Understanding the Atrioventricular Dissociation

Abstract

A 56-year old male patient was addressed to our intensive cardiac care unit with a diagnosis of “complete atrioventricular (AV) block”, 4 days following surgical treatment of symptomatic obstructive hypertrophic cardiomyopathy including large septal resection and mitral valve replacement. We noted no relevant medical history. On physical examination, at admission, the patient presented normal hemodynamic, regular heart rate about 60 bpm, no signs of right heart failure. An abnormal transient jugular vein pulse was noted. The EKG on admission is presented in Fig. 1. Do you agree with the diagnosis of complete AV block? Diagnosisof considered “complex”EKG features appears tobe reserved for cardiologists or even to electrophysiologists, although good sense and simple logical approach allowmaking the diagnosis yourself in a number of cases. Let‘s go for a typical case of a suspected AV conduction disorder following cardiac surgery. First, let’s review some elementary considerations on the cardiac conduction physiology. Sinoatrial node is a part of a specialized type of myocardial network which has the particularity of being also pacemaker myocyte cells. This specialized network is represented by sinoatrial node, AV node, His and bundle branch as well as distal Purkinje fibers. Unlike the rest of themyocytes, these pacemaker cells may initiate action potential through spontaneous diastolic depolarization. Because of a higher depolarization slope, the impulse initiated from sinoatrial node depresses the activity of subsidiary pacemaker sites, and also automatically generates electrical pulses that spread as waves of electrical excitation over the atria and then to ventricles via AV node. Interruption of the conduction at any level enables subsidiary pacemakers to emerge usually at a slower rate (the lower andmore distal is the block, the slower is the rate). On the opposite, enhanced automaticity in any part of this specialized myocardial network may overdrive normal sinus node activity, particularly when the latest is slower.