Medical Electronics Dr. Neil Townsend Michaelmas Term 2001 ( Pulse Oximetry: The story so far

Transcription

1 Medical Electronics Dr. Neil Townsend Michaelmas Term 2001 ( Oxygen is carried in the blood by haemoglobin which has two forms: Hb and HbO 2. These two forms have different absorptions at different wavelengths in the red to infra red frequency band. Lecture 6 By measuring the absorption of two different wavelengths and taking appropriate ratios it is possible, in theory, to evaluate the percentage of haemoglobin carrying oxygen. However, two problems arise: It is hard to differentiate between absorption due to arterial blood and absorption due to tissue and other blood. The Beer Lambert law does not hold because light travels in a very diffuse manner in flesh. The solution is to consider calibrate the absorption information based on empirical data. So, let us consider how one might build a pulse oximeter. In order to build a pulse oximeter several steps need to be in place. The probes need to be small and unobtrusive and this will affect how some of the steps are implemented.

2 Looking at the steps one at a time: Illuminating Small probes means that small light emitting devices need to be used. Red ( 660( nm) and Near Infra Red (NIR, 940 nm) wavelengths need to be emitted. Light Emitting Diodes (LEDs) are small and emit light at appropriate wavelengths. Illuminating Light Emitting Diodes (LEDs) are therefore, in principle, appropriate. However, standard LEDs are not sufficiently powerful. Special purpose LEDs have been designed: Internal lensing to give a high intensity output. Pulsed LEDs so that peak power is increased. Illuminating The LEDs need to be driven. Constant current through the LED (when on) ensures constant light output. How might this be achieved? Timing The LEDs should be pulsed for two reasons: Increased peak power for same average power. Careful timing of the on time for each LED allows a single photo detector to be used for both LEDs.

3 For example: Timing For example: Timing This can be achieved using an astable and a slight variation on the circuit we have just seen. Detecting Photodiodes are the simplest solid state optical detectors When light falls on the junction p n junction region an electron hole pair is created. The hole and the electron are swept in opposite directions. Detecting Photodiodes are the simplest solid state optical detectors When light falls on the junction p n junction region an electron hole pair is created. The hole and the electron are swept in opposite directions. The resulting light current is seen as a large increase in the reverse current. This current needs to be turned into a voltage. Detecting The light current can be converted into a voltage using a single op amp.

4 (d) Retain the received signal. Retaining A single photodetector is being used to provide two pieces of information. Therefore, it is important to know when it is giving information about absorption of the red and the NIR wavelengths respectively. Some form of sample and hold circuitry is therefore necessary. (d) Retain the received signal. (e) Control the amplitude of the transmitted light. (AGC) is when an attribute of a received signal is used to control the amplitude of the received signal. Commonly used in A.M. Radios. There are three reasons why an AGC circuit is used in pulse oximetry It allows the frequency response of the photodiode to be corrected. There are three reasons why an AGC circuit is used in pulse oximetry It allows the frequency response of the photodiode to be corrected. It keeps the a.c. signal (which varies between 0.1% and 2% of the total signal) within a pre defined range.

5 There are three reasons why an AGC circuit is used in pulse oximetry It allows the frequency response of the photodiode to be corrected. It keeps the a.c. signal (which varies between 0.1% and 2% of the total signal) within a pre defined range. It allows the d.c. level of both the NIR and the red signals to be kept at the same level (say 2V). This allows a new index, R, to be defined: R log 10 I ac log 10 I ac log (compared with: 10 I dc ac I dc 1 R ) log 10 I dc I ac dc (d) Retain the received signal. (e) Control the amplitude of the transmitted light. (f) Filter, store and interpret the information. Pulse Oximetry Instrumentation Finally, a CPU of some description will be used to process the data. It will: Do any necessary noise final reduction (possibly using averaging). Calculate R. Infer the SaO 2 using a look up table. ECG: And so... DONE And so... Respiration (via impedance plethysmography): DONE

6 SaO 2 (via pulse oximetry) And so... DONE And therefore... Blood Pressure: NEXT WEEK