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Non-Invasive Oxygen
Monitoring
Pulse Oximetry &
Conjunctival Oxygen Monitoring
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OBJECTIVES
At the end of this module, the student will
be able to
define terms associated with pulse oximetry. explain the theory of operation of a pulse
oximeter.
differentiate between functional and fractional
saturation. list the indications, limitations, advantages,
disadvantages and contraindications for use of a
pulse oximeter.
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OBJECTIVES
At the end of this module, the student
should be able to
list some of the things that affect the accuracy
of a pulse oximeter.
explain when co-oximetry would be more
appropriate than pulse oximetry.
describe patient application for each of thenoninvasive monitors.
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ASSIGNMENTS
Read:
Egan Chapter 16, pgs. 377 383
AARC Clinical Practice Guidelines - PulseOximetry
Articles on Pulse Oximetry (look on website)
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Circle of Oxygenation
Lungs
Uptake
PaO2
DissolvedOxygen
Blood
Carrying
SaO2
Carried
OxygenHeart
Pumping
DO2
Oxygen Delivery
Tissues
Utilization
O2
OxygenConsumption
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Hypoxia Develops Rapidly
SaO2%
PaO2mmHg
4 > 2 L/minA
Central Apnea
Obstructive Apnea
Hypoxia + Obstructive Apnea
from Hanning,"Oximetry and Anaesthetic Practice", 1985
0 1 2 3 4
0
20
40
60
80
100
12098
97
9695
90
8070
50
3010
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Pulse Oximetry
The Fifth Vital Sign
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Oxygen Content
The total amount of oxygen present inarterial blood.
Dissolved oxygen (PaO2)
Attached to hemoglobin (SaO2) Measured by co-oximeter
Estimated by pulse oximeter
Calculated by blood gas analyzer
Actual amount of hemoglobin (Hb)
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Oxygen Carrying Capacity
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The Science behind Oximetry
Spectrophotometry
Lambert-Beer Law
Light Transmission Spectra and Oximetry
Light Emitting Diode (LED) technology
Optical Plethysmography
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Spectrophotometry
Spectrophotometry is the measurement ofthe amount of light that is absorbed as it
passes through a substance.
Reflected Absorbed
Transmitted
Spectrophotometry is an application ofBeers (Beer-Lambert) law.
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Absorption Spectrum
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Schematic block diagram of a pulse oximeter
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Wavelength Selection
Two wavelengths are selected for analysis.
A red light at about 660 nanometers (nm).
An infrared light at about 940 nanometers.
So how do we get such specific
measurements?
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Light Emitting Diode
Technology A pulse oximeter sensor is made up of two
different parts:
An light emitting diode
A photodetector
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Misalignmentof the sensorby insertingfinger too far.
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Optical Plethysmography
Plethysmography is the measurement of
changes in volume in a particular part of
the body.
Originally designed to measure vascular
perfusion.
Changes in volume at a site is due to the
pulsation of arterial blood.
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Output signal generated by pulse oximeter. Saturationis based on the ratio of light absorption during
pulsatile and baseline phases.
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Functional vs. Fractional
Saturation SpO2 SaO2
Functional Saturation (SpO2)
Fractional Saturation (SaO2)
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Measurement vs. Monitoring
Measurement: A one time snapshot.
Example: ABG sample or a spot-check
SpO2.
Monitoring: Ongoing measurements.
Example: Continuous pulse oximetry
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Golden Rule For Safety
Monitors
SAFE ACCURATE
RELIABLE CONVENIENT
Always detect danger
and
No false goodness
A Safety Monitor must beA Safety Monitor must be::
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Technical Limitations
Calibration Assumptions
Optical Interference
Absorption Interference Signal Artifact
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Calibration Assumptions
Calibration curves
Accuracy range
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Optical Interference
Ambient Light
Optical Shunt
Optical Cross-Talk Edema
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Absorption Interference
Anemia
Skin Pigmentation
Nail Polish Carbon Monoxide
Methemoglobin
Intravascular Dyes
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Signal Artifact
Artifact can cause errors in measurement.
Artifact may obscure the signal
Causes: Low perfusion
Hypotension
Cold
Motion Artifact
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The Solution?
Improvements in technology that:
Improve the signal to noise ratio present in low
perfusion.
Improve the ability of the oximeter to readthrough motion.
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Sensor Application
Properly place sensor.
Single use sensor are meant for a single
patient.
Use sensor as designed.
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Patient Application - Sites
Adults & children Finger
Toe
Ear lobe
Cheek
Tongue
Nose
Small children & babies Wrist
Upper arm
Across foot
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Pulse oximetry probes
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Reporting Results
Four things to verify prior to reporting
results:
Waveform
Signal strength
Heart rate
Clinical Correlation
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Pulse oximeter tracing
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Oxyhemoglobin Dissociation Curve
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Contraindications (relative)
When arterial blood gases are indicated for
measurement of pH or PaCO2.
When CO-oximetry is indicated for
evaluation of total hemoglobin and
abnormal hemoglobins
HbCO%
HbMet%
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Maintenance
Pulse oximeters usually do a self-check
when they are first turned on.
They do not require regular routine
manual calibration.
Wipe probe and monitor with an alcohol
soaked cloth between patients.
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Mini Clini -Egan: Page
363
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Conjunctival
Principles of Operation
A sensor is inserted under the eyelid.
It contains a Clark PO2 electrode similar to a blood
gas analyzer.
It is used mainly in to OR for short-term application.
Conjunctival PO2 is reported and correlates to the
arterial PaO2.
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Conjunctival
PO2(PcjO2)