Dr. D.McG.Clarkson Determining the Accuracy of SpO2 Values

Transcription

1 Dr. D.McG.Clarkson Determining the Accuracy of SpO2 Values 1

2 Life saving technology The availability of the LED provided a convenient light which provides advantages of :- Compact size Modest power requirements Allows light modulation by electronic control Spectral range tailored to application source of

3 Light absorption of types of haemoglobin

4 Types of haemoglobin Oxyhaemoglobin - form with bound oxygen for oxygen transport - a key indicator of patient status. Reduced haemoglobin (de-oxyhaemoglobin) form with bound carbon dioixide Methaemoglobin is a form of haemoglobin where the haeme group contains Fe3+ rather than Fe2+ of normal haemoglobin. Normal levels are typically in the range 1 to 2 %. High levels of Methaemoglobin will drive Pulse Oximeter reading towards 85%. Carboxyhemoglobin (COHb) is a stable complex of carbon monoxide that is created by inhalation of carbon monoxide. Essentially it is making haemoglobin unavailable for oxygen transport in the blood. Levels are typically between 1.0% to 2% in normal population higher for smokers.

5 Origin of pulsatile waveforms

6 Detected RED & IR signal waveforms Sampling profile is based on time modulated signals of RED and IR LEDs

7 SATS value Pulse Oximetry The R Curve Fixed Reference Data Set for specific wavelengths R-Curve for specified Spectral characteristics e.g nm/907nm R value R value = (RED abs /IR abs)

8 Standard Masimo red LED

9 Masimo red and IR LED drive configuration

10 Detected waveforms of red and IR LEDs

11 What does the standard say? BS ISO :2011 Medical electrical equipment: Part 2-61: Particular requirements for the basic safety and essential performance of pulse oximeter equipment for medical use.

12 In Vitro Performance Test System (Reynolds et al, In vitro performance test systems for pulse oximeters, Med & Biol Eng. & Comput. 1992, Reynolds et al, Response of 10 pulse oximeters to an in vitro test system, B. J. Anaesth., 68, Identified requirement for validation of pulse oximeter devices at low saturations - eg less than 70%. Design based around simulation of blood in finger Inclusion of simulation of pulsatile flow Variable setting of oxygenation levels

13 In Vitro Performance Test System

14 Observations of the In Vitro test System: High correlation (r=0.979) between SpO2 measured by pulse oximeter and actual saturation. Identified difficulty of designing finger probe mimicking blood flow through finger - due to complexity of such flow Not practical for routine functional testing of pulse oximeter systems. Standard acknowledges the technical difficulties of using this approach.

15 Technique: LED Spectral Characterisation The focus of BS ISO : 2011 appears to related to consideration of devices to simulate function of pulsatile flow in the finger - rather than by the technique of determination of spectral output of LEDs in the probe - e.g the technique of the LightMan Submissions made to the standard to acknowledge the LightMan technique have not been incorporated into the revised standard. The key verification of the LightMan technique is the correlation of estimated pulse oximeter probe accuracy with breath down data where the spectral characterisation technique is related to the results of CO-oximeter evaluations on a sub set of subjects. (For the LightMan to function appropriately, a significant amount of unique intellectual property is required.)

16 Function of Lightman Unit

17

18 References Dugani S, Hodzovic I, Sindhakar S, Nadra A, Dunstan C, Wilkes AR, Mecklenburgh J. Evaluation of a pulse oximeter sensor tester. J Clin Monit Comput Jun;25(3): Milner QJ, Mathews GR, Powell RJ. Accuracy of the Lightman( ) pulse oximeter tester. Anaesthesia Jan;68(1): Milner QJ, Mathews GR. An assessment of the accuracy of pulse oximeters. Anaesthesia Apr;67(4):

19 BS ISO and functional testers

20 Appendix FF.6 of Pulse Oximeter standard: -- Several brands of FUNCTIONAL TESTER have an optomechanical finger containing a detector, which picks up light from the PULSE OXIMETER PROBE'S light emitter, and a light-emitting diode (LED) which delivers modulated light to the PULSE OXIMETER PROBE'S detector (see Figure FF.2). This is one example of the optically interfaced tester described above. If the PULSE OXIMETER PROBE'S red LED were of the wrong wavelength for the calibration curve in use, this would definitely cause the oximeter to be inaccurate in actual use on PATIENTS. The FUNCTIONAL TESTER would be unable to detect this error, as would the PULSE OXIMETER EQUIPMENT under test, so that inaccurate PULSE OXIMETER EQUIPMENT might well appear to be accurate.

21 The Pronk OxSim OX-1 Functional Tester

22 The Basket Case Sensor Working with an adapted Nellcor sensor where the red LED is swopped for a yellow LED - centre wavelength 580 nm - deviation of around 80 nm. The predicted percentage error using spectral analysis is >20% at 98% saturation value.

23 Comparative Measurements The Pronk says its OK On a patient it is not OK

24 Lightman evaluation of orange LED probe 2.00E E E E E E E E E E E

25 Manufacturer s Statement: The Pronk Simulator is one of the types described in the ISO Pulse Oximeter standard ISO that replaced ISO Both in ISO 9919 and ISO it is clear that simulators cannot and should not be used to assess accuracy of Pulse oximeter sensors. The Pronk looks at when the red and infrared are turned on. If the measured intensity is sufficient it then produces a red/infrared signal of an intensity set to mimic the intensity at specific SATs as if the wavelength is accurate. It does not measure and cannot measure the wavelength and the impact of a variation in wavelength on the system on a patient.

26 Summary of Comparison Function LightMan Pronk Ox-Sim detect RED LED active yes yes RED LED spectral measurement yes no detect INFRA RED LED active yes yes INFRA RED LED spectral measurement yes no determine probe clinical SPO2 accuracy yes no detect noise signal yes no heart rate simulation on monitor no yes inject (RED/IR) signal to monitor no yes

27 The Patient Safety Issue % unacceptable spectral errors in pulse oximeter sensors surveyed l Clinical demands on pulse oximetry have increased The accuracy of sensors in use is deteriorating. 5.0 Inaccurate sensors put patients at risk of receiving the wrong treatment

28 Summary Points BS ISO :2011 Sets the official approach to determination of pulse oximeter accuracy Confirms the inadequacy of functional testers Ignores technological development of the LightMan system Preoccupation with systems which mimic patient signals in artificial fingers. Clarity required on role of functional testers for SpO2

29 Thank you 29