Pulse Oximetry Dr.C.N.Chandra Sekhar M.D

Oxygenation Oxygen tension Oxygen content Oxygen saturation

Oxygen tension Clarke electrode Fuel cell Transcutaneous electrodes

Oxygen content (SpO 2 x Hb x 1.34) x PO 2

Oxygen Saturation ODC Curve: –Plot of degree of Hb saturation against oxygen tension

PULSE OXIMETRY Uses spectrophotometry based on the Beer- Lambert law Differentiates oxy- from deoxyhemoglobin by the differences in absorption at 660nm and 940nm Minimizes tissue interference by separating out the pulsatile signal

Absorption Spectrum

Signals

PULSE OXIMETRY Estimates heart rate by measuring cyclic changes in light transmission Measures 4 types of hemoglobin: deoxy, oxy, carboxy, and met Estimates functional hemoglobin saturation: oxyhemoglobin/deoxy + oxy

Beer-Lambert’s Law Beer’s Law: –The intensity of transmitted light decreases exponentially with the concentration of the substance Lambert’s Law: –The intensity of transmitted light decreases exponentially with the distance it travels

Pulse Oximeter probe

ABSORPTION SPECTRA

SOURCES OF ERROR Sensitive to motion Standard deviation is certified to 2-3% down to 70% saturation Sats below 85% increase the importance of error in the reading Calibration is performed by company on normal patients breathing various gas mixtures, so calibration is certain only down to 80%

SOURCES OF ERROR Skin Pigmentation –Darker color may make the reading more variable due to optical shunting. –Dark nail polish has same effect: blue, black, and green polishes underestimate saturations, while red and purple have no effect –Hyperbilirubinemia has no effect

SOURCES OF ERROR Low perfusion state Ambient Light Delay in reading of about 12 seconds Methylene blue and indigo carmine underestimate the saturation

SOURCES OF ERROR Dysfunctional hemoglobin –Carboxyhgb leads to overestimation of sats because it absorbs at 660nm with an absorption coefficient nearly identical to oxyhgb

SOURCES OF ERROR –Methgb can mask the true saturation by absorbing too much light at both 660nm and 940nm. Saturations are overestimated, but drop no further than 85%, which occurs when methgb reaches 35%.

Summary HbFNo significant clinical change Meth HbFalse low or high readings COHbFalse High readings Sulph HbNot a clinical problem BilirubinNot a clinical problem

Methylene BlueFalse low reading Indocyanine greenFalse low reading AnaemiaContraversial

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