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Using PPG Morphology to Detect Blood Sequestration

Stephen Linder Suzanne WendelkenSusan McGrath

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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.

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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

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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

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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

105

80

85

90

95

100

110

115

120

125

592 592.5 593 592 592.5 593

Time (sec)

PP

G

Pulse Height (PH)

Peak Threshold (PT)

Cardiac Period (CP)

Peak Width (PW)

Normalized Peak Width (NPW) is the ratio of PW to CP.

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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

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Results

Pulse amplitude decreases significantly for the ear probe, but not as much for the finger probe

Interesting differences in the pulse envelope

0 20 40 60 80 100 120

3.2

3.3

3.4

3.5 x 10 4 finger

0 20 40 60 80 100 120 3.22

3.26

3.3

3.34 x 10 4 ear

Time (sec)

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Results

30 30.5 31 31.5 32 32.5 33 33.5 34 34.5 35 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

PP

G

55 55.5 56 56.5 57 57.5 58 58.5 59 59.5 60 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

PP

G

finger ear

0.3 0.4 0.5 0.6 0.7 0.8 0.9

68 68.5 69 69.5 70 70.5 71 71.5 72 72.5 Time (sec)

PP

G

73

30 30.5 31 31.5 32 32.5 33 33.5 34 34.5 35 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Time (sec)

Peak stays the same even as heart rate increases

The troughs between peaks narrow

Supine Just after Standing Standing

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Change in Heart Rate

As expected heart rate goes up for most subjects

0 5 10 15 20 25 300

20

40

60

80

100

120

140

160

180

Trial Index

Tim

e(se

c)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Stand up

Lay downNormalized Heart Rate

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Change in PPG Amplitude

Ear PPG amplitude pinches

Stand up

Lay downNormalized Ear PPG Amplitude

0 5 10 15 20 25 300

20

40

60

80

100

120

140

160

180

Trial Index

Tim

e(s

ec)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

11

Change in Normalized Pulse Width

Pulse become a large percentage of cardiac cycle

Stand up

Lay downNormalized Pulse Width

0 5 10 15 20 25 300

20

40

60

80

100

120

140

160

180

Trial Index

Tim

e(se

c)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

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Results

NPW leads increase in heart rate which leads pinch in ear PPG amplitude

0 20 40 60 80 100 120 140 160 180 20

30

40

50

60

70

80

90

100

Time (sec)

HR NPWFINGER PHEAR

Standing Reclining

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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?

40 50 60 70 80 90

1

2

3

4

5

6

7

8

9

10

11

Time (sec)

Sub

ject

Ind

ex

Abrupt Change Detection

HRFINGER Peak PHEAR Constriction NPWFINGER Peak FWHMFINGER Peak

Prompt to stand

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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

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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

DisclaimerThis 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.

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Pulse Oximetry OverviewPulse 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

HbHbO

HbOp CC

COS

2

2

2

Extinction Curve

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

600 700 800 900 1000 1100

Wavelength (nm)

Absorp

tion

A(Hb)

A(HbO)

The pleth waveform consist of the IR tracing.

Indirect measurement of blood volume under the sensor