12

what affects vapor pressure?

Answer 12

• dependent only on the agent (and its characteristics) and temperature (NOT atm pressure)
• *temp is compensated for in vaporizer
• *increased temp, increased vapor pressure

Question 13

define boiling point.

Answer 13

temperature at which vapor pressure equals atm pressure

• lower atm pressure (higher altitudes), lower boiling pt.
• Des has lowest boiling point

Question 14

define latent heat of vaporization.

Answer 14

calories needed to convert 1g of liquid vapor without temperature change in the remaining liquid

• pulling heat from the liquid
• *temp of remaining liquid will drop as vaporization proceeds, lowering vapor pressure, unless prevented

Question 15

define specific heat.

Answer 15

calories needed to increase the temperature of 1g of a substance by 1 degree C
*vaporizers constructed with high specific heat materials
(copper) to minimize temp changes associated with
vaporization

Question 16

describe MAC.

Answer 16

-minimal alveolar concentration
-concentration that prevents movement in 50% of
patients in response to surgical stimuli
*mirrors partial pressure in the brain
*used to compare agent potencies
-used for experimental standard
*median value only
*at 1 atm pressure
*MAC values are additive
(0.5 MAC N2O [53%] + 0.5 MAC Sevo [1%] = 1 MAC)

Question 17

describe copper kettle (vernitrol) vaporizers.

Answer 17

-old technology (seen outside US, military)
*measured flow: operator controls flow bubbled through
agent from 2nd O2 flowmeter); amount of gas (O2)
going into vaporizer determines
what comes out
-outside of breathing system
-formula chart used
*manual temperature compensation
-copper has high specific heat and high thermal conductivity to preserve temp

Question 18

approx. how much vapor is one ml of agent?

Answer 18

200 ml of vapor

Question 19

why must saturated gas from copper kettle be diluted?

Answer 19

the vapor pressure (VP) of a volatile agent is greater than the partial pressure needed for anesthesia
-saturated gas must be diluted

Question 20

give an example of dilution using forane.

Answer 20

-VP of forane at 20 degrees C is 240
-concentration leaving the kettle is 240/760 (atm) or 32% (remember MAC is only 1.2%)
-100 ml O2 enters and leaves with 50 ml saturated
forane = 150 ml
-to deliver 1% forane, 100 ml from kettle must be diluted with 5000ml (5 L) FG
**FGF set at 5 L/min and temp remains stable, every 100
ml increase in O2 bubbled through liquid Forane in the
kettle increases anesthetic concentration 1%

Question 21

describe variable bypass vaporizers.

Answer 21

-amount of FG allowed to come in contact with liquid vapor is controlled
*what doesn’t come in contact “bypasses”
-gas entering the vaporizing chamber is “carrier gas”
-carrier gas/total gas = splitting ratio
-splitting ratio determined by internal resistance to flow
(controlled by vaporizer dial)
*wicks in vaporizing chamber cause full saturation (giving
a known concentration)
*saturated gas combines with FG at vaporizer outlet to
produce final desired concentration
**amt controlled by dial not gas flows
**temp compensated

Question 22

how is temp compensated in variable bypass vaporizers?

Answer 22

• two different metals welded together, each responding differently to temperature changing (bimetallic strip)
• temp changes causes strip bending (different ways for each strip) to either increase or decrease gas passing through the vaporizer

Question 23

Describe the Des vaporization.

Answer 23

-require electronic vaporizer
-Des almost boils at room temp (VP 681, 1 atm)
-Des vaporization requires cooling which would overwhelm conventional vaporizer (unable to maintain constant temp)
-with a conventional vaporizer, high volatility requires massive FGF to dilute carrier gas to usable concentration
**if Des heated: provides heat to the bottom to
compensate for massive latent heat of vaporization
(heat lost)

Question 24

describe Des vaporization.

Answer 24

*compensates for flow and atm; cannot compensate for
temp (compensate for temp when heated)
*no FGF enters Des vaporizer
-Tec 6
*require electric power and 10 min warm-up if cold
*can be filled during agent delivery (variable bypass
must be turned off)
*tilting can cause “No output” and shut off (tilting
allowing liquid above wicks can cause extra Des output
d/t low boiling pt.

Question 25

describe the electronic (Aladin cassette) vaporizer

Answer 25

-one central electronic control mechanism for all agents
-cassettes inserted into a port of anesthesia machine
-cassettes not being used stored in non-electronic ports
*no tilt risk
*hold less liquid agent (must be filled more often)
*heats Des with internal mechanisms only triggered by
Des cassette (heats quickly)

Question 26

what if Sevo (VP 157) vaporizer was filled with Halothane (VP 243)?
what if Sevo were placed in a Halothane vaporizer?

Answer 26

• too much Halothane will be given

- if reversed, too little Sevo will be given

Question 27

what are some safety mechanisms of vaporizers?

Answer 27

-color-coding (although not FDA required)
 Sevo-yellow; Forane-purple; Des-blue
-keyed fillers
-low filling ports
-secured vaporizers (minimizes tipping)
-concentration dial increases output in all when rotated
 counterclockwise 
*interlocks

Question 28

describe the interlock safety mechanism of vaporizers.

Answer 28

-ensures that only one vaporizer is turned on
-gas enters only the one which is on
-trace vapor output is minimized when the vaporizer is off
-vaporizers are locked into the gas circuit; ensures they
are seated correctly

Question 29

what are some risks of vaporizers?

Answer 29

• tipping: tilting > 45 degree vertical may need flushing and/or recalibrating to prevent OD
• out of calibration: breath to breath analysis should not be substituted for preventive maintenance
• leaks: negative pressure check should be done during daily machine check
• pumping: back pressure into vaporizer causes increased concentration into circuit; one way valve prevents in newer machines
• electronic dysfunction (Tec 6, Aladin)