Using PPG Morphology to Detect Blood Sequestration Stephen
Using PPG Morphology to Detect Blood Sequestration Stephen Linder Suzanne Wendelken Susan McGrath 1 Motivation 2 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. 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 3 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 4 One minute lying down followed by one minute standing up. Repeat. Grad student Beth Knorr with the Nonin pulse oximeter probes Methodology Data Instantaneous Hear Rate Pulse Height Normalized Peak Width Wilcoxon Rank Sum test for equal means to detect changes in features real-time 5 125 120 Cardiac Period (CP) 115 Pulse Height (PH)
110 PPG segmented by feature extractor Pulses characterized by features: 105 Peak Threshold (PT) 100 95 Peak Width (PW) 90 85 80 592 592.5 593 Time (sec) Normalized Peak Width (NPW) is the ratio of PW to CP. 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 6 Results
Pulse amplitude decreases significantly for the ear probe, but not as much for the finger probe Interesting differences in the pulse envelope 4 finger x 10 3.5 3.4 3.3 3.2 0 20 40 4 3.34 60 80 100 120 80 100 120 ear x 10 3.3 3.26 3.22 0 7 20 40
60 Time (sec) Results Supine 0.9 Just after Standing 0.9 0.9 0.8 0.8 0.7 0.7 0.8 Standing finger ear 0.6 PPG PPG PPG 0.7 0.6 0.6 0.5 0.5 0.5 0.4
140 160 180 11 10 Results Abrupt Change Detection HRFINGER Peak PHEAR Constriction NPWFINGER Peak FWHMFINGER Peak 9 8 Output of the feature detector NPW Increased before HR increased. 13 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) 21of the 33 trials the NPW begins to rise before the heart rate Prompt to stand causes a
statistically significant change in NPW why? 7 Subject Index 6 5 4 3 Prompt to stand 2 1 40 50 60 70 Time (sec) 80 90 Future Work Lower body negative pressure studies Sequesters approx. 3 Liters blood volume (60%) in the lower body (-90 mm Hg). Studies to compare supinestanding results to those from clinical tilt table tests Additional monitors: ECG with
Respiration tracing Develop low cost cardiac assessments 14 Subject in LBNP device. ISR, Brooks Army Medical Center Acknowledgements Thanks to Dr. Kirk Shelly for his valuable input All the volunteers who stood up for us so many times Collaboration? Contact: [email protected] Disclaimer This project was supported under Award No. 2000-DT-CXK001 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. 15 Pulse Oximetry Overview Uses the different light absorption properties of HbO2 and Hb to measure heart rate, oxygen saturation (SpO2) and pleth waveform Two LEDs of different wavelength Red 660 nm Infrared 940 nm
1.00E-03 8.00E-04 6.00E-04 A(Hb) 4.00E-04 A(HbO) 2.00E-04 0.00E+00 600 700 800 900 1000 1100 Wavelength (nm) HbO2 absorbs less red and more infrared than HB. Hb absorbs less infrared and more red The pleth waveform consist of the IR than HbO2. tracing. Two equations, two unknowns we Indirect measurement of blood volume can solve for SpO2 under the sensor S p O2 16 Extinction Curve Absorption C HbO2 C HbO2 C Hb
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