The following are the most common causes of first-degree AV block: Intrinsic AVN disease( every heart beat starts from a electrical souce ( here we will call it sino atrial node).from sinoatrial node it is conducted to another type of node called av node...its the work of av node to decide the rate at which you heart contract.first degree block means that the time taken to transfer ecectric impulse from sino atrial node to atrioventricular node is more then normal) Acute myocardial infarction (MI), particularly acute inferior wall MI Myocarditis( inflammation of heart) Electrolyte disturbances (eg, hypokalemia, hypomagnesemia) Drugs (especially those drugs that increase the refractory time of the AVN, thereby slowing conduction) in yourcase you are taking tablet maxzide which contains diuretics diuretics depletes body of electrolyte so my advise is to initally get your serum sodium potassium urea creatinine checked up to rule out any electrolyte blance and ask you dr to swith you over to other clases of anti hypertensive drugs