EKG, CO2 Elimination, ETCO2, SpO2, & StO2 Measurement Flashcards
Einthoven was responsible for
making the EKG more sensitive
The ECG is responsible for
creating a potential difference across the heart; it picks up the electrical activities of our cells
Where is lead II at?
60 degrees
Where is lead III at?
120 degrees
Where is lead I at?
0 degrees
Where is lead aVL at?
-30 degrees
Where is lead aVr at?
-150 degrees
Where is lead aVf at?
90 degrees
What is the view of chest leads?
transverse view of the heart
Leads V1 and V2 are said to be
anteroseptal
Leads V3 and V4 are said to be
anteroapical
Leads V5 and V6 are said to be
anterolateral
What are the lateral leads and where does the problem then lie?
Leads I, aVL, V5, and V6
problem with the left circumflex artery
What are the inferior leads and where does the problem then lie?
Leads II, III, and aVF
problem with the right coronary artery
What are the anterior leads and where does the problem then lie?
V1, V2, V3, and V4
left anterior descending artery
The Beer Lambert law says
more light is absorbed or absorbance increases as solute concentration increases
the amount of energy absorbed or transmitted by a solution is proportional to the solution’s molar absorptivity and the concentration of a solute
- a more concentrated solution absorbs more light than a more dilute solution
The relationship between light and absorbance
the amount of light that is transmitted is inversely proportional to the absorbance
-the more light that is transmitted, the less concentrated the solution, so the less that has been absorbed
The relationship between transmission and absorbance
transmission is inversely proportional to absorbance
if all light passes through a sample, none was absorbed so the absorbance would be zero and the transmission would be 100%. the solution would be pure solvent
Absorbance is
a measure of the quantity of light absorbed by a sample
Application of the Beer-Lambert law to pulse oximetry
the sensor shines two different wavelengths to determine how oxygenated and deoxygenated the blood is
we get a proportion of how much is oxygenated and how much is deoxygenated
What substances act as CO2 scrubbers?
Soda lime and calcium hydroxide lime
both are bases and will neutralize CO2 which is a volatile acid
At what volume are we completely reliant on the scrubbers to prevent CO2 rebreathing?
0.3-0.5 L/min
At what volume are not utilizing the scrubbers at all?
4-5 L
What are the end products of the scrubber?
carbonates, H20, and heat (exothermic reaction)
The scrubber is made up of
small and large granules and different size mesh which provides the maximum surface area without too much resistance
water makes our reaction work so we need moisture to prevent drying
The chemical reaction for soda lime is:
CO2 + H2O–> H2CO3
H2CO3+ NaOH–> Na2CO3 + H20 + heat (exothermic)
Na2CO3 + Ca(OH)2–>.CaCO3 + NaOH
The chemical reaction for calcium hydroxide is:
Ca(OH)2 + CO2–> CaCO3 + H20
compare soda lime and calcium hydroxide
both contain CaOH2 & H2O in similar percentages
soda lime also contains NaOH
Calcium hydroxide contains CaSO4 and CaCl2
What does the indicator dye tell us?
turns purple when pH <10.3
for calcium hydroxide the color indicator responds to
changes in granular hydration, not alkalization
Reverts to white when not in use or when moisture is removed
How long does the scrubber last?
each cannister lasts 8-10 hours assumes total rebreathing and no channeling
every 100 g of granules absorbs 15 L
so 1 kg is 150L of CO2
On average adults produce 15 L/hr of CO2
Describe capnometry, capnometer, and capnograph
Capnometry measures CO2 in respiratory gases
Capnometer is the device that measure the CO2
Capnograph is the graph that shows the CO2 measure
What are the air spaces within the canister?
inter-granular air space- increased surface area
intra-granular air space- porous
How to prevent dessication and CO formation:
use low gas flows
Change absorbent at least weekly
Shut off all flow meters when not in use
What are the two types of CO2 measurement in end-tidal?
mainstream: in line measurement/no gas removed
sidestream: aspiration of the respiratory gas/disposal
If we have a patient such as a neonate who is requiring a low TV, then we must turn down the volume to be sampled in the sidestream method
What are the measurement techniques of end tidal?
Infrared light absorption
-CO2 strongly absorbs infrared light at a
wavelength= 4300 nm
Mass spectrometry= utilizes CO2 chemical characteristics
Red is seen at
620-750 nm
A material that reflects all wavelengths of visible light appears
White
Black can be seen as a result of
The item absorbing all visible wavelengths of light
Deoxyhemoglobin absorbs light at
660 nm
Pulse oximetry relies on
The differences of oxygenated and deoxygenated hemoglobin in absorption of red and infrared light wavelengths
Deoxygenated hemoglobin absorbs 660 nm (red)
Oxygenated hemoglobin absorbs 940 nm (infrared)
Oxyhemoglobin absorbs
940 nm (infrared)
What is the normal difference expected between end tidal and PaCo2?
3-5 mmHg
What is the difference between end tidal and PaCO2 for someone with a lung disease?
10 mmHg
A patient with an obstructive lung disease will
have a plateau that is less flat (i.e. they will take longer to reach full expiration)
Visible wavelength is
500 nm
Pulse oximetry relies on
the differences of oxygenated and deoxygenated hgb in absorption of red and infrared light wavelengths
The SpO2 device is calibrated
O2 hgb/(O2 Hgb + Hgb)
estimates the arterial O2 saturation based on the ratio of O2 Hgb to total Hgb
We have more absorption of wavelength
during systole because the path length for the light was greater
Discuss blood only absorption for SpO2
The light also picks up bone, pigments, venous blood, and tissue so we have to determine blood flow only. We do this through taking the difference of systolic blood flow-diastolic blood flow to get blood only absorption
What is the accuracy of SpO2 in carbon monoxide poisoning?
in cases of CO poisoning, hemoglobin is still saturated but with CO instead of O2 and so it absorbs light at 940 nm so SpO2 readings will appear normal in CO poisoning
What does methemoglobin do to SpO2?
methemoglobin is a hemoglobin in the form of metalloprotein in which the iron in the heme group is in the Fe3+ state, not the Fe2+ state of normal hemoglobin
- methemoglobin cannot bind oxygen which means it cannot carry oxygen to tissues
- methemoglobin causes the pulse oximeter readout to tend towards 85%
What is functional versus fractional SpO2?
Functional SpO2 measures arterial blood saturation or the saturation of hemoglobin and fractional SpO2 refers to the fraction of hemoglobin which happens to be saturated
-in healthy people they are the same, in pathology such as carbon monoxide the fractional SpO2 would be a more accurate indicator
What wavelength does the cerebral oximetry use?
700 to 900 nm
What are the components of cerebral oximetry?
light source- produces NIR light of known wavelengths and intensity which passes through the skin, skull and cerebral tissue
Light detector- measures intensity of light exiting the cerebral tissue reflected back to the detector
Computer- converts the intensity of light exiting the cerebral tissue into the amount of O2hgb & it determines what is delivered vs. what is returned
What is a key difference between cerebral oximetry and Spo2 measurement?
measures non pulsatile/measures venous and arterial O2 Hgb and Hgb
The anatomy of the cerebral oximeter:
emitted light in the NIR wavelength range readily penetrates the skin and bone
some of the light is scattered, some of the light is
absorbed, and the rest of the light is reflected back
to a detector
How does the cerebral oximeter work?
photons of light transmitted through a sphere take an elliptical path
We subtract the skull absorption/transmission for the total, then we get brain transmission or absorption
Scalp detector (scalp transmission)
Brain detector (scalp + brain transmission)
Subtract the two to get brain transmission
For cerebral oximetry, the distance the detectors are away from each other and the light source
impacts how deeply the light penetrates
normal range is 30 to 50 nm or 1.5 cm
This is problematic because we are only able to
measure superficial tissue
Cerebral oximetry measures O2 hgb and hgb in
venous, arterial, and capillary blood
arterial to venous ratio of 30% to 70% and this is arbitrary and decided by manufacturer
calibrated by taking the SaO2 and the SvO2
What is the depth of penetration of the cerebral oximeter?
1.5 cm
1/3rd of the distance between the light emitter and detector
What are the similarities and difference between pulse oximetry and tissue oximetry?
Pulse: pulsatile, 660/940 nm, measures 100% arterial supply
Cerebral: non-pulsatile, similar wavelengths but different number of wavelengths, measures arterial, venous, and capillary supply and demand
What is the difference between LED vs. Laser light for cerebral oximetry?
LED light has broad wavelengths
Laser light has narrow wavelengths so some systems use multiple laser lights
somanetics LED light at 2 wavelengths: measures
changes in regional oxygen saturation (rSO2)
Casmed foresight uses laser light at 4 wavelengths: measures actual cerebral tissue oxygen saturation
How is the cerebral oximeter validated?
Measure SaO2 and SjbO2
uses arterial: venous ratio of 30%:70%
Calculated cerebral O2 saturation
SavO2= (0.3 x SaO2+ 0.7 x SjbO2)
The cerebral oximeter assesses relative
balance between oxygen supply and demand
What are factors that increase rSO2?
oxygen supply exceeds demand, increased blood pressure causes increased CBF, increased cardiac output, increased PaO2 SaO2 or CaO2, cerebral vasodilation increases CBF, increased hemoglobin/hematocrit increases CaO2
What are the factors that cause decreasing rSO2?
oxygen demand exceeds supply, increased metabolism without an increase in CBF, increased temperature, anesthetics (decreased CMR which decreases CBF)
What must be considered when interpreting SctO2 values in the clinical context?
cardiac output, blood pressure, CO2, fiO2, arterial pH (how much O2 can be unloaded), temperature, local blood flow, hgb concentration (content), hemorrhage (affects BP), embolism (affects flow), pre-existing disease, position changes (twisting of the neck can cause reduction in cerebral blood flow)
What are the advantages of cerebral oximetry?
non-invasive and requires no specialized training, real time oxygenation status of region of brain being monitored, by measuring predominantly venous versus arterial saturation it provides information about oxygen demand and supply balance
What are the disadvantages of cerebral oximetry?
does not measure global oxygenation (large area of the brain is not monitored), limited depth of penetration, measures only intravascular oxygenation (not a true reflection of intracellular oxygen availability), cannot differentiate the cause of neurologic dysfunction, electrocautery can interfere with cerebral oximetry
Intraoperative interventions that may improve cerebral oximetry:
adjust head position, increase anesthetic depth, decrease temperature, increase inspired oxygen, increase paCo2, increase MAP/CO, increase HCT
The magnitude of the de-/repolariziation vector depends upon
proportion of tissue depolarizing or repolarizing
The direction of the de-/re-polarization vector depends on
the orientation of the tissue in the body and the direction of depolarization
What determines the height of the voltage?
orientation of the tissue, direction of depolarization, proportion of tissue, and position of recording electrodes
Why might we have poor conduction of the ECG?
pericardial effusion or fibrosis
Magnitude of mean potential difference is denoted by
length of the vector
Where do we put the positive and negative electrons to assess lead I?
right arm negative –> left arm positive
Where do put the positive and negative leads to assess lead II?
right arm negative–> left leg positive
Where do we put the positive and negative leads to assess lead III?
left arm negative–> left leg positive
What limb is set to positive to assess aVr?
right arm
What limb is set to positive to assess aVl?
left arm
What limb is set to positive to assess aVf?
left leg
In regards to wavelengths, the more narrow the wavelength
the more energy particles per unit time
As path length increases,
absorption increases
How does the Co2 absorber work?
neutralization reaction (acid/base) Co2 is our volatile acid & soda lime or calcium hydroxide lime are the bases
Each 100 g of granules absorbs
15 L of Co2
Each absorbent canister has a total volume of
1500 mL
Each absorbent gas canister last about
8 to 10 hours assuming TOTAL rebreathing and no channeling
cannister air space is made up of
Intergranular air space: 50% of total air space
Intragranular air space: 8 to 10% of total air space
What inhaled anesthetic leads to the highest accumulation of CO?
desflurane
What is a concern with the absorber?
accumulation of CO d/t a reaction with volatile anesthetics and the absorbent
The relationship between PECO2 and PaCO2
PeCO2: high, PaCO2: high
PeCO2: Normal, PaCO2: normal or high
PeCO2: low, PaCO2: low, normal, or high
Emitted light in the cerebral oximetry metric can be
scattered, absorbed or reflected back to the detector
What is the normal range of regional tissue O2 saturation:
60-80%
What is the normal range of venous O2 saturation?
55-75%
What is the normal range of SaO2 saturation?
90-100%
How is the cerebral oximeter calculated?
Plot calculated SavO2 (arterial: venous O2 saturation-measured) against oximeter saturation
Visible light can be seen at a wavelength of
380 to 800 nm
More absorption occurs during systole because
the pathlength is longer; we can take the difference of systole and diastole absorption to determine blood only absorption
Toxins that can cause methemoglobinemia include
aniline dyes, nitrates, nitrites and medications (local anesthetics)
Methaemoglobin causes increased
oxygen binding affinity of normal hemoglobin, resulting in decreased unloading of oxygen to the tissues
