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The straining mentioned above is the Valsalva manoeuvre.

Intravenous adenosine given by fast intravenous infusion is the drug of choice for SVTs and works by causing heart block for a split second following injection - though it is contraindicated in asthma. Another drug that can be used is verapamil, a beta blocker, though this is contraindicated if other beta blockers have already been given or if there is a broad QRS complex on ECG.