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 cardiologist is studying how a new virus that infects the heart affects the electrical conduction system of the cardiac myocytes. He decides to obtain electrocardiograms on patients with this disease in order to see how the wave patterns and durations change over time. While studying these records, he asks a medical student who is working with him to interpret the traces. Specifically, he asks her to identify the part that represents initial ventricular depolarization. Which of the following characteristics is most consistent with this feature of the electrocardiogram?

Normal duration defined as less than 120 milliseconds

Elevated in patients with full thickness ischemic injury of the heart

Becomes peaked in states of hyperkalemia

Becomes prominent in states of hypokalemia

Normal duration defined as less than 200 milliseconds

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

A 42-year-old Caucasian woman is enrolled in a randomized controlled trial to study cardiac function in the setting of several different drugs. She is started on verapamil and instructed to exercise at 50% of her VO2 max while several cardiac parameters are being measured. During this experiment, which of the following represents the relative conduction speed through the heart from fastest to slowest?

Purkinje fibers > atria > ventricles > AV node

Purkinje fibers > ventricles > atria > AV node

Atria > Purkinje fibers > ventricles > AV node

AV node > ventricles > atria > Purkinje fibers

Purkinje fibers > AV node > ventricles > atria

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

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 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

Bradyarrhythmias and conduction disorders for USMLE Step 3: High-Yield Internal Medicine Lessons

Conduction System - The Heart's Electrical Grid

Heart Electrical Conduction System with ECG

Pacemaker Hierarchy: The heart's electrical signal originates and travels through specialized tissue.
- Sinoatrial (SA) node: Primary pacemaker (60-100 bpm).
- Atrioventricular (AV) node: Gatekeeper to ventricles (40-60 bpm).
- His-Purkinje system: Final pathway (20-40 bpm).

⭐ The AV node physiologically delays the impulse (~0.12s), allowing ventricles to fill completely before contraction. This delay creates the PR interval on an ECG.

AV Blocks - Communication Breakdown

Pathophysiology: Delay or interruption of the electrical signal from the atria to the ventricles at the AV node or His-Purkinje system.

ECG: Mobitz I, Mobitz II, and 2:1 AV Blocks

Type	ECG Findings	Clinical Significance
1st Degree	Fixed, prolonged PR interval (>0.20s). Every P wave is followed by a QRS.	Usually benign and asymptomatic.
2nd Degree (Mobitz I)	Progressive PR lengthening until a QRS complex is dropped.	📌 "Longer, longer, drop, then you have a Wenckebach." Low risk.
2nd Degree (Mobitz II)	Constant PR interval with intermittent, unexpected dropped QRS complexes.	High risk of progression to 3rd-degree block. Pacemaker indicated.
3rd Degree (Complete)	Complete AV dissociation; P waves and QRS complexes act independently.	Medical emergency. Requires immediate pacing due to risk of asystole.

Bundle Branch Blocks - Signal Detours

Pathophysiology: Conduction delay or block in the right or left bundle branch, widening the QRS complex to > 0.12s.
Right Bundle Branch Block (RBBB):
- ECG: RSR' pattern ("rabbit ears") in leads V1-V2; wide, slurred S wave in leads I, aVL, V5-V6.
- Often benign in asymptomatic individuals.
Left Bundle Branch Block (LBBB):
- ECG: Broad, notched, or slurred R wave in I, aVL, V5-V6; deep S wave in V1-V3.
- Often indicates underlying cardiac disease.

⭐ A new LBBB in the context of chest pain should be treated as a STEMI equivalent.

📌 Mnemonic (WiLLiaM MaRRoW):

In LBBB, see W-shape in V1 & M-shape in V6.
In RBBB, see M-shape in V1 & W-shape in V6.

ECG: Normal, LBBB, and RBBB patterns compared

Management - When to Shock or Pace

Assess Stability: First, determine if bradycardia is causing hemodynamic instability (hypotension, altered mental status, shock, chest pain, acute heart failure).

⭐ For high-degree AV blocks (Mobitz II, Third-Degree), transcutaneous pacing should be immediately prepared and is preferred over atropine, which may be ineffective or even paradoxical.

High‑Yield Points - ⚡ Biggest Takeaways

Sick Sinus Syndrome often presents as tachy-brady syndrome, causing syncope or dizziness in the elderly.

Mobitz II and Third-Degree (Complete) AV Block are high-risk for asystole and require a permanent pacemaker.

Mobitz I (Wenckebach) shows progressive PR prolongation before a dropped QRS; it is typically benign.

Third-Degree Block is defined by complete AV dissociation (P waves and QRS complexes are independent).

Atropine is first-line for symptomatic bradycardia; use transcutaneous pacing if unresponsive or for high-degree blocks.

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