Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 931: Which of the following is required for the Direct Fick method of measuring cardiac output?

A. O2 content of arterial blood
B. O2 consumption per unit time
C. O2 content of venous blood
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - The **Direct Fick method** calculates **cardiac output (CO)** using the formula: **CO = VO₂ / (CaO₂ - CvO₂)**, where VO₂ is oxygen consumption, CaO₂ is arterial oxygen content, and CvO₂ is mixed venous oxygen content. - Therefore, all three measurements—**O₂ content of arterial blood**, **O₂ consumption per unit time**, and **O₂ content of venous blood**—are essential components required for this calculation. - Each component plays a critical role in determining cardiac output: **O₂ content of arterial blood (CaO₂)** - Represents the oxygen delivered by the **arterial circulation** to the tissues - Essential for calculating the **arteriovenous oxygen difference (A-V O₂ difference)**, which reflects oxygen extraction by tissues - Typically measured from a systemic arterial sample **O₂ consumption per unit time (VO₂)** - Measures the body's **total oxygen utilization** per minute - Typically obtained through **spirometry** or metabolic cart measurements - Forms the **numerator** of the Fick equation, representing total oxygen uptake by tissues **O₂ content of venous blood (CvO₂)** - Indicates the **oxygen remaining in the blood** after tissue extraction - Must be measured from **mixed venous blood** (typically from pulmonary artery via right heart catheterization) - Combined with arterial O₂ content to determine the **A-V O₂ difference** (denominator of the equation) *Why other individual options are incomplete* - Selecting only one or two components would provide insufficient data to calculate cardiac output using the Direct Fick principle - The method fundamentally requires measuring both oxygen delivery (arterial content) and return (venous content), plus total consumption, to determine flow rate

Question 932: What does the first heart sound correspond to?

A. Mitral valve opening
B. Mitral valve closing (Correct Answer)
C. Aortic valve closing
D. Pulmonary valve closing

Explanation: ***Mitral valve closing*** - The **first heart sound (S1)** is primarily caused by the simultaneous **closure of the mitral and tricuspid valves** at the beginning of ventricular systole. - Mitral valve closure is the dominant component of S1 due to higher pressures in the left heart. *Mitral valve opening* - **Valve opening** is typically a silent event that does not produce an audible heart sound. - Abnormal sounds associated with valve opening, like an opening snap, occur in cases of **stenotic valves**. *Aortic valve closing* - The **aortic valve closes** at the end of ventricular systole, contributing to the **second heart sound (S2)**. - S2 also includes the closure of the **pulmonic valve**. *Pulmonary valve closing* - The **pulmonary valve closes** at the end of ventricular systole, contributing to the **second heart sound (S2)** along with the aortic valve. - This occurs after S1, not during it.

Question 933: At which point in the cardiac cycle does the closure of the mitral valve begin? (Refer to the diagram showing points A, B, C, and D)

A. Point A: Closure of the mitral valve (Correct Answer)
B. Point B: Opening of the aortic valve
C. Point C: Closure of the aortic valve
D. Point D: Opening of the mitral valve

Explanation: ***Point A: Closure of the mitral valve*** - As indicated in the Wigger's diagram, "Point A" directly corresponds to the event labeled "**Mitral Valve Closes**" in the "Pressure" section. - This closure signifies the beginning of **isovolumic contraction**, where ventricular pressure rises rapidly after filling and before the aortic valve opens. *Point B: Opening of the aortic valve* - The **opening of the aortic valve** occurs after the mitral valve has closed and the ventricular pressure has exceeded aortic pressure. - This point marks the beginning of the **ejection phase** of systole, not the closure of the mitral valve. *Point C: Closure of the aortic valve* - The **closure of the aortic valve** occurs at the end of ventricular ejection, initiating isovolumic relaxation. - This event is represented by the **dicrotic notch** and the second heart sound (S2), significantly later than mitral valve closure. *Point D: Opening of the mitral valve* - The **opening of the mitral valve** happens during ventricular diastole, allowing blood to flow from the atrium into the ventricle. - This occurs after the aortic valve has closed and ventricular pressure falls below atrial pressure, marking the beginning of **ventricular filling**.

Question 934: How would left ventricular failure affect the pressure-volume loop of the left ventricle?

A. Increased end-diastolic volume and decreased stroke volume (Correct Answer)
B. Increased end-diastolic volume with no change in stroke volume
C. Decreased end-diastolic volume with no change in stroke volume
D. Decreased end-diastolic volume and mildly decreased stroke volume

Explanation: ***Increased end-diastolic volume and decreased stroke volume*** - In **left ventricular failure**, the heart's pumping ability is impaired, leading to incomplete ejection of blood and a subsequent increase in the **end-diastolic volume (EDV)** as blood accumulates. - Due to the reduced contractility and inefficient ejection, the **stroke volume (SV)**, which is the volume of blood pumped out per beat, decreases significantly. *Increased end-diastolic volume with no change in stroke volume* - While **left ventricular failure** does lead to an increased **end-diastolic volume (EDV)** due to impaired pumping, it invariably results in a **reduced stroke volume (SV)**, not an unchanged one. - No change in stroke volume would imply that despite filling more, the heart maintains its ejection efficiency, which is contrary to the definition of heart failure. *Decreased end-diastolic volume with no change in stroke volume* - A **decreased end-diastolic volume (EDV)** would indicate less filling of the ventricle, often seen in conditions like hypovolemia or restrictive cardiomyopathy, which is not characteristic of **left ventricular failure**. - As explained, **left ventricular failure** is characterized by increased EDV and decreased SV. *Decreased end-diastolic volume and mildly decreased stroke volume* - **Left ventricular failure** predominantly causes an **increased end-diastolic volume (EDV)** due to incomplete emptying, rather than a decrease. - The decrease in **stroke volume (SV)** in heart failure is typically substantial rather than mild, reflecting the significant impairment in cardiac function.

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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