Pulse Oximetry. Signal du jour: pulse oximetry

Transcription

1 1/18/017 Pulse Oximetry BIOEN 468/568 a.k.a. BIOEN 498F/599G January 017 Signal du jour: pulse oximetry Pulse oximeter Purpose: monitor blood oxygenation Variable type: chemical Sensor type: optical Tissue oxygenation indicates overall physiological conditions of oxygen supply, blood circulation and tissue perfusion. 1

2 1/18/017 Optical Assessment of Blood Oxygenation Origin of optical signals: Oxyhemoglobin (HbO ) and deoxyhemoglobin (Hb) have distinct light absorption characteristics HbO absorbs more around 900nm Hb absorbs more around 750nm Modeling and Analysis Suppose we can measure the absorption coefficients at two wavelengths (750 nm and 900 nm) μ μ a,750 a,900 Hb,750 Hb,900 HbO HbO,750,900 [ HbO [ HbO ε s are the extinction coefficients of Hb and HbO, the absorption coefficient per unit concentration ] ]

3 1/18/017 Modeling and Analysis μ μ a,750 a,900 Hb,750 Hb,900 HbO HbO,750,900 [ HbO [ HbO Two equations in two unknowns: Solve for the two concentrations, [Hb] and [HbO ]. [ HbO ] Oxygen Saturation SO = + [ HbO ] ] ] μ a, μ a, μ a, μ a, μ a, More constituents require more equations Other tissue constituents absorb or scatter light For 5: fat, water, cartilage, bone, skin pigment λ1 λ λ3 λ 4 λ5 Hb, λ1 Hb, λ Hb, λ3 Hb, λ 4 Hb, λ5 HbO, λ1 HbO, λ HbO, λ3 HbO, λ 4 HbO, λ5 [ HbO ] [ HbO ] [ HbO ] [ HbO ] [ HbO ] HO, λ1 HO, λ HO, λ3 HO, λ 4 HO, λ5 [ H O] [ H O] [ H O] [ H O] [ H O] Fat, λ1 Fat, λ Fat, λ3 Fat, λ 4 Fat, λ5 We can then solve for [Hb] and [HbO ] [ Fat] [ Fat] [ Fat] [ Fat] [ Fat] Melanin, λ1 Melanin, λ Melanin, λ3 Melanin, λ 4 Melanin, λ5 [ Melanin], [ Melanin], [ Melanin], [ Melanin], [ Melanin]. 3

4 1/18/017 Or We can take advantage of the cardiac or arterial pulsation to measure arterial blood oxygenation Pulse Oximeter AC and DC components The amount of arterial blood does change over short periods due to pulsation Since it is the only light absorbing component which is changing over short periods of time, it can be isolated from other components 4

5 1/18/017 Method There are two unknown parameters: arterial [Hb] and [HbO ] Only two wavelengths are required, using LEDs: Red ( nm) and IR ( ) nm. 660 nm: red, more sensitive to reduced (de-oxygenated) hemoglobin, Hb 940 nm: infrared, more sensitive to oxygenated hemoglobin, HbO Source The Pulse Oximeter Pathlength, D Red & IR LEDs Photo Detector Detector 5

6 1/18/017 The Pulse Oximeter Silicon phototransistor: Sensitivity spectrum Source: Vishay Semiconductors, BPV11 data sheet 9/30/009 BIOEN

7 1/18/017 Segment of raw pulse oximetry signal Signal captured by A-to-D converter Time axis, labeled by sample 100 Hz Signal Modeling and Analysis Using Beer-Lambert Law: Absorption of transmitted light is exponential: Itrans = Iin exp( A) Where I trans and I in are the transmitted and incident light intensity, respectively, A is the wavelength dependent absorbance, and A = μ D = D( ε [ HbO ]) λ a, λ Hb, λ HbO, λ 7

8 1/18/017 Ideal Procedure 1. Measure both pulsatile (AC) and non-pulsatile (DC) component of the light intensity for each wavelength. Take the ratio for each wavelength, (I AC +I DC )/I DC, to remove the incident intensity and the DC absorbance: ( Iac + Idc )/ Idc = exp[ ( Aac + Adc ) + Adc ] = exp( Aac ) 3. Find the AC absorbance by taking the logarithm of the ratio above, i.e.: A ac = ln( Iac + Idc / Idc ) = D( ε Hb, λ[ Hb] HbO, λ[ HbO]) where [Hb] and [HbO ] are the arterial reduced and oxygenated hemoglobin concentrations, respectively. Ideal Procedure Note: the exact path length is difficult to find because light follows zigzag patterns (due to strong scattering). In practice, the ratio of AC and DC components is taken at each wavelength to remove the unknown path length D (step 3) 3. Take the ratio of the AC absorbance at two wavelengths to remove the path length: R = Aac,660 nm / Aac,940nm = ( ε [ HbO ])/( ε [ HbO ]) Hb,660nm HbO,660nm Hb,940nm HbO,940nm 4. Use a look-up or calibration table, to find the oxygen saturation SO for each measured ratio R. 8

9 1/18/017 SO vs. R Signal Processing Signal is low, and very noisy! Need analog and digital filtering. The incident light intensity needs to be adjusted automatically to ensure Transmitted signal is not too small so that it is measurable Transmitted signal is not too large so that it will not saturate the detector Need feedback mechanism, in real time. There are many artifacts that affect the measured signals: e.g. motion artifact & detector noise 9

10 1/18/017 The photodetector Phototransistor used for its speed Photodetectors Photocell (resistance decreases with light) Photodiode (current increases with light) Phototransistor (composed of two photodiodes) 1 10

11 1/18/017 Phototransistor: size and shape Source: BPV11 data sheet Phototransistor: current vs. light level Note radiometric (not photometric) quantity Source: Vishay Semiconductors, BPV11 data sheet 3 11

12 1/18/017 Detector circuit Similar to voltage divider May be buffered or amplified When might each of these be useful? Multiplex output via alternating LED wavelengths light V-out +5V BPV11 R ground 4 LED driver: current source LED Control voltage Voltage-controlled current source using operational amplifier, i LED = V 1 /R 1 Voltage regulation with zener diode; current control with potentiometer 5 1

13 1/18/017 Example circuit on the board Summary The oximeter is dependent on a pulsatile flow and produces a graph of the quality of flow. The computer within the oximeter is capable of distinguishing pulsatile flow from other more static signals (such as tissue or venous signals) to display only the arterial flow. We have discussed the modeling/analysis of the oximeter signal. What about processing? Do we need any and why? 13