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Pulse-Oximetry
Presented By
Moderated By
Dr Satish Negi
Dr Manoj Panwar
Defination
• A non invasive technology to monitor oxygen
saturation of haemoglobin.
History
• In early 1940.Glen Malkikan coined the term
oximeter.
• MATHEES- father of oximetry
20 papers in1934 –1944
• HERTZMAN 1937 –use of photoelectric finger
plethsmography
• 1975 –concept of pulse oximetry –Japan
• In 2008 modification continued and term High
Resolution Pulse Oximetry come into existence
Introduction
• Also called the fifth vital sign
• low SpO2 provide warning of hypoxemia before other signs
such as cyanosis or a change in heart rate are observed.
• Until the 1980s, noninvasive oximeters, known as ear
oximeters, were large, expensive, and cumbersome.They
required “arterialization” by heat or chemical treatment,
and their utility was limited by difficulties in differentiating
light absorbance of arterial blood from that of venous
blood and tissues.
• Technical advances, including light-emitting diodes (LEDs),
miniaturized photodetectors, and microprocessors, allowed
the creation of a new generation of oximeters, which were
smaller, less expensive, and easier to use.
• These differentiate the absorption of light by the pulsatile
arterial component from the static components, so they are
called pulse oximeters
• Oxygen Saturation
• Saturation is defined as ratio of O2 content to
oxygen capacity of Hb - expressed as percentage.
• Desaturation leads to Hypoxemia – a relative
deficiency of O2 in arterial blood. (PaO2 <
80mmHg – hypoxemia)
• Oxygen saturation will not decrease until
PaO2 is below 85mmHg.
• At SaO2 of 90%, PaO2 is already 60mmHg.
• Rough guide for PaO2 between saturation of
90%-75% is PaO2 = SaO2 - 30.
• (SaO2< than 76% is life threatening.)
Fractional Saturation
• This is Ratio of oxygenated Haemoglobin to
sum of all haemoglobin in blood.
Fractional saturation =
HbO2
-------------------------HbO2+ Hb+ Met Hb +CO Hb
Functional Saturation
• This is a measure of ratio between HbO2 and
sum of oxygenated and reduced Hb.
• Functional Saturation=
HbO2
-----------HbO2 + Hb
Cont..
• These must be determined by using an
in vitro oximeter.
• For patients with low dyshemoglobin levels,
difference between fractional and functional
saturation is very small.
• when dyshemoglobin levels are elevated, two
values can vary greatly, and pulse oximeter
readings may not agree with either the true
fractional or functional saturation values.
PRINCIPLES
• All atom and molecules absorbed specific wavelength
of light.this property is the basis for an optical
technique known as spectrophotometry.
• Beer Lambert Law
• Lambert’s Law- states that when a light falls on a
homogenous substance,intensity of transmitted light
decreases as the distance through the substance
increase
• Beer’s Law- states that when a light is transmitted
through a clear substance with a dissolved solute ,the
intensity of transmitted light decreases as the
concentration of the solute increases
Contd.
• Substances have a specific pattern of absorbing
specific wavelength – Extinction coefficient
Uses two lights of wavelengths
• 660nm –deoxy Hb absorbs ten times as oxy Hb
• 940 nm – absorption of oxyHb is greater
• Lab oximeters use 4 wavelengths to measure 4
species of haemoglobin
Optical Recognition of Hemoglobin
• Hemoglobin(like all protein)changes its structural
configuration when it participate in chemical
reaction,and each of the configuration has
distinct pattern of light absorption.
• Four different forms of hemoglobin represented
are :oxygenated
hemoglobin(Hbo2),Deoxygenated hemog
lobin(Hb),Methemoglobin(MetHb) and
Carboxyhemoglobin(CoHb)
Cont..
• Comparing the oxygenated and deoxygenated
form of hemoglobin in the Red region of light
spectrum(660nm)oxygenated hemoglobin does
not absorbed light as wel as deoxygenated
Hb.(that’s why oxygenated blood is more
intensely red than deoxygenated blood).
• While in infrared region(940nm)The opposite is
true,and oxygenated Hb absorbed more light
effectively than deoxygenatedHb.
Extinction
MetHb
coefficient
Oxy Hb
Deoxy
Hb
COHB
660nm
wavelength
940nm
Operating Principles
• The pulse oximeter computes the ratio
between these two signals and relates this
ratio to the arterial oxygen saturation, using
an empirical algorithm.
• Pulse oximeters discriminate between arterial
blood and other components by determining
the change in transmitted light caused by the
flow of arterial blood
• The oximeter pulses the red and infrared LEDs ON and
OFF several hundred times per second.
• The rapid sampling rate allows recognition of the peak
and trough of each pulse wave.
• At the trough, the light is transmitted through a
vascular bed that contains mainly capillary and venous
blood as well as intervening tissue.
• At the peak, it shines through all this plus arterial
blood. A photodiode collects the transmitted light and
converts it into electrical signals.
• The emitted signals are then amplified, processed, and
displayed on the monitor.
Accuracy
• A clinically acceptable level of arterial
oxygenation(SaO2 above 70%),the oxygen
saturation recorded by pulse oximeters(Spo2)
differs by less than 3% from the actual
saturation.
• pulse oximetry also show a high degree
consistency of repeated measurements.
Physiology
.• Oxygen – haemoglobin dissociation curve
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Pao2 mmhg
120
110
92
80
74
69
66
63
60
58
56
51
47
40
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Spo2
100
98
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95
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90
89
86
83
75
• Pulse oximetry uses light to work out oxygen
saturation. Light is emitted from light sources
which goes across the pulse oximeter probe and
reaches the light detector.
• If a finger is placed in between the light source
and the light detector, the light will now have to
pass through the finger to reach the detector.
Part of the light will be absorbed by the finger
and the part not absorbed reaches the light
detector.
• The amount of light that is absorbed by the finger
depends on many physical properties and these
properties are used by the pulse oximeter to
calculate the oxygen saturation.
• The amount of light absorbed depends on the
following:
• 1. concentration of the light absorbing substance.
• 2. length of the light path in the absorbing
substance
• 3. oxyhemoglobin and deoxyhemoglobin absorbs
red and infrared light differently
Type of Oximetry
• Transmission Pulse Oximetry
• light beam is transmitted through a vascular bed and is
detected on opposite side of that bed.
• Reflectance Pulse Oximetry
• relies on light that is reflected to determine oxygen saturation.
The probe has both an LED and a photodiode.
advantage - its signal in low perfusion is better.
limitations -The probe design must eliminate light that is
passed directly to the probe or is scattered in the outer surface
of the skin. The signals are weaker than those found in
transmission. this interfere in noisy enviorement and it is costly.
PLETHYSMOGRAPHY
• Pulse oximeters show pulsatile change in
absorbance in a graphical form. This is called
the "plethysmographic trace " or "pleth“
• It tells you how good the pulsatile signal is.
• If quality of pulsatile signal is poor- then
calculation of oxygen saturation may be
wrong.
• The pulse oximeter uses very complicated
calculations to work out oxygen saturation
Equipment
• Probes
• The probe (sensor, transducer) is the part that comes in
contact with the patient.
• It contains one or more LEDs (photodiodes) that emit
light at specific wavelengths and a photodetector
(photocell, transducer).
• The LEDs provide monochromatic light. This means
that they emit a constant wavelength throughout their
life, so they never need recalibration.
• Probes may be reusable or disposable. They have the
same accuracy.
• Self-adhesive probes are less susceptible to motion artifact
and are less likely to come off if the patient moves than
those that clip on.
• However, they are usually not as well shielded from
ambient light as clip-on probes.
• probes are available in different sizes. If a probe is too large
for the patient, some of the light output from the LED can
reach the photocell without passing through tissue, and
falsely high SpO2 readings will be produced .
• The photocell may not align with the probe, and readings
will not be possible.
• To reduce contamination, a glove, the finger of a glove,
or other covering may be used either over the
application site or over the probe .
• Cable
• The probe is connected to the oximeter by an electrical
cable. Cables from different manufacturers are not
interchangeable.
• Console
• Many different consoles are available . Most oximeters
that are used in the operating room are part of a
physiologic monitor. Most stand-alone units are line
operated but will work on batteries, making them
useful during transport.
• Most instruments provide an audible tone whose pitch
changes with the saturation. operator can be made
aware of changes in SpO2 without looking at the
oximeter.
• Alarms are commonly provided for low and high pulse
rates and low and high saturation. Many units
generate an alarm when the probe is not properly
applied to the patient or if the signal is inadequate.
• ASA standards for Basic Anesthetic Monitoring require
that the variable pitch pulse tone and low threshold
alarm be audible
• Oximeter Standards
• There must be a means to limit the duration of continuous
operation at temperatures above 41°C.
• The accuracy must be stated over the range of 70% to 100%
SpO2. If the manufacturer claims accuracy below 65%, the
accuracy must be stated over the additional range.
• If the manufacturer claims accuracy during motion, this and
the test methods used to establish it must be disclosed in
the instructions for use.
• If the manufacturer claims accuracy during conditions of
low perfusion, this and the test methods used to establish
it must be disclosed in the instructions for use.
• There must be an indication when the SpO2 or pulse
rate data is not current.
• If the pulse oximeter is provided with any physiologic
alarm, it must be provided with an alarm system that
monitors for equipment faults, and there must be an
alarm for low SpO2 that is not less than 85% SpO2 in
the manufacturer-configured alarm preset. An alarm
for high SpO2 is optional.
• An indication of signal inadequacy must be provided if
the SpO2 or pulse rate value displayed is potentially
incorrect.
• Any discoloration of the nail bed can affect the
transmission of light through the digit.
• dark nail polish, such as blue, green, brown, or
black colors, and bruising under the nail can
limit the transmission of light and result in an
artificially decreased Spo2 value.
• If the nail polish cannot be removed, the sensor
can be placed in a lateral side-to-side position
on the finger to obtain readings if no other
method of sampling the arterial bed is available
Sites of probe placement
• A disadvantage of placing a probe on an extremity is that detection
of desaturation and resaturation is slower than when probes are
placed more centrally . Response time may be quicker when the
probe is placed on the thumb.
• Motion artifacts are less frequent when the probe is placed on one
of the larger fingers.
• The probe should not be on the index finger during recovery. As a
patient awakens, the patient often will want to rub his or her eye,
usually with the index finger. If the oximeter probe is on that finger,
the cornea can be scratched.
• In general, the arm opposite from that on which the blood pressure
cuff is applied or in which an arterial catheter has been inserted
should be used.
• The pulse oximeter is sometimes integrated with the
noninvasive blood pressure monitor so that the pulse
oximeter will not alarm during the inflation cycle if
placed on the same arm as the blood pressure cuff.
• Occasionally, poor function may occur with probe
attachment to the same extremity as the intravenous
infusion, due to local hypothermia and
vasoconstriction
• Toe
Detection of desaturation or resaturation will not be as
rapid as with more centrally placed probes .
• The toe may provide a more reliable signal in patients who
have had an epidural block .
• Nose
• Nasal probes respond more rapidly to changes in
saturation than probes placed on extremities. The bridge,
the wings of the nostrils, and the nasal septum can be used
• It has been recommended under conditions such as
hypothermia, hypotension, and infusion of vasoconstrictor
drugs.
• If the patient is placed in the Trendelenberg position,
venous congestion may occur around the nose, causing the
pulse oximeter to display artificially low saturations
• Ear
• The earlobe should be massaged for 30 to 45 seconds with
alcohol or vasodilator or EMLA cream can be applied for 30
minutes prior to probe application to increase perfusion.
• useful when there is finger motion.
• A steep head down position may result in erroneous
readings.
• Tongue
• A tongue probe can be made by placing a malleable
aluminum strip behind the probe to allow it to bend around
the tongue.
• This site may be especially useful in patients who have burns
over a large percentage of their body surface .
• Desaturation and resaturation is detected by a probe at the
tongue quicker than one on the finger or toe.
• A lingual probe is more resistant to signal interference from
electrosurgery than probes placed on peripheral sites.
• Cheek
• A probe with a metal strip backing can be used to hold a disposable
probe around the cheek or lips.
• Buccal pulse oximetry is more accurate than finger pulse oximetry.
• effective during hypothermia, decreased cardiac output, increased
systemic vascular resistance, and other low pulse pressure states.
• Esophagus
• This probe uses reflectance oximetry. The esophagus, a core organ,
is better perfused than the extremities during states of poor
peripheral perfusion
• It reflects changes in arterial saturation more quickly than
peripheral sites such as the finger.
• useful for patients who have extensive burns
• Forehead
• A flat reflectance pulse oximeter sensor can be used on the
forehead. Pressure on the probe from a headband or
pressure dressing may improve the signal
• The forehead is less affected by vasoconstriction from cold
or poor perfusion than the ear or finger.
• Changes in saturation can be detected more rapidly at the
forehead than at the finger. However, pooling of venous
blood due to compromised return to the heart may cause
low saturation readings in supine patients
• Pharyngeal pulse oximetry by using a pulse oximeter
attached to a laryngeal mask may be useful in patients
with poor peripheral perfusion.
• Flexible probes may work through the palm, foot,
penis, ankle, lower calf, or even the arm in infants
• Pulse oximetry may be used to monitor fetal
oxygenation during labor by attaching a reflectance
pulse oximetry probe to the presenting part .
• A disadvantage is that the probe has to be placed
blindly and may be positioned over a subcutaneous
vein or artery, which will affect the reliability of the
readings.
uses
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Monitoring oxygenation: PACU,transport.
Detect inadvertent bronchial intubation
Managing one lung anaesthesia
Weaning from artificial ventilation
Controlling 02 administration(lowest safe flow
and concentration)
• Monitoring peripheral circulation(distal to
fracture) but not helpful in warning compartment
syndrome.
• Evaluation of sympathetic block
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SBP measurement
Locating vessels(axillary artery)
Avoiding hyperoxemia
Monitoring hypovolemia(skipping beats)
Monitoring sympathetic tone(dicrotic notch)
Pulse rate
Arrhythmias
pulse oximetry can balance blood gas analysis
neonatal care(one element of sugesting congenital
heart ds.)
• During thoracic anaesthesia
• Limitations and Disadvantages
• Failure to Determine the Oxygen Saturation
• Factors that are reported to contribute to higher failure rates
include ASA physical status 3, 4, or 5 patients, orthopedic, vascular,
and cardiac surgery; electrosurgery use; hypothermia; hypotension;
low hematocrit; and motion
• Poor Function with Poor Perfusion
• Pulse oximeters require adequate pulsations to distinguish light
absorbed from arterial blood from venous blood.
• Readings may be unreliable or unavailable if there is loss or
diminution of the peripheral pulse (proximal blood pressure cuff
inflation, external pressure, improper positioning,
hypotension,hypothermia, Raynaud's phenomenon, low cardiac
output, hypovolemia, peripheral vascular disease
• Erratic Performance with Dysrhythmias
• double- or triple-peaked arterial pressure waveform that confuses the pulse
oximeter, so it may not provide a reading
• Different Hemoglobins
• Most pulse oximeters are designed to detect only two species of hemogobin:
reduced and oxygenated. Whole blood often contains other moieties such as
carboxyhemoglobin, sulfhemoglobin, and methemoglobin.
• Methemoglobin
• Normally less than 1% of the total hemoglobin, methemoglobin (metHb) is an
oxidation product of hemoglobin that forms a reversible complex with oxygen
and impairs the unloading of oxygen to tissues.
• Methemoglobin absorbs light equally at the red and infrared wavelengths that
are used by most pulse oximeters. When compared with functional saturation,
most pulse oximeters give falsely low readings for saturations above 85% and
falsely high values for saturations below 85% .
• Carboxyhemoglobin
• Carboxyhemoglobin has an absorption spectrum similar to
that of oxyhemoglobin at 660nm, so most pulse oximeters
will over-read SpO2 by the percentage of
carboxyhemoglobin present
• An increase in HbCO may occur during laser surgery in the
airway, but the levels are not high enough to keep pulse
oximetry from reliably estimating saturation .
• Pulse oximeters that differentiate between oxyhemoglobin
and carboxyhemoglobin are now available
• Fetal Hemoglobin
fetal hemoglobin (Hb F) does not affect the accuracy of pulse
oximetry to a clinically important degree
• Sulfhemoglobin
• Sulfhemoglobinemia may be caused by drugs such as
metoclopramide, phenacetin, dapsone,and sulfonamides.
Sulfhemoglobin causes the pulse oximeter to display artifactually
low oxygen saturation.
• Bilirubinemia
• Severe hyperbilirubinemia can cause an artifactual elevation of
metHb and carboxyhemoglobin when using in vitro oximetry but
does not affect pulse oximetry readings.
• Low Saturations
• Pulse oximetry becomes less accurate at low oxygen saturations .
This inaccuracy is greater in patients with dark skin. It should be
used with caution in patients with cyanotic heart disease.
• Malpositioned Probe
• Prominent pulsations of venous blood may lead to
underestimation of the SpO2.
• High airway pressures during artificial ventilation may cause
phasic venous congestion, which may be interpreted by the
oximeter as a pulse wave. In some cases, it may be necessary
to turn the ventilator OFF to obtain a correct reading.
• Mixing Probes
• SpO2 measurements may not be accurate if one
manufacturer's probe is used with a different manufacturer's
instrument
• Severe Anemia
• The pulse oximeter may overestimate SpO2, especially at low
saturations, in patients with severe anemia.
• Skin Pigmentation
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Nail Polish and Coverings
Some shades of brown, black, blue, and green (but not red or purple) nail
polish may cause significantly lower saturation readings
In most cases, this problem can be overcome (without removing the polish
or the synthetic nail) by turning the probe 90 degrees so that it transmits
light from one side of the finger to the other side
Electrical Interference
Electrical interference from an electrosurgical unit can cause the oximeter
to give an incorrect pulse count . Some monitors display a notice when
significant interference is present. Some freeze the SpO2 display
Additional steps to minimize electrical interference include
locating the oximeter probe as far from, the surgical field as possible;
keeping the pulse oximeter probe and console as far as possible from the
surgical site. The electrosurgical apparatus and pulse oximeter should not
be plugged into the same power circuit
Problem of movement
• When we think of problems associated with pulse
oximeters it is important to remember that the
signal that is analyzed is really tiny. it is only
about 2 % of the total light that is analyzed.which
such a small signal, it is easy to see how errors
can occur.
• Pulse oximeters are very vulnerable to motion,
such as a patient moving his hand. As the finger
moves, the light levels change dramatically. Such
a poor signal makes it difficult for the pulse
oximeter to calculate oxygen saturation.
Problem of optical shunting
• The pulse oximeter operates best when all the light
passes through arterial blood.
• However, if the probe is of the wrong size or has not
being applied properly, some of the light , instead of
going through the artery, goes by the side of the artery
(shunting)(lower finger in image below).
• This reduces the strength of the pulsatile signal making
the pulse oximeter prone to errors. It is therefore
important to select the correct sized probe and to
place the finger correctly in the chosen probe for best
results.
Problem of too much ambient light
• in addition to the light from the LEDs, ambient (room) light
also hits the detector. For good functioning of the pulse
oximeter, the strength of the LED light falling on the
detector should be good when compared with the strength
of the ambient light falling on the detector.
• If the ambient light is too strong, the LED light signal gets
"submerged" in the noise of the ambient light. This can lead
to erroneous readings. Therefore, it is important to
minimise the amount of ambient light falling on the
detector.
• One can try and move away strong sources of room light.
One can also try and cover the pulse oximeter probe and
finger with a cloth etc.
Problem of not detecting hyperoxia
• all the hemoglobin are carrying oxygen, and
therefore the oxygen saturation is 100 %.
However, hemoglobin is not the only way oxygen
is carried in blood. Additional oxygen can also be
dissolved in the solution in which red blood cells
travel (plasma).
• The problem is that the pulse oximeter cannot
"see" the extra dissolved oxygen. So even though
this patients blood is full of extra oxygen, the
saturation still shows 100 %, instead of say 120 %.
Methods to improve signals
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Application of vesodilating cream(GTN cream)
Digital nerve block
Administration of intraarterial vesodilators
Placing a gloves filled with warm water over
patient hand.
Warm cool extremities
Try an alternative probe site.
Try a different probe
Try a different machine
Patient Complications
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Corneal Abrasions
Pressure and Ischemic Injuries
Burns
Injuries ranging from reddened areas to third-degree burns
under pulse oximeter probes have been reported
• Burns can result from incompatibility between the probe
from one manufacturer with the pulse oximeter of another.
A pulse oximeter probe may provide an alternate pathway
for electrosurgical currents
• When a probe is placed on a finger or toe, the light source
should be placed on the nail rather than on the pulp .
SET
• Masimo SET: begins with conventional R and IR
photoplethysmographic signals and then employs a
constellation of advanced techniques including
radiofrequency and light shielded optical sensors and
adaptive filteration to measure spo2
• Employs discrete saturation transform(DST) to isolate
individual saturation components in the optical
pathway
• When tissue under analysis is stationary, only single
saturation component(pulsatile arterial blood) is
present
• Masimo SET enabling the assessment
• Motion and Low Perfusion pulse oximetry.
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> Acoustic Respiration Rate
> Carboxyhemoglobin
> Methemoglobin
> Oxygen Content
> Pleth Variability Index
> Total Hemoglobin
> Oxygen Saturation
> Pulse Rate (PR)
> Perfusion Index
• Alternative to pulse oximeter are• 1)Band Co-oximeter 2)Blood gas analysis
• THANK YOU