Cardiac Resynchronization Therapy (CRT) and Anodal Stimulation Notes

Cardiac Resynchronization Therapy (CRT) and Anodal Stimulation

  • Common Ring Configuration: Unipolar coronary sinus leads can be programmed in a "common ring" configuration, utilizing the right ventricular (RV) lead's anodal electrode as the anodal electrode for the coronary sinus lead.
  • Anodal RV Stimulation: A left ventricular (LV) stimulus can cause anodal stimulation at the RV lead, leading to simultaneous biventricular activation.
  • Capture Thresholds: If the LV capture threshold > RV anodal capture threshold:
    • Biventricular capture OR
    • RV anodal only capture
  • Interference: This may interfere with programming of the interventricular (VV) pacing delay.
  • Triple-Site Pacing: It is hypothesized that when RV anodal stimulation occurs during biventricular (BiV) pacing, three wave fronts of ventricular depolarization occur:
    • LV tip
    • RV tip
    • RV proximal electrode
  • Hemodynamic Benefit: Pacing from three different sites may produce a hemodynamic benefit detected by echocardiography.
  • Ineffective VV Interval Timing: Anodal RV stimulation can make VV interval timing ineffective, especially when the LV is stimulated before the RV.
    • Results in unaltered simultaneous biventricular stimulation over a wide range of VV intervals.
  • Morphology Changes: At shorter V-V intervals, changes in morphology are observed due to anodal and cathodal RV stimulation.

The 12 Lead ECG

  • Baseline ECG: A non-paced 12 lead ECG (pre- or post-implant with intrinsic rhythm) is the best guide for assessing LV vs. RV pacing.
  • Comparison: Compare the baseline ECG with RV only, LV only, and BiV pacing to confirm capture.
  • RV Only Pacing:
    • Wide complex
    • Left bundle branch block (LBBB) configuration
    • Negative deflection of the QRS in leads V1-V4
  • LV Lead Position: The surface ECG may indicate a right bundle branch block (RBBB), LBBB, or a variation of either, depending on the LV lead's position.
  • LV Pacing Threshold Determination:
    • QRS complex widens with loss of LV capture.
    • Becomes less negative in lead I.
    • Becomes more negative in lead III.
    • Ventricular excitation originates from the inferior RV instead of the lateral LV.
  • Biventricular Pacing: The ECG tends to narrow somewhat (but not always) and may appear as a variant of LBBB, RBBB, or a normal impulse.

Device Based Intracardiac Electrogram (IEGM)

  • Utility: IEGMs can significantly help in assessing LV only capture.
  • IEGM Programming Suggestions:
    • RV Coil to Can (BiV capture verification in CRT-D patients)
    • SVC Coil to Can
    • Observation of changes in impulse to IEGM timing
  • Possible Ventricular Pacing States: At least seven possible states of ventricular pacing can occur in cardiac resynchronization systems that use an extended bipolar pacing configuration:
    • No capture (intrinsic conduction)
    • RV cathodal + LV cathodal (intended state)
    • LV (cathodal) only
    • RV cathodal only
    • RV anodal only
    • RV anodal + LV cathodal
    • RV (cathodal + anodal) + LV (“triple site” pacing)
  • Distinguishing Pacing Configurations: Most of these states can be distinguished by analysis of intracardiac electrograms combined with a 12-lead ECG.

ECG Lead Morphology

  • Axis Shift Examination: The morphology change associated with LV versus RV capture is best examined in the ECG lead that is perpendicular to the axis shift.
  • BV to LV Capture: Best identified as increasing positivity of the QRS in lead III.
  • BV to RV Capture: Best recognized as increasing positivity of the QRS in lead I.