Patients with arrhythmias may be asymptomatic or have only complaints of palpitations in mild cases, or may die suddenly in severe cases. Very fast (more than 180 beats per minute) or very slow heart rates (less than 40 beats per minute) can lead to congestive heart failure. Arrhythmias that are not meaningful in normal subjects but can produce heart failure in patients with heart disease, or worsen pre-existing heart failure. Arrhythmias can have significant effects on the blood supply to the heart, brain, kidneys, gastrointestinal tract, muscles and skin, but this damage depends largely on the nature and severity of the arrhythmia itself, the effect on cardiac output and the functional status of specific organs of importance. Since the hemodynamic impact of arrhythmias is mainly related to the nature and severity of the arrhythmia, episodic atrial and ventricular premature beats reduce coronary blood flow by only 5% and 12%, respectively, whereas frequent ventricular premature beats can reduce blood flow by 25%; in atrial fibrillation and ventricular tachycardia, this is serious, with coronary blood flow being reduced by as much as 40% and 60%, respectively. In cases of myocarditis with complete atrioventricular block, the reduction in coronary blood flow caused by an extremely slow heart rate can induce heart failure and even sudden death. In general, episodic premature beats do not affect blood flow in the cerebral circulation. During frequent atrial and ventricular premature beats, the average cerebral blood flow is reduced by 8% and 12%, respectively; extremely frequent ventricular premature beats can reduce cerebral blood flow by 25%; and during atrial tachycardia and atrial flutter or fibrillation with a rapid ventricular rate (faster than 100 beats per minute), cerebral blood flow can be reduced by as much as 40%. The reduction in cerebral blood flow due to arrhythmia is easily tolerated in patients with normal cerebral vessels, but in patients with lesions in the cerebral arteries, signs and symptoms of cerebrovascular insufficiency, such as vertigo, weakness, blurred vision, hemianopia, convulsions, syncope, partial paralysis or light paralysis, and even convulsions and mental abnormalities, will occur. Patients with complete AV block with extremely slow heart rate often have syncope and convulsions due to the reduction of cerebral blood flow, which is clinically and conventionally referred to as an A-Syndrome episode. Frequent atrial or ventricular premature beats can cause an average decrease in renal blood flow of 8% to 10%. Patients with atrial tachycardia and atrial fibrillation with rapid ventricular rate and ventricular tachycardia have a decrease in renal blood flow of 18%, 20%, and 60%, respectively, resulting in vasoconstriction of the renal circulation and causing renal ischemia, the symptoms of which include oliguria, proteinuria, and azotemia.