Introduction / Basic Anatomy



The electrocardiogram (ECG) that is interpreted today along with the names given to each component of the ECG and the electrical axis of the heart are all attributed to Willem Einthoven (1860-1927).


Positive Input

Negative Input

Bipolar Limb Leads
  I Left arm Right arm
  II Left leg Right arm
  III Left leg Left arm
Augmented Unipolar Limb Leads
  aVF Right arm Left arm + Left leg
  aVL Left arm Right arm + Left leg
  aVR Left leg Left arm + Right arm
Precordial Leads
  V1 Right sternal margin, fourth intercostal space Wilson Central Terminal
  V2 Left sternal margin, fourth intercostal space Wilson Central Terminal
  V3 Halfway between V2 and V4 Wilson Central Terminal
  V4 Left midclavicular line, fifth intercostal space Wilson Central Terminal
  V5 Left anterior axillary line, fifth intercostal space Wilson Central Terminal
  V6 Left midaxillary line, fifth intercostal space Wilson Central Terminal


Each lead shows the electrical activity of the heart from a different perspective. The P wave, PR interval, QRS complex, ST segment, T wave and sometimes U wave make up the ECG. Each of these components and their intervals are what we use to interpret the ECG. Here is a brief review of what each of these represents.

P Wave

The p wave represents atrial electrical activity. It is important to remember that electrical activity on the ECG does not necessarily represent mechanical activity. The p wave can be used to identify the source of the atrial activity and may be helpful in assessing atrial chamber size.

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PR Interval

The PR interval is the time of initial atrial activity to the beginning of the ventricular activity (QRS complex). This time includes the travel through the atria as well as through the AV node. Both a short (<0.08 seconds) and long PR (>0.20 seconds) interval can be diagnostic of disease processes.

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QRS Complex

The QRS complex is ventricular activation. If the initial deflection of the complex is negative, it is referred to as a Q wave (many QRS complexes have no Q wave). The first positive deflection is the R wave and a negative deflection after the R wave is the S wave. If there is no R wave present, the complex is referred to as a QS complex. Finally, if a second positive deflection occurs, it is an R’. The QRS complex gives insight to many clinical scenarios including bundle branch block, myocardial infarction and ventricular chamber enlargement or hypertrophy. (link to QRS complexes with all the various combinations of the above)

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ST Segment

The ST segment begins where the QRS complex ends (the j point). The level of the ST segment and the shape of the ST segment are the important components when looking for pathology.

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T Wave

The T wave represents ventricular repolarization. The direction of the T wave is normally upright in I, II, and V3 through V6. The T wave is normally inverted in aVR and V1. The direction is variable in the other leads.

The shape of the T wave is normally slightly asymmetric and rounded and the height is normally less than 5 mm in the standard leads and less than 10 mm in the precordial leads.

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QT Interval

The QT interval is measured from the beginning of the QRS complex to the end of the T wave and represents ventricular systole. In general, the QT interval should be less than half of the R – R interval. A prolonged QT interval has many causes including electrolyte abnormalities, drug effects, CNS pathology and congenital. A more detailed list and consequences is described later in the curriculum.

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U Wave

The presence of a U wave is normal and can be found in most tracings with close inspection. Prominent or inverted U waves can be pathologic.

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