A brief introduction to the standard 12-lead ECG (EKG)

A brief introduction to the� standard 12-lead ECG (EKG)

1. A brief introduction to the
standard 12-lead ECG (EKG)
by James W. Grier
Department of Biological Sciences
North Dakota State University
Fargo, ND 58105-5517
©2007, but may be copied and used without further permission
Reference: http://www.ndsu.edu/instruct/grier/eheart.html

3. First, to make sure we know where the heart is …

4. Sensing the heart’s
electrical activity
via electrodes
(contacts placed on
the surface of the
body)

5. Sensing the heart’s
electrical activity
via electrodes
(contacts placed on
the surface of the
body)
Note: anatomical
orientation is from the
subject’sperspective:

6. The basic four
limb electrodes:
right arm
left leg
left arm
right leg
electrical polarity:
neutral or ground
negative
positive
(manipulated by the EKG machine)

7. right arm
left leg
left arm
right leg
electrical polarity:
neutral or ground
negative
positive
Lead I(toward left)

8. for any given
viewing (positive)
electrode:
An approachingtrain of muscle fiber
depolarizations (or repolarizations moving away)
is seen as an upward traceon the recording
(opposite movement = downward trace)
Interpreting the view from an electrode
Note: the normal average direction for the heart’s electrical activity
is from the upper right, in the right atrium, to the lower left.

9. P
Q
R
S
T
(This particular tracing does not show a Q wave,
a downward wave just before the R wave.)
The main, typical waves of an EKG.

10. ATRIA: depol-pause-repol
(atrial repolarization is obscured by ventricular depolarization)
P

11. VENTRICLES: depol-pause-repolarize
Q
R
S
T
QRS complex

12. 1 mm =
0.1 mV
1 cm =
1 mV
1 mm = 0.04 seconds
5 mm = 0.20 seconds
25 mm/second
Standard calibration of EKG recordings

13. The appearance depends on the location of the electrode and
what the heart’s electrical activity is doing (resting or active,
normal vs various abnormalities, etc.).
In addition to Lead I, here are the others …(following pages)

14. right arm
left leg
left arm
right leg
electrical polarity:
neutral or ground
negative
positive
Lead II(toward left foot)

15. right arm
left leg
left arm
right leg
electrical polarity:
neutral or ground
negative
positive
Lead III(down & rightward)

16. right arm
left leg
left arm
right leg
electrical polarity:
neutral or ground
negative
positive
Leads I, II, & III together
(“Einthoven’s triangle”)

17. right arm
left leg
left arm
right leg
electrical polarity:
neutral or ground
negative
positive
Plus “augmented” leads, e.g.,
aVR

18. IIIII
aVR
I
aVF
aVL
Limb
Leads
Frontal view
of heart

19. V6
V5
V4
V3
V2
V1
Chest
leads
Cross sectional
view of heart

20. Summary: the12 standard leads are :
Limb leads –
I, from the right arm (-) toward the left arm (+) (taken together, these
II, from the right arm toward the left legthree form the classic
III, from the left arm toward the left leg"Einthoven's triangle")
aVR, augmented lead toward the right (arm) (note: aVR is approx.
aVL, augmented lead toward the left (arm) opposite of I and should
aVF, augmented lead toward the foot essentially mirror the
shape of I vertically)
Chest leads –
V1 through V6, starting over the right atrium with V1, and placed in
a semi-circle of positions leftwards, to the left side of the left ventricle

21. The normal progression of muscular contractions,
hence, electrical activity, travels from the upper right
part of the atria downward and leftwards to the
ventricles, with the left ventricle being the strongest.
Various combinations of limb leads and chest leads
taken together provide a three-dimensionalview into
the electrical activity and workings of the heart for
anyone who knows how to read an EKG.
Abnormalities, such as heart attacks, arrhythmias,
congenital problems, and a host of diseases and
factors that affect the heart will cause sometimes
major and sometimes subtle changes to the EKG
patterns, which can be interpreted by a trained,
experienced observer.

22. (plus the electrodes on the legs)
Positions of the electrodes:
right arm left arm
V1V2
V3
V4
V5
V6