- Expired carbon dioxide:
- Circle system: Components of the circle system which is what is used
for the anesthesia machines are the following: inspiratory and expiratory
limbs, unidirectional valves, and a reservoir bag. Circle systems are not
really closed ones and they reuse oxygen and anesthetic agents but get rid
of the CO2. Circle breathing allows you to use low flows – usually less
than one liter. It is based on O2 consumption so that you provide just
enough O2 to meet consumption. Normal consumption is 250cc. Advantages to
this are that you can use low flows which decrease room pollution where the
external scavenging units are out of the building, decrease cost, decrease
heat and moisture loss, and you can supply a steadier amt of anesthetic
gases. The true circle system has no outside extraction of gases. You must
have CO2 or agent monitoring, should have no detectable leaks and is not
common to use. Most now are the semi-closed or semi-open circle systems.
- Methods of CO2 removal: there are 3 ways to remove it from the
anesthetic loop. The most often used way is by chemical absorption. Second is
by dilution with fresh gas and third is to use valves to separate inhaled vs
exhaled – one way valves in each direction. These valves are plastic
or metal pieces. The valve closing causes gas to change direction going in or
out. Ie. An ambu- squeeze it and the valve opens gas goes in, release the
pressure on the bag and the valve closes and gas goes out. The dilution
with fresh gas method will depend on the type of circuit and what type of
breathing is being done for the patient – controlled vs spontaneous. A
general rule to remember is that you need 2 times the pts minute ventilation
to clear the CO2. In the Bain circuit you should use 3 times the minute vent
and also for use with the t-piece. These amts will assure you that the patient
has cleared the CO2 and isn’t rebreathing. If you use a t-piece (avg min.
vent. For 70kg pt is 8-12L) You would need to have fresh gas flow at 16-24L of
flow. If the flow is 6-8L then they will rebreathe at a rate of 12. You could
shorten the T on the t-piece to reduce the dead space and this will decrease
the CO2 rebreathing. You may also want the pt to rebreathe more CO2 like COPD’ers
so could add length to the t-piece. Usually there is rebreathing since we
normally use flows of 6-8L/min
- Components of baralyme and soda lime: CO2 is a gaseous nonmetal oxide
that we produce. It forms carbonic acid in the presence of water. We use this
principle in r/t getting rid of CO2 in anesthesia by a reaction with soda lime
to produce a substance that has little effect of people. Soda lime and
baralyme are the most common CO2 absorbers.
- Soda lime is composed of 4% Na Hydroxide, 1% K Hydroxide, 80%Ca
Hydroxide, 0.2% silica, and 14-19% water.
- Baralyme is composed of 20% Barium Hydroxide and 80% Ca Hydroxide and
occas. a small amt of K Hydroxide. Both have a dye indicator
- The difference btw the two is that soda lime has silica which is added
to give it hardness which will decr the amt of dust so pts don’t breathe
it in since it is very irritating to the airways. Baralyme is inherently
hard and doesn’t need the silica. Baralyme also doesn’t contain any
added water and so would work better in a dry, low humidity place like
- Reactions within absorbers: Soda lime reacts with the CO2 and water to
make carbonic acid which reacts with NaOH and KOH which form carbonates, heat
and water. Baralyme reacts with water, CO2, and Ba(OH)2 to form carbonates,
heat and water. The inicators within the limes are ethylene violet. The pH
change of becoming saturated with CO2 will activate the dye as the more acidic
pH will turn the lime purple. The soda lime can also change back to white-gray
if you allow enough time to pass. This dye indicates that the water content
has been saturated and water is no longer available for the chemical reaction.
The amt of CO2 absorbed is 26L/100 gm. Both soda and baralyme have dual
chambers for safety so that you have a backup canister available. The dual
chamber allows 2-3 times the tidal volume and allows for more surface area –
not every breath has to do through the reactions. This gives you a reservoir.
The size and shape of the granules make a difference. The smaller the granule
size the more surface area but then this increases resistance. The standard
size is 4 –8 mesh which is a combo of small and larger pieces and has less
tendency to stick together. A thing called channeling can occur where the gas
follows the path of least resistance and channels up through the lime. This
can burn out that one area and the rest isn’t being used. Another fact to
recall is that the hardness # is >75. When it is pulverized it takes this #
to break up into an alkaline dust.
- Exothermic reaction: All reactions occurring in the CO2 canister are
- Storage and handling: The canisters can be taken off the machines
easily and disposed of in the biohazardous waste for the hospital. Care must
be taken when installing a new canister since the metal rings surrounding each
connection can cause a leak in the system.
- Reactivity of components and anesthetics: Soda lime is neither
intrinsically toxic nor toxic when exposed to common anesthetics. However when
using an uncommon anesthetic, trichloroethylene, toxicity may result. In the
presence of alkali and heat, trichlorothylene degrades into the neurotoxin
dichloroacetylene. Phosgene, a potent pulmonary irritant, is also produced.
These toxicities can cause cranial nerve lesions, encephalitis, and ARDS.
Sevoflurane has been shown to produce degradation products on interaction with
CO2 absorbents which are fluromethyl-2, 2-difluoro-1-vinyl ether, and Compound
A which can be lessened by using low flow or closed circuit anesthesia
techniques, use of baralyme instead of soda lime, higher concentrations of
sevo in the anesthesia circuit, higher absorbent temps, and fresh absorbent.
- Carbon monoxide poisoning: The carbon monoxide will bind with the hgb
and not allow O2 to bind. For pulse oximetry the carboxyhemoglobin is
interpreted by the pulse ox as a mixture of ~ 90%oxyhemoglobin and
10%deoxyhemoglobin. Thus at high levels of COHb, the pulse ox will
overestimate true SaO2. A treatment for COHb could be treatment in a
hyperbaric chamber. Here the partial pressure of O2 increases in blood while
the saturation of hgb approaches 100%. Above this level all additional
O2-carrying capacity of blood is from O2 dissolved in plasma. This allows for
the preservation of aerobic metabolism in the face of severe anemia or
- Types of breathing circuits
: This is a type of low flow anesthesia in
which the fresh gas inflow is equivalent to the uptake of anesthetic gases and
oxygen and the relief valve on the anesthesia circle is closed. This is the
system where the amt of gas flowing in will equal consumption of O2 and
anesthetic gases. You need an accurate flowmeter capable of registering flows
as low as 100ml/min. Use of a reliable oxygen analyzer and CO2 monitor is
essential. The system should be free of leaks. A vent with bellows that rise
during expiration will detect leaks more readily than one with bellows that
descend during expiration. Induction with low flow is difficult since the
excretion of nitrogen will dilute the gases present in the system and maximal
uptake of the volatile agent during this period makes dosage difficult to
predict, and inadequate or excessive concentrations could frequently develop
in the circle. To counteract these problems, denitrogenation and induction are
first accomplished with high flows. Disadvantages to the closed system are
that inspired concentrations cannot be quickly altered, danger of hypercarbia
is greater when low flows are used, more attention is required to constantly
adjust fresh gas flow, and accumulation of high concentrations of undesired
gases and vapors in the system may occur. To use this system you must
calculate the vapor pressures of the agents. Another point is that you can
still spontaneously breathe with the APL valve closed but this is like a PEEP
Mapleson and Bain circuits: The Mapleson D is the most often used
system. It is comprised of a t-piece with an expiratory limb. The fresh gas
inlet is located near the pt and expiratory valve is close to the reservoir bag.
During the expiratory phase of spontaneous ventilation, fresh gas and alveolar
gas flow down the expiratory limb. The exp valve opens as pressure increases in
the circuit, and a portion of this mixture is expelled. The pt receives a combo
of fresh gas and mixed gas from the tubing during the next inspiration. The
content of this inspired mixture is determined by the rate of fresh gas flow,
the pt’s tidal volume, and the duration of expiratory pause. A long expiratory
pause allows the fresh gas to move down the tubing and flush the alveolar gas. A
short pause provides inadequate time to flush the alveolar gas and allows
rebreathing to occur. Mapleson determined that a fresh gas flow greater than two
times the minute ventilation was enough to prevent rebreathing. During the
inspiratory phase of controlled ventilation, alveolar gas and dead space gas,
instead of fresh gas, are forced out of the expiratory valve. The Bain circuit
also called the Jackson-Reis is a modification of the Mapleson D system. It is a
coaxial circuit in which the fresh gas flows through a narrow inner tube within
the corrugated outer tubing. The central tube originates near the reservoir bag,
but the fresh gas actually enters the circuit at the pt end. Exhaled gases enter
the corrugated tubing and are vented through the expiratory valve near the
reservoir bag. This circuit can be used for both spont and controlled
ventilation. Advantages are that it is lightwt, convenient, easily sterilized,
and reusable. Scavenging of the gases from the expiratory valve is facilitated
because the valve is located away from the pt. Exhaled gases in the outer
reservoir tubing add warmth to inspired fresh gases. The hazards of the Bain
circuit include unrecognized disconnection or kinking of the inner fresh gas
- Closed system anesthesia
- Monitoring modalities
- Pulse oximetry: This is determined by the relative proportions of
oxygenated and deoxygenated hgb. Deoxyhemoglobin absorbs more light in the red
band with a wavelength of 660 while oxyhemoglobin absorbs more light in the
infrared band with a wavelength of 910nm. The pulse ox emits lights in the two
wavelengths. The machine picks up a pulsatile component- plethmography and a
ratio of red to infrared is gathered and the machine estimates an O2 Sat.
Pulse oxes are dependent on a pulsatile waveform so any conditions of low or
absent pulse amplitude will fail to give a reading or give an inaccurate one.
These situations could be cardiac arrest, bp cuff inflation, tourniquet
application, hypovolemia, hypothermia, vasoconstriction, or cardiac bypass.
The pulse ox is also very sensitive to movement artifact and electrocautery.
Hematocrits less than 10% may adversely affect the accuracy by underestimating
the sat. An injection of methylene blue can cause a transient drop in your sat
as can indocyanine green and indigo carmine. In a few deeply pigmented people,
the sat may not be able to detect the red light thus not give a reading.
- Beer-Lambert Law: This r/t amt of light picked up by the SaO2 monitor.
The amt of it absorbed by the oxyhemoglobin is proportional to the concentration
of oxyhemoglobin and the length of the pathway it travels.
- Oxyhemoglobin dissociation curve: You need to know that for a PaO2 value
of 90% will give you a pO2 of 60. A PaO2 of 50% will give you a pO2 of 27. The
curve can shift to the left or right and this will cause changes in the amt of
O2 taken up by the tissues or offloaded in the lungs.
- Right Shift- O2 is offloaded easier to tissues but isn’t picked up
well in the lungs. Causes are Acidosis, and increases in the following:
temp, CO2, levels of 2-3 DPG
- Left Shift - O2 is harder to unload to the tissues since the hgb has
a higher affinity for the O2 but is easily picked up by the lungs. Causes
are alkalosis and a decrease in the following: temp, CO2, levels of 2-3 DPG.
A left shift will need better pO2 in order to offload O2 to the tissues.
Abnormal hgb can cause a left shift such as fetal hgb, carboxyhgb or carbon
monoxide poisoning, and methemoglobinemia
- A Way to Remember this: Left has an "L" in it as does
alkalotic as does HOLD as in hgb holds on to the O2. All the other arrows
in left are going down. A right shift is opposite this!
: The measurement of respiratory CO2 has become a standard.
It is used to confirm ETT placement, follow the adequacy of ventilation, and
estimate the partial pressure of CO2. Capnography is the visual representation
of the relative or absolute concentration of CO2 in the sample. It uses
infrared absorption, mass spectrometry, which exposes the sample to a magnetic
field and colorimetric detection. The collection device determines the amt%
and can give you several different gases at the same time. In most cases the
expired CO2 end tidal closely follows the PaCO2 with gradients less than
5mmHg. The CO2 waveforms are important for clinical. Phase 1 is the area near
the baseline and means end inspiration. Phase 2 is the sharp upswing and means
beginning of expiration. Phase 3 is the long plateau across the top and means
expiration. Phase 4 is the sharp downswing and means the start of inspiration.
Every breath on anesthesia machines should return to normal on general ETT pts
unless you are using a nonrebreathing system. This will mean that the CO2 is
totally absorbed and you begin with zero. See class notes for diagrams of the
Sources of CO2 rebreathing: This has been discussed already but they do
include: low fresh gas flow in systems without CO2 absorption, in a circle
system if the valves are incompetent or absent or if a nonrebreathing valve
allows significant back leak of exhaled gases or is assembled inappropriately.
Low tidal volume, wasted ventilation due to distention of system components,
compression of gases in the system, inappropriate adjustment of the pop-off
valve, leaks in the anesthesia machine, and disconnections can all lead to
hypoventilation and result in hypoxia and hypercarbia. It is important to
therefore monitor vital signs, observe chest motion, and use a precordial or
esophageal stethoscope to detect inadequate ventilation. End-tidal CO2
monitoring, measurement of exhaled volumes, and blood gas analysis are
CO2 dissociation curve: CO2 is the end product of aerobic metabolism.
This curve tells us the relationship btw the total CO2 content of blood and the
partial pressure of CO2. CO2 elimination depends on pulmonary bloodflow and
alveolar ventilation. CO2 is transported in the blood in 2 ways; in the plasma
and on the RBC. In the plasma it comes in 3 forms which are 1. CO2 combines with
AA group of plasma proteins to form a carbamino group, 2. Remains in solution
– dissolves in small amt since it is slightly soluble. And 3. As carbonic acid
where it combines with water. The majority of the CO2 is carried on the RBC. A
large portion of that combines to form carb-amino hgb which facilitates the
release of O2 from hgb. The largest % of CO2 is transported as hydrated as
carbonic acid. Carbonic anhydrase is the enzyme that facilitates the reaction of
bicarb and H and allows the reaction to occur 10.000 times faster. The CO2 can
go onto the RBC and the enzyme will speed up the process of breakdown of CO2 to
bicarb and H. 99.9% of carbonic acid dissociates into bicarb and H even without
carbonic anhydrase. The Hamburg phenomenon is that bicarb has a negative charge
and will diffuse out of the cell and into plasma while chloride will diffuse
from plasma to the RBC. This chloride shift is the Hamburg Phenom done to
maintain electrical neutrality. The major difference with the CO2 dissociation
curve from the oxyhemoglobin one is that the CO2 curve is much more linear and
has a steeper slope. For a given change in partial pressure significantly more
CO2 can be carried in the blood compared with O2. A part pres change – small
in CO2 will give a larger difference. Ie. A partial pres of 50 vs 55. 55 can
hold a lot more CO2 so you would see a larger jump here than a change from 50
–55 on the O2 curve. The O2 curve will affect the CO2 curve. The lower the sat
of hgb for O2, the larger the CO2 content for a given pCO2. Haldane effect is
that the deoxygenated blood (mixed venous) has a greater capacity to carry CO2
than does oxygenated blood. The slope of the CO2 curve is 3 times steeper than
the O2 one. Deoxygenated blood can carry more CO2 but also has a higher affinity
for H ions. If you didn’t have the Haldane effect the tissue pCO2 would have
to rise to 51mmHg to load the same CO2 found in mixed venous blood of 46mmHg.
You can offload CO2 at the tissues since the deoxygenated blood allows CO2 to
come out of the tissue easier and load up on the RBC’s. O2 gets onloaded in
the lungs. There is an affinity for O2 so CO2 goes off and O2 goes on.
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