Overview
This lecture covers the fundamental concepts and key steps in intracardiac electrogram (EGM) interpretation, including signal acquisition, filtering, measurement intervals, and correlation with the surface ECG in electrophysiology studies.
Surface ECG vs. Intracardiac EGM
- Surface ECG displays summed electrical activity from the heart recorded at the skin, typically at 25 mm/s and 10 mm/mV.
- EGM records localized electrical signals from within the heart chambers requiring amplification and rapid sweep speeds (100–200 mm/s).
- Intracardiac signals are smaller (<10 mV) and need filtering to remove noise for clear visualization.
Signal Filtering and Settings
- High pass filter removes low-frequency signals (e.g., baseline wander).
- Low pass filter removes high-frequency noise.
- Notch filter eliminates signals at a specific frequency (e.g., 60 Hz electrical interference), but it is not applied to the His channel.
- Standard EP lab filter settings: 30 Hz (high pass) and 500 Hz (low pass).
EGM Intervals and Measurements
- All cycle lengths are measured in milliseconds (ms) in EP labs.
- Cycle length (ms) = 60,000 ÷ heart rate (bpm).
- Main EGM intervals:
- PA interval: SA to AV node conduction (20–60 ms).
- AH interval: AV node conduction time (50–130 ms).
- HV interval: His-Purkinje/ventricular conduction (35–55 ms).
- PR interval on ECG = PA + AH + HV intervals.
Key EGM Signals and Channel Placement
- "A" wave (atrial depolarization) correlates with the P-wave on the ECG.
- "H" wave (His bundle activation) marks conduction through the AV node.
- "V" wave (ventricular depolarization) aligns with the QRS complex.
- Surface ECG channels are followed by recordings from RV apex, His, and coronary sinus (CS) electrodes.
- CS signals spread from proximal to distal as depolarization moves right to left.
Sinus Node Recovery and Dysfunction Testing
- Sinus Node Recovery Time (SNRT) measures sinus node automaticity after overdrive pacing.
- SNRT = time from last paced beat to first intrinsic beat; normal ≤ 1,500 ms.
- Corrected SNRT (CSNRT) accounts for baseline rate: CSNRT = SNRT – basic cycle length (normal ≤ 525 ms).
Relating EGM to ECG and Signal Sequence
- Each EGM signal can be mapped to a point on the ECG:
- A-waves correspond to P-wave,
- H-wave to PR segment,
- V-wave to QRS.
- Signal appears sequentially: atrial signal (A) on H channel/surface ECG, His channel, CS channels (proximal to distal), H signal, RV Apex/V signal, then V on His and CS.
- Activation sequence in CS catheter is always proximal to distal.
Key Terms & Definitions
- EGM (Electrogram) — Localized intracardiac recording of electrical activity.
- High Pass Filter — Allows frequencies above a set point to pass, removing baseline drift.
- Low Pass Filter — Allows frequencies below a set point to pass, removing high-frequency noise.
- Notch Filter — Removes a single frequency, typically 60 Hz.
- PA Interval — SA node to AV node conduction time (20–60 ms).
- AH Interval — AV node conduction time (50–130 ms).
- HV Interval — His to ventricular conduction time (35–55 ms).
- SNRT (Sinus Node Recovery Time) — Time for sinus node to resume after pacing.
- CSNRT (Corrected SNRT) — SNRT adjusted for cycle length (≤ 525 ms).
Action Items / Next Steps
- Review assignment instructions and complete the related task using the provided EGM-to-ECG correlation slides.
- Attend the weekly meeting for further clarification on identifying PA, AH, and HV intervals.