Supraventricular Tachycardia
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Direct answer
Supraventricular tachycardia is a narrow-complex tachycardia originating above the ventricles, most often from atrioventricular re-entry through either a dual nodal pathway in AVNRT or an accessory pathway in AVRT. Termination with vagal manoeuvres or adenosine suggests an atrioventricular node-dependent circuit, and catheter ablation offers a cure for recurrent symptomatic forms.
What you must remember
- AVNRT: re-entry within the AV node through slow and fast pathways; the commonest regular SVT, more frequent in women, with a short RP interval and a retrograde P wave buried in or just after the QRS.
- AVRT: re-entry through an accessory pathway; orthodromic AVRT conducts antegrade through the AV node and gives a narrow complex, while antidromic AVRT conducts down the accessory pathway and gives a wide complex.
- Pre-excitation: a delta wave in sinus rhythm points to an accessory pathway and therefore to AVRT, which changes the choice of drugs.
- Vagal manoeuvres: the modified Valsalva, with about 15 seconds of straining then lying flat with the legs raised, is effective; carotid sinus massage is avoided with carotid bruit or recent stroke.
- Adenosine: 6 mg rapid intravenous bolus, then 12 mg if required, with flushing and transient asystole; avoided in asthma and with dipyridamole, and it can unmask atrial flutter.
- Unstable patients: synchronised DC cardioversion, with sedation as the patient's condition allows.
- Ablation: slow-pathway modification cures AVNRT, and accessory pathway ablation cures AVRT; both are recommended for recurrent symptomatic episodes.
A young woman with sudden palpitations
A 28-year-old woman arrives with sudden palpitations at about 150 beats per minute, blood pressure 110 over 70 mmHg and a regular narrow-complex tachycardia. She is haemodynamically stable, so the first step is a modified Valsalva manoeuvre, which terminates the tachycardia in many patients. If it fails, adenosine is given as a rapid bolus with a flush, because the drug briefly blocks the AV node and interrupts the re-entry circuit. A persistent rhythm, if the adenosine test does not settle things, is treated with a calcium channel blocker or beta blocker, depending on blood pressure.
Once the rhythm is terminated, the 12-lead ECG is reviewed carefully for pre-excitation, which would change the long-term plan and the choice of drugs in future attacks. Ablation is offered after recurrence, since slow-pathway modification or accessory pathway ablation is curative in most suitable patients. For infrequent attacks in a patient with no pre-excitation, a pill-in-the-pocket approach using a beta blocker or calcium channel blocker may be discussed. The decision rests on attack frequency, quality of life and patient preference, rather than on the ECG alone.
Where the exam tries to catch you
The adenosine trap is the most common. Adenosine is safe for narrow-complex orthodromic tachycardia, but it is dangerous in pre-excited atrial fibrillation, where it can accelerate conduction down the accessory pathway and precipitate ventricular fibrillation. A second trap is atrial flutter with 2:1 block, which can look like SVT at a fixed rate of 150 beats per minute; adenosine transiently reveals the flutter waves and does not terminate the rhythm.
Carotid sinus massage is a favourite for questions about vagal manoeuvres: it is contraindicated with carotid bruit, recent stroke or transient ischaemic attack, because of the risk of embolism from plaque. In Indian clinical practice, the cheaper and safer first step is the modified Valsalva, and the viva answer names it before any drug is mentioned.
Frequently asked questions
What is the commonest mechanism of regular supraventricular tachycardia?
AV nodal re-entrant tachycardia, known as AVNRT, is the commonest mechanism of regular supraventricular tachycardia. It arises from a dual pathway within the AV node and is more frequent in women.
What is the adenosine dose for narrow-complex supraventricular tachycardia?
Adenosine is given as a rapid intravenous bolus of 6 mg, followed by 12 mg if needed, with a saline flush. Transient asystole and flushing are expected effects, and the drug should be avoided in asthma.
Why is carotid sinus massage avoided in some patients with SVT?
Carotid sinus massage is avoided in patients with carotid bruit, recent stroke or transient ischaemic attack because it may dislodge plaque and cause cerebral embolism. The modified Valsalva manoeuvre is the safer first-line vagal option.
Which ECG finding indicates an accessory pathway in SVT?
A delta wave, with a short PR interval and slurred QRS upstroke seen in sinus rhythm, indicates an accessory pathway and therefore AVRT. Its presence changes drug choice, because AV nodal blockers become dangerous if atrial fibrillation develops.