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

An ECG from an 8-year-old male with neurosensory deafness and a family history of sudden cardiac arrest demonstrates QT-interval prolongation. Which of the following is this patient most at risk of developing?

First degree atrioventricular block

A 40-year-old man is brought to the emergency department 20 minutes after his wife found him unconscious on the bathroom floor. On arrival, he is conscious and alert. He remembers having palpitations and feeling lightheaded and short of breath before losing consciousness. He takes captopril for hypertension and glyburide for type 2 diabetes mellitus. His vitals are within normal limits. Physical examination shows no abnormalities. Random serum glucose concentration is 85 mg/dL. An ECG shows a short PR interval and a wide QRS complex with initial slurring. Transthoracic echocardiography reveals normal echocardiographic findings with normal left ventricular systolic function. Which of the following is the most likely underlying cause of this patient's findings?

Accessory atrioventricular pathway

Ectopic foci within the ventricles

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

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 17-year-old girl suddenly grabs her chest and collapses to the ground while playing volleyball at school. The teacher rushes to evaluate the situation and finds that the girl has no pulse and is not breathing. He starts chest compressions. An automated external defibrillator (AED) is brought to the scene within 3 minutes and a shock is delivered. The girl regains consciousness and regular sinus rhythm. She is rushed to the emergency department. The vital signs include: blood pressure 122/77 mm Hg and pulse 65/min. The pulse is regular. An electrocardiogram (ECG) shows a shortened PR interval, a wide QRS complex, a delta wave, and an inverted T wave. Which of the following is the most likely pathology in the conduction system of this patient’s heart?

Accessory pathway from atria to ventricles

Impulse generation by tissue in atrioventricular node

Automatic discharge of irregular impulses in the atria

A 23-year-old man comes to the physician with a 1-week history of sharp, substernal chest pain that is worse with inspiration and relieved with leaning forward. He has also had nausea and myalgias. His father has coronary artery disease. His temperature is 37.3°C (99.1°F), pulse is 110/min, and blood pressure is 130/84 mm Hg. Cardiac examination shows a high-pitched rubbing sound between S1 and S2 that is best heard at the left sternal border. An ECG shows depressed PR interval and diffuse ST elevations. What is the most likely diagnosis?

Mycobacterium tuberculosis infection

A 65-year-old woman comes to the physician because of a 3-month history of intermittent palpitations and shortness of breath. Cardiopulmonary examination shows no other abnormalities. An ECG shows an absence of P waves, an oscillating baseline, and irregular RR intervals at a rate of approximately 95 beats per minute. The difference between atrial and ventricular rates in this patient is most likely due to which of the following?

Temporary inactivation of Na+ channels in the AV node

Prolonged influx through voltage-gated Ca2+ channels in the bundle of His

Transient activation of K+ current in Purkinje fibers

Inhibition of the Na+/K+-ATPase pump in ventricular cells

Limited speed of conduction through the left bundle branch

PR interval significance for USMLE Step 2 CK: High-Yield Physiology Lessons

PR Interval - The Basics

Represents the time from the start of atrial depolarization to the start of ventricular depolarization.
Components: P wave + PR segment.
Normal duration: $0.12 - 0.20$ seconds (3-5 small squares on ECG paper).
Clinically, it reflects the conduction time through the AV node.

ECG Waveform with PR Interval and Segment Highlighted

⭐ A consistently short PR interval (<0.12s) may suggest a pre-excitation syndrome, like Wolff-Parkinson-White (WPW), due to an accessory pathway bypassing the AV node.

Short PR Interval - Speedy Conduction

A PR interval < 0.12s indicates accelerated atrioventricular conduction, often by bypassing the AV node's physiological delay. This suggests ventricular pre-excitation.

Key Causes:
- Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway (Bundle of Kent) directly connects the atria to the ventricles. Look for the classic triad: short PR, a wide QRS, and a slurred upstroke on the QRS complex (Delta Wave).
- Lown-Ganong-Levine (LGL) Syndrome: An accessory pathway (James fibers) bypasses the AV node and connects to the His bundle. Results in a short PR but a normal QRS complex.
- Junctional Rhythm: The electrical impulse originates from the AV junction, leading to a shorter conduction time to the ventricles.

📌 Mnemonic: Wolves Pounce Low (WPW, P-Junctional, LGL).

ECG: Wolff-Parkinson-White syndrome

⭐ Exam Favorite: In a patient with WPW who develops atrial fibrillation, AV nodal blocking agents (adenosine, β-blockers, CCBs) are contraindicated. They can accelerate conduction through the accessory pathway, potentially precipitating ventricular fibrillation.

Long PR Interval - Conduction Delays

A PR interval exceeding 0.20 seconds (one large box on ECG) signifies a delay in the electrical impulse traveling from the atria to the ventricles. This is the defining feature of atrioventricular (AV) blocks.

ECG showing prolonged PR interval (First-Degree AV Block)

First-Degree AV Block:
- Characterized by a constant, prolonged PR interval > 0.20s.
- A 1:1 relationship between P waves and QRS complexes is maintained.
- Usually benign.
Second-Degree AV Block: Involves intermittently dropped QRS complexes.
- Mobitz I (Wenckebach): Progressive PR interval lengthening across several beats, culminating in a non-conducted P wave (a "dropped" beat).
  - 📌 Mnemonic: "Longer, longer, longer, DROP! Now you have a Wenckebach."
- Mobitz II: The PR interval remains constant for all conducted beats, but some P waves are unexpectedly not followed by a QRS.

⭐ Mobitz II is considered more clinically significant than Mobitz I because it carries a higher risk of progressing to a complete heart block. It often points to structural damage below the AV node.

The PR interval reflects AV conduction time; normal duration is 0.12-0.20 seconds.

A short PR interval (<0.12s) suggests pre-excitation (e.g., Wolff-Parkinson-White syndrome).

A prolonged PR interval (>0.20s) indicates an AV conduction block.

First-degree AV block: Consistently prolonged PR interval without dropped beats.

Second-degree AV block: Intermittent non-conduction of P waves (dropped QRS complexes).

Third-degree AV block: Complete AV dissociation; P waves and QRS complexes are independent.

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