/ Patient Care Solutions, Philips Research / Signal Processing Systems, Electrical Engineering Department TU/e

IEEE EMBS Benelux Symposium, Brussels, Belgium, 2 December 2011

Introduction

Figure 1: Pulse oximetryfinger clip used to measurea patient’s heart rate andblood oxygenation.

The use of pulse oximeters(Fig. 1) is spreading in lowacuity ambulatory settings,such as telemetric solutionsin general wards, or homemonitoring of patients withchronic lung disease. Inthese settings pulse oximetryis heavily affected by motion artifacts, because PPGs arehighly susceptible to motion. The goal of this researchis improving the robustness to motion of PPG measure-ments, enabling reliable ambulatory oximetry.

Measurement setup

Forehead sensor Laser diode + ball lens

LEDsPhotodiode

Tri-axial accelerometer

Figure 2: Customized forehead sensor to measure relative and globalsensor motion.

• A laser diode and an accelerometer measure rela-tive and global sensor motion respectively (Fig. 2).

• The forehead sensor has been attached by adhe-sives and a headband.

• A lead I ECG measurement provides a heart rate(HR) reference.

• A commercial pulse oximeter provides an oxygensaturation (SpO2) reference.

• One subject walked on a treadmill at differentspeeds to generate motion artifacts.

ResultsMonitor diode’s spectrogram [dB]

Time [s]

-180

-160

-140

-120

-100

-80

-60

-40-20

50 100 150 200 2500

10

20

30

70

80

90100

50

60

40 xn(t) ∼ cos(2π ∫ fd(τ )dτ )

yn(t) ∼ sin(2π ∫ fd(τ )dτ )

fd(t) =cos(θ)v(t)λ/2

yn(t): from carrier

xn(t): from second harmonic

φ(t) ∼ relative motion

“Doppler vector”

Figure 3: Relative sensor motion is measured via self-mixing interfer-ometry. The phase of the Doppler vector gives the relative motion.

90 100 110 120 130 140 150 160 170 180 190−0.01

0

0.01

0.02

Am

plitu

de [V

] Treadmill test at 6km/h − Forehead PPGs [0Hz − 8Hz] (offset adjusted for visibility)

RedInfrared

90 100 110 120 130 140 150 160 170 180 190100

110

120

130

HR

[bpm

]

Heart Rate

−10

0

10

Err

or [b

pm]

90 100 110 120 130 140 150 160 170 180 19096

98

100

SpO

2 [%

]

Oxygen Saturation

−2

0

2

Err

or [%

]

FH PPGsPicoSAT Ref.

90 100 110 120 130 140 150 160 170 180 1900

0.1

0.2

0.3

Time [s]

Fre

quen

cy [H

z] Beat Frequency: Heart Frequency − Pace Frequency (2.2Hz)

ECG Ref.

FH PPGs

Figure 4: The effect of walking on PPGs and the derived HR and SpO2.

94 96 98 100 102 104 106 108 110 112 114

0

10

20

x 10−3

Am

plitu

de [V

]Treadmill test at 6km/h − Forehead PPGs [0Hz − 8Hz] (offset adjusted for visibility)

RedInfrared

94 96 98 100 102 104 106 108 110 112 114

−100

0

100

# C

ycle

s

Relative sensor motion from Doppler signals (laser monitor diode) [0.5Hz − 30Hz]

94 96 98 100 102 104 106 108 110 112 114−0.5

00.5

1

Time [s]Acc

eler

atio

n [g

] Accelerometer [0Hz − 30Hz]

X (vertical)Y (arrow − sidewards)Z (orthogonal − forwards)

Figure 5: Relative and global sensor motion during walking.

Speed HR error: mean± std [BPM] SpO2 error: mean± std [%]

[km/h] Rest Motion Rest Motion

4 0.0576± 1.19 0.0958± 2.05 0.328± 0.507 0.294± 0.641

5 0.0414± 1.11 0.139± 2.92 0.704± 0.57 0.151± 0.891

6 0.0134± 1.10 0.361± 5.64 0.465± 0.599 0.291± 0.988

7 0.019± 1.16 0.328± 5.38 0.872± 0.604 0.234± 1.15

Table 1: HR and SpO2 error versus speed of walking.

Conclusions

• While walking at constant speed, a beat phe-nomenon at fbeat = |fHR − fpace| Hz is observedin forehead PPGs, affecting both HR and SpO2 ac-curacy.

• The beating phenomenon suggests an additivemotion artifact.

• During walking the relative sensor motion corre-lates best with the vertical accelerometer axis.

• Both measures of motion may perform comparablyas artifact reference to suppress the beating effectand improve HR and SpO2 accuracy.

Correlation of relative and global sensor motionwith motion artifacts in photoplethysmogramsRalph Wijshoff, Massimo Mischi, Alexander van der Lee,Pierre Woerlee, Paul Aelen, Niek Lambert, Ronald Aarts

contact: R.W.C.G.R.Wijshoff@tue.nl