A 21-year-old woman presents with palpitations and anxiety. She had a recent outpatient ECG that was suggestive of supraventricular tachycardia, but her previous physician failed to find any underlying disease. No other significant past medical history. Her vital signs include blood pressure 102/65 mm Hg, pulse 120/min, respiratory rate 17/min, and temperature 36.5℃ (97.7℉). Electrophysiological studies reveal an atrioventricular nodal reentrant tachycardia. The patient refuses an ablation procedure so it is decided to perform synchronized cardioversion with consequent ongoing management with verapamil. Which of the following ECG features should be monitored in this patient during treatment?

Amplitude and direction of the T wave

A 71-year-old man is brought to the emergency department by his wife because of increasing confusion, weakness, and vomiting for 1 day. He has had 5 episodes of vomiting and blurry vision; he told his wife that "everything appears in different colors." He has been unable to recall his wife's name or their address. His wife reports that his drug regimen was adjusted because of worsening tibial edema 1 week ago. He has congestive heart failure, atrial fibrillation, hypothyroidism, and osteoarthritis. Current medications include rivaroxaban, metoprolol, digoxin, levothyroxine, spironolactone, and furosemide. His temperature is 36.7°C (98°F), pulse is 56/min, and blood pressure is 98/68 mm Hg. He is confused and oriented only to person. Neurologic examination shows no focal findings. The abdomen is soft, and there is tenderness to palpation of both lower quadrants without guarding or rebound. There is 1+ pitting edema of both ankles. This patient is most likely to have which of the following ECG findings?

Mobitz type 2 atrioventricular block

A 72-year-old man comes to the physician because of a 2-month history of intermittent retrosternal chest pain and tightness on exertion. He has type 2 diabetes mellitus, osteoarthritis of the right hip, and hypertension. Current medications include insulin, ibuprofen, enalapril, and hydrochlorothiazide. Vital signs are within normal limits. His troponin level is within the reference range. An ECG at rest shows a right bundle branch block and infrequent premature ventricular contractions. The patient's symptoms are reproduced during adenosine stress testing. Repeat ECG during stress testing shows new ST depression of > 1 mm in leads V2, V3, and V4. Which of the following is the most important underlying mechanism of this patient's ECG changes?

Diversion of blood flow from stenotic coronary arteries

Transient atrioventricular nodal blockade

Reduced left ventricular preload

Ruptured cholesterol plaque within a coronary vessel

Increased myocardial oxygen demand

A 41-year-old woman presents to the emergency room with chest pain. She has had progressive substernal chest pain accompanied by weakness and mild shortness of breath for the past 2 hours. Her past medical history is notable for poorly controlled systemic lupus erythematosus (SLE), Sjogren syndrome, and interstitial lung disease. She was hospitalized last year with pericarditis presumed to be from SLE. Her temperature is 98.6°F (37°C), blood pressure is 106/56 mmHg, pulse is 132/min, and respirations are 26/min. On exam, the skin overlying the internal jugular vein fills at 9 cm above the sternal angle and distant heart sounds are appreciated. There is no friction rub. She is given 1000cc of intravenous fluids with no appreciable change in her blood pressure. An electrocardiogram in this patient would most likely reveal which of the following findings?

ST elevations in leads II, III, and aVF

Wide QRS complexes with no P waves

A 23-year-old man comes to the emergency department because of palpitations, dizziness, and substernal chest pain for three hours. The day prior, he was at a friend’s wedding, where he consumed seven glasses of wine. The patient appears diaphoretic. His pulse is 220/min and blood pressure is 120/84 mm Hg. Based on the patient's findings on electrocardiography, the physician diagnoses atrial fibrillation with rapid ventricular response and administers verapamil for rate control. Ten minutes later, the patient is unresponsive and loses consciousness. Despite resuscitative efforts, the patient dies. Histopathologic examination of the heart at autopsy shows an accessory atrioventricular conduction pathway. Electrocardiography prior to the onset of this patient's symptoms would most likely have shown which of the following findings?

Slurred upstroke of the QRS complex

Epsilon wave following the QRS complex

Prolongation of the QT interval

Cyclic alteration of the QRS axis

Two days after undergoing an uncomplicated total thyroidectomy, a 63-year-old woman has acute, progressive chest pain. The pain is sharp and burning. She feels nauseated and short of breath. The patient has a history of hypertension, type 1 diabetes mellitus, medullary thyroid cancer, multiple endocrine neoplasia type 2A, anxiety, coronary artery disease, and gastroesophageal reflux disease. She smoked half a pack of cigarettes daily for 24 years but quit 18 years ago. Current medications include lisinopril, insulin glargine, insulin aspart, sertraline, aspirin, ranitidine, and levothyroxine. She appears anxious and diaphoretic. Her temperature is 37.4°C (99.3°F), pulse is 64/min, respirations are 17/min, and blood pressure is 148/77 mm Hg. The lungs are clear to auscultation. Examination shows a 3-cm linear incision over the anterior neck with 1 mm of surrounding erythema and mild serous discharge. The chest wall and abdomen are nontender. There is 5/5 strength in all extremities and decreased sensation to soft touch on the feet bilaterally. The remainder of the examination shows no abnormalities. Which of the following is the most appropriate next step in management?

Obtain an ECG and troponin T levels

Administer IV pantoprazole and schedule endoscopy

Discontinue levothyroxine and obtain fT4 levels

Administer IV levofloxacin and obtain chest radiograph

Obtain urine and plasma metanephrine levels

A 40-year-old woman comes to the physician for a 6-month history of recurrent episodes of chest pain, racing pulse, dizziness, and difficulty breathing. The episodes last up to several minutes. She also reports urinary urgency and two episodes of loss of consciousness followed by spontaneous recovery. There is no personal or family history of serious illness. She does not smoke or drink alcohol. Vitals signs are within normal limits. Cardiopulmonary examination shows no abnormalities. Holter monitoring is performed. ECG recordings during episodes of tachycardia show a QRS duration of 100 ms, regular RR-interval, and absent P waves. Which of the following is the most likely underlying cause of this patient's condition?

AV node with slow and fast pathway

Pre-excitation of the ventricles

Mutations in genes that code for myocyte ion channels

Macroreentrant rhythm in the right atria through cavotricuspid isthmus

Fibrosis of the sinoatrial node and surrounding myocardium

ECG interpretation fundamentals for USMLE Step 2 CK: High-Yield Internal Medicine Lessons

ECG Basics - Heartbeat's Signature

ECG waveform with labeled waves, segments, and intervals

P wave: Atrial depolarization.
PR interval: AV conduction. Normal: 0.12-0.20s (3-5 small squares).
QRS complex: Ventricular depolarization. Normal: < 0.12s.
T wave: Ventricular repolarization.
QT interval: Total ventricular electrical activity. Varies with heart rate (use QTc).
Rate (Regular Rhythm): $300 / (\text{large squares between R waves})$.
Axis: Check Lead I & aVF. Normal is positive in both.

⭐ A prolonged QTc interval (men > 450ms, women > 470ms) is a critical risk factor for Torsades de Pointes.

Rate, Rhythm, & Axis - The Heart's Compass

Rate: For regular rhythms, use the formula: $Rate = 300 / (# \text{of large squares between R-R})$. Sequence: 300-150-100-75-60-50. For irregular, count R waves in a 6-second strip x 10.
Rhythm: Check if R-R intervals are regular (constant) or irregular.
Axis: Use Leads I & aVF to determine the quadrant.
- 📌 Mnemonic: "Two Thumbs Up" (I & aVF both positive) = Normal Axis.

ECG Hexaxial Reference System for Axis Deviation

⭐ The most common pathologic cause of Left Axis Deviation (LAD) is a Left Anterior Fascicular Block (LAFB).

Intervals & Blocks - Conduction Traffic Jams

PR Interval: Atrial to ventricular depolarization. Normal: <0.2s (5 small squares).
- Prolonged: AV block.
QRS Duration: Ventricular depolarization. Normal: <0.12s (3 small squares).
- Widened: Bundle branch blocks, ventricular rhythms.
QT Interval: Ventricular depolarization & repolarization. Rate-dependent (use QTc).
- Prolonged: ↑ risk of Torsades de Pointes.

ECG: 2nd Degree Type 1 Heart Block (Wenckebach)

AV Blocks (📌 Mnemonic Poem):
- 1st Degree: If the R is far from P. (PR >0.2s)
- 2nd Degree (Mobitz I): Longer, longer, longer, DROP! (Wenckebach)
- 2nd Degree (Mobitz II): If some P's don't get through. (Constant PR)
- 3rd Degree (Complete): If P's and Q's don't agree. (AV dissociation)

⭐ Mobitz II block has a high risk of progressing to a 3rd-degree block and often requires pacemaker implantation.

Hypertrophy & Enlargement - Swole Heart Problems

Atrial Enlargement: Focus on P-wave morphology.
- Right (RAE / P-pulmonale): Tall, peaked P-waves >2.5 mm in lead II.
- Left (LAE / P-mitrale): Broad, notched P-wave (≥0.12s) in lead II; biphasic P in V1 with a large negative deflection.
Ventricular Hypertrophy: Assesses QRS voltage & axis.
- Right (RVH): Right axis deviation, dominant R-wave in V1 (R>S), deep S-wave in V6.
- Left (LVH): Sokolow-Lyon criteria: S in V1 + R in V5/V6 >35 mm. Often shows a "strain pattern" (ST depression & T-wave inversion) in lateral leads.

ECG changes in left and right ventricular hypertrophy

⭐ The LVH "strain pattern" is a key finding; it indicates significant myocyte stress and is associated with a worse prognosis than voltage criteria alone.

High‑Yield Points - ⚡ Biggest Takeaways

Rate & Rhythm: Normal sinus is 60-100 bpm; ensure a P wave precedes every QRS.

Axis: Use Leads I & aVF. Upright in both is normal. Left axis: I(+) aVF(-). Right axis: I(-) aVF(+).

Intervals: PR (120-200 ms) reflects AV conduction. QRS (<120 ms) shows ventricular depolarization.

Key Pathologies: Long PR suggests AV block. Wide QRS indicates BBB or ventricular origin. Long QTc risks Torsades.

Ischemia/Infarct: Look for ST elevation (injury), ST depression (ischemia), and pathologic Q waves (prior infarct).

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