Lawand Hiwa Abdulla

S4/ Lecture 2 -> 2025/04121

PULSE OXIMETRY
1. it measures the O2 saturation of Hb in arterial blood

2. it measures the heart rate

3. give an idea about tissue perfusion by pulse wave form

-
is a non-invasive measurement of the arterial blood
oxygen saturation at the level of arterioles.
Pulse oximetry has proved to be a powerful monitoring
tool in the operative theatre, recovery wards, intensive
care units, general wards and during the transport of
critically ill patients.
Loading…
 Ideal site

1. well perfused
2. relatively immobile
3. comfortable for the patient
4. easily accessible
 Note/ Never
apply Bo cuff and pulse
oximetry the same have
on

apply The B3P Cuff and cannula hand and pulse

in theother hand
.
in a ,
oximetry
Components
1. A probe is positioned on the finger, toe, ear
lobe or nose Two light-emitting, diodes (LEDs)
-

produce beams at red and infrared frequencies

(660 nm and 940Loading…
nm respectively) on one side and
there is a sensitive photo detector on the other
side.
absorbs
- Reduced hemoglobin >
- Red.
Ideoxy hemoglobin)
oxyhemoglobineabs Infrared
2. The case houses the
microprocessor. There is a display
of oxygen saturation, pulse rate and
a plethysmographic
waveform of the pulse.
Alarm limits can be set for a low
saturation value and for both high
and low pulse rates.
 % X
·
j,
3

Mechanism of action s ⑧
~

1. oxygen saturation is estimated by measuring

the Transmission of light, through a pulsatile
vascular tissue bed (e.g. finger). T
d
,
%
·
This is based on Beer’s law (the relation between
the light absorbed and the concentration of solute
in the solution) and Lambert’s law (relation
between absorption of
light and the thickness of absorbing layer).
2. The amount of light transmitted
depends on many factors. The light
absorbed by non-pulsatile tissues
(e.g. skin, soft tissues, bone and
venous blood) is Constant (DC).
The non-constant absorption (AC) is
the result of arterial blood pulsations
Photo detector generates a
voltage proportional to the
transmitted light. The AC
Loading…
component of the wave is
about 1–5% of the total
signal.
3. The high frequency of the LEDs
allows the absorption to be
sampled many times during each
pulse beat. This is used to enable
running averages of saturation
to be calculated
 4. The microprocessor is
programmed to mathematically
analyze both the DC and AC
components at 660 and 940 nm
calculating the ratio of
absorption at these two
frequencies (R/IR ratio). The
result is related to the arterial
saturation.
The absorption of oxyhaemoglobin and
deoxyhaemoglobin at these two wave
lengths is very different. This allows these
two wavelengths to provide good
sensitivity.
805 nm is one of the isobestic points of
oxyhaemoglobin and deoxyhaemoglobin.
The OFF part allows a baseline
measurement for any changes in ambient
light.
5. A more recent design uses multiple
wavelengths to eradicate false readings from
carboxy haemoglobin and
methaemoglobinaemia.
Advanced oximeters use more than seven
light wavelengths. This has enabled the
measurement of haemoglobin value, oxygen
content, carboxyhaemoglobin and
methaemoglobin concentrations.
6. A variable pitch beep provides
an audible signal of changes in
saturation
Problems in practice and safety features
1. It is accurate (±2%) in the 70–100%
range. Below the saturation of 70%,
readings are extrapolated.
2. The absolute measurement of oxygen
saturation may vary from one probe to
another but with accurate trends. This is due
to the variability of the center wavelength of
the LEDs.
3. Carbon monoxide poisoning
(including smoking), coloured nail
varnish, intravenous injections of
certain dyes (e.g. methylene blue,
indocyanine green) and drugs
responsible for the production of
methaemoglobinaemia are all
sources of error .
4. Hypo perfusion and severe
peripheral vasoconstriction affect
the performance of the pulse
oximeter. This is because the AC
signal sensed is about 1–5% of the
DC signal when the pulse volume is
normal. This makes it less accurate
during vasoconstriction
when the AC component is reduced.
5. The device monitors the oxygen
saturation with no direct
information regarding oxygen
delivery to the tissues.
6. Pulse oximeters average their
readings every 10–20 s. They cannot
detect acute desaturation.
The response time to desaturation is
longer with the finger probe (more
than 60 s) whereas the ear probe has
a response time of 10–15 s.
7. Excessive movement or malposition of
the probe is a source of error.
8. Inaccurate measurement can be caused
by venous pulsation. This can be because of
high airway pressures, the Valsalva
manoeuvre or other consequences of
impaired venous return. Pulse oximeters
assume that any pulsatile absorption is
caused by arterial blood pulsation only.
9. The site of the application should be
checked at regular intervals as the probe can
cause pressure sores with continuous use.
Some manufacturers recommend changing
the site of application every 2 h especially in
patients with impaired microcirculation.
Burns in infants have been reported.
10. Pulse oximetry only gives
information about a patient’s
oxygenation. It does not give any
indication of a patient’s ability to
eliminate carbon dioxide
Advantages
1) Easy to use, safe, non-invasive.
2) Reliable and accurate (+/- 2%) between
saturations of 70% and 100% in sinus
rhythm.
3) Gives a continuous measurement of
oxygen saturation and pulse rate.
4) Is not affected by the presence of
different haemoglobins (e.g. HbF, HbA2,
HbS, etc.) or haemoglobin concentration.
Disadvantages of pulse oximetry
1. damage to skin caused by pressure from
probe
2. failure to detect hypoxemia in carbon
monoxide poisoning
3. failure to detect hypoventilation
4. slow response time, instrument and
circulatory delay
5. signal quality adversely affected by hypo
perfusion
6. inter-instrument variability
 ~
Sources of Error & ii
1. Strength of Arterial Pulse
Any factor that reduces arterial
pulsations will reduce the ability of
the instrument to obtain and analyze
the signal
A) Hypothermia
B) Hypotension
C) Vasopressor use and BP cuff
2) Body Movement
A) ShiveringLoading…
>
- Anesthesia
in Spinal

B) Parkinsonian tremors
3) Carboxyhemoglobin
4) Methemoglobin
5) Methylene Blue , indocyanin
green
6) High intensity light
7) Venous Pulsations secondary to
AV fistulas,Nail varnish
undrom

Wher free
strings
Complication
1) ischemic pressure necrosis
2) mechanical injury