Cardiac Electrophysiology — MCQs
A man presents with chest pain, and his ECG shows ST segment depression in leads V1 to V4. Which of the following medications is contraindicated?
Identify the ECG given below?
All are effects of the parasympathetic system on the heart except?
What characterizes the plateau (phase 2) of the ventricular myocyte action potential?
Which of the following statements about the Na-K pump is false?
All are cardiovascular system changes in pregnancy except.
A shift of posture from supine to upright posture is associated with cardiovascular adjustments. Which of the following is NOT true in this context?
Which part of the cardiac conduction system has the fastest conduction velocity?
The following data were obtained from a man weighing 70 kg: Aorta oxygen (O2) content is 20.0 vol%, femoral vein O2 content is 16 vol%, coronary sinus O2 content is 10 vol%, and pulmonary artery O2 content is 15 vol%. What is the cardiac output of this man, given a total body O2 consumption of 400 ml/min?
What does the T wave on an ECG represent?
Cardiac Electrophysiology Indian Medical PG Practice Questions and MCQs
Question 1: A man presents with chest pain, and his ECG shows ST segment depression in leads V1 to V4. Which of the following medications is contraindicated?
- A. Morphine
- B. Aspirin
- C. Beta blocker
- D. Thrombolytic (Correct Answer)
Explanation: ***Thrombolytic*** - The ECG findings of **ST segment depression** in leads V1-V4 suggest **ischemia** or possibly a **non-ST elevation myocardial infarction (NSTEMI)** or **posterior MI**, but not an **ST elevation myocardial infarction (STEMI)**. - **Thrombolytics** are contraindicated in NSTEMI and are primarily reserved for STEMI when PCI is not available or delayed, due to the risk of bleeding without significant benefit [1]. *Beta blocker* - **Beta blockers** are often indicated in chest pain suspected to be cardiac in origin to reduce myocardial oxygen demand and improve outcomes, unless contraindicated by **bradycardia**, **hypotension**, or **heart failure** [1]. - They help decrease **heart rate** and **blood pressure**, lessening the workload on the heart [2]. *Morphine* - **Morphine** is used for significant pain relief in acute coronary syndromes, including NSTEMI, as it can reduce pain and anxiety [3]. - It works by reducing **preload** and **afterload** through systemic venodilation, which in turn reduces myocardial oxygen demand [3]. *Aspirin* - **Aspirin** is a cornerstone of therapy for acute coronary syndromes, including NSTEMI, due to its **antiplatelet effects** [4]. - It inhibits **platelet aggregation**, preventing further thrombus formation and reducing the risk of ischemic events [4].
Question 2: Identify the ECG given below?
- A. Viral myocarditis
- B. Torsades de pointes (Correct Answer)
- C. Cardiac tamponade
- D. Pericarditis
Explanation: ***Torsades de pointes*** - The ECG shows a polymorphic ventricular tachycardia where the **QRS complexes appear to twist around the baseline**, a classic feature of Torsades de pointes. - This condition is often associated with **QT prolongation**, which is evident in some of the strips preceding the tachyarrhythmia. *Viral myocarditis* - While viral myocarditis can lead to various ECG abnormalities, it typically doesn't present with this specific **polymorphic ventricular tachycardia** morphology. - Common ECG findings in myocarditis include non-specific ST-T wave changes, sinus tachycardia, or conduction blocks, rather than the characteristic "twisting" pattern seen here. *Cardiac tamponade* - Cardiac tamponade is characterized by **electrical alternans** (alternating QRS amplitude), low voltage, and sinus tachycardia on ECG. - It does not cause a polymorphic ventricular tachycardia with the appearance of QRS complexes twisting around the baseline. *Pericarditis* - Pericarditis typically presents with **diffuse ST-segment elevation** (often concave up) and PR-segment depression. - It does not manifest as a polymorphic ventricular tachycardia like Torsades de pointes.
Question 3: All are effects of the parasympathetic system on the heart except?
- A. Negative chronotropic
- B. Negative dromotropic
- C. All are seen
- D. Negative inotropic (Correct Answer)
Explanation: ***Negative inotropic*** - While the parasympathetic system (via the **vagus nerve**) primarily affects the **sinoatrial (SA) and atrioventricular (AV) nodes** to decrease heart rate and conduction velocity, it has a **minimal direct effect on ventricular contractility** (inotropy) in humans. - The ventricles are less densely innervated by parasympathetic fibers compared to the atria, so acetylcholine's direct negative inotropic effect is **clinically insignificant** in a healthy heart. - This is the **EXCEPTION** - not a significant parasympathetic effect on the heart. *Negative chronotropic* - The parasympathetic system, primarily through **acetylcholine** acting on **muscarinic receptors** in the SA node, decreases the heart rate (chronotropy). - This slows the rate of spontaneous depolarization of pacemaker cells. - This **IS** a major parasympathetic effect. *Negative dromotropic* - Parasympathetic stimulation also slows the conduction velocity through the **AV node** (dromotropy). - This increases the PR interval on an ECG and can lead to various degrees of AV block in extreme cases. - This **IS** a major parasympathetic effect. *All are seen* - This option is incorrect because the **negative inotropic effect** is NOT a significant parasympathetic effect on the heart. - While negative chronotropic and negative dromotropic effects are prominent features of parasympathetic activity, the direct influence on ventricular contractility is minimal.
Question 4: What characterizes the plateau (phase 2) of the ventricular myocyte action potential?
- A. Describes when Ca2+ influx is predominant but K+ efflux is also significant.
- B. Describes when Ca2+ influx is balanced by K+ efflux.
- C. Describes when Ca2+ influx is balanced by K+ efflux, with Na+ channels inactivated. (Correct Answer)
- D. Can be influenced by sympathetic nerve stimulation.
Explanation: ***Describes when Ca2+ influx is balanced by K+ efflux, with Na+ channels inactivated.*** - During phase 2, the **influx of calcium ions** through L-type Ca2+ channels (maintaining depolarization) is roughly balanced by the **efflux of potassium ions** through delayed rectifier K+ channels. - The **inactivation of Na+ channels** after phase 0 prevents further Na+ influx and contributes to the plateau's stability, prolonging the action potential and allowing complete ventricular contraction. *Describes when Ca2+ influx is predominant but K+ efflux is also significant.* - While **Ca2+ influx is key** during phase 2, the unique characteristic is the **balance** between Ca2+ influx and K+ efflux, not the clear predominance of one over the other. - If Ca2+ influx were solely predominant, the membrane potential would continue to depolarize, not maintain a plateau. *Describes when Ca2+ influx is balanced by K+ efflux.* - This statement accurately describes a key aspect of phase 2 but is incomplete as it **omits the crucial role of inactivated Na+ channels**. - The inactivation of **fast Na+ channels** is fundamental to preventing premature repolarization and establishing the sustained plateau. *Can be influenced by sympathetic nerve stimulation.* - While sympathetic stimulation (via **catecholamines**) *can modulate* the duration and amplitude of the action potential, including the plateau, it is **not a *characterizing feature*** of the plateau phase itself. - Rather, it's an external regulatory mechanism that affects ion channel activity, not a fundamental description of the ion fluxes defining phase 2.
Question 5: Which of the following statements about the Na-K pump is false?
- A. It is not directly involved in the generation of action potentials.
- B. It is electrogenic
- C. It needs ATP for its functioning
- D. It is located on the apical membrane of cell (Correct Answer)
Explanation: ***It is located on the apical membrane of cell*** - The **Na-K pump**, or **Na+/K+-ATPase**, is primarily located on the **basolateral membrane** of epithelial cells, not **apical membrane**. - Its strategic placement on the basolateral membrane is crucial for maintaining cellular polarity and driving transepithelial transport processes, such as reabsorption in the kidneys. *It is electrogenic* - The Na-K pump is indeed **electrogenic** because it transports three **Na+ ions** out of the cell for every two **K+ ions** pumped in. - This unequal charge distribution creates a net movement of one positive charge out of the cell, contributing to the **resting membrane potential**. *It is not directly involved in the generation of action potentials.* - While the Na-K pump is essential for **maintaining the ion gradients** necessary for **action potentials**, it is not directly involved in their rapid depolarization or repolarization phases. - Action potentials are primarily generated by the rapid opening and closing of **voltage-gated ion channels** (e.g., Na+ and K+ channels). *It needs ATP for its functioning* - The Na-K pump is an **active transport mechanism** that moves ions against their concentration gradients, requiring **energy in the form of ATP hydrolysis**. - This **ATP-dependent process** ensures the continuous maintenance of the Na+ and K+ gradients, crucial for various cellular functions, including nerve impulse transmission and muscle contraction.
Question 6: All are cardiovascular system changes in pregnancy except.
- A. Increase in blood volume
- B. Increase in heart rate
- C. Increase in peripheral resistance (Correct Answer)
- D. Increase in cardiac output
Explanation: ***Increase in peripheral resistance*** - During normal pregnancy, **peripheral vascular resistance actually decreases** due to the effects of hormones like progesterone and the presence of the low-resistance uteroplacental circulation. - This decrease in resistance helps accommodate the increased blood volume and cardiac output. *Increase in cardiac output* - **Cardiac output increases significantly** during pregnancy (by 30-50%) to meet the metabolic demands of the growing fetus and maternal tissues. - This is primarily achieved through an increase in both stroke volume and heart rate. *Increase in blood volume* - **Blood volume increases substantially** (by 30-50%) during pregnancy, with plasma volume increasing more than red blood cell mass. - This expansion supports the increased cardiac output and placental perfusion. *Increase in heart rate* - **Heart rate increases** during pregnancy, typically by 10-20 beats per minute, contributing to the overall increase in cardiac output. - This physiological adaptation helps maintain adequate circulation.
Question 7: A shift of posture from supine to upright posture is associated with cardiovascular adjustments. Which of the following is NOT true in this context?
- A. Rise in heart rate
- B. Decrease in cardiac output
- C. Decrease in central venous pressure
- D. Rise in central venous pressure (Correct Answer)
Explanation: ***Rise in central venous pressure*** - When a person moves from a supine to an upright posture, gravity causes **blood pooling in the lower extremities**, leading to a *decrease* in venous return to the heart, not a rise in central venous pressure. - A decrease in central venous pressure is an expected physiological response to orthostasis due to the aforementioned venous pooling. *Decrease in central venous pressure* - This statement is physiologically *true* because gravity causes blood to pool in the lower limbs, reducing venous return and subsequently lowering the central venous pressure. - The **baroreflex** responds to this fall, attempting to restore blood pressure. *Rise in heart rate* - This is a normal physiological response to orthostatic stress, mediated by the **baroreflex**, to maintain cardiac output and blood pressure against gravity. - The sympathetic nervous system increases **heart rate** and contractility to compensate for reduced venous return. *Decrease in cardiac output* - Upon standing, the initial reduction in venous return leads to a transient decrease in **stroke volume**, which, despite the compensatory rise in heart rate, often results in a net *decrease* in cardiac output. - This is a normal and expected cardiovascular adjustment as the body adapts to the upright position.
Question 8: Which part of the cardiac conduction system has the fastest conduction velocity?
- A. Sinoatrial (SA) node
- B. Bundle of His fibers
- C. Purkinje fibers (Correct Answer)
- D. Atrioventricular (AV) node
Explanation: ***Purkinje fibers*** - They possess the **fastest conduction velocity** in the heart (2-4 m/s), necessary for rapid and synchronized ventricular contraction. - This rapid conduction ensures that all parts of the ventricles contract almost simultaneously, maximizing pumping efficiency. **Sinoatrial (SA) node** - The SA node is the primary pacemaker of the heart, setting the heart rate due to its intrinsic rhythmicity. - It has a relatively **slow conduction velocity** (0.05 m/s) which allows time for proper impulse generation and transmission. **Bundle of His fibers** - The Bundle of His transmits impulses from the AV node to the bundle branches. - While faster than the AV node (1-1.5 m/s), its conduction velocity is significantly **slower** than that of the Purkinje fibers. **Atrioventricular (AV) node** - The AV node introduces a crucial delay in impulse conduction, allowing sufficient time for atrial contraction and complete ventricular filling before ventricular systole. - It has the **slowest conduction velocity** (0.02-0.05 m/s) in the entire cardiac conduction system.
Question 9: The following data were obtained from a man weighing 70 kg: Aorta oxygen (O2) content is 20.0 vol%, femoral vein O2 content is 16 vol%, coronary sinus O2 content is 10 vol%, and pulmonary artery O2 content is 15 vol%. What is the cardiac output of this man, given a total body O2 consumption of 400 ml/min?
- A. 10 L/min
- B. 8 L/min (Correct Answer)
- C. 6 L/min
- D. 5 L/min
Explanation: ***8 L/min*** - The cardiac output is calculated using the **Fick principle**: CO = Total body O2 consumption / (Arterial O2 content - Mixed venous O2 content). - In this case, **Arterial O2 content is 20 vol%** and **Mixed venous O2 content (pulmonary artery) is 15 vol%**. So, CO = 400 ml/min / (20 vol% - 15 vol%) = 400 ml/min / 5 ml O2/100 ml blood = 400 / 0.05 = 8000 ml/min = **8 L/min**. *10 L/min* - This result would be obtained if the arteriovenous oxygen difference was smaller, specifically 4 vol% (400 / 0.04 = 10000 ml/min). - This calculation does not correctly use the given **mixed venous O2 content** from the pulmonary artery. *6 L/min* - This result would be obtained if the arteriovenous oxygen difference was larger, specifically 6.67 vol% (400 / 0.0667 ≈ 6000 ml/min). - This calculation misrepresents the **actual O2 extraction** from the arterial blood. *5 L/min* - This result would be obtained if the arteriovenous oxygen difference was 8 vol% (400 / 0.08 = 5000 ml/min). - This choice indicates an incorrect application of the **Fick principle** or misidentification of the relevant oxygen content values.
Question 10: What does the T wave on an ECG represent?
- A. Ventricular repolarization (Correct Answer)
- B. Atrial repolarization
- C. Atrial depolarization
- D. Ventricular depolarization
Explanation: ***Ventricular repolarization*** - The **T wave** signifies the electrical recovery of the **ventricles**. - This process involves the repolarization of ventricular myocardial cells, returning them to their resting potential. *Atrial repolarization* - **Atrial repolarization** occurs during the **QRS complex** and is usually masked by the larger ventricular depolarization. - It does not produce a distinct wave on a standard ECG tracing. *Atrial depolarization* - **Atrial depolarization** is represented by the **P wave** on the ECG. - This wave indicates the electrical activation of the atria, initiating atrial contraction. *Ventricular depolarization* - **Ventricular depolarization** is represented by the **QRS complex** on the ECG. - This complex signifies the electrical activation of the ventricles, leading to ventricular contraction.
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