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💔