Rick Schreurs

10 Chapter 1 Conduction disorders Disturbances in the physiologic electrical conduction of the heart may lead to diminished cardiac function. Conduction disturbances can take place at various places in the heart. When electrical currents are blocked or delayed at the level of the AV-node an AV-block is present. There are three types of AV-block to be distinguished: • First-degree AV-block: every P-wave is followed by a QRS complex with an interval >200ms. In other words, every atrial contraction is followed by ventricular contraction, but the interval between the two is delayed and longer than 200ms. • Second-degree AV-block: some P-waves are followed by a QRS complex while others are not. This blockage can be regular or at a random base. • Third-degree AV-block: Also known as total AV-block; conduction between the atria and ventricles is completely blocked. The heart relies on an escape rhythm which is usually slower than the normal sinus rhythm. AV-conduction delay leads to a mismatch between atrial and ventricular contraction and may cause a lower ventricular rate, which may alter ventricular filling and thereby induce heart failure. Conduction disturbances can also occur at the level of the rapid conduction system of the ventricles, of which right (RBBB) and left bundle branch block (LBBB) are important examples. Because in these cases a large proportion of the ventricular myocardium is electrically activated through slow cell-to-cell conduction, RBBB and LBBB lead to prolonged electrical activation between the RV and LV (interventricular dyssynchrony) and within the affected ventricle (intraventricular delay). Because each cardiac myocyte requires the electrical impulse to start the contraction, the abnormal electrical activation causes disturbances in the coordination of mechanical contraction between and within ventricles, which may result in reduced cardiac pump function [4, 5]. Heart failure Heart failure is a clinical syndrome characterized by a combination of signs and symptoms caused by structural and/or functional cardiac abnormalities, resulting in elevated intracardiac filling pressures and decreased cardiac output [6]. The prevalence of heart failure is 1-2% and predominantly affects older people. The etiology of heart failure is diverse with ischemic, valvular and hypertensive heart disease being most prevalent. Other causes include inflammation, toxic, immune and tachycardia, but also dyssynchrony and conduction disorders can lead to heart failure [6]. Often several causes interact in one patient making heart failure a challenging disease to treat. Different etiologies require different therapies, and although treatment has significantly improved over the last decades, prognosis of heart failure remains poor with high mortality (annual rate 7-17%) and hospitalization rate (32-44%) [7]. Besides trying to take away the underlying cause, treatment of heart failure primarily focusses on lifestyle changes and pharmacotherapy to reduce cardiac stress and filling