I&M I: Lecture 3 - Pulse Oximetry Flashcards
What is pulse oximetry?
Noninvasive measurement of % hemoglobin saturation
It measures the oxygen saturation of hemoglobin in the blood.
What does the oxyhemoglobin dissociation curve relate?
The oxygen saturation (SaO2) of hemoglobin to the oxygen tension (partial pressure, mmHg)
This curve is essential for understanding how oxygen is released from hemoglobin.
What causes shifts in Oxyhemoglobin Dissociation Curve
Oxygen Cascade
The process in which oxygen moves down a partial pressure gradient
Environment to respiratory tract to lungs to arterial blood to capillaries to mitochondria
Oxygen in the body
Some physically dissolved in plasma
Most chemically bound to hemoglobin (oxyhemoglobin)
What is the formula for arterial oxygen content (CaO2)?
CaO2 = (Hgb * 1.36 * SaO2) + (0.003 * PaO2)
Shows most of the Oxygen is dissolved in hemoglobin
Hgb represents hemoglobin concentration, and PaO2 is the partial pressure of oxygen.
What are the objectives of pulse oximetry during anesthesia?
To ensure adequate oxygen concentration in the inspired gas and blood during all anesthetics
This is a standard monitoring requirement by the ASA.
SaO2 vs SPO2 formulas
How does pulse oximetry work?
It transmits light through tissue, detecting light absorption to monitor arterial hemoglobin oxygen saturation
ASA Standard intraop an postop… Alarms have to always be audible
The method involves rapid light pulses and detection of troughs and peaks.
Wave lengths Measured by Pulse Ox?
Transmits red (660nm) and infrared light (940nm) into tissue
Determines the ratio b/n the signals and computes SpO2
Measures the “blueness” of arterial blood b/n light source and photodetector
Reduced Hb absorbs more red light
Oxyhemoglobin absorbs more infrared light
What are the two types of pulse oximetry?
- Transmission Pulse Oximetry
Transmits light beam through vascular bed to be read on opposite side - Reflectance Pulse Oximetry
Relies on backscattered light (can be placed on forehead: ideal for burn pts)
Uses LED and photodiode
Only effective on well-perfused tissue
Many limitations
Light transmitted through to the probe must be eliminated
Scattered light from skin surface must be eliminated
Large probe area necessary to pick up weak signals
Artificial highs: vasoconstriction, artificial lows: if probe is over an artery or vein
Each type uses different methods of light detection and is suited for varying patient conditions.
Pulse Ox Equipment
Probes
Contacts the patient
Disposable
More susceptible from ambient light artifact
Reusable
More susceptible to motion artifact
Cable
Connects probe to oximeter console
Console
Part of OR physiologic monitors, stand-alone units, or transport monitors
Displays %sats, pulse, and alarm limits
Alarms must be set and functioning (ASA Standard for Basic Anesthetic Monitoring)
Variable pitch tone
Vascular Volume and Sympathetic Tone Monitoring with Pulse Ox
Skipped beats or intermittent waveform: hypovolemia
Wavelength amplitude
Waveform amp becomes more stable or returns to normal when PPV interrupted: fluid therapy trial
Dicrotic notch
Descends: increased vasodilation
Rises: vasoconstriction
Vascular Volume and Sympathetic Tone Monitoring - Wavelengths
Skipped beats or intermittent waveform: hypovolemia
Wavelength amplitude
Waveform amp becomes more stable or returns to normal when PPV interrupted: fluid therapy trial
Dicrotic notch
Descends: increased vasodilation
Rises: vasoconstriction
Pediatrics and Pulse Ox
Advantages of Pulse Oximetry
Accurate
Noninvasive
Tone modulation
User can detect changes without staring at monitor
Compact
Transport monitors for pt travel
Low risk of thermal injury
What is the main disadvantage of pulse oximetry?
Built-in delay due to 10-15s cycle of SpO2 display
Pulse ox measures changes in R value beat-to-beat, but display averages those values over 10-15s before displaying that average
Poor function with poor perfusion
Difficulty detecting high O2 PP
>90mmHg O2, small changes in sats correlate to large changes in PaO2
Delayed hypoxic event detection
Centrally located probes have lower lag times than peripheral probes
Increased lag times: poor perfusion, venous obstruction, peripheral vasoconstriction, cold, motion artifacts
Dysrrhythmias
Irregular rhythms can lead to erratic readings
Augmented diastolic pressures in aortic balloon pulsation
This delay can affect timely clinical decisions.
What can cause inaccuracies in pulse oximetry readings?
- Methemoglobin (MetHb)
- Carboxyhemoglobin (HbCO)
- Hemoglobin S
- Sulfhemoglobin
Each of these hemoglobin moieties can lead to false saturation readings.
Methemoglobin (MetHb)
<1% of total hemoglobin
Oxidation product of hemoglobin that impairs unloading of O2 to tissues (Left shift)
As MetHb increases and fxnl hemoglobin decreases, discrepancy b/n SpO2 and fxnl sats increases.
The more metHb, the reading tends to show 85% (“maxes out” at 35% metHb in blood to hit 85% SpO2 reading)
Absorbs light equally at both pulse ox wavelengths
Gives falsely low readings when sats are >85% and falsely elevated when they are <85%
Congenital or drug-acquired
Drugs: nitrobenzene, benzocaine, prilocaine, dapsone
Carboxyhemoglobin (HbCO, COHb)
Hemoglobin exposed to CO
Pulse ox reads absorption as oxyhemoglobin
SpO2 reading is over-read by the percentage of HbCO present
Carbon monoxide is particularly potent, having 200–300 times the affinity of O2 for hemoglobin, combining with it to form carboxyhemoglobin. Carbon monoxide decreases hemoglobin’s O2-carrying capacity and impairs the release of O2 to tissues.
Hemoglobin S
Debates whether pulse oximetry is accurate in sickle cell patients exist
Sulfhemoglobin
Falsely low saturation readings provided in patients with elevated sulfhemoglobin
Drugs that can ⇧ sulfhemoglobin: reglan, phenacetin, dapsone, sulfonamides
What can lead to Inaccuracies with the Pulse Ox
Low Sats
Increasingly inaccurate the lower the saturation level
Mispositioning
Check position of probe frequently, secure it well if probe is unreachable during surgery, use proper size probes
Venous Pulsations
High PPV airway pressures may lead to venous congestion, which pulse ox may read as a pulse wave: artificially low reading may be given
Skin Pigmentation
No significant difference in accuracy with various skin pigmentations, but possible inaccurate readings on very dark skin with very low perfusion
Nail Polish
Dark blues, greens, and purples can give falsely low readings
Dyes
methylene blue, indocyanine green, lymphazurin, indigo carmine, nitrobenzene, and patent blue
Temporary dips (10-60 mins for MB, 1-15 mins for ICG) in saturation reading, but actual saturation unaffected
Hyperemia
Limb re-perfusion post-tourniquet may decrease SpO2
Electrical Interference
ECUs can cause false pulse count or falsely low SpO2 reading
Some monitors freeze when interference is detected
False security mid-case
Motion Artifacts
Shivering, Parkinson’s, neuromonitoring, nerve stimulators
False-positive and false-negative errors can be made
False positives: false alarm
False negatives: missed hypoxemia
True or False: Pulse oximeters can provide accurate readings in patients with sickle cell disease.
False
There is debate about the accuracy of pulse oximetry in sickle cell patients.
What is the risk associated with using pulse oximeters on poorly perfused tissue?
Poor function and difficulty detecting high O2 partial pressure
This can lead to delayed hypoxia detection.
