Clinical

Monitoring–
ANEST 307
College of Applied

MONITORING
Medical Science- Jubail

OXYGENATION
Dr. Radwa H Bakr
Assistant Professor
Anesthesia & Intensive Care
College of Medicine
Imam Abdulrahman Bin Faisal
University
Learning Outcomes
• Describe the physiology of oxygen transport in blood
• Compare the different types of cyanosis
• Discuss the principles of pulse oximetry
• Describe the advantages and disadvantages of pulse
oximeters
• Explain the sources of error in pulse oximetry and how to
troubleshoot them
 How does oxygen get into the
blood?
• One of the main functions of blood is to receive oxygen
from the lungs and transport it into the body’s tissues.
• At the same time, blood receives carbon dioxide from the
tissues, and brings it back to the lungs.
• The amount of gas dissolved in a liquid (blood, in this
case) is proportional to the pressure (partial pressure) of
the gas.
• In addition, each gas has a different solubility. Only about
0.3 ml of gaseous oxygen dissolves in 100 ml blood per
mmHg (pressure).
• This suggests that a human could not get sufficient oxygen
if solubility were the only way to get oxygen in the blood.
What is oxygen saturation?
• Hemoglobin bound to oxygen is called
oxygenated hemoglobin (HbO2). Hemoglobin not
bound to oxygen is called deoxygenated
hemoglobin (Hb). The oxygen saturation is the
ratio of the oxygenated hemoglobin to the total
hemoglobin in the blood, as defined by the
following equation.
• Oxygen saturation = HbO2 x 100
HbO2 ＋ Hb
Oxygen Saturation Physiology
• The relationship between O2 and Hgb is expressed in the following
equation:
Hgb + O2↔ HgbO2
• The reversibility of the reaction allows for the release of O2 to the
tissues
Oxyhemoglobin Dissociation Curve
• The relationship between oxygen tension and percent of
oxygen saturation is illustrated in the oxyhemoglobin
dissociation curve.
• It shows how the availability of O2 (PO2 in plasma) can
affect the reversible reaction between O2 and Hgb. The
curve demonstrates that the amount of O2 carried by Hgb
(percent saturated) increases rapidly to a PO2 of
approximately 50 and slows thereafter, as displayed by a
flattening of the curve
Oxyhemoglobin Dissociation Curve
Cyanosis
• Cyanosis is a bluish purple discoloration of the tissues
due to an increased concentration of deoxygenated
hemoglobin in the capillary bed
• Most easily seen in the lips, nail beds, earlobes, mucous
membranes, and locations where the skin is thin.
• May be enhanced or obscured by lighting conditions and
skin pigmentation.
Mechanisms
• Two mechanisms result in cyanosis:
• Systemic arterial oxygen desaturation  Central
cyanosis
• Increased oxygen extraction by the tissues  Peripheral
cyanosis.
Central cyanosis
• Central cyanosis is evident when systemic arterial concentration of
deoxygenated hemoglobin (Hb) in the blood exceeds 5 g/dL (3.1
mmol/L) (oxygen saturation ≤85 percent)
Peripheral cyanosis
• Patients with peripheral cyanosis have a normal systemic arterial
oxygen saturation.
• However, increased oxygen extraction results in a wide systemic
arteriovenous oxygen difference and increased deoxygenated
blood on the venous side of the capillary beds. The increased
extraction of oxygen results from sluggish movement of blood
through the capillary circulation.
• Causes:

• Vasoconstriction: caused by exposure to cold, venous

obstruction, elevated venous pressure, polycythemia, and low
cardiac output.
Relation between PaO2 and SaO2
• A rise in PaO2 (produced, for example, by breathing
supplemental oxygen) is accompanied by a rise in the arterial
oxygen saturation.
• However, because of the shape of the dissociation curve the
proportional increases are very different.
• Since the dissociation curve is relatively flat when the oxygen
saturation is >90%, increases in PaO2 have relatively little
impact on saturation or content in this range.
• As SaO2 approaches 100% (equivalent to a PaO2 128 mmHg),
there can be no further increase in saturation however high the
PaO2 rise.
• However, breathing increasingly higher oxygen concentrations
continues to increase the PaO2
Relation between PaO2 and SaO2
• When PaO2 falls (60 mmHg) there is a steep decline in
oxygen saturation (small reductions in PaO2 are
accompanied by disproportionately large reductions in
oxygen saturation)
• To some extent the reduced oxygen delivery is countered
by the greater ease with which oxygen can be “offloaded”
to the tissues.
• This portion of the dissociation curve could be regarded
as a “life-saving escalator” as a small increase in PaO2 in
a severely hypoxemic individual is likely to result in a
marked improvement in oxygen saturation, content and
supply
Pulse oximetry
• Pulse oximetry measures peripheral arterial oxygen
saturation (SpO2) as a marker for tissue oxygenation.
• It has become the standard for continuous, noninvasive
assessment of oxygenation and is often considered the
"fifth vital sign"

The difference between SpO2 and SaO2

• Oxygen saturation can be assessed by SaO2 or SpO2.

• SaO2 is oxygen saturation of arterial blood (called arterial

blood oxygen saturation)
• SpO2 is oxygen saturation as detected by the pulse
oximeter (called percutaneous oxygen saturation)
Principles and Equipment
• Pulse oximetry uses
spectrophotometry to determine the
proportion of hemoglobin that is
saturated with oxygen (ie,
oxygenated hemoglobin;
oxyhemoglobin) in peripheral
arterial blood.
•Light at two separate wavelengths illuminates oxygenated and
deoxygenated hemoglobin in blood.
•The ratio of light absorbance between oxyhemoglobin and the sum
of oxyhemoglobin plus deoxyhemoglobin is calculated and compared
with previously calibrated direct measurements of arterial oxygen
saturation (SaO2) to establish an estimated measure of peripheral
arterial oxygen saturation (SpO2)
Probes
• Pulse oximeter probes consist of two light-emitting diodes
and a photodetector.
• Emitters:
Deoxyhemoglobin absorbs light maximally in the red band of the
spectrum (600 to 750 nm), and oxyhemoglobin absorbs maximally
in the infrared band (850 to 1000 nm). Thus, the emitters emit light
at 660 nm and 940 nm for optimal detection of these two
substances.
• Detector
The detector (also known as sensor) detects the absorbance of
light from exposed tissue. The values are processed and a
saturation determined.
Probes
• In general, detectors and emitters are positioned facing each other
through interposed tissue.
• Probes are most frequently placed on the anterior and posterior
aspect of fingers, toes, ear lobes, nasal ala
• In infants, probes may also be placed on the palms, feet, arms,
cheeks, tongue, penis, nose, or nasal septum.
• Forehead probes have the emitter and detector adjacent to each
other so that saturation is measured from light that is reflected back
from (not through) exposed tissue.
• Ear probes and forehead probes respond more quickly to a change
than conventional finger probes
• Some probes are reusable clips, which are low in cost; others are
single-patient adhesive probes, which are higher in cost but have
less potential to transmit hospital-acquired infections.
Advantages
1. Rapid:
•Pulse oximetry is a rapid tool that accurately assesses
oxygenation, particularly in emergency situations.
Oxygenation is difficult to assess on the basis of physical
examination alone.
•Frank cyanosis does not develop until the level of
deoxyhemoglobin reaches 5 g/dL, which corresponds to an
arterial oxygen saturation (SaO2) of around 80 percent
•In addition, the threshold at which cyanosis becomes
apparent is affected by multiple variables including
peripheral perfusion, skin pigmentation, and hemoglobin
concentration
Advantages
2. Noninvasive
Blood gas analysis by arterial puncture or arterial line
sampling was for many years the only available method of
detecting hypoxemia, but this technique is painful and has
potential complications.
3. Provides continuous data
4. Pulse oximeters also display pulse rate and pulse
amplitude
Limitations
1. Inability to detect hyperoxemia
• Pulse oximetry is unable to detect significant
hyperoxemia. This is due to the shape of the oxygen-
hemoglobin dissociation curve, where large changes in
the partial pressure in arterial oxygen (PaO2) may result
in no change in oxygen saturation if the saturation is
already near 100 percent
Limitations
2. Inability to measure arterial oxygen tension
• Since pulse oximetry does not measure PaO2,
overreliance on pulse oximetry can miss detection of
clinically significant hypoxemia in adults but particularly
in children
• A large decrease in PaO2 will not produce a significant
fall in SaO2 until the steeper portion of the oxygen
hemoglobin dissociation curve is encountered at a PaO2
of approximately 60 to 70 mmHg.
Limitations
3. Inability to measure ventilation:
• While pulse oximetry is a convenient way of measuring
arterial oxygenation, it does not measure the arterial
carbon dioxide tension (PaCO2), which is a measure of
ventilation
• Supplemental oxygen administered on the basis of
oximetry alone in patients with hypercapnia may be
harmful by worsening hypercapnia (ie, oxygen-induced
hypercapnia).
• Therefore, when hypercapnia or hypoventilation is
suspected, an arterial blood gas should be obtained
COMPLICATIONS
• Extremely rare

1. Digital injury:
On rare occasions in critically ill patients
2. Burns have also been reported in patients undergoing
magnetic resonance imaging (MRI), although this
complication can be avoided by temporarily removing the
probe during MRI. This results from the generation of
electrical skin currents beneath the pulse oximeter cables
Applications
• Pulse oximetry is indicated in any clinical setting where hypoxemia may occur:
• Emergency departments

• Operating rooms

• Emergency medical services (EMS)

• Postoperative recovery areas

• Endoscopy suites

• Sleep and exercise laboratories

• Oral surgery suites

• Cardiac catheterization suites

• Facilities that perform conscious sedation

• Labor and delivery wards

• Inter-facility patient transfer units

• Patients' homes
Interpreting the Results
1. Peripheral oxygen saturation:
• As measured by pulse oximetry (SpO2) provides accurate
information on tissue oxygenation
2. Waveform analysis:
• Peripheral oxygen saturation can only be interpreted
when the waveform is normal. A normal pulse oximeter
waveform has a dicrotic notched appearance typical of an
arterial waveform that synchronizes with heart rate.
Pulse oximeter waveform

Common pulsatile signals on a pulse oximeter.

(A) Normal signal showing the sharp waveform with a clear dicrotic notch.
(B) Pulsatile signal during low perfusion showing a typical sine wave.
(C) Pulsatile signal with superimposed noise artifact giving a jagged
Correlation with arterial oxygen saturation
• Pulse oximetry provides an estimate of the proportion of
hemoglobin that is saturated with oxygen (ie, SpO2). In
most patients with SpO2 values of 90 percent or higher,
the value lies within 2 to 3 percent above or below the true
arterial saturation (SaO2) reference standard.
• However, the accuracy worsens when the SaO2 is <90
percent, and especially below 80 percent. Thus, SpO2 is
less reliable in critically ill patients where oxygenation can
rapidly fluctuate and desaturations are common.
Optimal oxygen saturation
• There is no optimal level of oxygen saturation, below
which tissue hypoxia occurs because of the large number
of variables that contribute to hypoxia at the tissue and
cellular level (temperature, pH, tissue blood flow).
• However, at sea level, resting oxygen saturation ≤95
percent or exercise desaturation of ≥5 percent as
abnormal.
Examples
• A resting oxygen saturation of 96 percent could be
abnormal if a patient previously had a resting oxygen
saturation of 99 percent.
• A target level of 88 to 92 percent may be sufficient in a
patient with an acute exacerbation of chronic obstructive
pulmonary disease (COPD) who is chronically
hypercapnic.
• A target saturation of >95 percent may be considered
optimal in a pregnant woman with acute respiratory
distress syndrome.
Tissue hyperoxia
• There is no optimal level of oxygen saturation above
which tissue hyperoxia occurs since hyperoxia cannot be
assessed with oximetry or arterial blood gas analysis.
• Example:
• In patients with an exacerbation of chronic obstructive
pulmonary disease (COPD) with chronic hypercapnia,
targeting levels ≥92 percent may worsen hypercapnia,
while patients with carbon monoxide (CO) poisoning or air
embolism should be treated with 100 percent oxygen or
hyperbaric oxygen to eliminate CO or air, respectively.
TROUBLESHOOTING SOURCES OF ERROR

• Pulse oximetry is subject to artifacts and patient-related

sources of error.
• The best defense against error is a high index of
suspicion.
• If a saturation reading is in doubt, a health care worker
can perform a quick check by putting the probe on his or
her own finger, this ensures that abnormal readings are
not due to equipment error.
TYPES OF ERROR
•Inadequate waveform
•Falsely normal or high readings
•Falsely low readings
A. Inadequate waveform
• A normal pulse oximeter waveform has a dicrotic notched
appearance similar to an arterial waveform.
• Waveforms are considered inadequate when the dicrotic
notched appearance is lost
• Many of the reasons for an inadequate waveform will also
give erroneously low readings.
1. Improper probe placement
• Improper probe placement is associated with loss of
amplitude of the waveform (ie decreased or flattened
pulse oximetry amplitude):
• Malposition or poor attachment to the skin can result in
either a falsely elevated or depressed reading
• These problems can be resolved by ensuring the probe is
properly attached with the light sources and detectors
opposite
• Placement of the sensor on the same extremity as a
blood pressure cuff or arterial line can cause false
readings
2. Motion artifact
• Motion artifact will cause falsely lower oximetry readings
• This most commonly results from motion due to
• Shivering

• Seizure activity

• Pressure on the sensor

• Transport of the patient by ambulance or helicopter

• The waveform will appear erratic and lose its normal

shape.
3. Hypoperfusion
• Pulse oximetry readings can be falsely low due to signal
failure due to hemodynamic instability or poor limb
perfusion
• In adults, the accuracy of standard pulse oximeters
decreases dramatically when systolic blood pressure falls
below 80 mmHg, resulting in underestimation of the actual
arterial oxygen saturation
• Repositioning the probe or using an alternate site may
help.
• When in doubt an arterial blood gas should be drawn.
4. Hypothermia
• Hypothermia may interfere with pulse oximetry because of
the associated peripheral vasoconstriction and shivering
• This can lead to a delay in the recognition of acute
hypoxemia, particularly if finger probes are used
Hypothermic patients should be monitored using an ear or
forehead probe
• Warming should correct the issue
B. Falsely normal or high reading
1. Carboxyhemoglobin

• High levels of carboxyhemoglobin are found in carbon

monoxide (CO) poisoning, or in chronic, heavy smokers
• Carboxyhemoglobin absorbs approximately the same amount
of 660 nm light (red light) as oxyhemoglobin the pulse
oximetry reading represents the sum of oxyhemoglobin and
carboxyhemoglobin
• This may mask life-threatening arterial desaturation.

• Arterial oxygen tension (PaO2) measurements tend to be

normal because PaO2 reflects O2 dissolved in blood, and this
process is not affected by CO.
• When carboxyhemoglobinemia is suspected, co-oximetry is
used to measure carboxyhemoglobin levels.
2. Glycohemoglobin A1c
• Glycohemoglobin A1c levels greater than 7 percent in
type 2 diabetics with poor glucose control have been
shown to result in overestimation of arterial oxygen
saturation (SaO2) by pulse oximetry.
• This may be due to an increased hemoglobin oxygen
affinity.
• If in doubt arterial blood gas analysis may be done
C. Falsely low readings
1. Methemoglobin
• Methemoglobinemia occurs when RBCs contain
methemoglobin at levels higher than 1%.
• Methemoglobin results from the presence of iron in the ferric
(Fe+3) form instead of the usual ferrous (Fe+2) form. This
results in a decreased availability of oxygen to the tissues.
• It can be congenital or acquired due to certain drugs (e.g
nitrates)
• Treatment : methylene blue and vitamin C

• Methemoglobin absorbs light at both 660 and 940 nm.

• High levels can falsely reduce the SpO2, trending towards

85 percent.
2. Sulfhemoglobin
• Sulfhemoglobinemia is most commonly caused by the
ingestion of oxidizing drugs (eg, sulfonamides,
metoclopramide, nitrates).
• High levels can falsely reduce the SpO2, trending towards
85 percent, similar to methemoglobin
3. Severe anemia
• Low hemoglobin concentrations cause falsely low
readings
• This effect is not clinically significant until the hemoglobin
level is less than 5 g/dL.

5. Nail polish
• The use of nail polish can potentially affect pulse oximeter
readings if the polish absorbs light at 660 nm and/or 940
nm
• Artificial acrylic nails may also affect the accuracy of pulse
oximetry readings.
Vital dyes
• Vital dyes include:
• Methylene blue (used to treat methemoglobinemia, or during
endoscopy)
• Indocyanine green (used for measuring cardiac output, for
ophthalmic angiography, or for measuring liver blood flow)
• Fluorescein (ophthalmic angiography) and isosulfan blue (used
intraoperatively to mark breast and melanoma tumors)
• They can cause falsely low pulse oximetry readings due
to absorption of light at 660 nm or 940 nm
• These effects are transient and resolve rapidly as the
dyes are diluted and metabolized
CO-OXIMETRY
• Multi-wavelength co-oximeters use four, rather than two,
wavelengths of light to detect oxyhemoglobin,
deoxyhemoglobin, carboxyhemoglobin, and
methemoglobin.
• They require a sample of arterial blood and a specific
laboratory request for its measurement.
References:
• Morgan and Mikhail's Clinical Anesthesiology
• http://www.uptodate.com