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Must-Know ECG Differential Diagnoses (DDx)
Key Points Definition: A differential diagnosis is the structured process of considering multiple possible conditions that could explain a patient’s symptoms and ECG findings. Purpose: The goal is to narrow…
Wide & Irregular Tachycardia: DDx
Key Points Definition: QRS >120 ms with an irregular ventricular rate. Clinical Priority: Always first consider AF with accessory pathway (AF + WPW) and polymorphic VT (PMVT, including torsades) —…
Narrow & Irregular Tachycardia: DDx
Key Points Narrow irregular tachycardia = QRS <120 ms with variable R–R interval. The big three DDx: Atrial fibrillation (AF) with RVR, atrial flutter with variable block, multifocal atrial tachycardia…
Narrow & Regular Tachycardia: DDx
Key Points Narrow regular tachycardia = QRS <120 ms with a steady R–R interval. The big three DDx: sinus tachycardia, SVT (AVNRT/AVRT/junctional), and atrial flutter with fixed AV conduction (commonly…
Right Axis Deviation + ST Elevation: DDx
Key Points RAD combined with ST-segment elevation (STE) is an uncommon but high-risk finding. While STE often suggests acute coronary occlusion, this pattern rarely reflects classic STEMI. Several non-ischemic conditions…
Extreme Axis Deviation: DDx
Key Points Definition: Extreme axis deviation is present when the QRS axis lies between –90° and ±180°. This is sometimes referred to as the “northwest axis.” ECG Pattern: QRS is…
Indeterminate Axis: DDx
Key Points An indeterminate axis, sometimes referred to as an “extreme” or “undetermined” axis, is suspected when the QRS complexes are isoelectric or nearly biphasic in both Lead I and…
Left Axis Deviation: DDx
Key Points Definition: Left axis deviation (LAD) is defined as a QRS axis between –30° and –90°. ECG Clue: QRS complex is positive in Lead I and negative in Lead…
Right Axis Deviation: DDx
Top 3 Emergent Causes of New Rightward Axis Deviation: When you encounter new right axis deviation on an ECG, immediately consider the following life-threatening conditions: Acute Right Heart Strain or…
LV Aneurysm vs STEMI: DDx
Key Points Left Ventricular Aneurysm (LVA): Causes persistent ST elevation lasting more than 2 weeks post-MI, typically seen in the anterior leads but can also affect the inferior leads. Occurs…
Not Enough P Waves (P:QRS < 1): DDx
Key Points Fewer P waves than QRS complexes: This means impulses are originating from below the atria (junctional, ventricular, or paced), or from ectopy interrupting sinus rhythm. Clinical relevance: In…
No Clear P Waves: DDx
Key Points Absence of Sinus Rhythm: The lack of clear P waves before the QRS complexes suggests loss or dysfunction of sinus activity. Potential Causes: This pattern can result from…
Too Many P Waves (P:QRS>1): DDx
Key Points Non-conducted P waves (too many P waves) occur when atrial impulses fail to conduct to the ventricles. Automated ECG interpretations are unreliable in irregular rhythms and with non-conducted…
STEMI vs Severe Hyperkalemia: DDx
Key Points The ECG Chameleon: Severe hyperkalemia can produce ST segment elevations, wide QRS complexes, and conduction delays that closely resemble STEMI or ventricular tachycardia. Early misinterpretation can lead to…
Left Bundle Branch Block (LBBB): DDx
Key Point Overview: Left bundle branch block (LBBB) is an intraventricular conduction delay that results from a block in the left bundle branch of the heart’s electrical conduction system. It…
T Wave Inversion DDx
Key Points Ischemia Isn’t Everything: While T-wave inversions often raise concern for ischemia, many non-ischemic processes (e.g., pulmonary pathology, neurologic conditions, repolarization variants) can produce similar findings. Clinical Correlation: Always…
Bradycardia: DDx
Key Points Common Offenders: Beta-blockers, calcium channel blockers, and digoxin often top the list of medication-induced bradycardia. High Vagal Tone or AV Block can arise from structural heart disease, ischemia,…
Abnormal P Wave Axis: DDx
Key Points Normal Sinus Rhythm: In a normal rhythm, the sinus node initiates atrial depolarization, resulting in a P wave with a normal axis: upright in lead II, inverted in…
Low Voltage QRS: DDx
Key Points: Definition: Low voltage QRS complexes reflect decreased myocardial electrical amplitude on ECG. Sensitive Definition (Preferred): Limb leads (I + II + III) sum < 15 mm OR precordial…
Wide QRS Bradyarrhythmia: DDx
Key Point Wide QRS bradycardias usually reflect infranodal conduction disease (His–Purkinje or below), making them much more dangerous than narrow QRS bradycardias. These rhythms often lead to hemodynamic instability and…
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