Diagnosis of SVT Type

(NEJM 2012;367:1438)

Onset

• Abrupt on/off argues against sinus tachycardia

Rate

• Not dx b/c most can range from 140-250 bpm, but: ST usually <150; AFL often conducts 2:1 → vent. rate 150; AVNRT & AVRT usually >150

Rhythm

• Irregular → AF, AFL w/ variable block, or MAT

P wave

• morphology
• Before QRS (ie, long RP) → ST, at ↣ (P ≠ sinus), MAT (≥3 morphologies)
• None (ie, buried in or deforming terminal QRS, eg, pseudo RSR′ in V1) → typical AVNRT, NPJT
• After QRS (ie, short RP) & inverted in inf. leads (ie, retrograde atrial) → AVNRT, AVRT (usually slightly after QRS; RP interval >100 ms favors AVRT vs. AVNRT), or NPJT
• Fibrillation or no P waves → AF
• Saw-toothed “F” waves (best seen in inferior leads & V1) → AFL

Response to vagal stim. or adenosine

• Slowing of HR often seen with ST, AF, AFL, AT, whereas reentrant rhythms (AVNRT, AVRT) may abruptly terminate (classically w/ P wave after last QRS) or no response. Occ at ↣ may terminate.
• In AFL & AF, ↑ AV block may unmask “F” waves or fibrillation