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 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 48-year-old female comes into the ER with chest pain. An electrocardiogram (EKG) shows a heart beat of this individual in Image A. The QRS complex best correlates with what part of the action potential of the ventricular myocyte shown in Image B?

Phase 0, which is primarily characterized by sodium influx

Phase 1, which is primarily characterized by potassium and chloride efflux

Phase 0, which is primarily characterized by potassium efflux

Phase 3, which is primarily characterized by potassium efflux

Phase 1, which is primarily characterized by calcium efflux

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 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 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

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

QRS complex analysis for USMLE Step 1: High-Yield Physiology Lessons

QRS Fundamentals - The Ventricular Voice

Represents: Ventricular depolarization.
Normal Duration: <0.12 seconds (3 small squares).
Components:
- Q wave: First negative deflection. Pathological if deep or wide.
- R wave: First positive deflection.
- S wave: Negative deflection following the R wave.
Axis: Overall direction of depolarization in the frontal plane; normal is -30° to +90°.

ECG Waveform Components and Intervals

⭐ A wide QRS (>0.12s) is a critical finding. It implies the electrical impulse originated in the ventricles or is conducted abnormally (e.g., Bundle Branch Block), a slower pathway than the normal His-Purkinje system.

Cardiac Axis - The Heart's Compass

Normal Axis: -30° to +90°. Represents the net direction of ventricular depolarization.
Quick Look Method: Check QRS deflection in Leads I & aVF.

Hexaxial reference system for QRS axis interpretation

LAD Causes:
- Left ventricular hypertrophy (LVH)
- Left bundle branch block (LBBB)
- Inferior MI
RAD Causes:
- Right ventricular hypertrophy (RVH)
- Pulmonary embolism
- Lateral MI

⭐ 📌 Two Thumbs-Up Rule: If the QRS in Lead I and aVF are both upright (positive), the axis is normal. Like two thumbs pointing up!

QRS Morphology - Size & Shape Clues

Amplitude (Voltage)
- Low Voltage: Limb leads < 5 mm; Precordial < 10 mm. Seen in obesity, COPD, pericardial effusion, hypothyroidism.
- Left Ventricular Hypertrophy (LVH): Sokolow-Lyon: S in V1 + R in V5/V6 > 35 mm.
Pathological Q Waves
- Wider than 0.04s or >25% of R-wave height.
- Signifies prior myocardial infarction.
Key Morphologies
- Delta Wave: Slurred QRS upstroke. Pathognomonic for Wolff-Parkinson-White (WPW).
- Bundle Branch Blocks (BBB):
  - 📌 WiLLiaM MaRRoW:
  - LBBB: W-shape in V1, M-shape in V6.
  - RBBB: M-shape in V1, W-shape in V6.

Normal vs. LBBB and RBBB QRS Complex Morphologies

⭐ A new LBBB in the context of chest pain is considered a STEMI equivalent until proven otherwise.

Wide QRS - The Slowpokes

Definition: QRS duration > 0.12 seconds (3 small squares). Indicates delayed ventricular depolarization, as the impulse travels through a slower pathway.
Key Causes & Morphology:
- Bundle Branch Block (BBB):
  - RBBB: Look for RsR' ("rabbit ears") in V1-V2 and a wide, slurred S wave in leads I and V6. 📌 Mnemonic: MaRRoW
  - LBBB: Broad, notched R waves in I, aVL, V5-V6. Broad, deep S waves in V1-V2. 📌 Mnemonic: WiLLiaM
- Ventricular Rhythms: Ventricular tachycardia, idioventricular rhythms, or paced rhythms.
- Other: Hyperkalemia, drug toxicity (e.g., TCAs, Class IA/IC antiarrhythmics).

ECG: Normal, LBBB, and RBBB in precordial leads

⭐ In a patient presenting with chest pain, a new or presumed new LBBB is treated as a STEMI equivalent until proven otherwise.

High‑Yield Points - ⚡ Biggest Takeaways

A wide QRS (>0.12s) implies a ventricular origin or an aberrant conduction, such as a bundle branch block.

Axis deviation provides key clues: LAD can indicate LVH or LAFB, while RAD suggests RVH or LPFB.

Poor R wave progression in the precordial leads is a significant sign of a prior anterior MI.

Pathological Q waves are markers of irreversible myocardial necrosis from a previous infarction.

LBBB shows a dominant S in V1; RBBB presents with an RsR' pattern in V1.

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