Ventricular tachycardia is indeed potentially lethal, and in just a few minutes. Depends on the heart's overall condition and history. SVT is not lethal, and many live with it largely untreated for years. It's the symptoms that make the difference. Some patients don't know they're fibrillating or in flutter, but the ECG will show it clearly. Others know because they feel faint, breathless, or they feel the thumping of the irregularly irregular beats of the chaotic atrium beating out of step with the ventricles below it.