Take the Test

The standard 12-lead electrocardiogram is a representation of the heart's electrical activity recorded from electrodes on the body surface. This section describes the basic components of the ECG and the lead system used to record the ECG tracings.

Topics for study:

1. ECG Waves and Intervals
   
2. Spatial Orientation of the 12 Lead ECG

click to view

This diagram illustrates ECG waves and intervals as well as standard time and voltage measures on the ECG paper.

1. ECG Waves and Intervals:

What do they mean?

 P wave: the sequential activation (depolarization) of the right and left atria

 QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously)

 ST-T wave: ventricular repolarization

 U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles

 PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex)

 QRS duration: duration of ventricular muscle depolarization

 QT interval: duration of ventricular depolarization and repolarization

 RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate)

 PP interval: duration of atrial cycle (an indicator or atrial rate)

2. Orientation of the 12 Lead ECG

It is important to remember that the 12-lead ECG provides spatial information about the heart's electrical activity in 3 approximately orthogonal directions:

Right		Left
Superior		Inferior
Anterior		Posterior

Each of the 12 leads represents a particular orientation in space, as indicated below (RA = right arm; LA = left arm, LF = left foot):

 Bipolar limb leads (frontal plane):

 Lead I: RA (-) to LA (+) (Right Left, or lateral)

 Lead II: RA (-) to LF (+) (Superior Inferior)

 Lead III: LA (-) to LF (+) (Superior Inferior)

 Augmented unipolar limb leads (frontal plane):

 Lead aVR: RA (+) to [LA & LF] (-) (Rightward)

 Lead aVL: LA (+) to [RA & LF] (-) (Leftward)

 Lead aVF: LF (+) to [RA & LA] (-) (Inferior)

 Unipolar (+) chest leads (horizontal plane):

 Leads V1, V2, V3: (Posterior Anterior)

 Leads V4, V5, V6:(Right Left, or lateral)

Behold: Einthoven's Triangle! Each of the 6 frontal plane leads has a negative and positive orientation (as indicated by the '+' and '-' signs). It is important to recognize that Lead I (and to a lesser extent Leads aVR and aVL) are right Ûleft in orientation. Also, Lead aVF (and to a lesser extent Leads II and III) are superior Ûinferior in orientation. The diagram below further illustrates the frontal plane hookup.

LOCATION OF CHEST ELECTRODES IN 4TH AND 5TH INTERCOSTAL SPACES:

V1: right 4th intercostal space

V2: left 4th intercostal space

V3: halfway between V2 and V4

V4: left 5th intercostal space, mid-clavicular line

V5: horizontal to V4, anterior axillary line

V6: horizontal to V5, mid-axillary line