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Using PPG Morphology to Detect Blood Sequestration

Using PPG Morphology to Detect Blood Sequestration. Stephen Linder Suzanne Wendelken Susan McGrath. Motivation. Is it possible to monitor the behavior of the cardiovascular system with a pulse oximeter?

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Using PPG Morphology to Detect Blood Sequestration

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  1. Using PPG Morphology to Detect Blood Sequestration Stephen Linder Suzanne WendelkenSusan McGrath

  2. Motivation • Is it possible to monitor the behavior of the cardiovascular system with a pulse oximeter? • Many studies have been done on the frequency characteristics of the pulse oximeter waveform, but not the spatial characteristics. • The morphology of the pulse oximeter waveform has not been thoroughly studied under conditions of orthostatic stress. • Pulse oximeters are cheap, easy to use and available off-the shelf. • Numerous applications have been developed in clinical or remote monitoring and assessment.

  3. Background • The photoplethysmogram (PPG) measures the temporal variation in blood volume of peripheral tissue, and thus blood flow • Used to detect Apnea and possibly airway obstructions • PPG has been used in mechanically ventilated patients to • Ascertain breathing status from the Respiratory Sinus Arrhythmia • Blood Volume

  4. Methods • Sensor: 3 FDA approved Nonin® pulse oximeters - Ear, finger, forehead • Supine-Standing experiment • We monitored 11 healthy subjects • 4 women, 7 men, ages 20-43 • 3 trials each • One minute lying down followed by one minute standing up. Repeat. Grad student Beth Knorr with the Nonin pulse oximeter probes

  5. Methodology • Data segmented by feature extractor • Pulses characterized by features: • Instantaneous Hear Rate • Pulse Height • Normalized Peak Width • Wilcoxon Rank Sum test for equal means to detect changes in features real-time Normalized Peak Width (NPW) is the ratio of PW to CP.

  6. Results • Significant changes were found during standing for the following parameters: • Heart rate • Normalized Pulse Width • Pulse Height from the ear probe • Full Width Half max

  7. Results • Pulse amplitude decreases significantly for the ear probe, but not as much for the finger probe • Interesting differences in the pulse envelope

  8. Results Supine Just after Standing Standing Peak stays the same even as heart rate increases The troughs between peaks narrow

  9. Change in Heart Rate • As expected heart rate goes up for most subjects Lay down Normalized Heart Rate Stand up

  10. Change in PPG Amplitude • Ear PPG amplitude pinches Lay down Normalized Ear PPG Amplitude Stand up

  11. Change in Normalized Pulse Width • Pulse become a large percentage of cardiac cycle Lay down Normalized Pulse Width Stand up

  12. Results • NPW leads increase in heart rate which leads pinch in ear PPG amplitude Reclining Standing

  13. Results • Output of the feature detector • HR increase detected in all subjects • NPW increase detected in 31/33 trials • Pulse height (ear probe) decrease detected in 9/11 subjects – no false alarms • One false alarm (Subject 5) • NPW Increased before HR increased. • 21of the 33 trials the NPW begins to rise before the heart rate • Prompt to stand causes a statistically significant change in NPW – why? Prompt to stand

  14. Future Work • Lower body negative pressure studies • Sequesters approx. 3 Liters blood volume (60%) in the lower body (-90 mm Hg). • Studies to compare supine-standing results to those from clinical tilt table tests • Additional monitors: ECG with Respiration tracing • Develop low cost cardiac assessments Subject in LBNP device. ISR, Brooks Army Medical Center

  15. Acknowledgements • Thanks to • Dr. Kirk Shelly for his valuable input • All the volunteers who stood up for us so many times • Collaboration? Contact: smw@dartmouth.edu Disclaimer This project was supported under Award No. 2000-DT-CX-K001 from the Office for Domestic Preparedness, U.S. Department of Homeland Security. Points of view in this document are those of the author(s) and do not necessarily represent the official position of the U.S. Department of Homeland Security.

  16. Pulse Oximetry Overview • Uses the different light absorption properties of HbO2 and Hb to measure heart rate, oxygen saturation (SpO2) and pleth waveform • Two LED’s of different wavelength • Red 660 nm • Infrared 940 nm • HbO2 absorbs less red and more infrared than HB. • Hb absorbs less infrared and more red than HbO2. • Two equations, two unknowns… we can solve for SpO2 • The pleth waveform consist of the IR tracing. • Indirect measurement of blood volume under the sensor

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