Non-Invasive Oxygen Monitoring

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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 the
  noninvasive monitors.

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ASSIGNMENTS

• Read
• Egan Chapter 16, pgs. 377 383
• AARC Clinical Practice Guidelines - Pulse
  Oximetry
• Articles on Pulse Oximetry (look on website)

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Circle of Oxygenation
Lungs Uptake PaO2 Dissolved Oxygen
Tissues Utilization ?O2 Oxygen Consumption
Blood Carrying SaO2 Carried Oxygen
Heart Pumping DO2 Oxygen Delivery
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Hypoxia Develops Rapidly
98
97
96
95
SaO2
PaO2 mmHg
? 4 gt 2 L/min
A
90
80
Central Apnea
70
Obstructive Apnea
50
30
Hypoxia Obstructive Apnea
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0
1
2
3
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from Hanning,"Oximetry and Anaesthetic Practice",
1985
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Pulse Oximetry

• The Fifth Vital Sign

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Oxygen Content

• The total amount of oxygen present in arterial
  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 of the
  amount of light that is absorbed as it passes
  through a substance.
• Reflected
• Absorbed
• Transmitted
• Spectrophotometry is an application of Beers
  (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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Misalignment of the sensor by inserting finger
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.
Saturation is based on the ratio of light
absorption during pulsatile and baseline phases.
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Functional vs. Fractional Saturation

• SpO2 ¹ 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

• A Safety Monitor must be

• RELIABLE
• CONVENIENT

• SAFE
• ACCURATE

Always detect danger and No false goodness
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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 read
  through 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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Plethysmograph 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)