How Pulse oximeter works- Principle and Limitations I SpO2 I Respiratory Physiology I COVID The slides will discuss about 1. Introduction 2. Principle of Pulse oximeter 3. Limitation of Pulse oximeter You can also watch the same topic on HM Learnings Youtube channel. You can also follow HM Learnings on facebook, instagram and twitter for daily updates

2. Pulse oximeter
• Pulse oximeter is used to measure peripheral O2 i.e. oxygen saturation
of hemoglobin in peripheral blood vessels.
• It is a non-invasive technique.
• The pulse oximeter has a probe that one attaches to the ear, finger, or
any part of the body at which pulsating blood vessels are accessible
externally.

3. Principle
• Pulse oximeter is based on two basic principle:
1. It differentiates between oxyhemoglobin (O2Hb) and
deoxyhemoglobin (HHb).
2. SpO2 is calculated only from the arterial compartment of blood.

4. Absorbance spectra of O2Hb and HHb
• Pulse oximeter emits two wavelengths of light- red
at 660 nm and near infrared at 940 nm from a pair
of small light-emitting diodes located in one arm
of the finger probe. The light that is transmitted
through the finger is then detected by a
photodiode on the opposite arm of the probe
• O2Hb absorbs greater amount of IR and less
amount of red light (scatters more red light)
whereas HHb absorbs more red light and less
infrared light.
• The relative amount of red and IR light absorbed
or not absorbed are used by the photodetector of
pulse oximeter to ultimately determine the
proportion of Hb bound to oxygen.

5. Calculation of arterial oxygen saturation of Hb
• The pulsatile blood flow is seen in arterioles and arteries (blood volume
increases during systole and decreases during diastole)and not seen in
capillaries and vein (blood volume remain constant).
• As a result, the amount of red and IR light absorbed fluctuates with the cardiac
cycle as the amount of O2Hb and HHb in the arterial blood fluctuates with
cardiac cycle.
• These total absorbances calculated by microprocessor and software of pulse
oximeter have two components:
1. a nonpulsatile component that arises from stationary tissues, including blood
inside capillaries and veins; and
2. a pulsatile component that arises from blood inside arterioles and arteries.
• The difference between the total and nonpulsatile absorbance is thus the
pulsatile component, which represents only arterial or oxygenated blood.

6. • SpO2 is thus calculated from the differential pulsatile absorbance of light at
two wavelength.

7. Limitations of Pulse oximeter
1. Since, it measures oxygen saturation only in arterial blood, it shows
totally normal SaO2 in case of peripheral cyanosis (e.g. purple
fingertips in case of cold induced vasoconstriction)
2. It shows normal SaO2 in carbon monoxide poisoning as absorption
spectrum of Hb-CO and Hb-O2 is same.

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