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1. PULSE OXIMETRY
Dr.ADITI BHATTACHARYYA
JR1DNB ANAESTHESIA

2. OXIMETRY
• Oximetry is the measurement of the o2 saturation of Hb.It is an
application of of the Beer-Lambert law,which relates the
transmission of light through a solution to the concentration of the
solute in the solution.

3. Early Oximeter
• Oximetry was introduced in the 1940s to detect hypoxemia .Early
oximetry ,which measured the transmission of red and infrared
light waves through the earlobe,had two major shortcomings:
• The transmission of light was influenced by factors other than
hemoglobin(e.g.skin pigments,and the thickness of earlobe),and
• It was not possible to differentiate between hemoglobin in
arteries and veins.

4. Pulse Oximetry
Basic principle
• When light beam passes through a pulsating artery,phasic changes
in arterial blood volume create pulsatile variations in the intensity
of the transmitted light beam.Therefore,restricting the analysis of
light transmission to pulsatile light waves will focus the analysis on
arterial blood,and will eliminate errors due to light absorbtion by
non pulsatile elements(e.g. hemoglobin in veins)

5. These probes are usually placed on the index or
middle finger,but can be placed on any digit ,including
the great toe,One side of the probe contains two light
emitting diodes that emit monochromic light at
wavelengths of 660nm and 940nm.these light waves
pass through the finger and sensed by a photo
detector on the opposite side of the probe.
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6. • The transmitted light waves are then passed through an AC
amplifier that amplifies the pulsatile light waves and blocks the
non pulsatile waves.The intensity of light transmission at 660nm
and 940nm is a reflection of the deoxygenated Hb and oxygenated
hemoglobin concentrations in arterial blood,respectively.The pulse
oximeter converts”light density” to “chemical
density”concentration for Hb and HbO2 using proprietary
algorithms.The ratio of Hbo2 to total hemoglobin(Hbo2+Hb)is then
used to define the fraction of hemoglobin that is saturated with
oxygen.The resulting “pulse oximeter saturation”(spo2) is
expressed as percentage:SpO2=HbO2/(HbO2+Hb)*100

7. RELIABILITY
• At clinically acceptable levels of arterial oxyhemoglobin saturation
(SaO2>70%),theO2 saturation recorded by pulse oximeters
(SaO2)differs by less than 3% from actual SaO2.In addition to
accuracy, the SpO2shows little tendency for spontaneous
variations.
• TIME PERIOD:60Min
• Mean variation:1%
• Range of variation:0-5%

8. Limitations and Errors
• Spo2 is an estimate of SaO2of circulating hemoglobin.As a
consequence,it does not provide information about tissue
oxygenation.BecauseSpO2 is a measurement of functional and not
fractionalSaO2,the presence of other Hb variants can significantly
affect its accuracy.The nonlinearity of Hb dissociation curve
prevents the detection of hyperoxia withSpO2 for high
SaO2,whereas for low saturations such as at altitude, small
changes in PaO2 can produce large changes in SpO2.Pulse
oximeter does not provide information about ventilation or acid
base status.

9. This Photo by Unknown Author is licensed under CC BY-SA-NC

10. Conditions that can lead to inaccuracies in
pulse oximeter readings:
• Hypoperfusion leads to a reduction in the amplitude of the
pulsatile component of the light absorbance waveform,the
essential signal for pulse oximetry.
• With continued clinical use,the performance of the LEDs in the
probe may be degraded,leading to inaccuracies.
• Venous pulsations may result in the detection of venous O2Hb
saturation by the pulse oximeter,resulting in artifactual reduction
of the presumed arterial SpO2 being measured.
• The presence of additional species of Hb can also generate
erroneous pulse oximeter readings.

11. • With normal SaO2,anemia has a little effect on
SpO2.However,in the presence of hypoxia,SpO2 readings
underestimate SaO2 in anemic patients with true hypoxemia.
• A relatively uncommon cause for redced SpO2 readings is the
presence of congenital variants of Hb,e.g.Hb Bassett,Hb
Rothschild,Hb canabiere have a reduced affinity for O2.
• Administration of intravenous dyes can result inaccurate SpO2
readings.
• Although all colors of nail polish can reduce the calculated value of
SpO2,black purple,and dark blue colors have the greatest effect.
• In the presence of severe hyperbilirubinemia (>30mg/dl)caused by
incread hemolysis or liver disease,the fraction of oxygenated HB
may be falsly low by artifactual increase in metHb and
COHb,resulting in more accurate SpO2than FO2Hb measurements.

12. • In patients with intra aortic balloon pump support ,SpO2 accuracy
depends on the brand of pulse oximeter used as well as the
support ratio.

13. Pulse Oximeter Probes
• Probes are usually applied to accessible body areas with high
vascularity,such as finger,nose,ear lobes or forehead.
• Different probe models may be advantageous in specific
conditions.
• Ear and forehead probes may be more reliable during
vasoconstriction compared with finger probes,given that the
arterial vessels of such regions are less responsive to circulating
catecholamines.

14. Venous Oximetry
• Specialized oximetry catheters are available that can monitor the
O2 saturation of hemoglobin in the SVC or pulmonary artery.
• Venous O2 Saturation;The O2 saturation in mixed
venous(pulmonary artery) blood and central venous(superior vena
cava)blood (SvO2 AND ScvO2respectively)
SvO2 or ScvO2=1-VO2/DO2
Mixed VENOUS O2Saturation:Measurements of SvO2 with the
pulmonary artery oximeter catheters are typically within 1-2% of in
vitro measurements.A greater than 5%change in svo2 that persists
for longer than 10 min is considered a significant change.
Central venous o2 satuation:
Central venous o2 Saturation:Measurements of Scvo2 wih central
venous catheter are slights lower than the Svo2,and this difference
is magnified in the presence of circulatory shock.

15. Dual Oxymetry
• The predictive value of svo2or Scvo2 can be increased by adding
the SpO2 from pulse oximetry.The difference(SpO2-SvO2) or
(SpO2-SCVO2)is roughly equivalent to the O2 extraction from
capillary blood

16. • Despite the recommendations of numerous boards and
guidelines,there is no evidence that the use ofpulse oximetry
improves patient outcome,not during transfers to ICUs nor in
terms of improving patient mortality.There is evidence that pulse
oximetry reduces the incidence of hypoxemia,and the duration
and the cost if ICUstay,suggesting it allows for early intervention.

17. Emerging Techniques
• Pulse spectroscopy is a new technique that utilizes hundreds of
wavelengths to assess normal and dysfunctional
hemoglobins,Initial results are promising with accurate
SpO2determinations,as well as COHb and MetHb assessment during
normoxia and hypoxia.

18. THANK YOU…