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Findings in ecg explained to a cardiologist
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"left bundle branch block" AND tachycardia ECG
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ECG Analysis - Cardiologist Level Report
Machine Interpretation (from printout)
- Tachycardia + Left Bundle Branch Block
- Abnormal ECG (confirmed)
- Rate: 156 ms RR intervals, HR ~156 bpm (the printed "156 ms" refers to cycle length; 60,000/386 ms = ~155 bpm based on standard conversion - verify directly)
- QT/QTc(Bz): 336/459 ms - QTc is borderline prolonged (Bazett corrected)
- PQRST: 72/-2/129 degrees (axis data)
- RV5/SV1: 1.823/2.178 mV
Systematic ECG Findings
1. Rate and Rhythm
- Ventricular rate ~155-156 bpm - frank tachycardia
- Rhythm appears regularly irregular (look for P wave regularity vs. dissociation - see below)
2. QRS Morphology - Wide Complex Tachycardia (WCT)
- QRS duration: 129 ms (>120 ms = wide), consistent with complete bundle branch block
- Classic LBBB morphology: broad notched R in lateral leads (I, aVL, V5, V6); broad QS or rS in V1-V2
3. QRS Axis
- Axis: -2 degrees - near normal/borderline left axis, which is atypical for typical LBBB (which usually shows left or normal axis). This warrants attention.
4. QTc
- QTc 459 ms (Bazett) - mildly prolonged, partly rate-dependent correction artifact at fast heart rates. Clinical significance depends on medications and electrolytes.
The Central Diagnostic Challenge: What is this WCT?
This ECG presents a wide complex tachycardia (WCT) at ~155 bpm with LBBB morphology. The primary differential is:
| Diagnosis | Key discriminating features |
|---|---|
| SVT with rate-dependent/pre-existing LBBB aberrancy | P waves visible, preceding QRS at fixed PR; QRS morphology identical to baseline LBBB |
| Bundle branch reentry VT (BBR-VT) | Classic LBBB configuration; associated with LV dysfunction, conduction disease (His-Purkinje disease); rapid (often >200 bpm); AV dissociation on close inspection |
| Monomorphic VT with LBBB morphology | AV dissociation, fusion/capture beats, NW axis, RS interval >100 ms in precordials |
| Antidromic AVRT | Pre-excitation pattern in sinus rhythm; full delta morphology |
Critically - all WCTs must be treated as VT until proven otherwise (Washington Manual of Medical Therapeutics, p. 254).
Applying Brugada Criteria (LBBB-morphology WCT)
From the ECG leads V1-V2, look for features diagnostic of VT in LBBB morphology (Kindwall criteria, per Braunwald's):
| Criterion | Favors VT |
|---|---|
| r wave in V1/V2 ≥0.04 s (40 ms) | If present → VT |
| Notched S downstroke in V1/V2 | If present → VT |
| Delayed S nadir >0.06 s from QRS onset | If present → VT |
| Any Q wave in V6 | If present → VT |
If none of the above features are present in V1/V2, and QRS morphology is a clean narrow r followed by smooth S, this strongly favors SVT with LBBB aberrancy.
Bundle Branch Reentry VT - A Specific Warning
Given LBBB morphology at this rate, BBR-VT must be actively considered. Per Braunwald's Heart Disease (7th ed., block7):
"Bundle branch reentry is the most common form... the circulating wavefront usually propagates down the right bundle branch, through the septum and up the left bundle branch... VT has a typical LBBB configuration. The VT is often rapid, faster than 200/min. It is associated with disease of the Purkinje system and often with severe LV dysfunction. Most patients have an interventricular conduction delay or even a pattern of complete LBBB during sinus rhythm."
At 155-156 bpm, this rate is consistent with BBR-VT (though usually faster). Check the baseline ECG - if the patient has pre-existing LBBB in sinus rhythm, BBR-VT becomes more likely. Catheter ablation of the right bundle branch is curative, but an ICD is usually warranted given associated LV dysfunction and other scar-related VT.
AV Dissociation Assessment
Scan lead II and V1 (the rhythm strip) for:
- Independent P waves marching through at a different (slower) rate than QRS - diagnostic of VT
- Capture beats - narrow complex appearing earlier than expected (supraventricular impulse captures ventricles during VT)
- Fusion beats - intermediate QRS morphology
Given the ventricular rate of ~155 bpm, P waves may be buried in QRS or T waves. A vagal maneuver or adenosine (with crash cart standby) can be diagnostic if hemodynamically stable - it will terminate/slow SVT but will not affect true VT.
Summary of Key Abnormalities
- Tachycardia at ~155 bpm - hemodynamically significant rate
- Wide QRS (129 ms) with LBBB morphology - classical appearance
- QTc 459 ms - mildly prolonged (Bazett, rate-dependent)
- Axis -2 degrees - near normal; slightly atypical for pure LBBB (left axis would be more typical - left axis deviation may suggest concurrent left anterior hemiblock or a VT origin)
- RV5/SV1 voltages elevated - may suggest LV hypertrophy as an underlying substrate
Immediate Clinical Actions
| Step | Action |
|---|---|
| Hemodynamic stability | If unstable → synchronized DC cardioversion immediately |
| 12-lead comparison | Compare with prior ECG - pre-existing LBBB vs. new WCT |
| Adenosine 6-12 mg IV | If stable - diagnostic + therapeutic for SVT; will not terminate VT |
| Echo | Assess LV function - severe LV dysfunction + LBBB morphology WCT = high BBR-VT probability |
| Electrolytes | K+, Mg2+ (correct any deficits) |
| Troponin | Exclude ischemia as trigger |
| If VT confirmed | IV amiodarone 150 mg over 10 min; EP study for BBR-VT (ablation consideration) |
Bottom Line
This is a wide complex tachycardia at ~155 bpm with LBBB morphology and mildly prolonged QTc. The machine correctly flags this as tachycardia + LBBB; however, differentiating SVT with LBBB aberrancy from VT (particularly BBR-VT) is the essential clinical task. Do not assume SVT. Apply Brugada/Kindwall criteria, assess for AV dissociation, compare with baseline, assess LV function, and if hemodynamically unstable - cardiovert first, diagnose later.
- Washington Manual of Medical Therapeutics, p. 253-254
- Braunwald's Heart Disease, p. 555 (Bundle Branch Reentry VT)
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