Cardiac muscle serves many necessary functions, leading to a specific structure that serves these functions. The structure highlighted is an important histology component of cardiac muscle. What would be the outcome if this structure diffusely failed to function?

Failure of propagation of the action potential from the conduction system

Failure of potassium channels to appropriately open to repolarize the cell

Ineffective excitation-contraction coupling due to insufficient calcium ions

Inappropriate formation of cardiac valve leaflets

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 58-year-old man presents to the emergency department for evaluation of intermittent chest pain over the past 6 months. His history reveals that he has had moderate exertional dyspnea and 2 episodes of syncope while working at his factory job. These episodes of syncope were witnessed by others and lasted roughly 30 seconds. The patient states that he did not have any seizure activity. His vital signs include: blood pressure 121/89 mm Hg, heart rate 89/min, temperature 37.0°C (98.6°F), and respiratory rate 16/min. Physical examination reveals a crescendo-decrescendo systolic murmur in the right second intercostal area. An electrocardiogram is performed, which shows left ventricular hypertrophy. Which of the following is the best next step for this patient?

Computed tomography (CT) chest scan without contrast

Cardiac chamber catheterization

Transesophageal echocardiography

An abnormal wave is noted on a routine ECG. The wave in question represents which of the following electrical events in the cardiac cycle?

Period between ventricular depolarization and repolarization

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

Cardiac axis determination for USMLE Step 2 CK: High-Yield Physiology Lessons

Cardiac Axis - The Heart's Compass

Definition: Net direction of ventricular depolarization, viewed from the frontal plane.
Normal Axis: -30° to +90°. The heart's electrical "compass" points down and to the left.
Quick Look Method: Check QRS deflection in Leads I & aVF.
- Normal: Positive in both I & aVF.
- 📌 Mnemonic: "Left is Leaving, Right is Reaching." If QRS in I & aVF leave each other (one positive, one negative) = LAD. If they reach for each other (both positive) = Normal. If they reach away (both negative) = Extreme RAD.

Cardiac Axis Determination using Lead I and aVF Quadrants

⭐ The most common cause of left axis deviation (LAD) in adults is a left anterior fascicular block (LAFB).

Quadrant Method - Two Thumbs Up

Rapidly estimates the heart's electrical axis by checking the net QRS deflection in just two leads: Lead I and Lead aVF.
📌 Two Thumbs-Up Rule:
- A positive QRS in Lead I is your left thumb pointing up (to the right).
- A positive QRS in Lead aVF is your right thumb pointing up (downwards).
Normal Axis (-30° to +90°): Lead I (+) and aVF (+) → Two thumbs up!
Left Axis Deviation (LAD): Lead I (+) and aVF (-) → Thumbs are "leaving" each other.
Right Axis Deviation (RAD): Lead I (-) and aVF (+) → Thumbs are "reaching" for each other.
Extreme Axis: Lead I (-) and aVF (-) → Two thumbs down.

⭐ The most common cause of right axis deviation (RAD) is Right Ventricular Hypertrophy (RVH), often secondary to chronic pulmonary disease (e.g., COPD).

ECG Cardiac Axis Quadrant Method Diagram

Isoelectric Lead - Pinpoint Precision

This two-step method offers a more exact axis calculation than the quadrant approach.
Step 1: Find the limb lead where the QRS complex is most isoelectric (biphasic). This means the positive (R) and negative (S) deflections are nearly equal, resulting in a net amplitude of zero.
Step 2: The cardiac axis is perpendicular ($90^\circ$) to this isoelectric lead. To determine the final direction, find the perpendicular lead and check its polarity. A positive QRS in the perpendicular lead confirms the axis direction.

⭐ A perfectly isoelectric lead is rare. This technique is most valuable for precisely locating an axis that falls on the border between two quadrants, such as confirming a left axis deviation at exactly -30° when Lead II is isoelectric.

Axis Deviations - Clinical Clues

Left Axis Deviation (LAD): Normal variant in obesity/pregnancy.
- Left Ventricular Hypertrophy (LVH)
- Left Bundle Branch Block (LBBB)
- Inferior wall Myocardial Infarction (MI)
- Wolff-Parkinson-White (WPW) syndrome
Right Axis Deviation (RAD): Normal in children/tall, thin adults.
- Right Ventricular Hypertrophy (RVH)
- Right Bundle Branch Block (RBBB)
- Anterolateral wall MI
- Chronic Obstructive Pulmonary Disease (COPD)
- Pulmonary Embolism (PE)

⭐ New onset RBBB with RAD is highly suggestive of a massive pulmonary embolism.

High‑Yield Points - ⚡ Biggest Takeaways

Normal axis is -30° to +90°. A quick check is a positive QRS in both Lead I and aVF.

Left Axis Deviation (LAD): Positive QRS in Lead I, negative in aVF. Key causes include LVH, LBBB, and inferior MI.

Right Axis Deviation (RAD): Negative QRS in Lead I, positive in aVF. Suspect RVH or acute pulmonary embolism.

Extreme Axis Deviation: Negative in both Lead I and aVF. Associated with ventricular tachycardia.

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