Ventricular escape beats occur when the rate of electrical discharge reaching the ventricles, transmitted to the atrioventricular node falls below the base rate determined by the rate of Phase 4 spontaneous depolarisation of ventricular pacemaker cells. An escape beat usually occurs 2–3 seconds after an electrical impulse has failed to reach the ventricles. This phenomenon can be caused by the sinoatrial node failing to initiate a beat, by a failure of the conductivity from the SA node to the atrioventricular node, or by atrioventricular block. Normally, the pacemaker cells of the sinoatrial node discharge at the highest frequency and are thus dominant over other cells with pacemaker activity. The AV node normally has the second fastest discharge rate. When the sinus rate falls below the discharge rate of the AV node, this becomes the dominant pacemaker, and the result is called a junctional escape beat. If the rate from both the SA and AV node fall below the discharge rate of ventricular pacemaker cells, a ventricular escape beat ensues. An escape beat is a form of cardiac arrhythmia, in this case known as an ectopic beat. It can be considered a form of ectopic pacemaker activity that is unveiled by lack of other pacemakers to stimulate the ventricles. Ventricular pacemaker cells discharge at a slower rate than the SA or AV node. While the SA node typically initiates a rate of 70 beats per minute, the atrioventricular node is usually only capable of generating a rhythm at 40-60 BPM or less. Ventricular contraction rate is thus reduced by 15-40 beats per minute. If there are only one or two ectopic beats, they are considered escape beats. If this causes a semi-normal rhythm to arise it is considered an idioventricular rhythm. The escape arrhythmia is a compensatory mechanism that indicates a serious underlying problem with the SA node or conduction system, and because of its low rate, it can cause a drop in blood pressure and syncope.
Diagnosis
An electrocardiogram can be used to identify a ventricular escape beat. The QRS portion of the electrocardiogram represents the ventricular depolarisation; in normal circumstances the QRS complex forms a sharp sudden peak. For a patient with a ventricular escape beat, the shape of the QRS complex is broader as the impulse can not travel quickly via the normal electrical conduction system. Ventricular escape beats differ from ventricular extrasystoles, which are spontaneous electrical discharges of the ventricles. These are not preceded by a pause; on the contrary they are often followed by a compensatory pause.
