Pulse Ox Quiz Questions Flashcards
(46 cards)
1
Q
What is pulse oximetry?
A
Noninvasive measurement of the percentage of hemoglobin saturated with oxygen in arterial blood
2
Q
What abbreviations are used to present the data obtained by a pulse ox?
A
Saturation value obtained in vivo by pulse ox: SpO2
Saturation value obtained in vitro from hemolyzed arterial blood: SaO2
3
Q
What principle of physics is pulse oximetry based on?
A
Beer-Lambert law: absorption of light by a homogenous solution is a function of the concentration of a solution
4
Q
What wavelength does oxygenated hemoglobin absorb?
A
Infrared light at 920-940nm
5
Q
What wavelength does deoxygenated hemoglobin absorb?
A
Red light at 660nm
6
Q
How does pulse oximetry work?
A
- Hgb absorbs red and infrared light at different wavelengths depending on whether light bound to O2 (infrared, 920-940nm) or deoxygenated (red light, 660nm)
- PO uses 2 light emitting diodes (LEDs) that pulse red, infrared light through perfused tissue several hundred times per second –> amt of light absorbed at each wavelength measured by sensitive photodetectors
- Absorption data expressed as a percentage of oxygenated to total hgb
7
Q
How does PO differentiate arterial from venous blood?
A
- Assumption: arterial blood pulsatile, venous blood/tissue beds are not
- With each pulse, influx of arterial blood into the capillary bed increases the total absorption of light –> allows pulse ox to distinguish systole (arterial blood + venous blood + tissue) from diastole (venous blood + tissue)
- Data collected during systole, diastole
- Diastolic absorption values subtracted from systolic absorption values –> allows PO to eliminate all data except that produced by arterial blood
- How acquired name “pulse” oximetry
8
Q
PaO2 27 = ? (normal O2 dissociation curve)
A
SpO2 50%
9
Q
PaO2 40 = ? (normal O2 dissociation curve)
A
SpO2 75%
10
Q
PaO2 60 = ? (normal O2 dissociation curve)
A
SpO2 90%
11
Q
SpO2 50%? (normal O2 dissociation curve)
A
PaO2 27mm Hg
12
Q
SpO2 75%? (normal O2 dissociation curve)
A
PaO2 40mm Hg
13
Q
SpO2 90%? (normal O2 dissociation curve)
A
PaO2 60mmg Hg
14
Q
What are advantages of PO?
A
- Easy to use
- Minimal site prep
- No special training required
- Non-invasive
- Does not require collection of blood –> no patient discomfort, no delay in data collection
- Virtually continuous measurement of saturation
- Provides information about resp, CV systems
- Affordable, mobile, easy to use
15
Q
Are there dangers, times when PO would be inadequate form of monitoring?
A
- High O2 partial pressures –> SpO2 may not adequately warn the anesthesiologist of impending hypoxic events
- Normal O2 saturation can be achieved in a hyperventilating patient breathing high inspired oxygen concentration
- Must assess adequacy of ventilation separately from adequacy of saturation
16
Q
What is the oxygen-dissociation curve?
A
Graphic representation of the relationship btw the percentage of oxygen saturation of hgb (SaO2, SpO2) and partial pressure of oxygen in arterial blood (PaO2)
17
Q
Is it necessary to calibrate the pulse ox?
A
No
18
Q
Response to low saturation: steps
A
- ensure adequate ventilation by eval patency of airway, RR, Tv. Ensure no interruptions of o2 flow or accumulation of CO2 in ax machine
- Increase FiO2 if <100%. DC N2O if used. Add mechanical ventilation if necessary
- Ensure cardiac output adequate –> MM, CRT, ABP, pulse quality
- Address other issues including positioning of the patient, surgical procedure (ex thoracotomy), medical conditions
- Confirm with co-oximeter/ABG
19
Q
How accurate are the saturation data provided by the pulse oximeter?
A
- Accurate within 2-6% in 80-100% range
- > 90% oxygen sat, SpO2 tends to be slightly lower than SaO2
- <70% oxygen sat, SpO2 tends to be slightly higher than SaO2
- Accuracy of SpO2 may deteriorate substantially as SaO2 continues to decrease
20
Q
How are pulse ox calibrated?
A
- Empirically calibrated by the manufacturer using absorption data from large group of healthy human volunteers (generally with SaO2 values > 70%)
- Optical properties of blood similar among mammalian individuals/between mammalian species, no individual instrument calibration required in the field
- Bc each brand of pulse ox calibrated using different data set, different brands may display slightly different saturation values for the same patient
21
Q
Does the PO always provide reliable saturation data?
A
No!
Numerous factors cause SpO2 to deviate from SaO2
Extrinsic: Patient motion, electrocautery, ambient light
Intrinsic: Dysfunctional hemoglobins, IV dyes
22
Q
What is meant by dysfunctional hgb?
A
Do not transport oxygen
COHb, MetHb
23
Q
Five types of Hgb?
A
- Fetal hgb (HbF)
- Oxyhemoglobin (O2Hb)
- Deoxyhemoglobin
- Carboxyhemoglobin (COHb)
- Methemoglobin (MetHb)
24
Q
How does fetal hgb affect the PO?
A
Does not affect accuracy of PO
Greater affinity for O2 than adult hgb
Absorption coefficients for HbF, adult hgb similar –> presence of HbF does not affect sat values
25
How does carboxyhemoglobin affect PO?
- Absorbance spectrum similar to that of HbO2
| - Most POs overestimate percentage sat by amount of COHb present
26
How does methemoglobin affect PO?
- Absorbance coefficient similar to both red and infrared bands
- Presence of large amounts of MetHb causes ratio of red to infrared absorbance to approach 1 --> sat value of 85%
- Large amounts of MetHb will drive SpO2 to 85% regardless of saturation value
27
How does the presence of IV dyes affect accuracy of the PO?
- Methylene blue used to treat methemoglobinemia --> large absorbance peak at 670nm
- Value close to 660nm wavelength of red light --> low SpO2 values recorded
- Dyes rapidly equilibrate in the blood and tissue bed --> become part of non-pulsatile background --> effects eliminated
28
How do extrinsic factors cause pulse ox inaccuracies? - Movement
- Movement of patient reduces/eliminates ability of PO to detect a pulse --> displays erratic HR
- If transient motion mimics a heart beat, instrument may be unable to distinguish btw pulsations DT motion artifact and normal arterial pulses
29
How can I be sure that the pulse ox is providing accurate saturation data?
- Can't always be sure
- Observe HR displayed by PO matches actual HR of the patient (or 2x HR in some horses)
- Strength of PO signal should be strong
30
Are there conditions that would cause the PO to completely fail to detect a pulse?
- Arterial pulses must be of adequate strength to be detected by PO
- Any significant decrease in peripheral vascular pulsation can produce a signal too weak to be detected
- Ex: hypotension, hypovolemia, VC, hypothermia
- Overall perfusion may not be poor but probe site may have poor intrinsic perfusion --> probe may need to be moved to a site with better perfusion
31
Are there different types of pulse oximeter probes?
1. Transmittance type
| 2. Reflectance Type
32
Where is the best site to place PO probe?
- Any pulsatile arteriolar bed may be used for attachment of probe
- Area must be without hair, non pigmented, fairly thin but not too thin
- Tongue of anesthetized patients produces most reliable, repeatable data
- Ex: lip, vulva, prepuce, toe web, fold of flank, ear pinna, metacarpus, digits, tail
- Can also use nose, nasal septum in large animals
- Animal skin often thicker than the skin of human beings making probe placement more difficult in veterinary species
33
Is PO useful in all species?
-Should be useful in most mammals - similarity in spectral properties of mammalian hgb btw species
34
Why do horses often show double pulse rate?
-Dicrotic notch in pulse wave large --> often counted as a second cardiac contraction
35
Is the PO useful in situations other than monitoring saturation during anesthesia?
- Critical care patients w questionable resp function
- Patients requiring/being weaned from oxygen/ventilatory support
- Movement interference - pulse ox in awake animals difficult --> lip, tail, toe best
- Measure fetal oxygen saturation during dystocia
- Assess intestinal viability during surgery
36
Transmittance type of PO probe
- LEDs and photodetector placed on opposite sides of the tissue bed
- Most common
- Clip type, c-clamp type, finger type
37
Downside of finger type pulse ox probes
Concave surface --> do not make adequate tissue contact --> pulse detection difficult in this type of probe
38
What can be done to alleviate interference from extrinsic factors?
- Averaging pulse readings over long periods of time (5-10sec)
- Decreasing size of the probe
- Designing better fitting probes
- Some POs gated on the R wave of an ECG - interpret data relayed only at the time of inscription of the R wave --> decreases motion artifact
- Increase CO to improve perfusion
39
What are the advantages of pulse oximetry?
- Easy to use, minimal site prep, no special training
- Non-invasive
- No blood collection --> non painful to patient
- No delay in data collection, continuous measurement of saturation
- Info about resp and CV systems
- Affordable, fairly mobile, easy to use
40
Dangers/times when PO inadequate form of monitoring?
- With high oxygen partial pressures, SpO2 may not adequately warn anesthesiologist of impeding hypoxic events
- Normal oxygen saturation can be achieved in a hypoventilating patient breathing a high oxygen concentration
- Impt to assess adequacy of vent separately from adequacy of saturation
41
Is PO used two monitor anesthetized human patients?
- 1/1/1990: ASA --> intraop SpO2 monitoring standard of care
- 1992: monitoring with PO added to post anesthesia recovery standard of care
42
If PO only evaluates deoxygenated and oxygenated hgb, how measure all moieties?
Co-oximeter
43
Describe shape of the oxygen-dissociation curve
- Flat part of curve on the upper right represents area where PaO2 so high that even changes in PaO2 result in minimal to no changes in SpO2/SaO2
- <90% sat, curve becomes steep --> minimal change in SpO2 represents large drops in PaO2 --> danger of rapid desaturation
44
What are dysfunctional hemoglobins?
Do not transport oxygen
45
How do extrinsic factors cause pulse ox inaccuracies? - Ambient Light
- Interfere with sensor - cause failure of PO to produce data or cause erroneous readings
- Most commonly show readings of 85%
- Some xenon arc surgery lights may cause 100% SpO2 readings and pulse rate 180-225bpm not attached to patient
- Eliminated by shielding probe from ambient light
- Some POs: effect of stray light mathematically eliminated by subtraction of background measurements taken when both LEDs in the sensor are turned off from measurement taken when either LED is turned on
46
What is the biggest limitation of pulse oximetry in both human and veterinary medicine?
Motion!
