The electrocardiogram (ECG or EKG) records the electrical activity of the heart. Electrodes placed on the skin detect currents produced by the propagation of action potentials through the heart muscle with each heartbeat. The ECG produces 12 tracings from different electrode combinations that can identify abnormalities in conduction pathways, enlarged heart structures, damaged heart regions, and causes of chest pain. A normal ECG shows three distinct waves - the P wave from atrial depolarization, the QRS complex from ventricular depolarization, and the T wave from ventricular repolarization.

1. 13.4 .2 , Electrocardiogram (ECG)
As action potentials propagate through the heart, they g~nerate electrical currents that
can be detected at the surface of the body. An electrocardiogram (ECG -or EKG) is a record-
ing of these electrical signals. ECG is a composite record of action potentials produced by all
th e heart muscle fibers during each heart beat. The instrument that records the changes is
called an e lcclroc.udiograph.
1n clinical practice, electrodes are positioned on the arms and legs (limb leads) and at six
ositions on the chest (chest leads) to record the ECG. The electroc~rdiograph e~pli~es the
heart's electrical signals and produces 12 different tracings from different combinations of

2. I352 f'-.1 A Text Book of Human·.Anatomy and Phy1lotogy-t
limb and chest leads. Each limb and chest electrode records slightly diff1!ru1it. ,,h•('t,i, ,ii ; I( tJv--
ity because of the difference in its position relative lo the he.irt. Hy ~·ompar!ng t.h1·•-t-;(• n•,·,>tdr;
with one another and with normal records, it is possible lo c.Jcl<~rrnitw (I) if Hw condu, lin,j
pathway is abnormal, (2) if the heart is enlarged, (3) if certain n •gions of Uw h1·;ut ,in:
damaged, and (4) the cause of chest pain.
Normal ECG
In a typical record, three clearly recognizalbe waves appear with each heartbt'.i.d. (J i1:,m·
13.10)
1
2.
3.
•
p wave : It is the first small upward deflection on the EC(; a ncJ repn:~cnh ,Jlri:d
depolarization, which spreads from the SA node through contractil<: fib<~r'> jn br)th
atria.
QRS complex : It is the second wave that begins as a downward deflection, continu,~,,
as a large, upright, triangular wave, and ends as a downward wave. The ()RS cornplf~X
represents rapid ventricular depo1arizalion, as the action potential spread1, throu~h
ventricular contractile fibers.
T wave : This third wave is a dome shaped upward deflection, which 1ndic.at1:-,
ventricular repolarization and occurs just as the ventricles are starting to relax. Th(; T
wave is smaller and wider than the QRS complex because repoJarization occurs mrJr~
slowly than depolarization. During the plateau period of a steady depolarizatfrm, tr1t
ECG tracing is flat.
Abnormal size of the waves : Larger P waves indicate enlargement of an atrium- Ar,
enlarged Q wave may indicate a myocardial infarction. An enlarged R wave gen£:TaI(r
indicates enlarged ventricles. The T wave is flatter than normal when the heart musd~
is receiving insufficient oxygen, eg, as in coronary artery disease. The T wave ma:,- 'c~
elevated in hyperkalemia (high blood K+ level).
PR Jr.terval
QRS
Complex
R
Q
s
T
Fig~13_10 NormaJelectrocardiogram or ECG (Lead ti).

3. cardiovascular System :lll:1 Iinte r,cls or Seg1nents
~naivsis of an ECG also in,·olvc~ n,~a~ttrt1't·,u '-l1o tl _
1
wc,v••t, c:allcid ,n..• .,. , _ .,_ • • , _ _ . ~ (", , 1~ tnc ~p,tnH ·1l'tW(•t-'n • ..~ , . L '
.., , ,J.; or ~e~nh nb. The l Q tnt<. n .11 (tron, bt:'ol ,i f 11 l 1 1 0 r,f thl' QR.,l d "'" . . , . • . . ~ nn n~ n . . wnve to )(} K a111 nP
ccmplex) is the tin1e requ1~"<~ for -~he action poh.•ntial to t·rnvd th1'ntlf-ih H1c• at-rlu, alriovt~n-
tricular node, and the retnauun~ hb~rs of t-hl' condtttl' l ·• '--= _ • ton tiYH ltH.
• The P~Q_lnlcrY.1; lengthens due to snn ti~~uc rnust•d hy dh,mrdt•rH Htu·h u111 c·oro1uJry
artery disease (CAD) and rh~un,ntic ft'VCt'.
• i:ne ~-T , eg1n cnl (fr?m the end of _s wave to lht' hl:'ginninK of T wave) n•prc•~t•nft-1 the•
rune ,vb.en the ventricular contractile fibers arc depnlnri:1.l'rl durlnH tht• ph-.t"cau pha,-u~
of the action potential.
•
•
•
•
•
The S-T seg1ncnl is elevate~ (abov~ the base line) in acull' myocurJial infarction and
depressed (beloi,r the base hne) ,-vhen tht:? henrl tnusd.c rl'ClivcH insuffkit~nt 07_.
The Q-T intcn·.1l (fron, the start of the QRS con1pll:'x t:o the Pnd of Lhe ·r wave) is thf~
tin,e fron1 the beginning of ventricular depolari7.ation to th<. end of vt-!ntricular
repolarization.
The Q-T int~n·.11 n1ay be lengthened by n1yocardial da1nagc, myocnrdi.al ischernia, or
conduction abnonnalities.
SLt ~ss testing : Sometimes it is helpful to evaluate the heart's response lo the stress of
physical exercise. For exrunple, narrowed coronary arteries will not be able to 1neet
the hearrs increased need for 0 2 during strenuous exercis~. Thh; situation creates
changes that can be seen on an ECG.
Continuous A1nbulatory Electrocardiogr;1phs , Abnonnal heart rhytluns and inadquatc
blood flow to the heart may occur briefly or unpredictably. These problen1s can be
detected by continuous an1bul.1tory dcctroc;-.rdiograph~ With this procedure,~ person
Vears a battery operated monitor (Holter monitor) that records an ECG continuously
for 24 hours. Electrodes, attached to the chest are connected to the 1nonitor and
information on the heart's activity is stored in the 1nonitor and retrieved later by
medical personnel.