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Monitoring > Oxymetry Monitoring > Flashcards

Oxymetry Monitoring Flashcards

1
Q

What is Co-oximetry monitoring?

A
  • use of multiple wavelengths of light to detect and quantify the oxygenated hemoglobin (oxyhemoglobin) and other nonfunctional types of hemoglobin (e.g. carboxyhemoglobin, methemoglobin)
  • possible because hemoglobin’s light absorption characteristics change when bound to O2 or other substances
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2
Q

How can haemoglobin oxygen saturation (SO2) of arterial blood be measured?

A
  1. co-oximetry (SaO2)
  2. pulse oximetry (SpO2)
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3
Q

How big is the % of the dissolved portion of O2, measured as the partial pressure of oxygen (PO2)?

A

2-3% of the total oxygen content of blood in health

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4
Q

What is haemoglobin called when oxygen is bound to the heme group in hemoglobin?

A

Oxyhaemoglobin

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5
Q

What is oxyhaemoglobin?

A

haemoglobin with oxygen bound at its heme group

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6
Q

What is haemoglobin called when no oxygen is bound to the heme group in hemoglobin?

A

deoxyhemoglobin (or reduced hemoglobin)

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7
Q

What is deoxyhaemoglobin (or reduced haemoglobin)?

A

haemoglobin with no oxygen is bound to its heme group

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8
Q

What determines the degree of haemoglobin saturation with oxygen?

A

PO2

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9
Q

What does the oxyhemoglobin dissociation curve describe?

A
  • relationship between PO2 and SO2
  • sigmoidal shape
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10
Q

Why does the oxyhemoglobin dissociation curve have a sigmoidal form? What is the result of that?

A

binding of the first oxygen molecule induces a conformational change in hemoglobin –> increases its affinity for additional oxygen molecules

–> Small changes in SpO2 correspond to significant changes in PaO2

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11
Q

What does accurate determination of SO2 from PO2 or vice versa from the oxyhemoglobin dissociation curve depend on?

A

on a normal position of the oxyhemoglobin dissociation curve

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12
Q

What causes a right shift of the oxyhemoglobin dissociation curve? What is the result of that?

A
  1. increased H+ (Acidosis)
  2. increased temperature
  3. increased CO2
  4. increased 2,3-DPG

Mneumonic: CADET (CO2, Acid/Altitude, 2,3 DPG, Exercise, Temperature)

–> unloading of oxygen (normally beneficial at the level of the tissues) is faciliated

Right = O2 is Released
Right = Rising

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13
Q

What causes a left shift of the oxyhemoglobin dissociation curve? What is the result of that?

A
  1. decreased H+ (alkalosis)
  2. decreased temperature
  3. decreased CO2
  4. decreased 2,3-DPG

Mneumonic: CADET (CO2, Acid/Altitude, 2,3 DPG, Exercise, Temperature)

–> oxygen loading (beneficial in the pulmonary capillaries) is promoted

Left = O2 is Locked up
Left = Lessening

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14
Q

What is P50?

A

PO2 at which hemoglobin is 50% saturated

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15
Q

What is the P50 of people, dogs and cats?

A

human: 26-27 mmHg
dogs: 28-31 mmHg
cats: 34-36 mmHg

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16
Q

What SO2 does a PO2 of 80 and 60 mmHg correspond to, respectively when derived from a human oxyhemoglobin dissociation curve? How does this change when derived from the canine one?

A

human oxyhemoglobin dissociation curve:
- 80mmHg: 95%
- 60mmHg: 90%

canine:
- 80mmHg: 93%
- 60mmHg: 86%

Feline curve has not been established.

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17
Q

What law is the technology of oximeters based on?

A

Beer–Lambert law

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18
Q

What does the Beer–Lambert law state?

A

the concentration of a substance can be determined by its ability to transmit light

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19
Q

How do oximeters work?

A

use spectrophotometry to determine the % of oxyhaemoglobin

20
Q

How do co-oximeters wokr?

A
  • utilize 4-8 different wavelengths of light to quantify multiple hemoglobin species in a blood sample
  • detect functional hemoglobin species (oxyhemoglobin + deoxyhemoglobin), as well as dyshemoglobins that are incapable of binding oxygen (e.g. carboxyhemoglobin, methemoglobin)
21
Q

What is carboxyhemoglobin?

A

hemoglobin preferentially binds carbon monoxide

22
Q

What is methemoglobin?

A
  • inactive form of hemoglobin
  • produced secondary to oxidative damage when the ferrous form of iron (Fe2+) in the heme group is oxidized to the ferric form (Fe3+)
23
Q

What is sulfhemoglobin?

A
  • hemoglobin reacts with sulfide and oxygen
  • usually not detected by co-oximeters
23
Q

What are functional haemoglobin species?

A
  1. oxyhaemoglobin
  2. deoxyhaemoglobin
24
Q

What is the functional hemoglobin saturation?
What is its equation?

A

% of functional hemoglobin that is oxygenated

Functinoal SO2 = [(HbO2/HbO2+HHb)] x 100

functional SO2 more closely corresponds with PaO2

25
Q

What is the fractional hemoglobin saturation?
What is its equation?

A

% of oxyhemoglobin compared with all hemoglobin species, including dyshemoglobins

Fractional SO2 = [(HbO2/HbO2+HHb + COHb + MetHb)] x 100

26
Q

Does pulse oximetry report functional or fractional hemoglobin saturation?

A

functional, as can only detect oxyhemoglobin and deoxyhemoglobin

27
Q

How does pulse oximetry work?

A
  • emits 2 wavelengths of light (red light at 660 nm and infrared at 940 nm) from the probe’s diodes into the patient’s tissue
  • evaluates only pulsatile (arterial) wavelength absorption in the calculations –> based on an optical technique known as photoplethysmography
  • Light absorption is then recorded by a photodetector + analyzed in a microprocessing unit
  • oxyhemoglobin absorbs more infrared light (940nm) and deoxyhemoglobin absorbs more red light (660nm)
  • their differential light absorption characteristics to calculate the arterial blood’s functional hemoglobin saturation
  • some also have plethysmograph: display a graphic waveform of a peripheral pulse wave –> amplitude corresponds to pulse quality
28
Q

Name 4 advantages of pulse oximetry over PaO2?

A
  • easy to use
  • minimal cost
  • continuous or long-term monitoring without the requirement for arterial catheter placement or repeated arterial puncture
29
Q

In what SO2 range does PO2 correlate well with SO2?

A
  • PO2 correlates well with SO2 in the 80% to 97% range
  • Relationship between SO2 and PO2 is non-linear above and below this range
  • Curve flattens out above an SO2 of approximately 97%
30
Q

Name 6 situations where pulse oximetry is indicated?

A
  1. anaesthesia
  2. mechanical ventilation
  3. triage
  4. on + off O2 supplementation to evaluate oxygen responsiveness
  5. patients in marked respiratory distress in which arterial blood gas analysis is not safe
  6. small patients in which arterial sampling may be very difficult
31
Q

Explain why SpO2 may be a sensitive marker for hypoxaemia in patients breathing room air, but not in patients on O2 supplementation?

A

A PO2 correlates well with SO2 in the 80% to 97% range. However, an SpO2 of 100% does not discriminate between normal and abnormal gas exchange efficiency in a patient on supplemental oxygen (in a patient on 100% oxygen, an SpO2 of 100% could indicate aything betweena PaO2 of 200mmHg (severely compromised oxygenation) to 500 mmHg (normal oxygenation).
An SpO2 ,99%–100% on oxygen supplementation, would indicate abnormal gas exchange.

32
Q

How does anaemia affect SpO2?

A

SpO2 is a measure of the saturation of hemoglobin with oxy- gen but gives no information regarding the quantity of hemoglobin. An anemic patient would be expected to have a normal SpO2, but the total oxygen content of the arterial blood would be low, potentially still compromising tissue oxygen delivery.
- severe anemia significantly affected bias and precision, finding accuracy to be unacceptable with hematocrits <10%

33
Q

Name 8 patient related factors influencing SpO2 readings?

A
  1. movement artifact
  2. dyshemoglobinemias
  3. ambient light
  4. skin pigment
  5. hypoperfusion
  6. hypothermia
  7. severe anemia
  8. Haired skin
34
Q

How can motion artifacts influence SpO2 readings and why?

A

erroneously low reading due to the monitor mistaking venous blood for arterial blood

35
Q

How would carboxyhemoglobin influence SpO2 readings and why?

A
  • falsely high SpO2 due to mistakenly read as oxyhemoglobin
  • Carboxyhemoglobin absorbs very little light in the infrared range (940nm), but as much light as oxyhemoglobin in the red range (660nm)
36
Q

How woud methemoglobin influenec SpO2 readings and why?

A
  • low SpO2 readings
  • when methemoglobin levels approach 30% –> SpO2 will plateau at approximately 85% regardless of the true level of oxygen saturation [ratio of absorbance (A660/A940) becomes 1.0 –> reads as a SpO2 of 85%]
  • high absorbance at both of these wavelengths
37
Q

What could be the reason for a gap between SpO2 and SaO2?

A

Dyshemoglobinemias

38
Q

Why may hypotension, hypoperfusion, or peripheral vasoconstriction affect the accuracy of SpO2 readings?

A

Because pulse oximetry requires analysis of the waveform of pulsatile blood flow

39
Q

What did Lee et al report in Anaesthetsiology (1991) on the effects of anemia on pulse oximetry and
continuous mixed venous hemoglobin saturation monitoring in dogs?

A
  • The accuracy as assessed by bias and precision for SoxO2 was similar for HCT of 40-15%. (Bias +/- precision was 2.1 +/- 5.7% for hematocrits greater than 40%, and -1.1 +/- 7.5% for hematocrits of 15% to 19%).
  • At HCT between 10-14%, the precision worsened to 12%,
  • HCT <10%: bias +/- precision were -11.5 +/- 11.8%.

–> pulse oximeter’s accuracy was similar, down to hematocrits of 10%

40
Q

What did Farrell et al report in JVECC (2019) when: evaluating pulse oximetry as a surrogate for PaO2 in awake dogs breathing room air and anesthetized dogs on mechanical ventilation?

A

SpO2 was not clinically suitable as a surrogate for PaO2 in awake dogs breathing room air, though it performed better in mechanically ventilated dogs on supplemental oxygen.
In both groups, there was a low sensitivity for the detection of hypoxemia.
The correlation coefficient between PaO2 /FiO2 and SpO2 /FiO2 ratios was 0.76

Optimal cutoff value for detecting hypoxemia (PaO2 < 80 mm Hg): SpO2 of 95% (sensitivity 77.8% + specificity 89.5%)
Using this cutoff, 6.9% of SpO2 readings failed to detect hypoxemia, whereas 7.2% predicted hypoxemia that was not present.

41
Q

In what range may SpO2/FiO2 (SF) ratios may correlate with PF ratios and why?

A
  • 80-97%
  • Because the oxyhemoglobin dissociation curve is relatively linear in that range
42
Q

What have recent human studies found about using SF ratios as reliable, noninvasive surrogates for PF ratio?

A

SF ratios can serve as reliable, noninvasive surrogates for PF ratio

43
Q

What did Calabro et al report in JVECC (2013) when evaluating the utility of comparing SpO2/FiO2 and PaO2/FiO2 ratios in dogs?

A
  • SF and PF in dogs spontaneously breathing room air have good correlation (ρ = 0.618, P < 0.01)
  • SF may be a useful, noninvasive surrogate for PF when assessing oxygenation in canine patients
44
Q

What is the major limitation when using SF ratios when assessing oxygenation?

A

ignore the impact of PaCO2 on oxygenation –> restricts their utility for the assessment of oxygenation of patients on room air
- influence of hypoventilation is negligible at an FiO2 >30%

45
Q

What did Farrell et al report in JVECC (2019) when evaluating the utility of comparing SpO2/FiO2 and PaO2/FiO2 ratios in mechanically ventilated dogs?

A

strong correlation of SF and PF ratio with SpO2 values between 80% to 97%

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