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Influence of Non-Physiological Blood Pressure Artifactson Cerebral Autoregulation

Erica. M. Rutter1 Adam Mahdi 2

1Center for Research in Scientific ComputationDepartment of Mathematics

North Carolina State University

2Institute of Biomedical EngineeringUniversity of Oxford

JMM, 2017

Erica. M. Rutter, Adam Mahdi Artifact Influence on ARI JMM, 2017 1 / 29

Outline

1 Cerebral AutoregulationBiological IntroductionMathematical Introduction

2 ArtifactsImplementationE↵ect on ARI

3 Conclusions and Further Directions


Outline





Cerebral Autoregulation

Cerebral Autoregulation: Combination of various physiological processesmodulating blood vessel properties in order to maintain constant cerebralblood flow despite changes in arterial blood pressure

Figure: Cerebral Autoregulation Zone. Source: Pires, Paulo W., et al. ”The e↵ects of

hypertension on the cerebral circulation.” American Journal of Physiology-Heart and Circulatory

Physiology 304.12 (2013): H1598-H1614.[4]


Cerebral AutoregulationWhy do we care?

Imparied Autoregulation seen in patients with:strokehead injurieschronic hypertension

Impaired Autoregulation is associated withbrain tissue injury by changes in blood pressuredisruption of blood-brain barrierdeath


Cerebral AutoregulationHow to measure?

ARI: Autoregulation Index (clinical tool) measures how well cerebralautoregulation is occurring

0 ! 9No Autoregulation Best Autoregulation


ARIHow to measure?

Raw ABP signals are pre-processed to obtain a beat-to-beat average

ARI is calculated using processed ABP and produces predicted CBFV


Data and Processing

36 one-minute baseline ABPand CBFV readings ofnormotensive subjects

low-pass filtered usingzero-phase 4th-orderButterworth filter in bothdirections with cuto↵ frequencyof 20 Hz

beat-to-beat detection

downsampled to 10 Hz foruniform signals


Outline





Mathematical ModelARI

Tiecks et. al [5] formulated the following phenomenological model:

dP =P � Pm

Pm � Pcr

x2 = x2 +x1 � 2Dx2

fT

x1 = x1 +dP � x2

fT

V̂ (T ,D,K ) = Vm (1 + dP � Kx2)

ARI = minARI2[0,9]

||V̂ (T ,D,K )� CBFV ||

whereP - ABP input, Pm - mean ABP

V̂ - CBFV output, Vm - mean CBFV

f - sampling frequency

Pcr - critical pressure (12 mmHG)Erica. M. Rutter, Adam Mahdi Artifact Influence on ARI JMM, 2017 10 / 29

Mathematical ModelVariations

Tiecks et. al [5] model was not specific of time points for x1 and x2:

dP(t) =P(t)� Pm

Pm � Pcr

V̂ (t) = Vm (1 + dP � Kx2(t))

Panerai et. al 1999 [3]

x1(t) = x1(t � 1)+

dP(t)� x2(t � 1)

fTx2(t) = x2(t � 1)+

x

1

(t)� 2Dx2(t � 1)

fT

Panerai 2010 et. al [1]

x1(t) = x1(t � 1)+

dP(t� 1)� x2(t � 1)

fTx2(t) = x2(t � 1)+

x

1

(t� 1)� 2Dx2(t � 1)

fT

Current Formulation

x1(t) = x1(t � 1)+

dP(t)� x2(t � 1)

fTx2(t) = x2(t � 1)+

x

1

(t� 1)� 2Dx2(t � 1)

fT


Mathematical ModelComparisons

We observe little di↵erence between the various formulations for models,so we stick with the 2016 “current” model


Mathematical ModelFinal Formulation

dP(t) =P(t)� Pm

Pm � Pcr

x1(t) = x1(t � 1) +dP(t)� x2(t � 1)

fT

x2(t) = x2(t � 1) +x1(t � 1)� 2Dx2(t � 1)

fT

V̂ (t,T ,D,K ) = Vm (1 + dP � Kx2(t))

ARI = minARI2[0,9]

||V̂ (t,T ,D,K )� CBFV (t)||

whereP - ABP input, Pm - mean ABPV̂ - CBFV output, Vm - mean CBFVf - sampling frequencyPcr - critical pressure (12 mmHG)


Outline





ABP MeasurementSetup

Figure: ABP measurement set-up. Source: “Arterial Line and Intra-arterial Blood

Pressure Monitoring.” Life in the Fast Lane Medical Blog. 14 June 2015. Web. 03 Jan. 2017


ABP MeasurementNon-Physiological Artifacts

Figure: Where non-physiological artifacts can occur in ABP measurement.Source: “Arterial Line and Intra-arterial Blood Pressure Monitoring.” Life in the Fast Lane

Medical Blog. 14 June 2015. Web. 03 Jan. 2017Erica. M. Rutter, Adam Mahdi Artifact Influence on ARI JMM, 2017 16 / 29

Non-Physiological Artifacts

Figure: Real examples of non-physiological artifacts in ABP measurements.Source: Li, Qiao et. al. ”Artificial arterial blood pressure artifact models and an evaluation of

a robust blood pressure and heart rate estimator.” Biomedical engineering online 8.1 (2009):.[2]


Non-Physiological ArtifactsImplementation

Only raw ABP data is altered to include non-physiological artifacts

CBFV data is not altered!


Non-Physiological ArtifactsImplementation

Each artifact is inserted in to the raw ABP measurements 5 secondsinto the time series

Severity of artifacts are measured in ‘levels’ where level 0 correspondsto no artifact and level 20 corresponds to maximal artifact.

For each artifact formulation, there may be multiple parametersgoverning the shape of the artifact. Upper and lower bounds will begiven for each parameter. Levels 0-20 correspond to linearlyincreasing all parameters from their lower to upper bounds


Artifact 1Saturation to maximal ABP

Amax(↵, L,Pmax) = tanh(↵⇡t)(Pmax � Pdias) + Pdias

Parameter Lower Bound Upper Bound Meaning↵ 0 0.1 saturation rate

Pmax (mmHG) 190 210 maximal ABPL (s) 0 5 total time of artifact


Artifact 2Square Wave Pulse

Asw (Pmax, L) =

⇢Pmax t 2 [0, L/2]0 t 2 [L/2, L].

Parameter Lower Bound Upper Bound MeaningPmax (mmHG) 190 210 maximal ABP

L (s) 0 10 total time of artifact


Artifact 3Pulse Pressure Reduction

Reduces systolic pressure linearly over the span of 45 seconds to the value

Pend = MPsys

Parameter Lower Bound Upper Bound MeaningM 1 0.1 ratio at end of artifact


Artifact 4Impulse

fL =

8<

:

L sin(2⇡t/L)

2⇡tt 2 [�L/2, L/2]

0 otherwise.

A

imp

(L) = P + (Psys � Pdia) fL, t 2 [�L/2, L/2]

Parameter Lower Bound Upper Bound MeaningL (s) 0 2 total time of artifact


Outline





Artifact E↵ects

50

100

150

200

AB

P [

mm

Hg

]A

max

Level 10Level 20

40

60

80

100

120

140

App

AB

P [

mm

Hg

]

0

50

100

150

200

Asw

AB

P [

mm

Hg

]

0 10 20 30 40 50

50

100

150

200

Aimp

AB

P [

mm

Hg

]

Time [s]

1

3

5

7

9

Amax

AR

I

Amax

mA

RI±

SD

1

3

5

7

9

App

AR

I

App

mA

RI±

SD

1

3

5

7

9

Asw

AR

I

Asw

mA

RI±

SD

0 4 8 12 16 20

1

3

5

7

9

Aimp

Art. Size

AR

I

0 4 8 12 16 20

Aimp

Art. Size

mA

RI±

SD


Artifact E↵ects10% ARI Change

Definition

The critical artifact size is the level of artifact which generates a change of10% change in the mean ARI of all patients.

Artifact Critical size ParametersA

max

5.21± 2.1 L=1.3 s Pmax = 195.2mmHg ↵ = 0.026A

sw

2.75± 2.1 L=1.4 s Pmax = 192.8mmHgA

pp

8.06± 5.1 M = 0.64A

imp

11.52± 3.9 L=1.05 s

Table: The mean critical artifact size ± standard deviation and the correspondingparameters that generate it.


Conclusions and Further Directions

ARI values can be sensitive to non-physiological artifacts present indata.

Saturation to maximal ABP and square wave pulses will alwayspredict an ARI value of 9 if present long enough.

Reduced pulse pressure and impulse artifacts are not as sensitive butcan still change ARI values up to 10% if present long enough.

Further DirectionsSensitivity of parameters of artifactsHow do multiple artifacts or repetition change ARI values?Implement detection system to filter out nonphysiological artifactsHypertensive vs. normotensive


Acknowledgements

Group 1 Participants:

P. Sang Chalacheva

Katrina Johnson

Greg Mader

Kevin O’Kee↵e

Grant Number: 1321794


Works Cited I

Dineen, Nicky E and Brodie, F G and Robinson, Thompson G and Panerai, Ronney B.Continuous estimates of dynamic cerebral autoregulation during transient hypocapnia andhypercapnia.Journal of applied physiology, 108(3):604–613, 2010.

Li, Qiao and Mark, Roger G and Cli↵ord, and Gari D.Artificial arterial blood pressure artifact models and an evaluation of a robust bloodpressure and heart rate estimator.Biomedical Engineering Online, 8(1):2009.

Panerai, Ronney B and Dawson, Suzanne L and Potter, and John F.Linear and nonlinear analysis of human dynamic cerebral autoregulation.American Journal of Physiology-Heart and Circulatory Physiology, 277(3): H1089–H1099,1999.

Pires, Paulo W and Ramos, Carla M Dams and Matin, Nusrat and Dorrance, and Anne M.The e↵ects of hypertension on the cerebral circulation.American Journal of Physiology-Heart and Circulatory Physiology, 204(12):H1598–H1614,2013.

Tiecks, Frank P and Lam, Arthur M and Aaslid, Rune and Newell, and David W.Comparison of static and dynamic cerebral autoregulation measurements.Stroke, 26(6):1014–1019, 1995.


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