Functional Saturation 

(SaO2)

Oxyhemoglobin Fraction 

(O₂Hb)

Oxygen Content 

(CaO2)

(1.37 x Hb x O₂Hb%/100) + (0.003 x PaO₂)

Measures O₂ content per 100mLs of blood 

Normal Oxygen Content 

(CaO₂)

(CaO2 x 10) x CO 

Converts deciliters to liters 

O₂ delivered - O₂ returned 

([CaO2 - CvO2) x 10]xCO

[1.37 x (Hb x O2sat/100- O2venous/100 x Hb)] x 10 x CO

Fick Equation

Normal VO₂

VO₂=[1.37 x (15 x100/100- 75/100 x 15)] x 10 x 5.0

VO₂= 250 cc/min

Makes use of the measurement of light absorbance to determine the concentration of solute in clear solutions (Beer-Lambert)

Estimate arterial hemoglobin saturation by measuring the transmission of two wavelengths of light through a pulsatile vascular bed 

Distinguish light absorbance of arterial blood from other tissues by looking for blood pulsations 

Based on the assumption that arterial blood is the only pulsatile absorpber 

By using two wavelengths at 660 nm and 940 nm as well as pulsatile flow, the oximeter is able to determine the concentrations of O₂Hb and RHb

These are used to form a ratio (R) 

Ratio

(R)

Directly related to hemoglobin saturation 

From experimental data, R is plotted to give corresponding O₂ saturation

Dyes

COHb and MetHb

Wavelength Uncertainty

Signal to Noise Ratio

If any light absorbing species other than RHb and O2Hb are present, the oximeter cannot accurately estimate saturation

Methylene Blue and Indigo Carmine

Absorbs both wave lengths equally

As both numerator and denominator are increased, R approaches 1.0

From the algorithm, if R=1 then the satration will be read as 85%

LED's may emit a non-monochromatic light

This is dealt with by having multiple algorithms in the software

The connector for the sensor cable is coded for the appropraite algorithm 

The detector may respond to room light

Weak pulses can make the pulsatile component extremely small and difficult to separate from tissue absorption

The Penumbra effect

If the detector is improperly placed so the light passes through the fingertip at a grazing incidence, a correct heart rate may be displayed falsely low saturation

May be a similar effect to R=1.0 

B/C of varying gain necessary in a variety of clinical situations changes in the signal do not correlate well with peripheral perfusion

Patient movement can cause what the oximeter interprets as pulsations 

Tricuspid Insufficiency

Gravitational effect 

Most claim +-2% x one standard deviation for values 70-100%

Uncertainity increases to +-3 with saturation values of 50-70%

For saturations above 70% the saturation should be within 2% of the actual saturation 68% of the time

Within 4% 95% of the time

Lung to finger circulation times are longer than lung to ear times

As a result response times are faster with ear probes