Patient Question:
Why is the QRS on a Kardia 6L ECG different from the QRS on a hospital ECG in a patient with an Aveir VR leadless pacemaker?
Claude AI Answer:
This is an excellent clinical observation that has a clear electrophysiologic explanation. The difference lies in whether the pacemaker is actively pacing or sensing the intrinsic rhythm at the time of each recording.
The Kardia 6L ECG: Paced Rhythm
A Kardia 6L recording taken at rest (for example, at 5:10 AM during sleep) showing a heart rate of 50 BPM captures the Aveir VR paced rhythm. The QRS complexes appear wide with a left bundle branch block (LBBB)-like morphology. This is the expected, characteristic pattern produced by right ventricular pacing.
Key features of the paced QRS on Kardia:
- Wide QRS complexes (typically 140–180 ms)
- LBBB-like morphology: broad R waves in Lead I, deep S waves in Leads II and III
- Heart rate at or near the programmed lower rate limit (e.g., 50 BPM during sleep)
- Kardia determination may read “Unclassified” because the algorithm does not recognize the paced pattern as a standard sinus rhythm
The Hospital ECG: Intrinsic (Native) Rhythm
A hospital 12-lead ECG may capture the patient’s intrinsic conduction—the native rhythm conducting normally through the His–Purkinje system. When the patient’s own heart rate exceeds the pacemaker’s programmed lower rate limit (for example, 75–85 BPM during daytime activity), the Aveir VR senses these native beats and appropriately inhibits its pacing output (VVI mode). The result is a narrow QRS with normal morphology.
Key features of the intrinsic QRS at the hospital:
- Narrow QRS complexes (normal duration)
- Normal QRS morphology with intact conduction
- Higher heart rate (75–85 BPM), driven by the patient’s own sinus node
- P waves visible preceding each QRS, confirming normal sinus rhythm
- Pacemaker is in sensing mode—no pacing artifacts visible
The Core Mechanism: Pacing vs. Sensing in VVI Mode
| Feature |
Kardia 6L (Paced) |
Hospital ECG (Intrinsic) |
| Pacemaker status |
Actively pacing |
Sensing & inhibited |
| Heart rate |
~50 BPM (lower rate limit) |
~75–85 BPM (native rate) |
| QRS width |
Wide (140–160 ms) |
Narrow (normal) |
| QRS morphology |
LBBB-like pattern |
Normal conduction |
| Depolarization pathway |
Cell-to-cell spread from RV pacing site |
Normal His–Purkinje system |
| P waves |
May be dissociated or absent |
Present, preceding each QRS |
| Context |
Rest/sleep (low intrinsic rate) |
Awake/active (adequate intrinsic rate) |
Why Does the Paced QRS Look So Different?
When the Aveir VR fires from its position in the right ventricle, the electrical impulse does not travel through the fast-conducting His–Purkinje fiber network. Instead, it spreads cell-to-cell through the ventricular myocardium, which is inherently slower. This slower, non-physiologic depolarization produces:
- A wider QRS complex — because the wavefront takes longer to depolarize both ventricles without the rapid conduction system
- An LBBB-like pattern — because the right ventricle is activated first (pacing site), and the left ventricle is activated later as the impulse crosses the interventricular septum
- Altered axis and morphology — because the direction of depolarization is fundamentally different from normal top-to-bottom, right-to-left activation
Clinical Significance: This difference is entirely normal and reassuring. It confirms the Aveir VR is functioning properly in VVI mode: pacing when needed (when the intrinsic rate drops below the programmed lower rate), and appropriately inhibiting when the patient’s native rhythm is adequate. The QRS morphology difference is simply the electrophysiologic fingerprint of where the electrical impulse originates.
Follow-Up Question:
Can you confirm that the paced QRS on the Kardia 6L ECG is less than 220 ms?
Claude AI Answer:
To answer this question precisely, the Kardia 6L ECG recording was analyzed using calibrated grid measurement at the standard paper speed of 25 mm/s (where each minor gridline = 1 mm = 40 ms, and each major gridline = 5 mm = 200 ms).
Measurement Methodology
The QRS duration was measured across all available limb leads (I, II, III, aVR, aVL, aVF) using the maximum QRS duration method—identifying the lead with the earliest QRS onset and the lead with the latest QRS offset to capture the true total ventricular depolarization time.
Calibration verification: Minor gridlines were confirmed at 43 pixels apart in the digital image, corresponding exactly to the 1 mm = 40 ms standard. This was cross-validated against major gridlines (5 mm = 200 ms intervals).
Results
Maximum Paced QRS Duration
140 – 160 ms
Confirmed: Well under 220 ms
The QRS complexes consistently measured approximately 3.5 to 4 minor grid boxes in width across all leads, corresponding to 140–160 ms. This measurement was reproducible across multiple complexes on pages 2 through 5 of the Kardia recording.
Lead-by-Lead Observations
- Lead I: Low-amplitude, predominantly positive QRS. Narrow appearance; QRS spans approximately 3–3.5 small boxes.
- Lead II: Biphasic morphology with initial positive deflection (r wave) followed by a sharp deep S wave. Total QRS width approximately 3.5–4 small boxes.
- Lead III: Widest-appearing complexes with deep QS/rS pattern. QRS width approximately 3.5–4 small boxes. The upward deflection following the S wave represents the T wave, not a terminal QRS component.
- aVR: Low-amplitude complexes consistent with the paced morphology.
- aVL: Biphasic complexes; QRS width consistent with other leads.
- aVF: Similar to Lead II morphology; QRS duration approximately 3.5 small boxes.
Clinical Interpretation of QRS Width
| QRS Duration Range |
Clinical Significance |
| < 120 ms |
Normal intrinsic QRS; not expected with RV pacing |
| 120–150 ms |
Typical for RV septal pacing; lower risk of pacing-induced cardiomyopathy |
| 140–160 ms ✔ |
This patient’s measurement — within the expected range for Aveir VR RV pacing |
| 160–200 ms |
Wider paced QRS; may warrant monitoring for ventricular dyssynchrony over time |
| > 200 ms |
Significantly prolonged; higher concern for pacing-induced cardiomyopathy |
| ≥ 220 ms |
Markedly prolonged; suggests superimposed conduction disease or suboptimal pacing site |
Clinical Significance of the 140–160 ms QRS:
- A paced QRS under 200 ms is generally associated with a lower risk of pacing-induced cardiomyopathy, particularly when pacing burden is not excessively high.
- The measured QRS at 140–160 ms is typical and expected for the Aveir VR leadless pacemaker positioned in the right ventricle.
- If the QRS were approaching or exceeding 200–220 ms, that would strengthen the clinical argument for upgrading to Left Bundle Branch Area Pacing (LBBAP) to achieve more physiologic ventricular activation.
- At 140–160 ms, the QRS width alone does not indicate an urgent need for pacing system upgrade, though LBBAP may still offer benefits through more physiologic activation for long-term ventricular function preservation.
Important Note on Kardia 6L Limitations: The Kardia 6L records only limb leads (I, II, III, aVR, aVL, aVF). A full 12-lead ECG with precordial leads (V1–V6) may reveal subtle additional QRS components not visible in limb leads alone. However, for the purpose of confirming whether the QRS is less than 220 ms, the limb leads provide adequate information, and the measurement is unequivocal.