The QRS complex is a sequence shown on electrocardiograms (ECGs). In an ECG test, the heart’s rhythm and function are mapped out on graph paper. The QRS complex, a specific sequence of deflections seen on the printout, represents the depolarization of the right and left ventricles of the heart.

An ECG printout shows five deflections, or waves, that are arbitrarily named P, Q, R, S and T. Q waves are displayed as small downward deflections following the P wave, and the R wave is shown as a large upward deflection, or spike. The S wave is the small downward deflection following the R wave.

The time intervals between the QRS complex determine the ventricular rate. A normal QRS complex lasts 80 to 120 milliseconds. Wide QRS complexes, or those lasting longer than the normal rate, indicate ventricle impairment such as bundle branch blocks. Often with bundle branch blocks, a second upward deflection occurs within the QRS complex. Other causes for a wide QRS complex include ectopic foci or abnormal pacemaker sites that slow conduction within the heart and increase the time for depolarization of the heart muscle.

Narrow QRS complexes are often seen in children and in patients with tachycardia. Short QRS complexes indicate an arrhythmia within or above the atrioventricular node. Sinus tachycardia, atrial fibrillation, atrial flutter, and supraventricular tachycardia cause narrow QRS complexes. Cardiologists and other medical professionals find the QRS complex useful in diagnosing a variety of disorders. Cardiac arrhythmias, ventricular hypertrophy, myocardial infarction, conduction abnormalities and electrolyte imbalances can all be diagnosed by analyzing the QRS complex on an ECG.

Most supraventricular tachycardia (SVT) are not life-threatening. Some SVT indicated by wide or narrow QRS complexes, however, might require treatment. Physical maneuvers activating the parasympathetic nervous system, also called vagal maneuvers, increase intrathoracic pressure and affect the pressure sensors within the aorta’s arch.