ventricular tachycardia
Ventricular tachycardia is a fast rhythm that takes over the heart from the wrong place — the ventricles themselves rather than the natural pacemaker above. Because the impulse no longer follows the heart's efficient wiring, the pumping chambers contract quickly but inefficiently, like a powerful engine racing out of gear. It is far more dangerous than the supraventricular tachycardias because it can collapse blood pressure or degenerate into a fatal rhythm.
On the ECG it appears as a run of wide, abnormally shaped QRS complexes at a rate usually above 100 — and often 150 to 250 — beats a minute. A run lasting under 30 seconds is called non-sustained; a longer or symptomatic run is sustained and demands urgent attention. The most common underlying cause is scarred muscle after a heart attack, which provides the substrate for a re-entry circuit.
Management depends on whether the patient is stable. A pulseless or collapsing patient needs immediate defibrillation, while a stable patient may be treated with antiarrhythmic drugs or synchronized cardioversion. For those at lasting risk, an implantable cardioverter-defibrillator stands guard, and catheter ablation can target the responsible scar.
Months after a large heart attack, a man feels faint and his monitor shows a wide-complex tachycardia at 180; he is still conscious with a pulse, so synchronized cardioversion restores his normal rhythm and an ICD is later implanted.
Post-infarction scar is the most common substrate for sustained ventricular tachycardia.
Any wide-complex tachycardia should be treated as ventricular tachycardia until proven otherwise — assuming the safer diagnosis avoids the harm of mistakenly treating it as a benign SVT.