Serial Kardia 6L Tracings in Patients with LBBAP: What Surface ECG Can and Cannot Tell You About Supine Discomfort

When serial single-lead-grade tracings show stable left bundle branch area pacing capture but the patient still reports positional symptoms, the answer is not in the surface morphology. It is in the device timing and the hemodynamics.

Comparing Two Serial 6-Lead Recordings

The Kardia 6L provides leads I, II, III, aVR, aVL, and aVF at 25 mm/s and 10 mm/mV with enhanced filtering. It is a useful longitudinal home tool for paced patients, but its interpretive ceiling is narrow. The comparison below illustrates a typical scenario in a patient three months post-LBBAP upgrade.

Morphologic Findings

In the earlier tracing, every QRS is preceded by sharp, narrow vertical deflections — pacing artifacts are clearly visible in II, III, aVF, and aVR. Two-component artifacts in lead II are consistent with sequential atrial then ventricular pacing in a DDD configuration. QRS complexes are relatively narrow with a left-superior frontal axis (positive in I and aVL, negative in II, III, aVF), the expected morphology of LBBAP capture engaging proximal left bundle fibers or LV septum.

In the later tracing, QRS complexes look morphologically very similar: same axis, similar width, similar T-wave polarity. Pacing spikes appear less obvious, but this almost certainly reflects recording conditions — finger contact, filter behavior on the small LBBAP stimulus — rather than loss of capture, because the QRS vector is unchanged.

The Limits of Surface ECG for Positional Symptoms

This is the critical interpretive boundary: stable paced QRS morphology and absence of supine symptoms are not the same thing. A 30-second resting 6-lead surface tracing is essentially silent on the mechanisms that produce positional discomfort in paced patients.

Mechanisms Not Visible on a Kardia 6L

1. AV timing mismatch in the supine state — sensed AV delay, paced AV delay, or rate-dependent AV behavior producing suboptimal LA-to-LV filling when preload rises on lying flat. Requires device interrogation with timing parameters or supine echo with Doppler inflow.
2. Loss of AV synchrony at specific rates — mode switches, atrial undersensing, or pseudofusion that manifests intermittently, especially at rest or low rates. Requires device diagnostics.
3. Diastolic dysfunction unmasked by increased venous return — preload-sensitive symptoms in a remodeled ventricle. An echo-level question.
4. Pulmonary venous congestion or orthopnea-equivalent physiology — hemodynamic, not electrical.
5. Mechanical and positional factors — lead tension, pocket discomfort, pectoral irritation in a recently upgraded system. Entirely extra-electrical.
6. Autonomic shifts on recumbency — vagal tone changes affecting AV conduction, sensor behavior, or rate response.
7. Subclinical pericardial irritation or effusion in the post-upgrade window — requires imaging.

What the ECG Comparison Does and Does Not Tell You

What it tells you

• The pacing system is electrically stable across the interval.
• There is no new bundle branch behavior, no obvious loss of capture, no AF, no ventricular ectopy in the recorded windows.
• The capture vector (and therefore the location of conduction system engagement) is preserved.

What it does not tell you

• Whether AV intervals are hemodynamically optimized.
• Whether the patient has appropriate atrioventricular synchrony in the supine state, at rest, or overnight.
• Whether diastolic function or LA pressures are improving, stable, or worsening.
• Whether pacing burden, mode switches, or sensor-driven rate behavior are appropriate.
• Whether the patient is still symptomatic and why.

Heart Rate Differences Between Snapshots

A change from 50 bpm to 60 bpm between two single 30-second resting recordings has limited interpretive value without context. Possible explanations include: programming change to the lower rate limit, sinus tracking on a higher intrinsic rate, sensor-driven rate response, sleep or rest rate hysteresis active in one tracing and not the other, or simple time-of-day autonomic variation. None of these can be discriminated from the surface tracing alone.

Appropriate Next Investigations When Supine Symptoms Persist

If supine discomfort persists after an LBBAP upgrade despite morphologically clean surface tracings, the productive diagnostic moves are:

• Full device interrogation — rate histograms, percent atrial and ventricular pacing, mode-switch episodes, AV timing intervals, and any high-rate episodes correlated to the time of symptoms.
• Programming review — sensed and paced AV delays, rate hysteresis, sleep rate, rate response curve, and whether AV optimization (echo-guided or device-algorithm-guided) was performed post-upgrade.
• Supine vs. upright Doppler echocardiography — E/A, E/e′, mitral inflow timing, LA strain, and any change in functional MR with position.
• Ambulatory monitoring overnight with patient-recorded symptom timestamps for correlation against rhythm and pacing diagnostics.
• Targeted imaging if mechanical or pericardial etiologies remain on the differential.

Practical Caveats on the Kardia 6L Itself

• "Unclassified" is the expected determination for any paced rhythm; the algorithm is trained on intrinsic rhythms and does not classify pacing morphologies.
• Paced QRS duration — the metric that matters most for LBBAP "health" — cannot be precisely measured at 25 mm/s on a single-finger lead. A 50 mm/s 12-lead with calipers, or the device's own measured paced QRSd, gives a far better answer.
• Pacing spike conspicuousness varies with finger contact and filter behavior. Spike visibility is not a reliable marker of capture status when bipolar LBBAP outputs are small.