Synopsis
Use QRS width, regularity and haemodynamic state to diagnose and safely terminate adult SVT without giving AV-nodal blockers to a dangerous mimic.
- SVT is a mechanism group, not one ECG diagnosis: common regular narrow-complex causes are AVNRT, orthodromic AVRT, atrial tachycardia and flutter with fixed conduction.
- An abrupt-onset, very regular narrow tachycardia with hidden or retrograde P waves suggests AVNRT/AVRT, but obtain a 12-lead during symptoms whenever this does not delay emergency care.
- Classify first by stability, QRS width below or at least 120 ms, and regularity.
Key red flags
Shock, syncope with severe/ongoing hypotension, myocardial ischaemia, severe heart failure with pulmonary oedema, or immediately post-ROSC.
Investigation priorities
Identify instability and make deterioration immediately treatable.
Management branches
Adverse features attributable to SVT/tachyarrhythmia.
- Call expert/resuscitation help, attach pads, treat reversible causes and prepare careful sedation/anaesthesia if conscious without delaying shock.
- Deliver synchronised cardioversion: RCUK initial 70-120 J for atrial flutter or paroxysmal SVT, with stepwise energy increases and up to three attempts.
Regular QRS below 120 ms without life-threatening features and no evidence of pre-excitation.
Key medicines
Adenosine—RCUK March 2026 algorithm regimen6 mg rapid IV bolus with an immediate rapid saline flush; if unsuccessful, 12 mg rapid IV; if still unsuccessful, 18 mg rapid IV. RCUK does not mandate a fixed inter-dose interval on the algorithm: assess the immediate response and maintain continuous ECG.
Adenosine—product-label comparison3 mg rapid IV bolus over 2 seconds; if SVT persists after 1-2 minutes, 6 mg rapid IV; if it persists after a further 1-2 minutes, 12 mg rapid IV. The cited SmPC says additional or higher doses are not recommended.