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
One minute lying down followed by
one minute standing up. Repeat.
4
Grad student Beth Knorr
with the Nonin pulse
oximeter probes
Methodology

Data segmented by feature
extractor
Pulses characterized by
features:
Instantaneous Hear Rate
Pulse Height
Normalized Peak Width
125
120
Cardiac
Period
(CP)
115
Pulse
Height
(PH)
110
PPG

105
Peak
Threshold
(PT)
100
95
Peak
Width
(PW)
90
85
80
592
592.5
593
Time (sec)

5
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.
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
0.9
Just after Standing
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
0.4
0.3
0.3
0.4
0.3
0.2
30
30.5
31
31.5
32
32.5
33
33.5
34
34.5
0.2
55
35
73
55.5
56
56.5
57
57.5
58
0.8
0.7
0.6
59.5
60
68
68.5
69
69.5
70
70.5
71
Time (sec)
71.5
72
72.5
The troughs between
peaks narrow
0.5
0.4
0.3
8
59
Peak stays the same
even as heart rate
increases
0.9
0.2
58.5
30
30.5
31
31.5
32
32.5
Time (sec)
33
33.5
34
34.5
35
Change in Heart Rate

As expected heart rate goes up for most subjects
180
0.9
160
0.8
140
0.7
Lay down
120
Time(sec)
0.6
100
0.5
80
0.4
Stand up
60
0.3
40
0.2
20
0
9
0.1
0
5
10
15
Trial Index
20
25
30
Normalized
Heart Rate
Change in PPG Amplitude

Ear PPG amplitude pinches
180
1
160
0.9
0.8
140
0.7
Lay down
120
Time(sec)
0.6
100
0.5
80
0.4
Stand up
60
0.3
40
0.2
20
0
10
0.1
0
5
10
15
20
Trial Index
25
30
0
Normalized
Ear PPG
Amplitude
Change in Normalized Pulse Width

Pulse become a large percentage of cardiac cycle
180
1
160
0.9
0.8
140
Lay down
Time(sec)
Stand up
0.7
120
0.6
100
0.5
80
0.4
60
0.3
40
0.2
20
0
11
0.1
0
5
10
15
20
Trial Index
25
30
0
Normalized
Pulse
Width
Results

NPW leads increase in heart rate which leads pinch in ear PPG
amplitude
Reclining
Standing
100
HR
NPWFINGER
PHEAR
90
80
70
60
50
40
30
12
20
0
20
40
60
80
100
Time (sec)
120
140
160
180
11
10
Results
Abrupt Change
Detection
HRFINGER Peak
PHEAR Constriction
NPWFINGER Peak
FWHMFINGER Peak
9
8
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?
13
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).

14
Studies to compare supinestanding 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
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-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

Extinction Curve
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
1.00E-03
Absorption

 Red 660 nm
 Infrared 940 nm



16
6.00E-04
A(Hb)
4.00E-04
A(HbO)
2.00E-04
0.00E+00
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
S pO2 
8.00E-04
CHbO2
CHbO2  CHb
600
700
800
900
1000
1100
Wavelength (nm)


The pleth waveform consist of the IR
tracing.
Indirect measurement of blood volume
under the sensor