Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 901: Vagal stimulation of the heart causes

A. Increased R-R interval in ECG (Correct Answer)
B. Increased cardiac output
C. Increased heart rate
D. Increased force of heart contraction

Explanation: ***Increased R-R interval in ECG*** - Vagal stimulation releases **acetylcholine**, which acts on **M2 muscarinic receptors** in the heart, particularly in the SA and AV nodes. - This leads to a decrease in heart rate, which manifests as an **increase in the R-R interval** on an ECG. *Increased heart rate* - Vagal stimulation is part of the **parasympathetic nervous system**, which generally **decreases heart rate**. - **Sympathetic stimulation**, not vagal stimulation, is responsible for increasing heart rate. *Increased force of heart contraction* - While vagal innervation affects atrial contractility, its primary effect on ventricular contractility is minimal compared to **sympathetic stimulation**. - Increased force of contraction is mainly mediated by **catecholamines** from the sympathetic nervous system. *Increased cardiac output* - Cardiac output is the product of heart rate and stroke volume; a decrease in heart rate due to vagal stimulation would generally **decrease cardiac output**, assuming stroke volume remains constant or does not significantly increase to compensate. - Vagal stimulation primarily aims to **conserve energy** and *slow down heart activity*, not increase overall output.

Question 902: 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 903: Which of the following factors does not directly influence oxygen delivery to tissues?

A. Type of fluid administered (Correct Answer)
B. Cardiac output
C. Oxygen saturation
D. Hemoglobin concentration

Explanation: ***Type of fluid administered*** - While fluid administration can indirectly affect oxygen delivery by altering blood volume and cardiac output, the **type of fluid itself (e.g., crystalloid vs. colloid)** does not directly influence the oxygen-carrying capacity of the blood or its release to tissues. - The direct effect of fluid resuscitation is on **hemodynamic parameters**, which then influence delivery. *Oxygen saturation* - **Oxygen saturation** directly reflects the percentage of hemoglobin binding sites occupied by oxygen, thus determining the amount of oxygen carried by each unit of blood. - A decrease in oxygen saturation significantly reduces the **total oxygen content** available for tissue delivery. *Cardiac output* - **Cardiac output** (heart rate × stroke volume) is a primary determinant of blood flow to tissues, and therefore directly influences the rate at which oxygenated blood is delivered throughout the body. - A lower cardiac output leads to **reduced oxygen delivery** despite adequate oxygen content in the blood. *Hemoglobin concentration* - **Hemoglobin concentration** directly dictates the blood's capacity to carry oxygen, as hemoglobin is the main oxygen-carrying molecule in red blood cells. - A low hemoglobin concentration (anemia) results in **decreased oxygen-carrying capacity** and thus impaired oxygen delivery to tissues.

Question 904: Depolarization of the ventricular muscles starts at which site?

A. Right side of the septum
B. Apex of the heart
C. AV groove
D. Left side of the interventricular septum (Correct Answer)

Explanation: ***Left side of the interventricular septum*** - The **Purkinje fibers** rapidly transmit the electrical impulse down the **Bundle of His** and then branch out to depolarize the myocardium. - Depolarization typically begins on the left side of the **interventricular septum**, spreading outwards and upwards. *Right side of the septum* - While the septum depolarizes, the earliest activation generally initiates on the **left septal wall**, not the right. - The spread of activation from the right side of the septum occurs slightly later than from the left because the left bundle branch is shorter and reaches the septum more quickly, establishing earlier depolarization. *Apex of the heart* - Although the Purkinje fibers extend towards the apex, the initial site of ventricular depolarization is the **septum**, not the apex itself. - Depolarization then proceeds from the septum towards the apex and the ventricular free walls. *AV groove* - The **AV groove** is the junction between the atria and ventricles, containing the **AV node**. - The **AV node** delays propagation, but ventricular depolarization originates from the **His-Purkinje system** beyond the AV node, specifically in the septum.

Question 905: What is the reason for the higher V wave in the left atrium compared to the right atrium?

A. The right atrium receives more blood.
B. Lower compliance (higher stiffness) of the left atrium. (Correct Answer)
C. Increased blood return to the left atrium from the pulmonary veins.
D. Decreased blood return to the left atrium from the pulmonary veins.

Explanation: ***Lower compliance (higher stiffness) of the left atrium.*** - The **left atrium** typically has **lower compliance** (higher stiffness) than the right atrium. - This anatomical difference means that for the same volume of blood return, the pressure increase (and thus the **V wave**) in the less compliant left atrium will be greater than in the more compliant right atrium. *Increased blood return to the left atrium from the pulmonary veins.* - While the **left atrium** receives a significant volume of blood from the **pulmonary veins**, the absolute volume of blood return alone doesn't explain the *higher V wave* relative to the right atrium unless compounded by compliance differences. - The overall cardiac output is the same for both sides of the heart; therefore, the absolute blood return to both atria is roughly equal over time. *The right atrium receives more blood.* - The **right atrium** receives venous blood from the **superior and inferior vena cava**, representing the entire systemic circulation. - The **left atrium** receives blood from the pulmonary circulation, and in a healthy individual, the **cardiac output** is the same for both ventricles, implying equal venous return to both atria over cycles. *Decreased blood return to the left atrium from the pulmonary veins.* - **Decreased blood return** would lead to a *lower V wave* (lower pressure) in the left atrium, not a higher one. - A higher V wave indicates increased volume or pressure during atrial filling.

Question 906: Which of the following structures in the heart are known for their rapid conduction of electrical impulses?

A. Sinoatrial (SA) node
B. Atrioventricular (AV) node
C. His bundle
D. Purkinje fibers (Correct Answer)

Explanation: ***Correct: Purkinje fibers*** - **Purkinje fibers** have the **fastest conduction velocity** among all cardiac tissues, approximately **4 m/s** - These specialized myocardial fibers ensure **rapid and synchronized depolarization of the ventricles**, allowing for efficient and coordinated ventricular contraction - Their rapid conduction is essential for simultaneous contraction of ventricular myocardium from apex to base *Incorrect: Sinoatrial (SA) node* - The SA node is the natural **pacemaker** of the heart, initiating electrical impulses at a rate that determines heart rate - However, its conduction velocity is **very slow** (~0.05 m/s), much slower than Purkinje fibers - Its role is impulse generation, not rapid conduction *Incorrect: Atrioventricular (AV) node* - The AV node has the **slowest conduction velocity** in the heart (~0.05 m/s) - It **delays electrical impulses** from the atria to the ventricles (AV delay ~0.1 seconds) - This delay allows for **complete ventricular filling** before ventricular contraction begins *Incorrect: His bundle* - 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), it is still **significantly slower than Purkinje fibers** - Its conduction velocity is intermediate between the AV node and Purkinje fibers

Question 907: When recording Lead I on an ECG, the right arm is the negative electrode. Which electrode serves as the positive electrode?

A. Right arm + left arm
B. Left leg
C. Right leg
D. Left arm (Correct Answer)

Explanation: ***Left arm*** - In a standard 12-lead ECG, **Lead I** is a **bipolar limb lead** that measures the electrical potential difference between the right arm and the left arm. - The convention for Lead I dictates that the **right arm** is the **negative electrode** and the **left arm** is the **positive electrode**. *Left leg* - The **left leg** serves as the **positive electrode** for **Lead III** (with the left arm as negative) and for **aVF** (with the average of right arm and left arm as negative). - It does not serve as the positive electrode for Lead I. *Right leg* - The **right leg electrode** typically serves as a **ground electrode** in the 12-lead ECG system. - Its primary function is to minimize electrical noise and interference, not to measure potential differences for standard leads. *Right arm + left arm* - Combining the signals from the right and left arm electrodes does not result in a standard ECG lead or a designated positive electrode for Lead I. - Lead I specifically measures the potential difference *between* these two electrodes, with the left arm being positive and the right arm being negative.

Question 908: According to the Frank-Starling law of the heart, stroke volume is directly related to which parameter?

A. Cardiac output
B. End-diastolic volume (Preload) (Correct Answer)
C. Stroke volume
D. Arterial BP

Explanation: ***End-diastolic volume (Preload)*** - The Frank-Starling law of the heart states that **stroke volume is directly proportional to end-diastolic volume (preload)**. - Increased venous return leads to greater end-diastolic volume, which stretches the cardiac muscle fibers, resulting in a more forceful contraction and increased stroke volume. - This relationship explains the intrinsic ability of the heart to adapt to varying amounts of venous return. *Cardiac output* - Cardiac output is the product of stroke volume and heart rate (CO = SV × HR). - While stroke volume affects cardiac output, the Frank-Starling law specifically describes the relationship between preload and stroke volume, not cardiac output directly. *Arterial BP* - Arterial blood pressure is influenced by cardiac output and systemic vascular resistance. - The Frank-Starling law does not directly describe the relationship between stroke volume and arterial blood pressure. *Stroke volume* - Stroke volume is the dependent variable (outcome) in the Frank-Starling relationship, not the independent variable (cause). - The law describes how preload affects stroke volume, not how stroke volume affects itself.

Question 909: Least conduction velocity is seen in which of the following?

A. AV node (Correct Answer)
B. Purkinje fibres
C. Bundle of His
D. Ventricular myocardial fibers

Explanation: ***AV node*** - The **atrioventricular (AV) node** has the **slowest conduction velocity (~0.02-0.05 m/s)** in the cardiac conduction system. - This deliberate delay allows for complete **ventricular filling** before contraction, which is crucial for efficient cardiac function. - The slow conduction creates the **PR interval** on ECG, representing the AV nodal delay. *Purkinje fibres* - **Purkinje fibers** have the **fastest conduction velocity (~2-4 m/s)** in the heart, enabling rapid, synchronized ventricular depolarization. - Their extensive network ensures that both ventricles contract almost simultaneously for effective blood ejection. *Bundle of His* - The **Bundle of His** exhibits a relatively fast conduction velocity **(~1-1.5 m/s)**, transmitting the impulse from the AV node to the bundle branches. - While not as fast as Purkinje fibers, it's significantly faster than the AV node. *Ventricular myocardial fibers* - **Ventricular myocardial fibers** conduct impulses at an intermediate speed **(~0.3-1.0 m/s)**, facilitating the contractile process throughout the ventricular muscle. - Their conduction velocity is slower than the specialized conduction system but faster than the AV node.

Question 910: Which cell type is primarily targeted by shear stress in vascular structures?

A. Endothelial cells (Correct Answer)
B. Fibroblasts
C. Smooth muscle cells
D. Pericytes

Explanation: ***Endothelial cells*** - **Endothelial cells** form the inner lining of blood vessels and are directly exposed to and sense **shear stress** and **hemodynamic forces** from blood flow. - Their response to these forces is crucial for regulating vascular tone, permeability, and angiogenesis through mechanotransduction pathways. *Fibroblasts* - **Fibroblasts** are primarily involved in synthesizing the **extracellular matrix** and are found in the adventitia, or outer layer of blood vessels. - While they contribute to vascular integrity, they are not the primary target of direct hemodynamic shear stress within the vessel lumen. *Smooth muscle cells* - **Smooth muscle cells** are located in the media, or middle layer of blood vessels, and are responsible for regulating **vascular tone** and blood pressure. - They respond primarily to circumferential stretch and various vasoactive substances, but **endothelial cells** are the initial sensors of intraluminal shear stress. *Pericytes* - **Pericytes** are contractile cells that wrap around endothelial cells of capillaries and venules, contributing to **microvascular stability** and blood flow regulation. - While they interact closely with endothelial cells, they are not the primary cells directly targeted by shear stress from blood flow within vascular structures.

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