A 39-year-old female with poorly controlled systemic lupus erythematosus (SLE) presents to the emergency room with a cough and pleuritic chest pain. She states that she developed these symptoms 2 days prior. The pain appears to improve when the patient leans forward. She currently takes hydroxychloroquine for her systemic lupus erythematosus but has missed several doses recently. Her temperature is 99°F (37.2°C), blood pressure is 135/80 mmHg, pulse is 115/min, and respirations are 22/min. Physical examination reveals a rise in jugular venous pressure during inspiration. In addition to tachycardia, which of the following EKG patterns is most likely to be seen in this patient?

PR depressions and diffuse ST elevations

Peaked T waves with flattened P waves

Irregularly irregular QRS complexes with no P waves

ST segment depressions in leads II, III, and aVF

Prolonged PR interval with normal QRS complexes

A researcher measures action potential propagation velocity in various regions of the heart in a 42-year-old Caucasian female. Which of the following set of measurements corresponds to the velocities found in the atrial muscle, AV Node, Purkinje system, and ventricular muscle, respectively?

1.1 m/s, 0.05 m/s, 2.2 m/s, 0.3 m/s

0.05 m/s, 1.1 m/s, 2.2 m/s, 3.3 m/s

2.2 m/s, 0.3 m/s, 0.05 m/s, 1.1 m/s

0.3 m/s, 2.2 m/s, 0.05 m/s, 1.1 m/s

0.5 m/s, 1.1 m/s, 2.2 m/s, 3 m/s

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 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

An investigator develops a new drug that decreases the number of voltage-gated potassium channels in cardiac muscle cell membranes. Which of the following is the most likely effect of this drug on the myocardial action potential?

Decreased resting membrane potential

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 55-year-old man presents to his physician with weakness and fatigue for 1 week. There is no significant past medical history. He mentions that he is very health conscious and has heard about the health benefits of juices. He is following a juice-only diet for the last 2 weeks. His physical examination is completely normal, except for depressed deep tendon reflexes. The only abnormality in a complete laboratory evaluation is a serum potassium level of 6.0 mEq/L (6.0 mmol/L). There are significantly peaked T-waves on ECG. Which of the following pathophysiologic mechanisms best explains the patient’s symptoms?

Decreased resting membrane potential of skeletal muscle cells

Prolonged release of Ca2+ ions after stimulation of Ryanodine receptors

Hyperpolarization of skeletal muscle cells

Dysfunction of dystrophin-glycoprotein complex

Bundle branch blocks for USMLE Step 2 CK: High-Yield Physiology Lessons

Cardiac Conduction - The Electrical Highway

Initiation: Sinoatrial (SA) node, the heart's primary pacemaker.
Transmission: Atrioventricular (AV) node (gatekeeper, creates PR interval) → Bundle of His.
Distribution: Splits into Right Bundle Branch (RBB) and Left Bundle Branch (LBB).
- LBB further divides into anterior and posterior fascicles.
Final Pathway: Purkinje fibers ensure rapid, coordinated ventricular depolarization.

⭐ The Bundle of His and Purkinje fibers are the fastest conducting tissues in the heart, crucial for synchronized ventricular contraction.

Right Bundle Branch Block - MaRRoW's Pattern

Wide QRS complex: Duration > 0.12 seconds.
Characteristic Pattern:
- V1/V2 (Right-sided leads): Shows an RSR' pattern, often described as "rabbit ears." This forms the 'M' of the mnemonic.
- V6, I, aVL (Left-sided leads): Displays a wide, slurred S wave. This forms the 'W' of the mnemonic.
📌 Mnemonic: MaRRoW
- M pattern in V1
- W pattern in V6

ECG patterns in Right and Left Bundle Branch Blocks

⭐ In a patient with a new RBBB and acute symptoms like shortness of breath or chest pain, consider acute right heart strain from a pulmonary embolism (PE).

Left Bundle Branch Block - WiLLiaM's Wave

ECG Criteria:
- QRS duration > 0.12 seconds.
- Dominant S wave in V1 (forming a "W" shape).
- Broad, notched, or slurred R wave in lateral leads I, aVL, V5, V6 (forming an "M" shape).
- Absence of normal septal Q waves in lateral leads.
- Appropriate discordance: ST segments and T waves are typically directed opposite to the major QRS deflection.
📌 Mnemonic: WiLLiaM
- W-shape in V1
- M-shape in V6

LBBB ECG: WiLLiaM pattern in V1 and V6

⭐ New-onset LBBB in a patient with symptoms consistent with acute coronary syndrome (e.g., chest pain) should be considered a STEMI equivalent.

Block Comparison - Clinical Crossroads

QRS Duration: >0.12s for both LBBB and RBBB.
Key Distinction: Axis deviation and morphology in precordial leads.

Feature	Left Bundle Branch Block (LBBB)	Right Bundle Branch Block (RBBB)
V1 Lead	Deep, broad S wave	RSR' pattern ("Rabbit Ears")
V6 Lead	Broad, notched R wave	Wide, slurred S wave
Clinical Imp.	Often indicates underlying pathology	Can be a normal variant

ECG: Normal, LBBB, and RBBB QRS Morphology

⭐ New-onset LBBB in the setting of acute chest pain is treated as a STEMI equivalent until proven otherwise.

High‑Yield Points - ⚡ Biggest Takeaways

Bundle Branch Blocks are defined by a wide QRS complex (> 0.12s).

RBBB shows an RsR' pattern ("bunny ears") in leads V1-V2 and a wide S wave in V6.

LBBB presents with a broad, notched R wave in V5-V6 and a deep S wave in V1.

A new LBBB is highly suspicious for myocardial infarction as it can mask ischemic changes.

LBBB often indicates underlying coronary or structural heart disease.

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