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3. Characteristics of the Normal ECG

It is important to remember that there is a wide range of normal variability in the 12 lead ECG. The following "normal" ECG characteristics, therefore, are not absolute. It takes considerable ECG reading experience to discover all the normal variants. Only by following a structured "Method of ECG Interpretation" (Lesson II) and correlating the various ECG findings with the particular patient's clinical status will the ECG become a valuable clinical tool.

Topics for Study:

  1. Measurements
  2. Rhythm
  3. Conduction
  4. Waveform description




Normal sinus rhythm

The P waves in leads I and II must be upright (positive) if the rhythm is coming from the sinus node.


Normal Sino-atrial (SA), Atrio-ventricular (AV), and Intraventricular (IV) conduction

Both the PR interval and QRS duration should be within the limits specified above.

Waveform Description

(Normal ECG is shown below - Compare its waveforms to the descriptions below)

image 03-01

P Wave

It is important to remember that the P wave represents the sequential activation of the right and left atria, and it is common to see notched or biphasic P waves of right and left atrial activation.

QRS Complex

The QRS represents the simultaneous activation of the right and left ventricles, although most of the QRS waveform is derived from the larger left ventricular musculature.

ST Segment and T wave

In a sense, the term "ST segment" is a misnomer, because a discrete ST segment distinct from the T wave is usually absent. More often the ST-T wave is a smooth, continuous waveform beginning with the J-point (end of QRS), slowly rising to the peak of the T and followed by a rapid descent to the isoelectric baseline or the onset of the U wave. This gives rise to an asymmetrical T wave. In some normal individuals, particularly women, the T wave is symmetrical and a distinct, horizontal ST segment is present.

The normal T wave is usually in the same direction as the QRS except in the right precordial leads. In the normal ECG the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR.

Normal ST segment elevation: this occurs in leads with large S waves (e.g., V1-3), and the normal configuration is concave upward. ST segment elevation with concave upward appearance may also be seen in other leads; this is often called early repolarization, although it's a term with little physiologic meaning (see example of "early repolarization" in leads V4-6):

image 12-05

Convex or straight upward ST segment elevation (e.g., leads II, III, aVF) is abnormal and suggests transmural injury or infarction:

image 12-09

ST segment depression is always an abnormal finding, although often nonspecific (see ECG below)

image 12-06

ST segment depression is often characterized as "upsloping", "horizontal", or "downsloping".

image 01-18

The normal U Wave: (the most neglected of the ECG waveforms)