Ambulation Analysis in Wearable ECG

165 Pages·2009·3.72 MB·English

by  Subhasis Chaudhuri

EEAE mbulation Analysis in Wearable ECG
.
Subhasis Chaudhuri Tanmay D. Pawar
Siddhartha Duttagupta
Ambulation Analysis in Wearable ECG
Subhasis Chaudhuri Tanmay D. Pawar
Department of Electrical Engineering Department of Electrical Engineering
Indian Institute of Technology, Bombay Indian Institute of Technology, Bombay
Powai, Mumbai-400076 Powai, Mumbai-400076
India India
[email protected]
Siddhartha Duttagupta
Department of Electrical Engineering
Indian Institute of Technology, Bombay
Powai, Mumbai-400076
India
ISBN978-1-4419-0723-3 e-ISBN978-1-4419-0724-0
DOI10.1007/978-1-4419-0724-0
Springer Dordrecht Heidelberg London New York
Library of Congress Control Number: 2009928038
©SpringerScience + BusinessMedia,LLC2009
Allrightsreserved. Thisworkmaynotbetranslatedorcopiedinwholeorinpartwithoutthewritten
permissionofthepublisher(SpringerScience+BusinessMedia,LLC,233SpringStreet,NewYork,NY
10013,USA),exceptforbriefexcerptsinconnectionwithreviewsorscholarlyanalysis.Useinconnection
withanyformofinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilar
ordissimilarmethodologynowknownorhereafterdevelopedisforbidden.
Theuseinthispublicationoftradenames,trademarks,servicemarks,andsimilarterms,eveniftheyare
notidentifiedassuch,isnottobetakenasanexpressionofopinionastowhetherornottheyaresubject
toproprietaryrights.
Illustrations used in the text: Figures 1.3, 3.6, 3.7, 6.2, 6.3, 6.4, 6.6, 7.1, 7.6, 7.7, 7.8, 7.9, 7.12, 8.4, 8.5,
8.6, 8.8, 8.11, 8.12, 8.13, © Institute of Electrical and Electronics Engineers. Reproduced with permission.

Printedonacid-freepaper
Springer is part of Springer Science+Business Media (www.springer.com)
To
My father for casting an everlasting shadow of his life on mine, and
my father-in-law for teaching me the value of simplicity.
-SC
My parents with love.
-TP
My closest circle of friends: my family.
-SD
Preface
We live in a fast-paced world today and, very often, we do not even have
enough time for ourselves to recuperate fully after su(cid:11)ering from ailments.
We also su(cid:11)er from many lifestyle diseases, inconveniencing us slowly and
silently. These are proving to be a big burden to our health care system.
Quite naturally, researchers in the health care sector are trying their best to
come up with technologies that provide help to those who would like to live
a normal life despite their health problems.
Pervasive health care in the area of cardiac abnormalities is perhaps the
mostimportantone.Onerequiresconstantmonitoringofthecardiaccondition
of a subject who is known to have had di(cid:14)culties earlier. Holter monitoring
had been one of the major steps in this direction, although it is not meant
to be a fully wearable yet pervasive system. Over the last decade, attempts
arebeingmadetobuildadevicewhichistrulywearableandprovidesvarious
pre- and post-processing facilities.
Electrocardiography is the simplest non-invasive method of collecting sig-
nal from the beating heart that provides extremely useful information to the
doctors.HenceECGdatacollectionisalmostroutineinanyclinicdealingwith
heart patients. The procedure is highly standardized and the recorder is well
calibrated so that any doctor anywhere in the world can interpret the data.
The protocol requires that the patient lies down and be completely immobile
sothatskintoelectrodecontactisneverdisturbed.Anysuchdisturbancewill
producespurioussignalsintheECGdataobliterating thetrueinterpretation
bythedoctor,makingECGpresumablyunsuitableforwearableapplications.
E(cid:11)orts have been made to improve the quality of ECG recorders. The
weight and size have come down drastically. Data storage and on-chip pro-
cessingpowerhaveimprovedmanifold.Thepowerconsumptionlevelhasbeen
reduced substantially such that the battery life is now prolonged to several
days.Thefunctionality nowalsoincludeswirelessconnectivity among others.
All these recent developments make the wearable ECG recorder an ideal can-
didate for 24(cid:2)7 monitoring of cardiac patients trying to maintain an active
lifestyle.
VIII Preface
Notwithstanding above, various types of body movement activities, called
BMA in this monograph, continue to plague the usefulness of such recorders
due to the motion artifact it generates in the collected ECG data. The skin
stretches and contracts due to body movement, changing the contact resis-
tance at the electrode. Any change in skin humidity would also a(cid:11)ect the re-
sistivity of the skin but it is not considered in this monograph. The recorded
signalisthusverydi(cid:11)erentfromthesedentaryECGwhenthesubjectisunder
ambulation. Since the true ECG signal and the motion artifact overlap in the
spectrum, it is not possible to (cid:12)lter out these artifacts without a(cid:11)ecting the
morphology of the ECG signal.
IspervasivecardiacmonitoringusingW-ECGrecorderatallfeasible?This
isthekeyfocusofthismonograph.Althoughtheindustryfocushasprimarily
been in developing such recorders, the signal processing aspects of the corre-
sponding recordings have been mostly overlooked and the corresponding area
is still largely unexplored. Needless to say, the (cid:12)nal utility of such devices
would depend on how useful are these signals to the clinicians.
Weobservethatmostphysicalactivitiesinvolvecertainrepetitivemotions
of our muscles and hence there must be some structureto the motion artifact
signal which is superposed on the true cardiac signal. We ask the question
if such a structure can at all be discovered from the recorded ECG signal.
If it is so then that particular body movement can be recognized from the
recorded ambulatory ECG signal itself. If the structure of the artifact signal
isknownforagivenactivity,thisshouldalsohelpusincleaningthecorrupted
ECG signal. Further, in order that the above statement makes sense to the
clinicians, there has to be some experimental validation that must bring out
explicit correlation between this structured noise or artifacts and the actual
motion (as measured by some device, say, an accelerometer) of the limb(s).
The primary aim of the monograph is to demonstrate through development
ofappropriatesignalprocessingtechniquesandexperimentalproceduresthat
it is, indeed, possible to develop a truely pervasive, W-ECG based, cardiac
monitoring system.
As mentioned earlier, the current state of research in this area of ambu-
latory ECG analysis is still in its infancy. A lot remains to be done before a
(cid:12)rm recommendation of the new technology can be made for absorption by
the health care industry. This book is meant to initiate the necessary dialog
among researchers and practicing engineers to carry the concept further.
Any technical document in the form of a book or a monograph may su(cid:11)er
from the problem of being super(cid:13)uous and unsubstantiated as there is very
little opportunity for complete reviewing by peers. In order to avoid such a
problemwemadesurethatthekeyconceptsare(cid:12)rstpeerreviewedintheform
ofpublicationsinappropriatejournals.Thepublicationsthatoverlapwiththe
contents of the book are given in the references [99, 100, 101, 102, 103]. We
havemodi(cid:12)edandenhancedthemtosuitthedemandofthisbook.Needlessto
say, the copyrights of various (cid:12)gures reproduced from our earlier publications
rest with the original publishers.
Preface IX
The book is addressed to a broad audience. It should be useful to both
practitioners and researchers in the area of biomedical engineering. We have
tried to make the book self-contained and hence there is no speci(cid:12)c pre-
requisite. Any one with basic familiarity with digital signal processing and
linear algebra, will (cid:12)nd the content quite readable. In order to bridge the gap
between hardware and algorithmic developments, we have included chapters
that speci(cid:12)cally discuss the hardware aspects and the corresponding calibra-
tion issues. We have invited our colleagues who have actually developed such
a hardware to write a chapter on hardware details. This particular aspect
should be speci(cid:12)cally useful to the practitioners in technology development.
Wehopethatthereaderswill(cid:12)ndthebookuseful.Wewelcomecomments
and suggestions from readers.
Mumbai Subhasis Chaudhuri
March 2009 Tanmay Pawar
Siddhartha Duttagupta
Acknowledgments
The authors are truly indebted to Prof. Soumyo Mukherji at IIT Bombay for
suggesting to us that we investigate the possibility of identifying the nature
ofmotionartifactsinECGsignals.Ourdiscussionswithhimhadalwaysbeen
very fruitful. The authors are also thankful to Profs. Harish Pillai and S. N.
Merchant,bothatIITBombay,fortheirinsightfulcommentsandsuggestions.
Thanks are also due to Profs. Maryam Shoejai-Baghini, Dinesh K. Sharma,
both at IIT Bombay and Prof. Rakesh Lal who is currently at University of
California,SantaBarbara,forhavingprovideduswithallnecessaryhardware
support as well as for having accepted to write an invited chapter for this
monograph.
The authors thank N. S. Ananthakrishnan for his help in data collec-
tion and running some experiments for developing the contents of Chapter 8.
Thanks are due to Dr. Gandhi, Dr. Mohanty and their sta(cid:11) at Nanavati Hos-
pital in Mumbai for their enthusiastic help and support in collecting relevant
data from cardiac patients. We are also thankful to Prof. G. G. Ray of IITB
for his help in data collection. Needless to say, the authors are thankful to all
subjects who volunteered in the experiments.
JenniferEvansandhersta(cid:11)atSpringer,particularlyKellyMoritz,deserve
to be thanked for making sure that we do complete the manuscript. Authors
are also thankful to IEEE and Springer for their generous policy of allowing
the authors to use (cid:12)gures published in their journals in the monograph.
The authors thank the Bharti Center for Communication in EE depart-
ment at IIT Bombay for help and support.
The authors thank the Department of Science and Technology (DST) for
the (cid:12)nancial help in the form of J. C. Bose National Fellowship to the (cid:12)rst
author. The acknowledgment is incomplete without mentioning the constant
support and encouragement received by the authors from their family mem-
bers.
IIT Bombay Subhasis Chaudhuri
March 2009 Tanmay Pawar
Siddhartha Duttagupta
Contents
1 Introduction............................................... 1
1.1 Basics of Electrocardiogram (ECG) ........................ 1
1.2 Artifacts in ECG ........................................ 5
1.2.1 EMG Noise....................................... 5
1.2.2 Baseline Wandering................................ 5
1.2.3 Powerline Interference.............................. 6
1.2.4 Motion Artifacts .................................. 6
1.3 Ambulatory Monitoring .................................. 6
1.4 Challenges in Ambulatory ECG Processing ................. 8
1.5 Mathematical Model of Ambulatory ECG Signal ............ 9
1.6 Tour of the Book ........................................ 10
2 Review of ECG Analysis................................... 15
2.1 QRS Detection Methods.................................. 16
2.2 Delineation of Wave Boundaries ........................... 18
2.3 Beat Alignment ......................................... 20
2.4 Noise Reduction in ECG ................................. 21
2.5 Detection of Body Posture Changes........................ 23
2.6 Overview of Wearable ECG Recorders ..................... 24
2.7 Analysis of Ambulation in ECG ........................... 25
3 Hardware Development of Wearable ECG Devices......... 27
3.1 Introduction ............................................ 27
3.2 Basics of Personal ECG Instruments ....................... 28
3.2.1 System Modules and Operation ..................... 28
3.2.2 System Requirements .............................. 29
3.3 Electrodes .............................................. 29
3.4 Signal Conditioning...................................... 30
3.4.1 Implementation Using General-Purpose ICs........... 31
3.4.2 ASIC (Application-Speci(cid:12)c Integrated Circuit) Design
for Signal Conditioning............................. 33