I. Pulse Oximetry Flashcards
A noninvasive & continuous means estimating the percentage of Hb saturated with oxygen in arterial blood at the peripheral capillary level.
Pulse Oximetry
Normal Reading 94-100%
Pulse Oximetry determines SpO2, which is a estimation of ____.
SaO2
(arterial O2 saturation)
Benefits of Pulse Oximetry
- Non-invasive
- Cheap
- Compact & portable
- Detects hypoxemia sooner than visual signs of cyanosis.
- No contraindications!
two types of hemoglobin
- Oxyhemoglobin
- Deoxyhemoglobin
% of O2 in blood bound to Hgb?
97%
(3% dissolved in plasma)
____ is the % of of Hgb carrying O2 molecules
O2 saturation
O2 carrying capacity and delivery are a function of the ____ and ____.
- O2 saturation
- Hgb concentration
Pulse oximetry is more sensitive than cyanosis for detection of ____.
Desaturation
Visible cyanosis occurs at a SpO2 of ____.
~80% 😨
Uses of Pulse Oximetry:
- Estimating SaO2 & Hypoxemia
- Desaturation Detection
- ID Pulmonary & Airway Problems
- Measures HR & Diagnose Arrhythmias
- Indicates Tissue Perfusion (strength or size of waveform)
What are the two techniques Pulse Oximetry utilizes IOT function properly?
- Spectrophotometry
- Photoplethysmography (PPG)
Measures how much a substance absorbs light by measuring the intensity of light that passes through a sample solution
Spectrophotometry
Spectrophotometry describes O2 saturation as a function of ____
light absorption detected🔦🕵🏻
____ employs an optical technique that detects volumetric changes in blood & circulation (i.e., detects volume change of pulsatile artery)
Photoplethysmography 🩸🕵🏻
creates a Pulse & Pleth wave
Pulse Ox has LED probes that send light impulses ____/sec
2000-3000
With regard to spectrophotometry, the amount of light absorbed depends on:
- Concentration (Beer’s Law)
- Length of Path (Lambert’s Law)
- Differences in Light Absorption (Absorption Variation)
The physical property of concentration, as it pertains to Pulse Oximetry, is based on what principle?
Beer’s Law: The amount of absorbed light is porportional to the solution concentration
🔦=🍺
Pulse Ox
Which physical property states the amount of light absorbed is proportional to the length of the path that the light has to travel in the absorbing distance?
Lambert’s Law
↓light path = ↓absorption = ↑ light pass through
↑light path = ↑absorption = ↓ light pass through
Pulse Ox
The Absorption Variation property describes how Hgb variants differ in ____.
light absorption
The Pulse Ox employs what two light (& wavelengths)?
Red (660 nm) 🔴
Infrared (940 nm)
Which Hgb absorbs greater amounts red light (660nm)?
Deoxyhemoglobin
not weighed down w/ O2, therefore has higher energy
Which Hgb absorbs greater amounts of infrared light (940nm)?
Oxyhemoglobin
weighed down with O2 molecules, therefore has (lower) energy
Pulse Ox
What variable represents how much light is absorbed when all three principles are taken into account?
Extinction Coefficient
Beer’s & Lambert’s Laws cannot be applied strictly due to ____.
scattering of light through the blood
Because scattering of light occurs as light passes through blood, what was developed to ensure accurate readings?
Calibration Curve
To develop the Calibration Curve, the pulse ox had to be calibrated to related SpO2 reading to ____; thus the ____ was developed.
- R:IR ratio
- Calibration Curve Algorithm
Because of the limits posed by human testing, SpO2 measurements below ____ have to be computationally estimated. As a result, these readings contain a greater degree of reading error.
70%
Light absorbing tissue is divided into what two components?
- Pulsatile Arterial Blood (changing absorbance = A/C)
- Systole ↑ Light Absorption 1-2% - Non-Pulsatile Blood (non-changing absorbance = D/C)
- non-pulsatile arterial, venous, capillary, tissues, dyes, & dyshemoglobins
Why does systole ↑ light absorption?
- ↑ diameter of arteries & arterioles
- shift in erythrocytes axis
Diastole (both arteries & veins): non-pulsatile ∴ no changing absorbance
Instrument that measures variations is the size of body part on the basis of amount of blood passing through/present.
Plethysmograph
systole = expansion of artery
diastole = contraction of artery
What is the application of photoplethysmography?
photodetector measures the pulsatile changes in the absorbance of the 2 light wavelengths (R & IR) and then plots a waveform
The peak of the pleth waveform represents ____, while the trough represents ____.
- systole
- diastole
Dicrotic Notch is also observable on waveform
A proper pleth waveform suggests what?
the SpO2 is reliable
When the R value is = 1, the SpO2 is ____.
85%
As the R:IR ↓ (i.e., as R value decreases), SpO2 ____.
R-value = x-axis of Calibration Curve
Increases
↑absorbance at 940nm = ↑ HgbO2 saturation = ↓ R-value = ↑SpO2
Pulse oximetry calculates a/an ____ and uses the calibration curve to estimate ____.
- R-value
- SaO2
between 70-100%, the SpO2 standard deviation is +/- ____%.
2%
What are four sources of signal inaccuracies when using a Pulse Ox?
- Ambient Light: ↑ D/C signal
- Low Perfusion: ↓A/C signal
- Motion/Shivering: Artifact ↑ A/C signal
- Addtional Light Absorber in the Blood: IV Dyes & Hgbs
What source of error increases the DC signal, leading to a falsely high SpO2 reading.
Ambient light interference
If the non-pulsatile component becomes dominant, the pulse oximeter may mistakenly interpret this as a stronger pulsatile signal, potentially resulting in an erroneously high SpO2 reading.
Pulse Ox
Describe the sequence of LED activation
- Red LED ON → measures Red & Room light → Red LED Off
- IR LED ON → measures IR & Room light → IR LED Off
- Both LEDs OFF → measures only Room light
sequence repeated 2000-3000x/second‼️
(helps photdector adjust for ambient light)
What can be done to minimize ambient light interference?
cover the pulse ox with opaque material (i.e., blanket)
What could be the source of low perfusion and signal dropouts
Low A/C signal → Low Pleth Amplitude → Innacurate SpO2
Vasoconstriction
- Vasoactive agents💉
- Hypothermia 🥶
- BP Cuff
- PVD
Hypovolemia
- Distributive or hypovolemic shock😱
- blood loss🩸
- decreased CO💔
Artifactual large AC signal creates what type of reading?
Falsely Low
excessive movement, moving hand, shivering:
Causes venous, stationary tissue to be measured (“Venous Pulsation”) along with SaO2 = ↑R:IR =↓SpO2 reading (false)
A greater volume of venous blood is measured ∴ more deoxyHgb blood is measured ∴ more red light is measured ∴ R value ↑ (1:1 → 2:1) ∴ R:IR ↑ = SpO2 ↓
↓R:IR = ↑SpO2
↑R:IR = ↓SpO2
What are some causes of “Venous Pulsation” to occur, which occurs as part of Motion Artifact leading to False Low Pulse Ox readings?
- SvO2 measured along with SaO2 (unable to properly distinguish between the two)
- Tight Finger probes
- Severe tricuspid regurgitation
- Probe location/patient position: forehead & t-burg
- Distributive shock: vasodilation → A/V Shunting
What are the 3 IV Dyes?
-
Methylene Blue
- Endo
- *668 nm *(close to ‘red’ @ 660 = looks like deoxyHgb)
- short-lived falsely low SpO2 reading (10-60 min with dose 2-5mg/kg) - Indocyanine Green (ICG)
- Tissue Perfusion Tests/Plastics/Vascular
- Similar effect as MB, but shorter lived (30 sec) - Indigo Carmine
- GYN/Urology - Fluorescein
- does NOT effect Pulse Ox
What are the range of Hgb abnormalities?
- CarboxyHgb
- MetHgb
- SulfHgb
- Sickle Cell Hgb
- Fetal Hgb
- Genetically acquired Hgb abnormality
Hint: Carl Met Susie’s Sick GrandFather
____ SaO2 is the fraction of effective Hgb which is oxygenated
Functional SaO2
(HgbO2, Hgb)
Approximates SpO2
Normally Used
____ is the fraction of total Hgb which happens to be oxyenated
Fractional SaO2
(HgbO2, Hgb, MetHgb, HgbCO)
Requires blood draw for accuracy
Functional SaO2 Formula
HgbO2/(HgbO2 + Hgb)
x 100%
What is the pathophysiologic reason MetHgb prevents the Pulse Ox from working properly?
- Fe++ is oxidized to Fe+++
- Fe+++ cannot bind O2 (this causes other 3 O2 molecules to bind more strongly to Hgb, NOT releasing to tissues)
- O2 content is reduced & O2 delivery may be reduced w/o compensatory changes in CO
- Light is absorbed equally at 660 & 940nm ∴ wavelength averaging ↑ R:IR to 1 → causes false low SpO2 reading
What approximate SpO2 reading is typically observed when MetHgb levels are present?
85%
(under or overestimated)
MetHgb Clinical Signs & Symptoms
1. SpO2 that does NOT respond to 100% FiO2
2. Severity of symptoms closely correlates to MetHgb level and severity of impaired O2 delivery
3. Symptoms will be worse for
any MetHb level in patients with baseline impairment of O2 delivery (i.e. CHF, pneumonia, COPD, anemia)
MetHgb Signs & Symptoms Chart
Common Causes of MetHgb
- Congenital
- Acquired/Drug Induced (Most Common)
- Local Anesthetics (EMLA, Lido, Benzo: 🌪spray/topical)
- Phenytoin
- Sulfonamides
- Metoclopramide
- NTG or SNP
Treatment of MetHgb
GOAL: ↓Fe+++ → Fe++
Start 💊Tx at:
- 20% MetHgb when pt is symptomatic
- 30% when asymptomatic
💊Tx: Methylene Blue
- 1-2 mg/kg over 3-5 minutes (resolved by 30 minutes)
CO competes with O2 for binding site on Hgb with an affinity ____ times greater than O2.
210x
∴ ↓ O2 delivery & ↓O2 utilization (blocking ETC)
To the pulse ox, ____ looks just like O2Hgb, but actual O2 delivery is much lower
COHgb
- absorbs as much red light as O2Hgb (∴ mimics O2Hgb appearance)
- Absorbs very little IR (940nm)
- Overestimates SpO2 (remains ≥90%)
S&S of COHgb
Mild (0-30%): Headaches & Dizziness
Moderate (30-40%): Disorientation & Syncope
Severe (>40%): Cariac Dysarrhythmias & Death
cherry lips
COHgb Causes
Incomplete combustion of carbon containing materials
COHgb Tx
- Administer 100% O2 to compete with CO for binding sites
- CO half-life: 1-2 hours - Hyperbaric Chamber (2.5-3 atm)
- CO half-life: 22 min
CO normal half-life 4-5.5 hours
Pulse Ox
What are some other sources of error?
- Nail Polish (Blue>Green>Black)
- EM Radiation
- Electrosurgical Unit (Bovie)
- Placement of Probe
- foreheads/T-berg: Impeded venous drainage (more distensibility of venous blood at site of probe is significant)
👍🏻Jaundice, skin tone, and anemia have little to no effect on Pulse Ox
Why doesn’t anemia affet SpO2?
- Pulse Ox is just measuring the ratio of oxygenated vs deoxygenated Hgb
- During mild anemia, the reduced amount of functioning Hgb is still saturated with O2 ∴ SpO2 will be unchanged
- However, once Hct levels reach <15%, SPO2 reading may fail or become innacrurate due to low perfusion (low oxygen delivery occurring at the tissues)
What are the three applications for using SpO2 & Plethysmography?
- Assessment of Oxygenation
- Monitoring Vascular Sympathetic Tone
- Monitoring Circulation
How does Plethysmography provide insight to vascular sympathetic tone?
By observing Pleth Amplitude, we know:
- the amplitude is directly proportional to the vascular distensibility
- ** Inversely proportional to vascular tone**
What information does the Pleth provide with regard to monitoring circulation?
- HR
- Rhythm Analysis
- Determining Systolic BP
- Monitoring Vascular Volume (respiratory variability & preload) [baroreceptor reflex]
- ↑ Fluid Volume = ↑ BP & ↑CO (↑Preload)
- ↓ Fluid Volume = ↓ BP & ↓CO (↓Preload)
- 𝝙 Fluid Volume → ↓↑ O2/CO2 → ↓↑ RR
Clinical Assessments of Oxygenation
S&S (w/o Pulse Ox)
- Hypoxia:
- ↑ RR (tachypnea), but may be depressed by anesthetics - Cyanosis:
- observer variability and lighting
- Cyanosis & arterial saturation do not always correlate (i.e., ↓CO or Anemia)
characteristics
Pulse Ox is ____, ____, and ____.
continuous, reliable, objective
The amplitude of the Pleth is directly proportional to ____ over a wide range of CO.
vascular distensibility
(may be reduced by auto-gain function, automatically increases size of waveform for better viewing ∴ disable if necessary)
Pleth is ____ proportional to vascular tone
tone = contraction
inversely
Decreased vascular tone = ____ amplitude of pleth waveform
Increased
(due to vasodilation)
↓Tone = VD = ↑Distensibility = ↑Amp
What can cause decrease vascular tone?
- Pharmacologic (VD)
- Physiologic (warming & sedation) sauna → VD & sedation/relaxation
- Anesthetic (Regional Sympathetic Blocks: spinal & epidural)
____ vascular tone leads to decreased amplitude
Increased
(due to VC)
↑Tone = VC = ↓Distensibility = ↓Amp
What can cause increased vascular tone?
- Pharmacologic (phenylephrine)
- Physiologic (Surgical stress or stimulation; cold)
increased vascular tone = vasoconstriction
How does the Pleth help with monitoring circulation?
- HR & Rhythm Analysis
- Useful in detecting cardiac arrhythmias
- Max benefit: use with EKG If Pulse Ox is normal, but EKG is abnormal → likely erroneous
- Helps with artifact from movement/Bovie - Determining Systolic BP
- similar to info from NIBP
- Systolic = return to flow - Monitoring Vascular Volume
- respiratory variabilty
Changes in vascular volume can also affect cardiac output and hemodynamics. Reduced vascular volume can lead to decreased stroke volume and cardiac output.In response, the body may try to compensate by increasing heart rate and respiratory rate to maintain tissue perfusion and oxygen delivery.Also, a decrease in vascular volume can lead to decreased oxygen delivery to tissues (hypoxia). This can result in increased ventilation (respiratory rate and depth) as a compensatory response to try to improve oxygenation. Increased sympathetic activity can stimulate the respiratory centers in the brainstem, leading to increased respiratory rate and depth.
With regard to patient positioning, how does a Pulse Ox help?
- detects arm positions that compromise circulation
- during shoulder injury, can alert to brachial artery compression
- Limb fractures can have compromised flow
During a mediastinoscopy, compression of ____ can occur, precipitating the use of an A-line.
brachiocephalic artery
Sympathetic blocks ____ blood flow
increase
why do sympathetic blocks increase blood flow?
By blocking or disrupting the sympathetic signals with a sympathetic block, vasodilation (widening of blood vessels) can occur. This dilation allows more blood to flow to the affected area, increasing blood flow.
The pulse ox wave waveform (pleth) closely parallels what other waveform?
BP waveform
only visible with an A-line
↑ Thoracic Pressure = Compression of SVC & IVC = Low Volume
Low volume causes what to occur on the Pleth?
Large variations in Pleth amplitudes
what are the highly vascular sites that may be good site for Pulse Ox?
- Finger
- Ear Lobe
- Nose
- Forehead
(lip, tongue, toe, foot)
What is one way to verify successful block using a pulse ox?
- Place on toe
- Increase in pulse amplitude indicates successful block
Because: sympathetic blocks cause vasodilation, by blocking sympathetic VC responses → vasodilation = increased vascular distensibility = ↑ Pleth Amplitude
Pulse Ox location:
- low failure rate
- ↑ accuracy (better than earlobe)
- sensitive to VC
- Response time may be slower than more central location
finger
Pulse Ox location:
- Longer delay (than finger/central location)
- Helpful for epidural
toe
Pulse Ox location:
- rapid response to change
- accuracy more scrutinized
- useful for hypothermia, hypotension, VC drugs
Nose
if T-berg, venous congestion could cause artificially low readings
bridge or septum
Pulse ox location:
- useful when ↑ motion
- faster response than finger
- relatively immune to vasoconstriction
Ear
if T-berg, venous congestion could cause artificially low readings
Pulse Ox Location:
- very accurate
- uses either a disposabe probe or ear probe
Tongue
Pulse Ox location:
- More accurate than finger
- Detects ↑↓ quickly
- uses ear probe
- artifact during induction
Cheek/lip
not enjoyed awake pts
Which type of plethysmography is utilized by a forehead pulse ox?
Reflective Plethysmography
light source is next to detector
standard is transmission plethysmography
Pulse Ox location:
- Easily accessible
- less affected by VC
- ↓↑ detected faster than finger
- venous congestion may cause low readings in suping patients
- do NOT use in patients in T-berg = Inaccurate
Forehead
placed just above eyebrow
forehead vs finger
Blue Line:
Orange Line:
Blue: forehead
Orange: Finger
the relationship between SaO2 & PaO2 is ____.
nonlinear
P50: the PaO2 at which Hgb is 50% saturated → PaO2 = 27mmHg
The saturation of mixed-venous blood is about 75% →PaO2 = 40mmHg
SpO2 of 95% → PaO2 = 75mmHg
Limitations of SpO2
- cannot detect hyperoxia
- May not warn of decreasing PaO2 (until reaches <120mmHg)
- Esophageal or intubation
Left shift of Oxy-Hgb Dissociation Curve
Left = “Load Up” (O2)
- ↑ affinity for O2
- ↓ [H+]
- ↑pH
- ↓ T°
- ↓ 2,3-DPG
- HbF
Right Shift of Oxy-Hgb Dissociation Curve
Right = “Remove” O2
- ↓ affinity for O2
- ↑[H+]
- ↓pH
- ↑ 2,3-DPG
- ↑T°
standard deviations of SpO2 readings
Above 70%:
+/- 2% → 68% of time = 1SD
+/-4% → 95% of time = 2SD
Below 70%
+/- 3% → 1SD
Finger response to SpO2 changes vs Ear response
Finger: 24-50 seconds
Ear: 10-20 seconds
finger slower, but more reliable during hemodynamic instability
Averaging Concept used by Pulse Ox
Averaging prevents erroneous measurements from artifact ✅
True reductiuons likewise delayed 👎🏻
↓ Averaging Time: quick response, but artifact may be displayed as SpO2
↑ Averaging Time: slow response, but most artifact will be averaged out
“Stabilizing the Signal”
- When initially applied to patient, various light
intensities try to find strong enough signal to transmit through tissue - Once pulse found, usually several second delay to average several pulses
** In general, if the waveform on the Pleth is good, then the reading will be accurate.*
While 70% is the threshold for accuracy in normal patients, when does accuracy decrease for a critically ill patient?
<90%
poor waveform
complications of using Pulse Ox
- misinterpretation of data
- Critically ill patients
- Pressure on sensor
- Damage to skin (sensors & skin should be checked every 2 hours)
- Corneal abrasion
options to remedy low quality signal:
Site may be poorly perfused
- warm extremity (could be VC)
- check position of arm
- reposition to nose,ear, or tongue
No signal:
- check connection
- cable defective
- probe defective
What should be checked if pulse ox is sitting at 93-94% but everything else looks normal?
Mainstem intubation
Does capnography help with rapid assessment of mainstem ID?
No
