7.1 12-Lead ECG in the Cath Lab
Key Takeaways
- ST elevation in contiguous leads localizes acute transmural ischemia: anterior (V1–V4), inferior (II, III, aVF), lateral (I, aVL, V5–V6), and posterior (reciprocal ST depression V1–V3 with tall R waves)
- New LBBB with compatible symptoms meets STEMI criteria because ST-segment analysis is unreliable; RBBB does not mask ST elevation but can complicate axis and chamber interpretation
- Diffuse ST depression with ST elevation in aVR suggests left main or severe multivessel ischemia and warrants urgent hemodynamic and angiographic evaluation
- Electrical alternans, low QRS voltage, and sinus tachycardia on a pre-procedure ECG raise suspicion for pericardial effusion or tamponade before vascular access
- RCIS staff must recognize acute ischemic patterns, communicate territory and rhythm findings to the physician, and correlate ECG changes with hemodynamics and symptoms during the case
12-Lead ECG in the Cath Lab
Quick Answer: In the cath lab, the 12-lead ECG is used to confirm the ischemic territory before angiography, detect conduction abnormalities that change reperfusion criteria, monitor for evolving ischemia during the case, and flag electrical signs of tamponade or pericardial compromise. RCIS staff are expected to recognize patterns, document changes, and communicate findings promptly to the physician.
Every diagnostic and interventional case begins with rhythm and ischemia assessment. The ECG complements hemodynamics: ST-segment shifts may appear before pressure changes, and new conduction block can explain sudden hypotension or heart failure during catheter manipulation. Your role is not to make the final diagnosis, but to ensure the team never misses a time-critical pattern.
Localizing Acute Infarct Patterns (STEMI)
ST-segment elevation in anatomically contiguous leads indicates acute transmural injury. The leads reflect the area of myocardium facing the electrode.
| Territory | Lead Group | Typical Vessel | Cath Lab Priority |
|---|---|---|---|
| Anterior | V1–V4 (± V5–V6) | LAD | Emergent left heart cath; watch for cardiogenic shock |
| Inferior | II, III, aVF | RCA (often) or circumflex | Right ventricular involvement possible; preload-dependent |
| Lateral | I, aVL, V5–V6 | Circumflex or diagonal | May coexist with anterior or inferior patterns |
| Posterior | Reciprocal changes: ST depression V1–V3, tall R in V1–V2 | Posterior descending / circumflex | Obtain posterior leads (V7–V9) when suspected |
| Septal | V1–V2 (± aVR) | Proximal LAD septal branches | Often part of larger anterior territory |
Contiguity rule: STEMI diagnosis requires ST elevation meeting criteria in at least two contiguous leads. Isolated ST changes in non-contiguous leads suggest a different etiology until proven otherwise.
Hyperacute T waves—tall, symmetric, broad-based T waves—may precede ST elevation and signal very early occlusion. Document and repeat the ECG if the clinical picture fits acute coronary syndrome.
Bundle Branch Blocks in the Cath Lab
Conduction system disease changes both diagnosis and hemodynamic tolerance during catheterization.
| Finding | ECG Clues | Cath Lab Implications |
|---|---|---|
| Right bundle branch block (RBBB) | rsR' in V1, wide S in I and V6, QRS ≥120 ms | Does not mask ST elevation; inferior STEMI with RBBB still meets criteria. New RBBB with ischemic symptoms warrants urgent angiography. |
| Left bundle branch block (LBBB) | Broad notched R in I, aVL, V5–V6; deep S in V1; QRS ≥120 ms | ST segments are normally discordant—standard STEMI criteria do not apply. New LBBB with ischemic symptoms is treated as a STEMI equivalent. |
| Bifascicular block | RBBB plus left anterior or posterior fascicular block | High-grade AV block risk; have transvenous pacing available for complex cases. |
| Complete heart block | AV dissociation, escape rhythm | May reflect inferior MI (RCA), drug effect, or catheter-related trauma; correlate with rate and blood pressure. |
Sgarbossa criteria (modified versions used in practice) help identify MI in the setting of existing LBBB when ST elevation is concordant or excessively discordant. When in doubt during an acute presentation, treat as STEMI equivalent and expedite angiography.
Ischemia Without ST Elevation
Not all ischemia presents with ST elevation. Recognizing subtler patterns prevents delayed intervention.
| Pattern | ECG Appearance | Clinical Context |
|---|---|---|
| Subendocardial ischemia | Horizontal or downsloping ST depression | NSTEMI, demand ischemia, reciprocal change to remote STEMI |
| T-wave inversion | Deep symmetric T inversion in contiguous leads | Evolving infarction, Wellens syndrome (critical LAD stenosis), post-reperfusion |
| Wellens syndrome | Biphasic or deeply inverted T waves in V2–V3 | High-risk LAD lesion; do not stress test—urgent angiography |
| Left main / multivessel | Diffuse ST depression in multiple territories plus ST elevation in aVR | Severe global subendocardial ischemia; hemodynamic instability common |
| De Winter T waves | Upsloping ST depression with tall symmetric T waves in precordial leads | LAD occlusion variant; may occur without classic ST elevation |
During the procedure, new ST depression or T-wave changes may signal wire-induced ischemia, embolization, no-reflow, or spasm. Compare to the baseline tracing, note the time, and notify the physician immediately.
Tamponade and Pericardial Clues on ECG
Cardiac tamponade is a cath lab emergency—post-PCI ventricular perforation, post-ablation, post-structural intervention, or post-device closure can all accumulate pericardial blood quickly. The ECG offers early clues before pulsus paradoxus and equalization of diastolic pressures are fully established.
| ECG Finding | Mechanism | Action in the Cath Lab |
|---|---|---|
| Electrical alternans | Heart swings within effusion; alternating QRS amplitude | High suspicion for tamponade; prepare for pericardiocentesis |
| Low QRS voltage | Fluid insulating the myocardium | Compare to prior ECG; correlate with echo if available |
| Sinus tachycardia | Compensatory response to reduced stroke volume | Do not attribute solely to anxiety or sedation |
| PR depression | Pericardial inflammation | May appear in acute pericarditis; less specific in tamponade |
Tamponade physiology produces hypotension, jugular venous distension, and muffled heart sounds (Beck triad)—but in the lab you may see hypotension and tachycardia first. If electrical alternans appears with falling blood pressure after a wire or device manipulation, treat it as tamponade until excluded by imaging.
Integrating the ECG With Cath Lab Workflow
Pre-procedure: Confirm indication (STEMI, NSTEMI, unstable angina, structural evaluation), identify baseline conduction disease, and note pacemaker spikes or artifact. Compare to EMS or ED tracings—lead placement errors mimic ischemia.
Intraprocedure: Monitor for arrhythmias during catheter advancement (especially RV and LV entry), transient ST changes during balloon inflation, and bradyarrhythmias with inferior or RCA territory disease. Document rhythm strips when complications occur.
Post-procedure: Reperfusion arrhythmias (accelerated idioventricular rhythm) may appear after successful PCI. Persistent ST elevation suggests failed reperfusion or no-reflow. New conduction block after septal alcoholization, TAVR, or complex PCI requires serial ECGs and hemodynamic monitoring.
Common Pitfalls for RCIS Candidates
- Misplacing leads — Swapped arm leads invert limb patterns; V1–V2 placed too high mimic anterior changes.
- Ignoring reciprocal changes — ST depression in V1–V3 may be the only clue to posterior STEMI.
- Applying standard STEMI criteria in LBBB — Use STEMI-equivalent rules for new LBBB with symptoms.
- Attributing alternans to baseline variability — Compare beat-to-beat QRS amplitude systematically.
- Delaying notification — Any new ST elevation, symptomatic arrhythmia, or tamponade pattern requires immediate verbal report.
Mastering territory localization, bundle branch interpretation, ischemic equivalents, and tamponade electrical signs ensures the cath lab team acts on the ECG as a real-time safety tool—not just a pre-procedure checkbox.
A patient presents with chest pain and ST elevation in leads II, III, and aVF. Which coronary territory is most likely involved, and what additional ECG finding would suggest right ventricular involvement?
A 68-year-old with acute chest pain has a new left bundle branch block and no prior ECG for comparison. What is the appropriate cath lab interpretation and response?
During a complex PCI, the patient becomes hypotensive and the monitor shows beat-to-beat variation in QRS amplitude. The most appropriate immediate concern is: