Tachyarrhythmias
AV nodal re-entrant tachycardia
Regular narrow complex tachycardia around 180 with P waves hidden inside or just after the QRS.
What you see
- Regular, narrow QRS, typically 140–220 bpm.
- P waves absent, or seen as a pseudo-R′ in V1 and a pseudo-S in the inferior leads.
- Abrupt onset and offset.
- ST depression during the tachycardia is common and does not by itself mean coronary disease.
Why it looks like that
The AV node contains a fast pathway and a slow pathway with different refractory periods. A well-timed atrial ectopic blocks in the fast pathway, travels down the slow one, and returns up the fast one — a circuit small enough that atria and ventricles depolarise almost simultaneously, which is why the P wave hides in the QRS.
What to do
- Vagal manoeuvres first — the modified Valsalva (supine with passive leg raise) is markedly more effective than the standard one.
- Adenosine 6 mg, then 12 mg, by rapid push with a flush; warn the patient about the sensation.
- Synchronised DC cardioversion if haemodynamically unstable.
- Definitive treatment is slow pathway ablation, with a small risk of AV block.
The trap
Adenosine is contraindicated in asthma and in pre-excited AF, and is potentiated by dipyridamole while being blocked by theophylline. If the rhythm is regular and broad, treat it as VT until proven otherwise rather than reaching for adenosine.
More in tachyarrhythmias
Atrial fibrillation
Irregularly irregular narrow complexes with no P waves and a wandering baseline.
Atrial flutter
Sawtooth flutter waves at about 300 per minute, usually conducted 2:1 to give a ventricular rate near 150.
Monomorphic ventricular tachycardia
Broad, regular, identical complexes — assume VT in any broad complex tachycardia until you have proved otherwise.
Torsades de pointes
Polymorphic VT twisting around the baseline, on a background of a long QT.