Stable narrow-complex SVT: vagal manoeuvres then adenosine
In stable (alert, adequate blood pressure) SVT (supraventricular tachycardia: fast rhythm above the ventricles) without chest pain or heart failure, vagal manoeuvres (stimulating the vagus nerve) come first, then adenosine.
Remember
In stable SVT, try vagal manoeuvres first, then give adenosine 6 mg as a rapid intravenous bolus (given fast into a large vein), followed by 12 mg if needed. DC cardioversion (a shock timed to the heartbeat) is kept for the unstable patient, such as one with hypotension or chest pain, or for when drugs fail.
Memory aid
adenosine is the heart’s reset button
Adenosine stops the AV node for a moment so that the rhythm can restart.
A regular narrow-complex tachycardia is a fast, regular rhythm with narrow QRS complexes (a normal-width QRS, meaning the ventricles are being activated normally). When it starts suddenly in a young patient, it is typical of paroxysmal SVT. Paroxysmal means it starts and stops abruptly. It is typically AV-nodal re-entrant tachycardia (AVNRT), in which the electrical impulse circles round a loop involving the AV node, the electrical gateway between the atria and the ventricles.
In a stable patient the first step is a vagal manoeuvre. It boosts the activity of the vagus nerve, which slows the AV node and can break the loop. The modified Valsalva manoeuvre has the patient strain against a closed airway for 15 seconds, then lie flat with the legs raised. Vagal manoeuvres are quick and need no drug, so they come before any drug.
If they fail, the next step is adenosine: 6 mg as a rapid intravenous bolus into a large vein, followed by 12 mg if needed. It causes a brief block of the AV node, which breaks the loop, and its effects last only seconds. Warn the patient to expect a brief but unpleasant feeling of impending doom and chest tightness.
Adenosine is avoided in severe asthma, because it can cause bronchospasm (narrowing of the airways). If adenosine cannot be used, the alternative is intravenous verapamil, a calcium channel blocker that slows the AV node, or an intravenous beta-blocker, which does the same. In severe asthma, verapamil avoids the risk of bronchospasm that adenosine and beta-blockers carry. If the patient is unstable (for example, with hypotension, chest pain, acute heart failure or reduced consciousness), synchronised DC cardioversion (an electric shock timed to the heartbeat) is used instead of drugs. For a stable patient, remember the order as nerve, then drug, then shock.
Key facts
- First step
- vagal manoeuvres in a stable patient, such as the modified Valsalva manoeuvre, where the patient strains against a closed airway for 15 seconds and then lies flat with the legs raised.
- Next step
- adenosine 6 mg as a rapid intravenous bolus, then 12 mg if needed.
- Why it works
- a brief block of the AV node breaks the loop (the re-entry circuit).
- Warn the patient
- expect a brief but unpleasant feeling of impending doom and chest tightness.
- Avoid adenosine in
- severe asthma, because it can cause bronchospasm.
- If adenosine cannot be used
- intravenous verapamil or an intravenous beta-blocker is the alternative; in severe asthma, verapamil avoids the risk of bronchospasm that adenosine and beta-blockers carry.
- Unstable patient
- synchronised DC cardioversion rather than drugs.
Exam traps
- Synchronised DC cardioversion is kept for SVT that causes instability, such as hypotension, chest pain or acute heart failure, or for when drugs fail. A stable, alert patient with a normal blood pressure does not need a shock while safer measures are still available.
- Adenosine before a vagal manoeuvre gets the order wrong: in a stable patient adenosine comes second, because the manoeuvres are quick, free of drug side effects and sometimes end the rhythm on their own.
- Starting adenosine at 12 mg is the right drug at the wrong dose: the regimen starts at 6 mg, and 12 mg is a repeat dose if the first dose fails.
Read the full notes
Tested in 13 SimpleMed+ questions
Practice it in Pharmacology, with the answer and a full explanation after every question.
Practice these questions