8.2 Diagnostic Pacing Maneuvers: Entrainment, PPI-TCL, V-A-V vs V-A-A-V & His-Refractory PVCs

8.2 Diagnostic Pacing Maneuvers: Entrainment, PPI-TCL, V-A-V vs V-A-A-V & His-Refractory PVCs

While baseline intervals and activation sequences offer essential diagnostic hypotheses, definitive identification of supraventricular tachycardia mechanisms frequently requires active diagnostic pacing maneuvers. By systematically perturbing the tachycardia with timed pacing trains and premature extrastimuli, the cardiac electrophysiology specialist can directly probe circuit anatomy, identify critical components of the reentrant loop, and differentiate between Atrioventricular Nodal Reentrant Tachycardia (AVNRT), Orthodromic Atrioventricular Reentrant Tachycardia (AVRT), and Atrial Tachycardia (AT).

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Ventricular Overdrive Pacing (VOP) & Entrainment Fundamentals

Transient entrainment is the continuous resetting of a reentrant circuit by an external pacing train delivered at a cycle length slightly faster than the intrinsic tachycardia cycle length (TCL). When pacing is delivered, the wavefront propagates into the reentrant circuit in two directions:

1. Antidromic wavefront: Collides with and extinguishes the circulating tachycardia wavefront.
2. Orthodromic wavefront: Penetrates the excitable gap, continues in the direction of the circuit, and resets the tachycardia to the pacing cycle length.

Clinical VOP Protocol in SVT

1. During stable, sustained SVT, identify the intrinsic Tachycardia Cycle Length (TCL) (e.g., $360\text{ ms}$).
2. Program the stimulator to deliver a ventricular drive train from the Right Ventricular Apex (RVA) or Right Ventricular Base at a cycle length $10\text{ to }30\text{ ms}$ shorter than the TCL (e.g., pacing cycle length of $330\text{ to }350\text{ ms}$).
3. Deliver a train of 15 to 25 pacing stimuli ($S_1$) to achieve continuous ventricular capture and accelerate the atrial electrograms to the pacing cycle length.
4. Abruptly cease pacing and analyze the transition response on the surface ECG and intracardiac channels.

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The Return Sequence: V-A-V vs. V-A-A-V Response

The activation sequence of intracardiac electrograms immediately following cessation of ventricular overdrive pacing is one of the most reliable diagnostic discriminators in cardiac electrophysiology.

V-A-V Response (AVNRT or AVRT):         V-A-A-V Response (Atrial Tachycardia):

Paced V -> Last Paced Stimulus          Paced V -> Last Paced Stimulus
   |                                       |   
   v                                       v   
Retrograde A (Entrained)                Retrograde A (Entrained)
   |                                       |   
   v                                       v (Concurrently)
Antegrade V (First Return Beat)         Spontaneous AT Beat (A)
                                           |   
                                           v   
                                        Antegrade V (First Return Beat)
Sequence: Paced V -> A -> V             Sequence: Paced V -> A -> A -> V

1. The V-A-V Response

• Electrophysiological Mechanism: The final paced ventricular stimulus ($V_{paced}$) captures the ventricle, enters the retrograde limb of the circuit, and conducts retrogradely to depolarize the atrium ($A$). That atrial wavefront then turns around and conducts antegradely down the AV node-His-Purkinje system to depolarize the ventricle ($V_{return}$).
• Diagnostic Significance: The resulting activation sequence is $V - A - V$. This establishes that the atrium and ventricle are linked components of the circuit, proving that the tachycardia is an AV-junction dependent rhythm (either AVNRT or AVRT).

2. The V-A-A-V Response

• Electrophysiological Mechanism: In Atrial Tachycardia, the atrial pacemaker or microreentrant focus fires independently of the ventricle. When ventricular overdrive pacing is conducted, the last paced ventricular beat conducts retrogradely across the AV node into the atrium ($A_{entrained}$). However, the autonomous atrial tachycardia circuit has already discharged its next scheduled spontaneous impulse ($A_{spontaneous}$). Both atrial electrograms occur before antegrade conduction across the AV node can produce a ventricular depolarization ($V_{return}$).
• Diagnostic Significance: The resulting sequence is $V - A - A - V$. A true V-A-A-V response is 100% specific for Atrial Tachycardia!
• Diagnostic Caveat (Pseudo V-A-A-V): In rare cases, typical AVNRT can manifest an apparent $V-A-A-V$ sequence if dual AV nodal pathway conduction occurs on the return beat (conduction down the fast pathway and simultaneous slow pathway conduction producing a second atrial beat) or if pacing terminates AVNRT with an A wave followed by a sinus beat. True entrainment with fixed atrial acceleration must be confirmed before interpreting the response.

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Post-Pacing Interval (PPI) and Corrected PPI ($ ext{PPI - TCL}$)

The Post-Pacing Interval (PPI) is the time interval measured in milliseconds from the last pacing stimulus artifact ($S_{last}$) to the first return electrogram recorded at the pacing catheter ($V_{return}$ at the RVA).

$\text{PPI} = \text{Time from Pacing Site to Circuit} + \text{Circuit Revolution Time (TCL)} + \text{Time from Circuit to Pacing Site}$

Subtracting the Tachycardia Cycle Length (TCL) removes the intrinsic circuit revolution time, isolating the total conduction time required for the wavefront to travel from the pacing catheter to the reentrant circuit and back:

$\Delta(\text{PPI} - \text{TCL}) = \text{PPI} - \text{TCL}$

+------------------------------------------------------------------------------------+
| DIAGNOSTIC THRESHOLDS FOR PPI - TCL                                                |
+------------------------------------------------------------------------------------+
| PPI - TCL < 115 ms  ===> Orthodromic AVRT (Pacing site is IN / NEAR circuit)       |
| PPI - TCL > 115 ms  ===> Typical AVNRT (Pacing site is OUTSIDE / REMOTE from loop) |
+------------------------------------------------------------------------------------+

Worked Clinical Example 1: Orthodromic AVRT

• Tachycardia Cycle Length (TCL): $340\text{ ms}$
• Ventricular Pacing Cycle Length: $320\text{ ms}$ (at RVA)
• Measurement: Timing from last RV pacing spike ($S_{last}$) to the onset of the first return electrogram on the RV catheter ($V_{return}$) = $410\text{ ms}$ ($PPI = 410\text{ ms}$).
• Calculation: $\text{PPI} - \text{TCL} = 410\text{ ms} - 340\text{ ms} = 70\text{ ms}$
• Interpretation: Because $70\text{ ms} < 115\text{ ms}$, the pacing site in the ventricle is an obligate, direct limb of the reentrant circuit. This finding definitively confirms Orthodromic AVRT.

Worked Clinical Example 2: Typical AVNRT

• Tachycardia Cycle Length (TCL): $340\text{ ms}$
• Ventricular Pacing Cycle Length: $320\text{ ms}$ (at RVA)
• Measurement: Timing from last RV pacing spike ($S_{last}$) to the first return RV electrogram ($V_{return}$) = $500\text{ ms}$ ($PPI = 500\text{ ms}$).
• Calculation: $\text{PPI} - \text{TCL} = 500\text{ ms} - 340\text{ ms} = 160\text{ ms}$
• Interpretation: Because $160\text{ ms} > 115\text{ ms}$, the pacing site is remote from the reentrant circuit. The pacing wavefront had to travel out-of-circuit up through the right bundle branch and His bundle to reach the nodal loop and return. This confirms AVNRT.

The Corrected Post-Pacing Interval ($PPI_{corr} - TCL$)

During ventricular overdrive pacing, antegrade conduction through the AV node on the return beat often experiences decremental delay, artificially prolonging the return $AH$ interval relative to the tachycardia $AH$ interval:

$\Delta AH = AH_{return} - AH_{tachycardia}$ $\text{PPI}_{corr} - \text{TCL} = (\text{PPI} - \text{TCL}) - \Delta AH$

• Diagnostic Cutoff: An adjusted value $< 85\text{ ms}$ indicates AVRT; a value $> 85\text{ ms}$ indicates AVNRT.

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Stimulus-Atrial (SA) vs. Ventricular-Atrial (VA) Differences

A complementary measurement to PPI is the difference between the Stimulus-Atrial ($SA$) interval during entrainment and the Ventricular-Atrial ($VA$) interval during spontaneous tachycardia:

$\Delta(SA - VA) = SA_{paced} - VA_{tachy}$

Where:

• $SA_{paced}$: Measured from the last ventricular pacing stimulus to the earliest retrograde atrial electrogram on the His or CS catheter.
• $VA_{tachy}$: Measured from the onset of the surface QRS (or intrinsic RV electrogram) to the same atrial electrogram during SVT.

+------------------------------------------------------------------------------------+
| DIAGNOSTIC THRESHOLDS FOR SA - VA                                                  |
+------------------------------------------------------------------------------------+
| SA - VA < 85 ms  ===> Orthodromic AVRT (Direct retrograde conduction via Kent)     |
| SA - VA > 85 ms  ===> Typical AVNRT (Slow travel through RV to reach AV node)      |
+------------------------------------------------------------------------------------+

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His-Refractory Premature Ventricular Complexes (PVCs)

The introduction of a single premature ventricular complex (PVC) timed precisely when the His bundle is refractory is among the most elegant and decisive diagnostic maneuvers in electrophysiology.

Biophysical Principle & Timing

• The PVC must be delivered coincident with or within $30\text{ to }40\text{ ms}$ prior to the His bundle deflection recorded on the His catheter.
• At this exact microsecond window, the His bundle is depolarized and rendered in its absolute refractory period. Therefore, the paced ventricular wavefront cannot propagate retrogradely up the His-Purkinje system or AV node.

Tachycardia Wavefront:     ... V ---> His (Refractory) ---> AV Node ---> Atrium ...
                                          ^
                                          | (Blocked! Cannot enter His)
PVC Delivered at Refractoriness:        Paced PVC
                                          |
                                          +==================> Accessory Pathway ===> Atrium (RESET!)

Diagnostic Outcomes of a His-Refractory PVC

1. Advancement (Resetting) of the Atrium:
   • If the His-refractory PVC depolarizes the atrium earlier than the scheduled tachycardia A wave (advancing the atrial electrogram by $\ge 10\text{ to }20\text{ ms}$) with an identical retrograde atrial activation sequence, an accessory pathway MUST be present!
   • Because the His bundle was refractory, the electrical impulse could only reach the atrium via an extranodal bypass tract.
2. Tachycardia Termination without Depolarizing the Atrium:
   • The PVC blocks in the accessory pathway, terminating the tachycardia without conducting to the atrium. This proves that the accessory pathway is not merely a bystander, but an indispensable component of the reentrant circuit (AVRT).
3. Tachycardia Termination with Retrograde Atrial Conduction:
   • The PVC conducts retrogradely up the accessory pathway, reaches the atrium, but finds the AV node refractory, extinguishing the tachycardia. This also confirms an accessory pathway.
4. No Effect (Tachycardia Continues Unaltered):
   • Inconclusive. The pacing impulse may not have reached a remote accessory pathway (e.g., a left lateral pathway when pacing from the RV apex) before the intrinsic tachycardia wavefront entered it.

Para-Hisian Pacing Maneuver

Para-Hisian pacing is performed in sinus rhythm or during ventricular pacing to differentiate retrograde conduction over the normal AV node-His Purkinje system from retrograde conduction over an anteroseptal, midseptal, or concealed septal accessory pathway.

Catheter Placement and Technique

• A quadripolar or decapolar catheter is positioned across the tricuspid annulus immediately adjacent to the His bundle, where a clear His deflection is visible on the distal electrogram.
• Pacing is delivered at a fixed cycle length (e.g., $600\text{ ms}$) while titrating pacing current output to transition between two distinct capture states:
  1. High Output Pacing (His Bundle + Myocardial Capture): Captures both the specialized His bundle and adjacent local right ventricular septal myocardium simultaneously. The resulting surface QRS complex is relatively narrow, with a short stimulus-to-ventricular onset time.
  2. Low Output Pacing (Pure Myocardial Capture): Pacing output is lowered until His bundle capture is lost, capturing only the local ventricular myocardium. The surface QRS complex becomes significantly wide, with a slurred upstroke and delayed ventricular activation.

High Output (His + Myocardial Capture):    Low Output (Pure Myocardial Capture):
Narrow QRS                                  Wide QRS

  Stim ----> His Bundle ====> AV Node        Stim ----> Myocardium --(slow)--> Purkinje -> His -> AV Node
      \----> Myocardium                          (Conduction to AV Node Delayed by 40-60 ms!)

Diagnostic Responses in Para-Hisian Pacing

• Physiological Basis of Normal AV Nodal Response: When His capture is lost at low output, the electrical impulse must propagate cell-to-cell across working ventricular myocardium before it can enter the Purkinje network and reach the His bundle and AV node. This adds $40\text{ to }60\text{ ms}$ of myocardial transit time, causing the retrograde $VA$ interval to prolong significantly ($VA_{wide} > VA_{narrow}$).
• Pathological Basis of Accessory Pathway Response: If a septal accessory pathway is present, the ventricular myocardium is directly connected to the atrium via the pathway. The wavefront reaches the bypass tract directly through local myocardium without requiring His bundle participation. Consequently, the retrograde $VA$ interval does not prolong, and may even paradoxically shorten because direct myocardial depolarization enters the pathway sooner.