The rate you claim is merely an electrical discharge rate of the SA node compounded by the extra focus or re-entrant, including through the AV node. It does not reflect the effort of the myocardium in pumping blood. In fact, when in an arrhythmia, the heart becomes de facto inefficient due to an inability to move blood quickly in response to demand. That is why some feel faint and wobbly when experiencing fibrillation; their heart in that state cannot keep up with demand. If the output is weak and diminished, then so must the workload be diminished.

Anyone reading: if you are in arrhythmia, and KNOW it is only fibrillation or flutter (and not ventricular tachycardia, which is exceedingly dangerous!), you can live for days and weeks in fibrillation or flutter. That state IS associated with early morbidity, so you do want to have it controlled....particularly if the rate exceeds 100 BPM for more than about 24 hours. Also note that, even if you are not highly symptomatic, and feel quite well thanks very much, you should not allow your heart to beat faster than 100 BPM at rest for more than a day. Seek help if it won't correct because you'll want a longer life than a permanently rapidly beating heart will allow you.

Note that cardiologists will prescribe beta-blockers or calcium channel blockers that limit cardiac output if there is hypertension involved. That is why it is important for triage nurses to determine a fibrillating patient's BP, and to look at non-fibrillating BP as well. If the pressure is too high, those drugs like metoprolol reduce cardiac effort, and slow the heart rate some (depending on dose). If it's just the rate that is too high, then you'll find the blockers do a reasonably good job as well.