What is the difference between a narrow and a wide QRS complex?

Answer 1

The simplest way to answer this question is that a wide QRS complex refers to the heart being paced by a site of ,cardiac tissue in the ventricles rather than the atria.

There are more complex things to consider however, but the above is your most basic answer if that is what you are looking for. The heart is normally paced by the SA node or the sinoatrial node located in the right atrium. Where there is damage to the heart, say for example you had a previous heart attack which caused the death of heart muscle, this can damage the ability of a region of the heart to conduct. You can have ectopic pacing, where different areas of the atria other than the SA node take charge in pacing the heart. You can even have random regions of the atria pacing and electrical signals, for lack of a better description, running into each other causing the atria to quiver rather than pump which can lead to atrial fibrillation. Pulseless ventricular tachycardia is a wide QRS complex with relatively regular QRS patterns, but even though there is rapid conduction by the ventricles, there is no appropriate time or relaxation of the muscle to fill or pump blood effectively to the rest of the body and this is where emergency response teams will initiate CPR and use defibrillation. Polymorphic ventricular tachycardia is a wide QRS complex with a ribbon like appearance and is often due to electrolyte abnormalities like low magnesium levels (in fact Mg is provided for patients with this rhythm without laboratory confirmation of electrolyte abnormalities). You can have wide QRS complexes with bundle branch blocks, and for this just think of the physiology of the heart and the bundle branches which split from the bundle of his in the interventricular septum. Think of decreased or blocked conduction to the right or left branches. You can distinguish by looking in different leads of the EKG, primarily V2, V3, V5 and V6 for an interesting, wide QRS complex which actually form a characteristic RSR' wave.

Note that a patient can be stable and relatively asymptomatic with both wide and narrow QRS complexes.

Answer 2

A QRS complex on an electrocardiogram refers to ventricular depolarization. Depolarization is a term used to describe the movement of ions across a membrane which results in changes in charge potential on either side. When this happens in a neuron or cardiac cell an action potential travels down the length of the nerve cell or cardiac cell which then lead to a muscle contraction at the synapse. In the heart, the ventricles respond to the depolarization signal by physically contracting. This is referred to the as the mechanical component of the heart. Therefore, a QRS complex is often referred to as "ventricular contraction" assuming that the heart is functioning normally. A wider complex tells you that it is taking longer for the ventricles to receive the signal to contract. This can be from an interventricular conduction delay like a bundle branch block or because the signal is originating in an abnormal part of the heart where the signal takes longer to travel to each cell.