QRS Complex

The QRS complex represents ventricular depolarization. Atrial repolarization occurs during this time, but the atrial repolarization wave is hidden in the QRS complex and is not able to be evaluated. The QRS complex is often the largest portion of the EKG as the ventricles are much larger than the atria. It is important to evaluate the width, height, and morphology. It is often divided into 3 segments: the q wave, r wave, and s wave; however, all 3 segments are not commonly present in all leads. They are capitalized when they are the dominant feature of the QRS complex; if two are equally dominant (large), then they are both capitalized. The q wave is a negative deflection that occurs before the r wave. The r wave is the positive deflection. This may then be followed with an s wave, which is another negative deflection. Sometimes, there is a second positive deflection, which is called an r' wave.

It is important to remember the normal pattern of ventricular depolarization. The septum is initially depolarized, followed by depolarization down towards the apex, then leftwards, then to the upper left to finish depolarizing the superior-most portions of the left ventricle. This produces the usual QRS complexes seen throughout the EKG and it is very important to remember this. The QRS is considered to be positive if the area under the positive deflections is larger than the area under the negative deflections (and vice versa). The direction of the QRS complex is very important when discussing changes to the ST segment or T waves to evaluate for concordance (ST segment/T wave in same direction as the QRS complex) or discordance (opposite directions).

The normal width of the QRS complex is 70-100ms. Wide complexes (>120ms) are almost always of ventricular origin or aberrant conduction; the main exception is that junctional rhythms (from the AV node) tend to be minimally widened (usually between 100-120ms). Aberrant conduction may occur secondary to bundle branch blocks, rate-related aberrancy (due to the refractory period of the ventricles), metabolic/tox (acidosis, hyperkalemia, hypothermia, sodium channel blocker toxicity), pre-excitation, or ventricular pacing. It is common to have repolarization abnormalities with depolarization abnormalities; therefore, it is common to find ST-T changes associated with these QRS changes.

The voltage (or amplitude) of the QRS complex and each individual portion of the QRS complex should also be evaluated. Ventricular hypertrophy is a common finding on EKG's. They manifest as increased voltage of the QRS complex and also commonly have repolarization abnormalities. Right ventricular hypertrophy is much more rare but findings of hypertrophy are seen in the rightward leads. Left ventricular hypertrophy is a relatively common finding and the criteria for hypertrophy are seen in the leftward leads. The specific criteria are somewhat complex (and numerous with LVH) and not discussed on this page. Low voltages are seen when the heart is farther away from the EKG leads: obesity, COPD, and pericardial effusion. Low voltage definitions are <5mm in the limb leads and <10mm in the chest leads.