Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 151: What causes the third heart sound (S3)?

A. Closure of the atrioventricular valves
B. Closure of the aortic and pulmonary valves
C. Mid-diastolic flow into the ventricle (Correct Answer)
D. Atrial contraction

Explanation: **Explanation:** The **Third Heart Sound (S3)**, also known as the "ventricular gallop," occurs during the **early to mid-diastolic phase** of the cardiac cycle. It is caused by the rapid rush of blood from the atria into a compliant (or overfilled) ventricle. This sudden deceleration of blood flow against the ventricular wall causes vibrations that produce the low-pitched sound. **Analysis of Options:** * **Option C (Correct):** S3 occurs during the **rapid filling phase** of diastole. It is physiological in children, young adults, and pregnant women, but pathological in older adults, indicating volume overload. * **Option A:** Closure of the AV valves (Mitral and Tricuspid) produces the **First Heart Sound (S1)**. * **Option B:** Closure of the Semilunar valves (Aortic and Pulmonary) produces the **Second Heart Sound (S2)**. * **Option D:** Atrial contraction (atrial kick) produces the **Fourth Heart Sound (S4)**, which occurs in late diastole and is always pathological (associated with stiff, non-compliant ventricles). **NEET-PG High-Yield Pearls:** * **Best heard with:** The **bell** of the stethoscope at the apex (left lateral decubitus position). * **Pathological S3:** A classic sign of **Congestive Heart Failure (CHF)**, Dilated Cardiomyopathy, or Mitral Regurgitation. * **Rhythm:** It creates a "Kentucky" gallop rhythm (S1-S2-S3). * **Timing:** Occurs just after S2, when the AV valves open.

Question 152: During inhibition of the SA node by vagal stimulation, which of the following occurs?

A. Acting on K+ channels resulting in K+ efflux (Correct Answer)
B. Acting on K+ channels resulting in K+ influx
C. Acting on Ca2+ channels resulting in K+ efflux
D. Acting on Ca2+ channels resulting in K+ influx

Explanation: ### Explanation **1. Why Option A is Correct:** Vagal stimulation involves the release of **Acetylcholine (ACh)** from the parasympathetic nerve endings. ACh binds to **Muscarinic (M2) receptors** on the SA node. This binding triggers the activation of a specific type of G-protein-coupled inward rectifier potassium channel called **$K_{ACh}$ channels**. * **Mechanism:** Activation of these channels increases the permeability of the nodal cell membrane to Potassium ($K^+$). * **Result:** Since the intracellular concentration of $K^+$ is higher than the extracellular concentration, $K^+$ moves out of the cell (**Efflux**). This loss of positive ions causes **hyperpolarization** of the resting membrane potential, making it more negative and further away from the firing threshold. This slows down the heart rate (negative chronotropic effect). **2. Why Other Options are Incorrect:** * **Option B:** $K^+$ influx would require moving against its concentration gradient under normal physiological conditions. Influx of positive ions would cause depolarization, not inhibition. * **Option C & D:** While vagal stimulation does indirectly inhibit L-type $Ca^{2+}$ channels (decreasing the slope of Phase 4 depolarization), the primary and most immediate ionic mechanism for hyperpolarization in the SA node is the direct opening of $K^+$ channels, not $Ca^{2+}$ channels. Furthermore, $Ca^{2+}$ channels do not facilitate $K^+$ movement. **3. High-Yield Facts for NEET-PG:** * **Vagal Escape:** If vagal stimulation is intense and prolonged, the ventricles may start beating at their own intrinsic rate (Purkinje rhythm); this is known as "Vagal Escape." * **Neurotransmitter:** Parasympathetic = Acetylcholine; Sympathetic = Norepinephrine. * **Receptor Subtype:** M2 receptors are Gi-coupled (inhibitory), leading to decreased cAMP levels. * **Effect on Slope:** Vagal stimulation **decreases the slope of Phase 4** (pre-potential) in the SA node, thereby increasing the time required to reach the threshold.

Question 153: What is the normal cerebral blood flow per 100 grams of brain tissue per minute?

A. 55 ml/100 gm/min (Correct Answer)
B. 400 ml/100 gm/min
C. 100 ml/100 gm/min
D. 200 ml/100 gm/min

Explanation: **Explanation:** The brain is one of the most metabolically active organs in the body. In a healthy adult, the total cerebral blood flow (CBF) is approximately **750–800 ml/min**, which accounts for about **15% of the total cardiac output**. When calculated per unit of tissue, the average value is **50–55 ml/100 gm/min**. This flow is tightly regulated by autoregulation (maintaining constant flow between MAP 60–140 mmHg) and chemical factors, primarily the partial pressure of CO₂ ($PCO_2$). **Analysis of Options:** * **Option A (55 ml/100 gm/min):** This is the standard physiological value. While grey matter receives more (~80 ml/100g/min) and white matter receives less (~20 ml/100g/min), the average for the whole brain is 54–55 ml/100g/min. * **Option B (400 ml/100 gm/min):** This value is characteristic of the **Kidneys**, which receive the highest blood flow per unit weight to facilitate filtration. * **Option C (100 ml/100 gm/min):** This is closer to the blood flow of the **Heart (Myocardium)** at rest, which is approximately 70–80 ml/100 gm/min. * **Option D (200 ml/100 gm/min):** This is significantly higher than normal brain perfusion and does not correspond to a standard resting organ value, though the **Carotid Bodies** have the highest flow per 100g of any tissue (~2000 ml/100g/min). **High-Yield Clinical Pearls for NEET-PG:** * **Critical Threshold:** Consciousness is lost if CBF falls below **30 ml/100 gm/min**. Irreversible damage occurs below **10–12 ml/100 gm/min**. * **Most Potent Stimulator:** $CO_2$ is the most potent regulator of CBF. An increase in $PaCO_2$ causes vasodilation and increases flow. * **Monro-Kellie Doctrine:** The cranial vault is a fixed volume; an increase in blood or brain tissue must be compensated by a decrease in CSF or venous blood to prevent rising ICP.

Question 154: When does the pre-ejection ventricular phase begin?

A. Aortic valve open, pulmonary valve closed
B. Aortic valve closed, pulmonary valve open
C. Aortic and pulmonary valves are open (Correct Answer)
D. Aortic and pulmonary valves closed

Explanation: ### Explanation The **pre-ejection ventricular phase** (also known as the proto-systolic phase) is the brief period at the very beginning of ventricular systole, occurring immediately after the isovolumetric contraction phase. **1. Why Option C is Correct:** During the isovolumetric contraction phase, the intraventricular pressure rises sharply while all valves remain closed. Once the pressure in the left ventricle exceeds the pressure in the aorta (approx. 80 mmHg) and the pressure in the right ventricle exceeds the pressure in the pulmonary artery (approx. 8 mmHg), the **semilunar valves (Aortic and Pulmonary) open**. The pre-ejection phase marks the transition where the valves have just opened, but the actual rapid displacement of blood (ejection) has not yet reached its peak. Therefore, at the start of this phase, both valves are open to allow blood to exit the heart. **2. Why Other Options are Incorrect:** * **Options A & B:** In a healthy heart, the right and left ventricles contract almost simultaneously. Therefore, the pulmonary and aortic valves open nearly at the same time to initiate ejection into the systemic and pulmonary circulations. One valve being open while the other is closed would indicate a pathological delay or a specific pressure abnormality. * **Option D:** This describes the **Isovolumetric Contraction** phase. During this phase, the ventricles are contracting as closed cavities to build up pressure; no blood is ejected because the semilunar valves are still shut. **Clinical Pearls for NEET-PG:** * **Isovolumetric Contraction:** The phase with the steepest rise in ventricular pressure ($dP/dt$). * **First Heart Sound ($S_1$):** Occurs at the beginning of systole due to the closure of AV valves (Mitral and Tricuspid). * **Second Heart Sound ($S_2$):** Occurs at the beginning of diastole (isovolumetric relaxation) due to the closure of Semilunar valves. * **Ejection Fraction:** Normally 55–65%; it is the fraction of the End-Diastolic Volume (EDV) ejected during the ejection phase.

Question 155: All of the following statements regarding arteries are true EXCEPT?

A. Artery walls are thicker than vein walls.
B. Arteries generally carry oxygenated blood.
C. Arteries experience higher blood pressure than veins.
D. Arteries possess valves. (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Arteries possess valves.** **Why Option D is the correct (false) statement:** Valves are a characteristic feature of the **venous system**, not the arterial system. Arteries are high-pressure vessels that receive blood directly from the heart; this pressure is sufficient to keep blood flowing in one direction. In contrast, veins are low-pressure vessels that often work against gravity; they require **semilunar valves** to prevent the backflow of blood (retrograde flow). The only "valves" associated with the arterial system are the aortic and pulmonary valves at the very beginning of the great arteries, but the arterial vessels themselves do not contain valves. **Analysis of Incorrect Options:** * **Option A:** True. Arteries have a much thicker **tunica media** (smooth muscle layer) compared to veins to withstand and regulate high systemic pressures. * **Option B:** True. Most arteries carry oxygenated blood. The notable exceptions are the **pulmonary arteries** and **umbilical arteries**, which carry deoxygenated blood. * **Option C:** True. Arteries are known as "resistance vessels." They experience high, pulsatile pressure (e.g., 120/80 mmHg), whereas veins are "capacitance vessels" with much lower, steady pressure (e.g., <10 mmHg). **High-Yield Clinical Pearls for NEET-PG:** * **Arterioles** are the primary site of peripheral resistance and the main determinants of systemic blood pressure. * **Veins** contain approximately 65-70% of the total blood volume at any given time (Capacitance vessels). * **Exception to the Valve Rule:** The **Venae Cavae** and the **Portal Vein** are major veins that do *not* have valves. * **Histology Tip:** In a cross-section, arteries maintain a circular shape due to their thick walls, while veins often appear collapsed or irregular.

Question 156: Cardiac output is measured by all methods EXCEPT?

A. Doppler
B. Oscillometry (Correct Answer)
C. Thermo-dilution method
D. Fick's principle

Explanation: **Explanation:** **1. Why Oscillometry is the correct answer:** Oscillometry is a technique used primarily for the measurement of **Blood Pressure**, not Cardiac Output (CO). It works by detecting the magnitude of oscillations caused by blood flow against an automated inflating/deflating cuff. While it provides systolic, diastolic, and mean arterial pressures, it does not directly measure the volume of blood pumped by the heart per minute. **2. Analysis of Incorrect Options:** * **Doppler (Option A):** This is a non-invasive method using ultrasound. By measuring the flow velocity of blood across the aortic or pulmonary valve and multiplying it by the cross-sectional area of the vessel, CO can be calculated (CO = Stroke Volume × Heart Rate). * **Thermo-dilution (Option B):** This is the **clinical gold standard** for measuring CO. It involves injecting a cold saline bolus via a Swan-Ganz catheter and measuring the temperature change downstream. The rate of temperature change is inversely proportional to the CO (based on the Stewart-Hamilton equation). * **Fick’s Principle (Option D):** This is the **theoretical gold standard**. It states that the uptake of a substance (usually Oxygen) by an organ is equal to the product of the blood flow to that organ and the arteriovenous concentration difference of that substance. Formula: $CO = \text{O}_2 \text{ consumption} / (\text{Arterial } \text{O}_2 - \text{Mixed Venous } \text{O}_2)$. **Clinical Pearls for NEET-PG:** * **Indicator Dilution Method:** Uses substances like Indocyanine green; it is the precursor concept to thermo-dilution. * **Echocardiography:** The most common non-invasive bedside method to estimate CO/Ejection Fraction. * **Mixed Venous Blood:** For Fick's principle, mixed venous blood must be sampled from the **Pulmonary Artery** to ensure proper mixing.

Question 157: All of the following are physiological effects of the manoeuvre shown below except?

A. Fall in BP
B. Rise in HR
C. Decreased venous return
D. Increased stroke volume (Correct Answer)

Explanation: ***Increased stroke volume*** - During the **Valsalva manoeuvre**, increased **intrathoracic pressure** compresses the venous system, reducing **venous return** and **preload**. - This leads to **decreased stroke volume** due to reduced left ventricular filling, making this the exception among the listed effects. *Fall in BP* - The **decreased venous return** and reduced **stroke volume** during the manoeuvre result in a significant **drop in blood pressure**. - This occurs particularly during **phase 2** of the Valsalva manoeuvre when cardiac output is compromised. *Rise in HR* - The fall in blood pressure triggers **baroreceptor-mediated reflexes** that increase **sympathetic activity**. - This compensatory mechanism leads to **tachycardia** to maintain cardiac output despite reduced stroke volume. *Decreased venous return* - Increased **intrathoracic pressure** compresses the **inferior vena cava** and other venous structures. - This mechanical compression significantly reduces the **venous return** to the right heart, decreasing preload.

Question 158: What is the normal cardiac index in an adult?

A. 5.9 L/min/m2
B. 2.3 L/min/m2
C. 3.2 L/min/m2 (Correct Answer)
D. 4.6 L/min/m2

Explanation: ### Explanation **1. Understanding the Correct Answer (C):** Cardiac Index (CI) is a hemodynamic parameter that relates the Cardiac Output (CO) to a person’s Body Surface Area (BSA). This provides a more accurate assessment of cardiac performance than CO alone, as it accounts for the individual's body size. * **Formula:** $CI = \frac{\text{Cardiac Output}}{\text{Body Surface Area}}$ * In a healthy adult, the average Cardiac Output is approximately **5 L/min** and the average BSA is **1.7 m²**. * Calculation: $5 / 1.7 \approx 2.9 \text{ to } 3.2 \text{ L/min/m}^2$. * The standard physiological range is **2.5 to 4.2 L/min/m²**, making **3.2 L/min/m²** the most accurate representative value among the options. **2. Analysis of Incorrect Options:** * **Option A (5.9 L/min/m²):** This value is significantly higher than normal. Such a high index would indicate a hyperdynamic state (e.g., severe thyrotoxicosis, sepsis, or strenuous exercise). * **Option B (2.3 L/min/m²):** This is below the normal threshold. A CI less than **2.2 L/min/m²** is a clinical marker for **cardiogenic shock** or significant heart failure. * **Option D (4.6 L/min/m²):** While closer to the upper limit, it is generally considered above the average resting baseline for a healthy adult. **3. NEET-PG High-Yield Pearls:** * **Peak Age:** Cardiac Index is highest at age 10 (approx. 4 L/min/m²) and gradually declines with age. * **Clinical Significance:** CI is vital in the ICU setting to differentiate types of shock. * **BSA Calculation:** Most commonly calculated using the **Mosteller formula** or **DuBois formula**. * **Key Value:** A CI $< 2.2 \text{ L/min/m}^2$ in the presence of high pulmonary capillary wedge pressure (PCWP) defines cardiogenic shock.

Question 159: Closing of the mitral valve begins at which point?

A. A (Correct Answer)
B. B
C. C
D. D

Explanation: ***Point A*** - Point A marks the **beginning of ventricular systole** when **left ventricular pressure** first exceeds **left atrial pressure**, causing the **mitral valve** to close. - This corresponds to the start of **isovolumetric contraction** phase, where ventricular pressure rises rapidly with all valves closed. *Point B* - Point B typically represents **aortic valve opening** when left ventricular pressure exceeds aortic pressure. - At this point, the mitral valve is already **fully closed** and ventricular ejection begins. *Point C* - Point C usually corresponds to **peak systole** during the ejection phase when ventricular pressure is at its maximum. - The mitral valve remains **closed throughout systole** and does not begin closing at this point. *Point D* - Point D represents **aortic valve closure** at the end of systole when ventricular pressure falls below aortic pressure. - This marks the beginning of **diastole**, but mitral valve closure occurred much earlier at point A.

Question 160: What is true about troponin?

A. It has 3 subunits.
B. Troponin C binds to calcium.
C. Troponin I binds actin and tropomyosin.
D. All of the above. (Correct Answer)

Explanation: Troponin is a complex of three regulatory proteins integral to muscle contraction in skeletal and cardiac muscle. It is located on the thin (actin) filament and works in conjunction with tropomyosin to regulate the interaction between actin and myosin. **Explanation of Options:** * **Option A:** Troponin is indeed a heterotrimeric complex consisting of **three subunits**: Troponin T, Troponin I, and Troponin C. * **Option B:** **Troponin C (TnC)** is the calcium-binding subunit. In cardiac muscle, it has one high-affinity binding site for $Ca^{2+}$. When calcium binds to TnC, it induces a conformational change that moves the troponin-tropomyosin complex, exposing the myosin-binding sites on actin. * **Option C:** **Troponin I (TnI)** is the inhibitory subunit. It binds to **actin** to inhibit the ATPase activity of the actomyosin complex. It also interacts with **tropomyosin** to keep it in a position that blocks the cross-bridge cycle during relaxation. * **Troponin T (TnT):** (Though not a separate option) It binds to **tropomyosin**, anchoring the entire troponin complex to the thin filament. **Clinical Pearls for NEET-PG:** 1. **Cardiac Biomarkers:** Cardiac-specific isoforms (cTnI and cTnT) are the "gold standard" biomarkers for diagnosing Myocardial Infarction (MI) due to their high sensitivity and specificity. 2. **Troponin C Exception:** Unlike TnI and TnT, Troponin C is identical in both cardiac and slow-twitch skeletal muscle; therefore, it is **not** used as a specific diagnostic marker for MI. 3. **Smooth Muscle:** Troponin is **absent** in smooth muscle. Instead, calcium binds to **calmodulin**, which then activates Myosin Light Chain Kinase (MLCK).

Practice by Chapter

Cardiac Electrophysiology

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Cardiac Cycle

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Cardiac Output and Its Regulation

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Hemodynamics and Blood Flow

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Arterial System Physiology

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Microcirculation and Lymphatics

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Venous Return and Central Venous Pressure

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Cardiovascular Reflexes

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Regional Circulations

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Cardiovascular Responses to Exercise and Stress

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