6.2 Intracardiac Electrogram Interpretation & Normal Baseline Intervals (PA, AH, HV)

6.2 Intracardiac Electrogram Interpretation & Normal Baseline Intervals (PA, AH, HV)

Following successful diagnostic catheter navigation, the electrophysiology team acquires baseline intracardiac electrograms during stable sinus rhythm. Systematic interval analysis dissects the surface electrocardiogram (ECG) into discrete, anatomical conduction segments: the intra-atrial conduction time (PA interval), the atrioventricular nodal conduction time (AH interval), and the infranodal His-Purkinje conduction time (HV interval).

Accurate measurement of these baseline intervals requires optimal recording system setup, knowledge of electrogram morphology, and adherence to strict electrographic measurement endpoints. Infranodal conduction delays often masquerade as benign first-degree AV block on the surface 12-lead ECG; intracardiac interval analysis provides definitive localization and risk stratification for progressive conduction system failure.

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Recording System Calibration & Channel Configuration

To resolve rapid, millisecond-level intracardiac potentials, the EP recording system is configured with standardized display parameters:

• Display Sweep Speed: Standard diagnostic interval measurements are performed at a sweep speed of 100 mm/s to 200 mm/s (compared to standard clinical ECG speeds of 25 mm/s). A sweep speed of 100 to 200 mm/s expands the time axis, allowing caliper measurement precision within 1 to 2 milliseconds.
• Channel Hierarchy: Channels are standardly stacked from top to bottom on the acquisition monitor:
  1. Surface ECG leads (typically leads I, II, III, V1, and V6 for vector confirmation).
  2. High Right Atrium (HRA proximal and distal).
  3. His Bundle Electrogram (HBE proximal, mid, and distal pairs).
  4. Coronary Sinus (CS 1-2 distal to CS 9-10 proximal).
  5. Right Ventricle (RV apex or RVOT).
• High-Frequency Filtering: HBE channels utilize a bandpass filter of 30 Hz to 100 Hz (high-pass) and 300 Hz to 500 Hz (low-pass). Increasing the high-pass cutoff up to 100 Hz filters out low-frequency atrial and ventricular muscle repolarization waves, accentuating the sharp, high-frequency His spike.

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Components of the His Bundle Electrogram (HBE)

The His bundle channel represents the temporal and anatomical cornerstone of the diagnostic EP study. A correctly positioned bipolar pair records three consecutive potentials:

Surface Lead II:       [P wave]-------------------[QRS Complex]--------------
                                                    |
Intracardiac HBE:            (A)         (H)        (V)
                              |           |          |
                              +--- PA ----+          |
                              |           |          |
                              +---- AH ---+          |
                                          +--- HV ---+

Timing Landmarks:     Earliest P       Earliest HBE-A   His Spike Onset    Earliest Ventricular (Surface/EGM)

1. Atrial Electrogram ($A$ wave): Represents local depolarization of the low right atrial interatrial septum adjacent to the compact AV node. It is typically a medium-frequency, biphasic deflection.
2. His Potential ($H$ spike): A rapid, narrow, high-frequency biphasic or triphasic spike representing the synchronous depolarization of the common bundle of His. Normal His duration is less than 30 ms (typically 10 to 20 ms). Its steep slew rate ($\frac{dV}{dt}$) distinguishes it from slower myocardial potentials.
3. Ventricular Electrogram ($V$ wave): A large, sharp deflection representing local depolarization of the basal interventricular septum.

The A:V Amplitude Ratio

• Target Ratio: An optimal recording on the distal His channel displays an $A:V$ ratio of 1:2 to 1:1.
• Proximal Displacement: If the $A$ wave is significantly larger than the $V$ wave ($A:V > 2:1$), the catheter is positioned too high in the atrium near the compact AV node; the His potential may be small or absent.
• Distal Displacement: If the $V$ wave dominates and the $A$ wave is small ($A:V < 0.5:1$), the catheter has advanced too far across the tricuspid valve into the right ventricle. The His potential may merge into the onset of the $V$ wave, or the catheter may record a right bundle branch (RBB) potential rather than a true His potential.

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Baseline Intracardiac Intervals: Definitions & Normal Values

1. The PA Interval (Normal: 25 - 55 ms)

• Measurement Endpoints:
  • Onset: The earliest onset of the surface P wave on any of the 12 leads (or the onset of the high right atrial electrogram if it precedes the surface P wave).
  • Offset: The earliest high-frequency onset of the low right atrial deflection ($A$) on the His bundle electrogram.
• Electrophysiological Substrate: Measures intra-atrial conduction time—the duration required for an electrical impulse originating in the sinoatrial node to propagate through the crista terminalis, right atrial pectinate muscle, Bachmann's bundle, and interatrial septum to reach the compact AV node.
• Clinical Significance:
  • Prolongation (>55 ms): Indicates intra-atrial conduction delay (IACD). Commonly observed in patients with significant right atrial dilation, extensive atrial fibrosis, prior extensive atrial surgical maze incisions, or toxicity from Class IC antiarrhythmic agents (e.g., flecainide, propafenone).
  • Shortening (<25 ms): Observed during ectopic low right atrial rhythms or coronary sinus rhythms where atrial activation originates near the AV junction, depolarizing the low septum before the rest of the atria.

2. The AH Interval (Normal: 50 - 120 ms)

• Measurement Endpoints:
  • Onset: The earliest deflection of the low right atrial electrogram ($A$) on the His bundle tracing.
  • Offset: The initial onset of the His bundle potential ($H$).
• Electrophysiological Substrate: Measures atrioventricular nodal conduction time—the transit time from the low right atrial inputs through the transitional zone and compact AV node to the common bundle of His.
• Physiological & Biophysical Characteristics:
  • Conduction through the AV node is slow, mediated predominantly by slow inward calcium currents ($I_{Ca,L}$) through L-type calcium channels.
  • Decremental Conduction: The hallmark of AV nodal tissue. As the pacing rate increases (shorter pacing cycle lengths) or when premature atrial extrastimuli are introduced, conduction slows, and the AH interval progressively prolongs.
  • Autonomic Sensitivity: The AV node is densely innervated by sympathetic and parasympathetic nerve fibers. Parasympathetic (vagal) stimulation, carotid sinus massage, and negative dromotropic drugs (beta-blockers, non-dihydropyridine calcium channel blockers [diltiazem, verapamil], digoxin, adenosine) prolong the AH interval. Sympathetic stimulation, isoproterenol, and vagolytic agents (atropine) shorten the AH interval.
• Clinical Significance:
  • Prolongation (>120 ms): Represents first-degree atrioventricular block localized to the AV node. In the absence of symptoms, isolated AH prolongation is benign.
  • Shortening (<50 ms): Seen in states of high adrenergic tone, Lown-Ganong-Levine syndrome (enhanced AV nodal conduction), or pre-excitation via an atriofascicular/atrioventricular bypass tract that inserts directly into the His bundle.

3. The HV Interval (Normal: 35 - 55 ms)

• Measurement Endpoints:
  • Onset: The earliest rapid deflection of the His bundle spike ($H$) on the HBE channel.
  • Offset: The earliest onset of ventricular activation ($V$) detected on ANY recorded channel—whether on the 12-lead surface ECG (earliest Q/R wave onset) or on an intracardiac ventricular electrogram (such as the RV or HBE $V$ wave).
• Electrophysiological Substrate: Measures His-Purkinje system conduction time—the transit time from the common bundle of His down through the right and left bundle branches, the anterior and posterior fascicles, and the peripheral Purkinje arborization up to the working ventricular myocardium.
• Physiological & Biophysical Characteristics:
  • Conduction in the His-Purkinje network is rapid, driven by fast inward sodium currents ($I_{Na}$, Phase 0 of the action potential).
  • Non-Decremental (All-or-None): Conduction velocity through the normal His-Purkinje system is inelastic and fixed. At physiological heart rates, the HV interval does not prolong with rapid pacing. It maintains a constant duration until abrupt conduction failure occurs when the refractory period of the bundle branch is reached.
  • Autonomic Independence: The His-Purkinje system has negligible parasympathetic innervation; its conduction velocity is unaffected by vagal maneuvers or atropine.

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Normal Baseline Intracardiac Conduction Intervals

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Clinical Significance of Abnormal HV Intervals

The HV interval is the most critical prognostic measurement acquired during a baseline diagnostic EP study. Conduction failure within the His-Purkinje system produces wide-complex, unstable asystolic pauses or hemodynamic collapse.

Abnormally Short or Negative HV Intervals (<35 ms)

An HV interval measuring less than 35 ms—or where ventricular activation precedes the His deflection (negative HV)—occurs under specific pathological conditions:

1. Ventricular Pre-Excitation (Wolff-Parkinson-White Syndrome):
   • An antegrade-conducting atrioventricular accessory pathway (Kent bundle) bridges the atrium and ventricle, bypassing the delayed AV node and His-Purkinje system.
   • Ventricular myocardium is depolarized early by the accessory pathway. Consequently, the earliest surface ventricular deflection (the delta wave) occurs before or simultaneously with the His potential, yielding an HV interval < 35 ms, zero ms, or a negative HV interval.
2. Ventricular Ectopy / Ventricular Tachycardia (VT):
   • Depolarization originates within ventricular myocardium and propagates retrogradely toward the conduction system. The working ventricular muscle depolarizes prior to retrograde invasion of the His bundle, producing a negative or retrogradely directed HV interval.
3. Fascicular Ventricular Arrhythmias / Pacing:
   • Origin within the left anterior or posterior fascicle activates local ventricular tissue with a short delay (<30 ms) relative to retrograde His activation.

Normal Conduction:          (A) ------> (H) ----------------> (V)        HV = 35 - 55 ms

Pre-Excitation (WPW):       (A) ------> (H)
                             |           |                              HV < 35 ms (or Negative)
                             +-------------[Accessory Pathway]-----> (V)

Abnormally Prolonged HV Intervals (>55 ms)

Prolongation of the HV interval reflects diseased, damaged, or sclerodegenerative tissue within the common His bundle, both bundle branches, or all three fascicles (bifascicular/trifascicular disease):

• HV 56 to 69 ms: Mild-to-moderate infranodal conduction disease. Indicates latent conduction disease that requires close clinical observation, particularly in patients presenting with unheralded syncope.
• HV 70 to 99 ms: Severe infranodal conduction disease. Reflects high risk of spontaneous progression to complete, paroxysmal infranodal heart block. When identified in patients with documented syncope where other causes have been excluded, permanent pacemaker implantation is indicated (Class IIa).
• HV $\ge$ 100 ms: Extremely advanced infranodal disease. Represents imminent, catastrophic failure of the His-Purkinje system. According to ACC/AHA/HRS guidelines, an HV interval of 100 ms or greater at baseline is a Class I indication for permanent pacemaker implantation, even in completely asymptomatic patients.

Pharmacological Stress Testing: The Procainamide Provocation Test

In patients presenting with recurrent, unexplained syncope and a normal or borderline baseline HV interval (55 to 69 ms), latent infranodal disease can be unmasked using intravenous procainamide (a Class IA sodium channel blocker):

• Protocol: Procainamide is infused intravenously at 10 to 15 mg/kg at a rate of 50 mg/min (maximum 1,000 mg) under continuous hemodynamic and electrogram monitoring.
• Positive Diagnostic Endpoints for Pacemaker Implantation:
  1. The HV interval prolongs to $\ge$ 100 ms.
  2. The HV interval increases by greater than 100% of baseline.
  3. Provocation of spontaneous or atrial-pacing-induced second- or third-degree infranodal block (an $H$ deflection not followed by a $V$ wave).