Approach to Narrow QRS Tachycardia

Introduction

Narrow QRS tachycardia

• Regular, rate > 100, QRSd < 120
• AT, AVNRT, orthodromic AVRT
• Preexcitation in sinus rhythm / with atrial pacing
• Dual AV nodal physiology
• Manner of induction

Narrow QRS tachycardia, central atrial activation, very short VA

Preexcitation with narrow QRS tachycardia

Narrow QRS tachycardia with VA > 70 ms and central VA

General approach

• What is the situation ?
• What are the differential diagnoses ?
• What are the electrophysiologic differences ?
• What pacing maneuvers can differentiate ?

How to do

• Setup the stimulator
• How to measure
• How to interpret
• Practice

Scenarios

Commonest Setting

• Regular narrow QRS tachycardia
• 1:1 VA
• Atrial activation central
• VA > 70 ms

SVT

Differential diagnoses (important ones)

• AVNRT
• Orthodromic AVRT
• Atrial tachycardia

EP differences

• A / V essential to tachycardia
• Focal versus reentry
• Presence of extranodal pathway
• VA linking
• Distance of circuit from RV apex
• V and A activation - serial versus simultaneous

Chamber essential to circuit

Focal / reentry

Extranodal pathway

VA linking

Distance from RV apex

Sequential vs simultaneous activation

Pacing maneuvers

Single most useful maneuver ?

RV overdrive pacing - most useful maneuver

• Easy to perform and interpret
• Can distinguish AT from AVNRT / AVRT (VAAV versus VAV)
• Can distinguish between AVNRT / AVRT (cPPI-TCL, SA-VA)
• Onset of entrainment can provide clues
• Can help even if not entrained !

Setting up

• Burst pacing from right ventricle
• Sync on
• Tachycardia CL - 30 ms
• Pace until atrium entrained
• Stop pacing

What to look for ?

• Don't 'eyeball'
• Does tachycardia continue ?
• Was the atrium entrained ?
• Which is the last entrained A ?
• Sequence - VAV or VAAV

What to look for ?

• corrected PPI - TCL
• SA - VA
• Is there fusion during entrainment ?
• Beginning of entrainment - A or His ?
• How many beats to entrain ?

RV pacing in narrow QRS tachycardia - Rule out (or in) AT

RV pacing in narrow QRS tachycardia - Rule out (or in) AT

RV pacing in narrow QRS tachycardia.

Why all this fuss about measurement ?

RV pacing in AT - VAAV

RV pacing in narrow QRS tachycardia - AVNRT / AVRT

SA / VA intervals

Fusion during entrainment ?

Entrainment

Beginning of entrainment can give a clue

• Atrial acceleration in transition zone
• A entrainment before H entrainment

Could not entrain - useful information ?

Summary

• VAAV identifies AT, VAV rules out
• SA - VA and PPI - TCL are longer for AVNRT
• Can use basal pacing if responses are equivocal
• A entrained earlier for AVRT, entrained before His

His refractory PVC

• Little more difficult to perform and interpret
• Very useful maneuver
• Especially differentiate septal AP from AVNRT

His refractory PVC - setting up

• R synchronised single extrastimuli
• Check that sync is working
• Start 30 ms less than RR
• Decrement by 10 ms
• Continue until refractoriness or tachycardia termination

His refractory PVC - measurement and interpretation

• Confirm cycle length is stable
• Measure AA around each PVC to find longest CI at which PVC preexcites A
• Decide if His is refractory at this time

His refractory PVCs - responses

• Advance atrial activation without change in sequence
• Delay atrial activation without change in sequence
• Terminate tachycardia without conduction to atrium
• Does not alter atrial activation

PVC during tachycardia

PVC terminates tachycardia

Other clues during narrow QRS tachycardia

• VA unlinking - spontaneous or after atrial burst
• Cycle length and VA changes with bundle branch block
• Spontaneous termination

Spontaneous termination

Bundle branch block

Bundle branch block

Bundle branch block

Maneuver in sinus rhythm - Central VA conduction with ventricular pacing

Scenario

Central VA conduction

Setting

• During ventricular pacing, 1:1 VA conduction
• Central atrial activation

Differential diagnoses

• Nodal conduction
• Septal accessory pathway

EP differences

• Decremental conduction
• His is a waypoint
• Distal insertion (entry point)
• Adenosine sensitivity

Decremental single ventricular extrastimuli

• Easy to perform (must be routine)
• Atrial activation pattern
• VA interval - Decremental conduction
• VA relation to VH

Setting up and measurements

• Pace from RV apex
• His catheter and RA / CS catheters
• Decrement by 10-20 ms
• VA interval measured to earliest A

Decremental conduction

• Classical property of nodal conduction
• Can also be seen with AP
• How to differentiate

RV 500/300 ms

RV 500/280 ms

RV 500/260 ms

Parahisian pacing

• Somewhat difficult to perform
• Often difficult to interpret
• Still very useful sometimes

Parahisian pacing

Parahisian pacing - setting up

• His catheter, slightly pushed in (small A)
• Start with low output and increase gradually
• Watch QRS morphology for intermittent His capture

Parahisian pacing - interpretation

• His capture - narrower QRS, His not seen, RVA early
• Beware of atrial capture
• Beware of pure His capture
  • Narrower QRS
  • Isoelectric interval from pacing spike to QRS

Parahisian pacing - interpretation

• Identify beats with and without His capture
• Look at atrial activation sequence
• Measure VA interval

Parahisian pacing - with His capture

• Nodal conduction - same sequence, shorter VA
• AP conduction - same sequence, same VA
• Mixed response - different sequence, shorter VA

Parahisian

Parahisian

Pre-ablation

Post ablation

Other maneuvers

Response to adenosine

Differential pacing

• VA interval during pacing from apex and base
• Shorter from apex for nodal conduction
• Shorter from base for AP conduction

RV apex pacing

RV base pacing

Summary

• Understanding of basic electrophysiology of arrhythmias
• Practice is important
• Although not required at most time, will prove critical in select cases