A brief introduction to the standard 12-lead ECG

A brief introduction to the standard 12-lead ECG

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 Reference: http://www.ndsu.edu/instruct/grier/eheart.html 2007, but may be copied and used without further permission

First, to make sure we know where the heart is Sensing the hearts electrical activity via electrodes (contacts placed on the surface of the body)

Sensing the hearts electrical activity via electrodes (contacts placed on the surface of the body) Note: anatomical orientation is from the subjects perspective:

The basic four limb electrodes: right arm left arm electrical polarity:

neutral or ground negative positive (manipulated by the EKG machine) right leg left leg Lead I (toward left)

right arm left arm electrical polarity: neutral or ground negative positive right leg

left leg Interpreting the view from an electrode for any given viewing (positive) electrode: An approaching train of muscle fiber

depolarizations (or repolarizations moving away) is seen as an upward trace on the recording (opposite movement = downward trace) Note: the normal average direction for the hearts electrical activity is from the upper right, in the right atrium, to the lower left. R T

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

ATRIA: depol-pause-repol (atrial repolarization is obscured by ventricular depolarization) QRS complex R T Q

S VENTRICLES: depol-pause-repolarize Standard calibration of EKG recordings 1 mm = 0.1 mV 1 mm = 0.04 seconds

25 mm/second 5 mm = 0.20 seconds 1 cm = 1 mV The appearance depends on the location of the electrode and

what the hearts electrical activity is doing (resting or active, normal vs various abnormalities, etc.). In addition to Lead I, here are the others (following pages) Lead II (toward left foot) right arm left arm

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

right arm left arm electrical polarity: neutral or ground negative positive right leg

left leg Leads I, II, & III together (Einthovens triangle) right arm left arm

electrical polarity: neutral or ground negative positive right leg left leg Plus augmented leads, e.g.,

aVR right arm left arm electrical polarity: neutral or ground negative positive

right leg left leg Frontal view of heart aVR aVL

I Limb Leads III aVF

II Cross sectional view of heart V1 Chest leads

V2 V3 V4 V5V6 Summary: the 12 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 leg three 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

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-dimensional view 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. Positions of the electrodes: right arm

left arm V1 V2 V3 V4 V5 V6 (plus the electrodes on the legs)

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