What are the characteristic ECG (electrocardiogram) findings for premature ventricular beats, hypokalemia (low potassium levels), hyperkalemia (high potassium levels), atrial fibrillation, hypermagnesemia (elevated magnesium levels), hypomagnesemia (low magnesium levels), acute pericarditis, Brugada's syndrome, STEMI (ST-elevation myocardial infarction), hypertrophic obstructive cardiomyopathy, and bundle branch blocks?

ECG Characteristics for Diagnosing Cardiac Conditions

To accurately diagnose these conditions on ECG, focus systematically on QRS morphology and width, ST-segment changes, T-wave abnormalities, P-wave presence/morphology, and specific lead patterns as outlined below.

Premature Ventricular Beats (PVCs)

Look for wide QRS complexes (>120 ms) occurring earlier than expected, without preceding P waves, followed by a compensatory pause. 1

• Examine all 12 leads, as PVCs can originate from any ventricular location 1
• The QRS morphology differs from normal sinus beats and appears bizarre 1
• If you identify >2,000 PVCs in 24 hours on monitoring, perform comprehensive cardiac evaluation including echocardiography and cardiac MRI to exclude cardiomyopathy 1

Atrial Fibrillation

Atrial fibrillation shows irregularly irregular RR intervals with absent P waves, replaced by chaotic fibrillatory waves. 1

• Focus on leads V1, II, III, and aVF where fibrillatory waves are best visualized 1
• Measure ventricular rate over 6 seconds and multiply by 10 for average rate 1
• The absence of organized P waves distinguishes this from other atrial arrhythmias 1

Electrolyte Abnormalities

Hypokalemia

Hypokalemia produces prominent U waves, QT prolongation, and predisposes to torsades de pointes. 1

• Focus on precordial leads where changes are most evident 1
• Look for flattened or inverted T waves with prominent U waves 1
• Critical caveat: Hypokalemia (even at 2.9 mmol/L) can unmask Type 1 Brugada pattern ECG with coved ST-segment elevations in V1-V2, which resolves with potassium correction 2

Hyperkalemia

Hyperkalemia creates peaked, narrow-based "tented" T waves initially, progressing to widened QRS, flattened P waves, and potentially Brugada-like patterns in severe cases. 1

• Early changes: peaked T waves visible in multiple leads 3, 4
• Progressive changes: PR prolongation, P wave flattening, QRS widening 4
• Critical caveat: Severe hyperkalemia can produce ST-segment elevation mimicking Brugada pattern or pseudomyocardial infarction, which resolves with potassium correction 3, 4

Hypomagnesemia

Hypomagnesemia causes QT prolongation, prominent U waves, and predisposes to torsades de pointes. 1

• Changes are most evident in precordial leads 1
• Often coexists with hypokalemia, compounding arrhythmia risk 1

Hypermagnesemia

Hypermagnesemia produces prolonged PR and QT intervals with widened QRS, though ECG changes are less specific than other electrolyte disorders. 1

Acute Pericarditis

Acute pericarditis shows diffuse ST elevation with upward concavity (saddle-shaped) in multiple leads, accompanied by PR depression, without reciprocal ST depression. 1

• Look for widespread ST elevation with upward concavity in leads I, II, aVL, aVF, V2-V6 1
• PR depression occurs in most leads except aVR (which shows PR elevation) 1
• Key distinguishing feature: The absence of reciprocal ST depression differentiates pericarditis from STEMI 1
• The ST elevation has upward concavity ("saddle-shaped"), unlike the convex ST elevation of STEMI 1

Brugada Syndrome

Brugada syndrome Type 1 pattern shows coved ST elevation ≥2 mm in V1-V2 with downsloping ST segment and inverted T wave. 5, 1

• Focus specifically on leads V1-V2 in the 4th intercostal space 1
• The pattern shows high take-off ST-segment elevation ≥2 mm with downsloping ST-segment elevation followed by a negative symmetric T wave 5
• Often accompanied by right bundle branch block pattern 1
• Critical diagnostic maneuver: Record V1-V2 in the 2nd intercostal space to unmask the Brugada pattern if initially negative 1
• Important caveat: Both hypokalemia and hyperkalemia can induce reversible Brugada-like patterns that resolve with electrolyte correction 2, 3, 4

STEMI (ST-Elevation Myocardial Infarction)

STEMI requires ST elevation at the J-point in two or more contiguous leads: ≥0.1 mV in all leads except V2-V3, where thresholds are higher and age/sex-dependent. 1

Inferior STEMI

• Look for ST elevation in leads II, III, and aVF 1
• These are contiguous inferior leads 1

Lateral STEMI

• Look for ST elevation in leads I, aVL, V5, and V6 1
• Lateral changes often accompany anterior or inferior infarctions 1

Anterior STEMI

• Look for ST elevation in precordial leads V1-V4 1
• V2-V3 have higher thresholds for ST elevation (age and sex-dependent) 1

Obtain ECG within 10 minutes of presentation and repeat every 15-30 minutes if initially non-diagnostic. 1

Hypertrophic Obstructive Cardiomyopathy (HOCM)

HOCM shows deep, narrow Q waves in lateral and inferior leads, giant inverted T waves in precordial leads, and voltage criteria for left ventricular hypertrophy. 1

• Focus on lateral leads (I, aVL, V5-V6) for Q waves and ST-T changes 1
• Look for giant T wave inversion in V2-V4 1
• Pathological Q waves defined as Q/R ratio ≥0.25 or ≥40 ms duration in two or more contiguous leads (except III and aVR) 5
• Important distinction: Deep lateral or inferior Q waves in athletes with physiological LVH can be normal, requiring Q/R ratio assessment 5

Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

RBBB shows QRS ≥120 ms with RSR' pattern ("M-shaped") in V1-V2 and wide S waves in lateral leads I, aVL, V5-V6. 1

• Focus on V1-V2 for the characteristic "M-shaped" or RSR' pattern 1
• Examine lateral leads (I, aVL, V5-V6) for wide S waves 1

Left Bundle Branch Block (LBBB)

LBBB shows broad, notched R waves in lateral leads (I, aVL, V5-V6) and deep S waves in V1-V2, without Q waves in lateral leads. 1

• QRS duration ≥120 ms 1
• Absence of Q waves in lateral leads is a key feature 1
• Critical clinical point: LBBB is found in <1 in 1,000 athletes but is common in patients with cardiomyopathy and ischemic heart disease, requiring thorough investigation with echocardiography and cardiac MRI with perfusion study 5