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?
Q2
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?
Q3
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?
Q4
A researcher is studying how electrical activity propagates across the heart. In order to do this, he decides to measure the rate at which an action potential moves within various groups of cardiac muscle tissue. In particular, he isolates fibers from areas of the heart with the following characteristics:
A) Dysfunction leads to fixed PR intervals prior to a dropped beat
B) Dysfunction leads to increasing PR intervals prior to a dropped beat
C) Dysfunction leads to tachycardia with a dramatically widened QRS complex
D) Dysfunction leads to tachycardia with a sawtooth pattern on electrocardiogram
Which of the following is the proper order of these tissues from fastest action potential propagation to slowest action potential propagation.
Q5
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?
Q6
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?
Q7
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?
ECG interpretation US Medical PG Practice Questions and MCQs
Question 1: 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?
A. Delayed repolarization (Correct Answer)
B. Delayed depolarization
C. Accelerated repolarization
D. Decreased resting membrane potential
E. Accelerated depolarization
Explanation: ***Delayed repolarization***
- **Voltage-gated potassium channels** are primarily responsible for the efflux of potassium ions during the **repolarization phase** (phase 3) of the cardiac action potential.
- A decrease in the number of these channels would reduce potassium efflux, thus slowing down the repolarization process and prolonging the **action potential duration**.
*Delayed depolarization*
- **Depolarization** (phase 0) of the cardiac action potential is primarily mediated by the rapid influx of **sodium ions** through voltage-gated sodium channels.
- Changes in potassium channels do not directly affect the speed of depolarization.
*Accelerated repolarization*
- Accelerated repolarization would occur if there were an *increase* in the number or activity of **potassium channels**, leading to a faster efflux of potassium ions.
- A *decrease* in these channels would have the opposite effect.
*Decreased resting membrane potential*
- The **resting membrane potential** is primarily maintained by the **leak potassium channels** and the **Na+/K+ ATPase pump**, not directly by voltage-gated potassium channels involved in repolarization.
- A decrease in voltage-gated potassium channels would not significantly alter the resting membrane potential.
*Accelerated depolarization*
- Accelerated depolarization would result from an *increase* in the speed or magnitude of **sodium influx** during phase 0.
- A reduction in potassium channels has no direct impact on the rate of sodium channel activation or current.
Question 2: 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?
A. Impulse generation by tissue in atrioventricular node
B. Accessory pathway from atria to ventricles (Correct Answer)
C. Automatic discharge of irregular impulses in the atria
D. Wandering atrial pacemaker
E. Blockage in conduction pathway
Explanation: ***Accessory pathway from atria to ventricles***
- The ECG findings of a **shortened PR interval**, **delta wave**, and **wide QRS complex** are characteristic of **Wolff-Parkinson-White (WPW) syndrome**, which involves an **accessory pathway** (Bundle of Kent) bypassing the AV node.
- This accessory pathway allows for pre-excitation of the ventricles, predisposing patients to **tachyarrhythmias** like the one experienced by the patient (sudden cardiac arrest).
*Impulse generation by tissue in atrioventricular node*
- This describes a **junctional rhythm**, which would present with a **normal or long PR interval** and a **narrow QRS complex**, contrasting with the given ECG findings.
- A junctional rhythm typically results in a slower heart rate and is not generally associated with sudden cardiac arrest in healthy individuals.
*Automatic discharge of irregular impulses in the atria*
- This typically refers to **atrial fibrillation** or multifocal atrial tachycardia, which would show an **irregularly irregular rhythm** or multiple P-wave morphologies, not the specific PR and QRS abnormalities seen.
- While atrial fibrillation can occur with WPW, the primary pathology described by the ECG findings is the accessory pathway itself.
*Wandering atrial pacemaker*
- A **wandering atrial pacemaker** is characterized by varying P-wave morphology and PR intervals as the pacemaker shifts between different atrial sites, but it generally maintains a normal QRS duration.
- It is typically a benign arrhythmia and does not cause the pre-excitation or the risk of sudden cardiac death seen in this patient.
*Blockage in conduction pathway*
- A **blockage in the conduction pathway** (e.g., AV block) would result in a **prolonged PR interval** or dropped QRS complexes, which is the opposite of the shortened PR interval observed.
- While heart block can cause syncope, it wouldn't explain the pre-excitation pattern (delta wave, wide QRS) seen in the ECG.
Question 3: 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?
A. Decreased resting membrane potential of skeletal muscle cells (Correct Answer)
B. Prolonged release of Ca2+ ions after stimulation of Ryanodine receptors
C. Hyperpolarization of skeletal muscle cells
D. Dysfunction of Na+ channels
E. Dysfunction of dystrophin-glycoprotein complex
Explanation: ***Decreased resting membrane potential of skeletal muscle cells***
- The patient's **hyperkalemia** (serum potassium 6.0 mEq/L), evidenced by peaked T-waves, reduces the electrochemical gradient for potassium, making the **resting membrane potential less negative (more depolarized)**.
- While seemingly contradictory, a persistent partial depolarization due to high extracellular potassium can lead to inactivation of voltage-gated sodium channels, preventing the generation of new action potentials and causing **muscle weakness and depressed reflexes**.
*Prolonged release of Ca2+ ions after stimulation of Ryanodine receptors*
- This mechanism is associated with conditions like **malignant hyperthermia** or certain myopathies, characterized by muscle rigidity, cramps, or excessive heat production, which are not seen here.
- Hyperkalemia primarily affects **membrane excitability** rather than intracellular calcium release pathways directly.
*Hyperpolarization of skeletal muscle cells*
- **Hyperpolarization** would make the resting membrane potential more negative, making it harder to reach the threshold for an action potential, leading to weakness.
- This typically occurs in conditions causing **hypokalemia**, as a lower extracellular potassium concentration increases the electrochemical gradient and causes a net efflux of potassium ions.
*Dysfunction of Na+ channels*
- Dysfunction of **sodium channels** can cause various neuromuscular disorders, including periodic paralysis or myotonic conditions.
- While hyperkalemia indirectly affects sodium channel function by altering the resting membrane potential, the primary pathophysiologic insult here is the altered potassium gradient, not an intrinsic channel defect.
*Dysfunction of dystrophin-glycoprotein complex*
- This complex is crucial for maintaining muscle fiber integrity and is defective in **muscular dystrophies** (e.g., Duchenne muscular dystrophy).
- Such conditions cause progressive muscle degeneration and weakness, which develop over a much longer period than the acute symptoms described here and are not related to electrolyte imbalances.
Question 4: A researcher is studying how electrical activity propagates across the heart. In order to do this, he decides to measure the rate at which an action potential moves within various groups of cardiac muscle tissue. In particular, he isolates fibers from areas of the heart with the following characteristics:
A) Dysfunction leads to fixed PR intervals prior to a dropped beat
B) Dysfunction leads to increasing PR intervals prior to a dropped beat
C) Dysfunction leads to tachycardia with a dramatically widened QRS complex
D) Dysfunction leads to tachycardia with a sawtooth pattern on electrocardiogram
Which of the following is the proper order of these tissues from fastest action potential propagation to slowest action potential propagation.
A. B > D > C > A
B. D > C > A > B
C. B > C > D > A
D. A > D > C > B (Correct Answer)
E. A > C > D > B
Explanation: ***A > D > C > B***
* **Purkinje fibers (A)** have the fastest conduction velocity in the heart to ensure rapid and synchronous ventricular depolarization. The description of "fixed PR intervals prior to a dropped beat" in **Mobitz type II second-degree AV block** indicates an issue with conduction distal to the AV node, often in the His-Purkinje system, while still maintaining typical conduction through the atria and AV node for conducted beats.
* **Atrial muscle (D)** has a faster conduction velocity than the AV node but slower than Purkinje fibers. The "sawtooth pattern on electrocardiogram" unequivocally points to **atrial flutter**, which is characterized by rapid, regular depolarization of the atria.
* **Ventricular muscle (C)** has a conduction velocity slower than Purkinje fibers but faster than the AV node. "Tachycardia with a dramatically widened QRS complex" is characteristic of **ventricular tachycardia (VT)**, which arises from abnormal electrical activity within the ventricles.
* **AV node (B)** has the slowest conduction velocity in the heart, which allows for proper ventricular filling. "Increasing PR intervals prior to a dropped beat" describes **Mobitz type I second-degree AV block (Wenckebach phenomenon)**, which is due to progressive prolongation of conduction delay within the AV node itself.
*B > D > C > A*
* This order incorrectly places the **AV node (B)** as the fastest and **Purkinje fibers (A)** as the slowest, which is contrary to the known conduction velocities in the heart.
* The AV node is critical for delaying the impulse, making it the slowest, while Purkinje fibers are designed for rapid spread, making them the fastest.
*D > C > A > B*
* This option incorrectly places **atrial muscle (D)** as faster than **Purkinje fibers (A)**. Purkinje fibers have the fastest conduction velocity in the heart, considerably faster than atrial muscle.
*B > C > D > A*
* This arrangement incorrectly lists the **AV node (B)** as the fastest and **Purkinje fibers (A)** as the slowest. The AV node is the slowest for its physiological role of delaying ventricular contraction, while Purkinje fibers are optimized for rapid conduction.
*A > C > D > B*
* While placing **Purkinje fibers (A)** as the fastest and the **AV node (B)** as the slowest is correct, this order incorrectly places **ventricular muscle (C)** as faster than **atrial muscle (D)**. Atrial muscle generally conducts faster than ventricular muscle in normal physiology.
Question 5: 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?
A. Elevated in patients with full thickness ischemic injury of the heart
B. Becomes peaked in states of hyperkalemia
C. Becomes prominent in states of hypokalemia
D. Normal duration defined as less than 120 milliseconds (Correct Answer)
E. Normal duration defined as less than 200 milliseconds
Explanation: ***Normal duration defined as less than 120 milliseconds***
- The question asks for the representation of **initial ventricular depolarization**, which corresponds to the **QRS complex** on an ECG.
- The normal duration of the **QRS complex** is typically less than **0.12 seconds (120 milliseconds)**, reflecting efficient ventricular depolarization.
*Elevated in patients with full thickness ischemic injury of the heart*
- This description refers to the **ST segment elevation** seen in **ST-segment elevation myocardial infarction (STEMI)**, which represents myocardial injury, not initial ventricular depolarization.
- While related to cardiac electrical activity, **ST segment elevation** is a consequence of injury and refers to repolarization abnormalities, not the QRS complex itself.
*Becomes peaked in states of hyperkalemia*
- **Peaked T waves** are characteristic of **hyperkalemia**, indicating altered ventricular repolarization, not ventricular depolarization.
- The T wave represents ventricular repolarization, and its morphology changes significantly with potassium imbalances.
*Becomes prominent in states of hypokalemia*
- A **prominent U wave** is sometimes observed in **hypokalemia**, which follows the T wave and is thought to represent repolarization of Purkinje fibers.
- The U wave is distinct from the QRS complex and does not represent initial ventricular depolarization.
*Normal duration defined as less than 200 milliseconds*
- A duration of less than 200 milliseconds (0.20 seconds) typically refers to the normal duration of the **PR interval**, which represents atrial depolarization and conduction through the AV node.
- The **QRS complex** (initial ventricular depolarization) has a shorter normal duration, typically less than 120 milliseconds.
Question 6: 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?
A. Prolonged influx through voltage-gated Ca2+ channels in the bundle of His
B. Transient activation of K+ current in Purkinje fibers
C. Inhibition of the Na+/K+-ATPase pump in ventricular cells
D. Limited speed of conduction through the left bundle branch
E. Temporary inactivation of Na+ channels in the AV node (Correct Answer)
Explanation: ***Temporary inactivation of Na+ channels in the AV node***
- The ECG findings are classic for **atrial fibrillation**, characterized by a rapid, irregular atrial rhythm (oscillating baseline with no P waves) and an irregularly irregular ventricular response.
- The **AV node's refractory period** and the number of sodium channels available for conduction dictate the rate at which atrial impulses can pass to the ventricles, preventing a dangerously fast ventricular rate.
*Prolonged influx through voltage-gated Ca2+ channels in the bundle of His*
- The **bundle of His** primarily conducts impulses rather than primarily regulating the rate difference between atria and ventricles through calcium channel kinetics.
- Prolonged calcium influx would generally **slow conduction** or decrease excitability, but it's not the primary mechanism explaining the ventricular rate control in atrial fibrillation.
*Transient activation of K+ current in Purkinje fibers*
- **Purkinje fibers** are involved in rapid ventricular depolarization, but their primary role is not to mediate the rate difference between atria and ventricles in atrial fibrillation.
- Activation of K+ current typically leads to **repolarization**, affecting action potential duration, not the overall filtering of atrial impulses.
*Inhibition of the Na+/K+-ATPase pump in ventricular cells*
- Inhibition of the **Na+/K+-ATPase pump** would lead to intracellular sodium accumulation and depolarization, potentially causing arrhythmias, not regulating the ventricular rate in atrial fibrillation.
- This is the mechanism of action for **digoxin**, which can slow AV nodal conduction but through a different primary pathway affecting the pump.
*Limited speed of conduction through the left bundle branch*
- While conduction system abnormalities can occur, a **limited speed of conduction** specifically in the left bundle branch would cause a wide QRS complex or bundle branch block, not the inherent rate-limiting seen in atrial fibrillation.
- The AV node is the primary regulator of ventricular response rate in atrial fibrillation due to its inherent physiological properties.
Question 7: 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?
A. Purkinje fibers > ventricles > atria > AV node
B. Purkinje fibers > atria > ventricles > AV node (Correct Answer)
C. Atria > Purkinje fibers > ventricles > AV node
D. AV node > ventricles > atria > Purkinje fibers
E. Purkinje fibers > AV node > ventricles > atria
Explanation: ***Purkinje fibers > atria > ventricles > AV node***
- The **Purkinje fibers** have the fastest conduction velocity, ensuring rapid and synchronous ventricular depolarization.
- The **atria** conduct impulses faster than the ventricles, but slower than the Purkinje fibers, allowing for atrial contraction before ventricular systole.
*Purkinje fibers > ventricles > atria > AV node*
- This option correctly identifies the **Purkinje fibers** and **AV node** at the fastest and slowest ends, respectively, but incorrectly orders the atria and ventricles.
- While Purkinje fibers are fastest, cardiac muscle cells (atria then ventricles) conduct slower than Purkinje fibers.
*Atria > Purkinje fibers > ventricles > AV node*
- This option incorrectly places the **atria** as having the fastest conduction speed, which is not true as Purkinje fibers are significantly faster.
- It also misorders the Purkinje fibers relative to the atria in terms of speed.
*AV node > ventricles > atria > Purkinje fibers*
- This option is incorrect as it places the **AV node** as the fastest conductor and the **Purkinje fibers** as the slowest, which is the exact opposite of their actual conduction speeds.
- The AV node is known for its slow conduction to allow for ventricular filling.
*Purkinje fibers > AV node > ventricles > atria*
- This option incorrectly places the **AV node** as the second fastest conductor, and the ventricles as slower than the atria.
- The AV node is specifically designed to slow the impulse to allow for proper ventricular filling.
