Extreme tachycardia is a rate that is very, very fast. There is no precise defintion for this. Very fast rates can be dangerous to the heart. Causes of extreme tachycardia include ventricular tachycardia, accessory conduction pathways and re-entrant tachycardia. Infants can have very fast heart rates too.
See also: Rate
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This ECG is from a man in his 60s who presented with intermittent shortness of breath and chest pressure over the past few days. This was his initial ECG.
This ECG shows a regular narrow complex tachycardia around 200 bpm, without P waves, with marked ST elevation in inferior leads and ST depression in anterior leads and aVL. The cause was thought to be atrial flutter from newly started flecainide. He also underwent angiography and had stents inserted to the left circumflex and right coronary arteries.