Often young patients with normal hearts.
AV nodal re-entry or WPW syndrome.
P waves usually not visible (hidden by QRS complexes).
Vagal manoeuvres: valsalva or carotid sinus massage. The patient should be supine and as relaxed as possible, to avoid competing sympathetic reflexes.
If vagal manoeuvres fail:
• Adenosine, rapid IV bolus, 6 mg.
Follow by a bolus of 10 mL sodium chloride 0.9% to ensure that it reaches the heart before it is broken down.
Half-life: ± 10 seconds.
Run the ECG for 1 minute after the injection.
If 6 mg fails, repeat with 12 mg.
If this fails, repeat with another 12 mg.
If the drug reaches the central circulation before it is broken down the patient will experience flushing, sometimes chest pain, wheezing and anxiety.
If the tachycardia fails to terminate without the patient experiencing those symptoms, the drug did not reach the heart.
If none of the above is effective, or if the patient is hypotensive, consider DC shock.
Long term therapy
Teach the patient to perform vagal manoeuvres. Valsalva is the most effective.
For infrequent, non-incapacitating symptoms:
Atenolol, oral, 50–100 mg daily.
If asthmatic, but normal heart:
Verapamil, oral, 80–120 mg 8 hourly.
Verapamil and digoxin are contraindicated in WPW syndrome.