Various QRS complexes with nomenclature.

Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.

The QRS complex is a recording of a single heartbeat on the ECG that corresponds to the depolarization of the right and left ventricles. Ventricles contain more muscle mass than the atria, therefore the QRS complex is considerably larger than the P wave. The His/Purkinje specialised muscle cells coordinate the depolarization of both ventricles, the QRS complex is 80 to 120 ms in duration represented by three small squares or less, but any abnormality of conduction takes longer and causes widened QRS complexes.

The atrial repolarization wave, which resembles an inverse P wave, is buried inside the QRS wave. The atrial repolarization wave is obscured by the QRS because it is far smaller in magnitude.

[edit] Terminology

Not every QRS complex contains a Q wave, an R wave, and an S wave. By convention, any combination of these waves can be referred to as a QRS complex. However, correct interpretation of difficult ECGs requires exact labeling of the various waves. Some authors use lowercase and capital letters, depending on the relative size of each wave. For example, an Rs complex would be positively deflected, while an rS complex would be negatively deflected. If both complexes were labeled RS, it would be impossible to appreciate this distinction without viewing the actual ECG.

[edit] Clinical significance

• The duration, amplitude, and morphology of the QRS complex is useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.
  • For example, a prolonged QRS complex can be associated with hyperkalemia.^[1]

• Q waves can be normal (physiological) or pathological.
  • Pathological Q waves refer to Q waves that have a height of 25% or more than that of the partner R wave and/or have a width of greater than 0.04 seconds.
  • Normal Q waves, when present, represent depolarization of the interventricular septum. For this reason, they are referred to as septal Q waves, and can be appreciated in the lateral leads I, aVL, V5 and V6.

• Q waves greater than 1/4 the height of the R wave, greater than 0.04 sec (40 ms) in duration, or in the right precordial leads are considered to be abnormal, and may represent myocardial infarction.

[edit] See also

• Electrophysiology
• Electrocardiography

[edit] References