Sinus tachycardia is due to an increase in cardiac impulses arising in the sino-atrial node. It is almost always due to a secondary cause, either cardiac disease or systemic illness. Long-standing severe tachycardia may be deleterious to cardiac function due to reduction of cardiac output and reduced coronary blood flow.

The following table lists common conditions resulting in sinus tachycardia.

All antipsychotics cause some degree of tachycardia, mainly due to cardiac cholinergic blockade. Clozapine with the strongest relative affinity for this receptor can cause more than a 20% increase in heart rate . Even so, this is rarely clinically relevant. Slow titration of clozapine may prevent development of tachycardia. If the tachycardia is treated, it responds to beta blockers and calcium channel blockers. The tachycardia resolves if clozapine is stopped.

The possibility of myocarditis with clozapine should be considered when tachycardia occurs. Tachycardia with clozapine is common while myocarditis is rare. Both develop in the first few days to weeks of medication initiation. Clozapine is also associated with cardiomyopathy [1] and this may be related to acute myocarditis or due to tachycardia-mediated ventricular dysfunction. Generally, if tachycardia is the only presentation, there is no need for evaluation for myocarditis or cardiomyopathy. See table for clinical and diagnostic features of clozapine-induced myocarditis. Symptoms of cardiomyopathy such as dyspnea and fatigue may develop more insidiously if it occurs without preceding myocarditis.