Cardiovascular System — MCQs

Cardiovascular System Indian Medical PG Practice Questions and MCQs

Question 321: The duration of a ventricular myocyte action potential is approximately:

A. Twice as long as the relative refractory period
B. As long in duration as the QRS complex
C. Nearly as long as the QT interval (Correct Answer)
D. Twice as long in skeletal muscle

Explanation: **Explanation:** The ventricular action potential (AP) is a prolonged event (lasting approximately 200–300 ms) characterized by a plateau phase (Phase 2) due to the influx of calcium ions. **Why Option C is correct:** On an Electrocardiogram (ECG), the **QT interval** represents the total time for ventricular depolarization and repolarization. Since the ventricular action potential encompasses both these phases (depolarization at Phase 0 and repolarization ending at Phase 3), the duration of a single ventricular myocyte AP is nearly identical to the QT interval. **Analysis of Incorrect Options:** * **Option A:** The Absolute Refractory Period (ARP) in cardiac muscle is exceptionally long, lasting almost as long as the entire contraction. The Relative Refractory Period (RRP) is much shorter, occurring during the final stages of repolarization. The AP duration is significantly longer than twice the RRP. * **Option B:** The **QRS complex** represents only ventricular depolarization (Phase 0). The action potential continues through the ST segment and T wave, making it much longer than the QRS complex. * **Option C:** In reality, cardiac action potentials are **10 to 100 times longer** than skeletal muscle action potentials (which last only 2–5 ms). **High-Yield NEET-PG Pearls:** * **Phase 2 (Plateau):** Mediated by L-type $Ca^{2+}$ channels (DHPR); this phase prevents tetanization of cardiac muscle. * **QT Interval Variability:** It varies with heart rate (shortens as HR increases). The "Corrected QT" (QTc) is calculated using **Bazett’s Formula**: $QTc = QT / \sqrt{RR \text{ interval}}$. * **Clinical Link:** Drugs that block $K^+$ channels (Class III antiarrhythmics) prolong the action potential duration, which manifests as a prolonged QT interval on ECG, increasing the risk of *Torsades de Pointes*.

Question 322: Systolic blood pressure is directly proportional to which of the following factors?

A. Compliance of the arterial wall
B. Stroke volume (Correct Answer)
C. Radius of the peripheral vessels
D. Elasticity of the arterial wall

Explanation: ### Explanation **1. Why Stroke Volume is the Correct Answer:** Systolic Blood Pressure (SBP) represents the maximum pressure exerted in the arteries during ventricular contraction. It is primarily determined by two factors: **Stroke Volume (SV)** and the **Compliance/Distensibility** of the large arteries. * **The Relationship:** SBP is **directly proportional** to Stroke Volume. When the left ventricle ejects a larger volume of blood into the aorta (increased SV), the arterial walls are stretched more significantly, leading to a higher peak pressure (SBP). **2. Analysis of Incorrect Options:** * **A & D. Compliance/Elasticity of the arterial wall:** SBP is **inversely proportional** to compliance. Compliance refers to the ability of a vessel to distend. In conditions like atherosclerosis (decreased compliance/increased stiffness), the aorta cannot expand to accommodate the stroke volume, leading to a sharp rise in SBP. * **C. Radius of the peripheral vessels:** The radius primarily determines **Total Peripheral Resistance (TPR)**. While TPR significantly influences **Diastolic Blood Pressure (DBP)**, it has a minimal direct effect on SBP compared to stroke volume. **3. NEET-PG High-Yield Pearls:** * **SBP Determinants:** Stroke Volume (Major) and Large Artery Compliance. * **DBP Determinants:** Total Peripheral Resistance (Major) and Heart Rate. * **Pulse Pressure (PP):** Defined as SBP minus DBP. It is directly proportional to Stroke Volume and inversely proportional to Compliance. * **Windkessel Effect:** The elastic recoil of the aorta during diastole that maintains continuous blood flow; loss of this effect (in old age) increases SBP and decreases DBP, leading to **Isolated Systolic Hypertension**.

Question 323: Which of the following factors affect pulse pressure?

A. Stroke volume
B. Compliance of aorta
C. Ejection fraction
D. All of the above (Correct Answer)

Explanation: **Explanation:** Pulse Pressure (PP) is defined as the difference between Systolic Blood Pressure (SBP) and Diastolic Blood Pressure (DBP). Mathematically, it is expressed as: **Pulse Pressure ≈ Stroke Volume / Arterial Compliance** **1. Why "All of the above" is correct:** * **Stroke Volume (SV):** This is the primary determinant of pulse pressure. An increase in SV (the amount of blood ejected into the aorta per beat) leads to a proportional increase in SBP, thereby widening the pulse pressure. * **Compliance of Aorta:** Compliance refers to the distensibility of the vessel. In a healthy aorta, high compliance buffers the pressure rise during systole. If compliance decreases (e.g., in atherosclerosis or aging), the aorta becomes "stiff," leading to a sharp rise in SBP and a wider pulse pressure. * **Ejection Fraction (EF):** EF is the ratio of SV to End-Diastolic Volume. Since SV is a direct component of EF, any change in the heart's contractile efficiency (EF) directly influences the volume of blood entering the aorta, subsequently affecting the pulse pressure. **2. High-Yield Clinical Pearls for NEET-PG:** * **Widened Pulse Pressure:** Seen in conditions like **Aortic Regurgitation** (classic "water-hammer pulse"), Hyperthyroidism, Patent Ductus Arteriosus (PDA), and Atherosclerosis. * **Narrowed Pulse Pressure:** Seen in **Aortic Stenosis**, Cardiac Tamponade, and Severe Heart Failure. * **Key Concept:** While Mean Arterial Pressure (MAP) is determined by Cardiac Output and Total Peripheral Resistance, **Pulse Pressure** is primarily determined by **Stroke Volume** and **Vessel Compliance**.

Question 324: A patient has a blood pressure of 130/80 mmHg, a cardiac output of 5000 ml, and a heart rate of 70 beats per minute. Calculate the volume of blood pumped with each beat.

A. 70 ml (Correct Answer)
B. 110 ml
C. 50 ml
D. 90 ml

Explanation: **Explanation** The correct answer is **70 ml**. This question tests the fundamental physiological relationship between Cardiac Output (CO), Heart Rate (HR), and Stroke Volume (SV). **1. Why the Correct Answer is Right:** The volume of blood pumped with each beat is defined as the **Stroke Volume (SV)**. The relationship is expressed by the formula: * **Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR)** * Rearranging for SV: **SV = CO / HR** * Calculation: $5000\text{ ml/min} \div 70\text{ beats/min} \approx 71.4\text{ ml}$ * Rounding to the nearest option gives **70 ml**, which is the standard physiological average for a healthy adult. **2. Why the Incorrect Options are Wrong:** * **B (110 ml):** This value is closer to the **End-Diastolic Volume (EDV)**, which is the total volume in the ventricle before contraction, not the amount ejected. * **C (50 ml):** This represents a reduced stroke volume, often seen in heart failure or tachyarrhythmias where filling time is compromised. * **D (90 ml):** While possible in athletes or during exercise (due to increased contractility), it does not fit the mathematical parameters provided in the question. **3. NEET-PG High-Yield Clinical Pearls:** * **Pulse Pressure:** In this patient, it is $130 - 80 = 50\text{ mmHg}$. Pulse pressure is directly proportional to stroke volume and inversely proportional to aortic compliance. * **Ejection Fraction (EF):** $EF = (SV / EDV) \times 100$. Normal range is 55–70%. * **Fick’s Principle:** Remember that CO can also be calculated as $\text{Oxygen consumption} / (\text{Arterial } O_2 \text{ content} - \text{Venous } O_2 \text{ content})$. * **Preload & Afterload:** Stroke volume is increased by preload (Frank-Starling law) and contractility, but decreased by excessive afterload.

Question 325: All are cardiovascular effects of parasympathetic stimulation except?

A. Decreased heart rate
B. Decreased conduction
C. Increased automaticity (Correct Answer)
D. Increased refractive period

Explanation: **Explanation:** The parasympathetic nervous system (PNS) influences the heart primarily through the **Vagus nerve (Cranial Nerve X)**, which releases **Acetylcholine (ACh)**. ACh acts on **M2 muscarinic receptors**, leading to inhibitory effects on cardiac tissue. **Why "Increased Automaticity" is the correct answer:** Parasympathetic stimulation **decreases automaticity**, particularly in the SA node. It does this by increasing K+ conductance (hyperpolarization) and decreasing the slope of Phase 4 spontaneous depolarization. Therefore, "Increased automaticity" is the opposite of what the PNS does; increased automaticity is a hallmark of **sympathetic** (adrenergic) stimulation. **Analysis of Incorrect Options:** * **Decreased heart rate (Negative Chronotropy):** ACh slows the firing rate of the SA node by hyperpolarizing the resting membrane potential. This is a classic PNS effect. * **Decreased conduction (Negative Dromotropy):** Vagal stimulation significantly slows conduction through the AV node by increasing the PR interval. This can lead to a physiological heart block in cases of high vagal tone. * **Increased refractive period:** By slowing the recovery of ion channels and slowing conduction velocity (especially in the AV node), the PNS increases the effective refractory period (ERP), protecting the ventricles from rapid atrial rates. **High-Yield Clinical Pearls for NEET-PG:** * **Vagal Escape:** If the vagus nerve is overstimulated, the heart may stop, but a latent pacemaker (like the Purkinje fibers) will eventually take over. * **Ventricular Effect:** Parasympathetic innervation to the ventricles is sparse compared to the atria; thus, its effect on ventricular contractility (Inotropy) is minimal. * **Atropine:** A muscarinic antagonist used to treat symptomatic bradycardia by blocking these parasympathetic effects.

Question 326: What is shown by the QT interval?

A. Hypocalcemia
B. Hypokalemia
C. Hypercalcemia (Correct Answer)
D. Hyperkalemia

Explanation: The **QT interval** represents the total time for ventricular depolarization and repolarization. Its duration is primarily determined by the length of the **ventricular action potential plateau (Phase 2)**, which is mediated by the inward movement of Calcium ($Ca^{2+}$) ions. ### Why Hypercalcemia is Correct In **Hypercalcemia**, the increased extracellular calcium concentration shortens the duration of the action potential plateau. Because the plateau phase ends sooner, ventricular repolarization occurs earlier, leading to a **shortened QT interval**. This is a classic high-yield ECG finding. ### Explanation of Incorrect Options * **Hypocalcemia:** Low serum calcium levels prolong the Phase 2 plateau of the action potential. This results in a **prolonged QT interval**, the opposite of the effect seen in hypercalcemia. * **Hypokalemia:** Characteristically causes **ST-segment depression, T-wave flattening/inversion, and the appearance of U-waves**. While it may appear to prolong the "QU" interval, it does not typically shorten the QT interval. * **Hyperkalemia:** The earliest sign is **tall, peaked "tented" T-waves**. As levels rise, it leads to PR prolongation, loss of P-waves, and widening of the QRS complex (forming a sine wave pattern), but not a shortened QT interval. ### NEET-PG High-Yield Pearls * **QT Interval Rule of Thumb:** Hypercalcemia = Short QT; Hypocalcemia = Long QT. * **Digoxin Effect:** Digoxin toxicity also causes a shortened QT interval, often accompanied by the characteristic "reverse tick" or "scooped" ST-segment depression. * **Correction:** Because the QT interval varies with heart rate, clinicians use the **Bazett formula** to calculate the **Corrected QT (QTc)**. * **Congenital Long QT Syndromes:** Romano-Ward (autosomal dominant) and Jervell and Lange-Nielsen (autosomal recessive + sensorineural deafness).

Question 327: What is the normal QT interval in seconds?

A. 0.12 - 0.20
B. 0.40 - 0.43 (Correct Answer)
C. 0.08 - 0.10
D. None of the above

Explanation: The **QT interval** represents the total time required for **ventricular depolarization and repolarization**. On an ECG, it is measured from the beginning of the Q wave to the end of the T wave. ### **Explanation of Options** * **Correct Answer (B) 0.40 – 0.43 seconds:** In a healthy adult with a normal heart rate (approx. 60–100 bpm), the QT interval typically ranges between 0.35 and 0.44 seconds. The value 0.40 – 0.43 seconds falls squarely within this physiological range. * **Option A (0.12 – 0.20 s):** This represents the normal **PR interval**, which is the time taken for an impulse to travel from the SA node to the ventricles. * **Option C (0.08 – 0.10 s):** This represents the normal duration of the **QRS complex**, indicating the time taken for ventricular depolarization. ### **Clinical Pearls for NEET-PG** 1. **Heart Rate Dependency:** The QT interval varies inversely with heart rate (shortens as HR increases). Therefore, clinicians use the **Corrected QT (QTc)**, most commonly calculated using **Bazett’s Formula**: $QTc = \frac{QT}{\sqrt{RR \text{ interval}}}$. 2. **Long QT Syndrome (LQTS):** A QTc > 0.44s in men or > 0.46s in women is considered prolonged. This predisposes patients to a life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes**. 3. **Electrolyte Correlation:** * **Hypocalcemia** prolongs the QT interval. * **Hypercalcemia** shortens the QT interval. 4. **Drug-Induced Prolongation:** Common culprits include Class IA and III antiarrhythmics, Macrolides, Fluoroquinolones, and Antipsychotics.

Question 328: In a patient with a transplanted heart, which of the following mechanisms contributes to the increase in cardiac output during exercise?

A. Reinnervation of the transplanted heart by the vagus nerve
B. Intrinsic contractile mechanisms of the transplanted heart
C. Circulating epinephrine released from the adrenal medulla (Correct Answer)
D. The Bainbridge reflex

Explanation: **Explanation:** In a **transplanted heart**, the organ is surgically **denervated**. This means it lacks direct autonomic (sympathetic and parasympathetic) nerve supply. Consequently, the heart cannot respond to immediate neural impulses that typically increase heart rate and contractility at the onset of exercise. **1. Why Option C is Correct:** Since the heart is denervated, it relies on **humoral mechanisms** rather than neural ones. During exercise, the sympathetic nervous system triggers the adrenal medulla to release **epinephrine** into the bloodstream. This circulating epinephrine acts on the $\beta_1$-adrenergic receptors of the transplanted heart, leading to a delayed but significant increase in heart rate (chronotropy) and force of contraction (inotropy), thereby increasing cardiac output. **2. Why Incorrect Options are Wrong:** * **Option A:** Reinnervation of the vagus nerve is generally incomplete or absent in the clinical timeframe of most transplant patients. The heart remains functionally denervated. * **Option B:** While the Frank-Starling mechanism (intrinsic) does help increase stroke volume via increased venous return, the primary driver for the sustained increase in cardiac output during exercise in these patients is the hormonal response. * **Option C:** The **Bainbridge reflex** requires intact afferent and efferent vagal pathways to increase heart rate in response to increased atrial pressure. Since the heart is denervated, this reflex is abolished. **High-Yield Clinical Pearls for NEET-PG:** * **Resting Heart Rate:** A transplanted heart has a higher-than-normal resting heart rate (90–110 bpm) because the inhibitory influence of the **Vagus nerve** is lost. * **Exercise Response:** There is a **"lag" phase** at the start of exercise (slow rise in HR) and a delayed recovery to baseline after exercise because the heart must wait for circulating catecholamines to rise and fall. * **Drugs:** Atropine will **not** increase the heart rate in a transplant patient (no vagal tone to block).

Question 329: Cardiac output depends on all of the following except?

A. Cardiac rate
B. Body surface area (Correct Answer)
C. Stroke volume
D. Cardiac contractility

Explanation: **Explanation:** The fundamental equation for cardiac output is **Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR)**. **Why Body Surface Area (BSA) is the correct answer:** Cardiac output is an absolute measurement of the volume of blood pumped by the heart per minute (typically ~5 L/min). While CO is often *normalized* to a person's body size to allow for clinical comparison between individuals of different sizes—a parameter known as the **Cardiac Index (CI = CO/BSA)**—the actual generation of cardiac output itself does not depend on the body surface area. BSA is a scaling factor used for interpretation, not a physiological determinant of the heart's pumping capacity. **Analysis of incorrect options:** * **Cardiac Rate (HR):** As per the formula (CO = SV × HR), any change in heart rate directly impacts the cardiac output. * **Stroke Volume (SV):** This is the volume of blood ejected per beat. It is a primary determinant of CO. * **Cardiac Contractility:** This is a major determinant of Stroke Volume (along with preload and afterload). Increased contractility (inotropy) increases the ejection fraction and stroke volume, thereby increasing cardiac output. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Index (CI):** Normal range is **2.5 to 4.0 L/min/m²**. It is a more accurate indicator of whether the CO is sufficient for an individual's metabolic needs. * **Fick’s Principle:** The gold standard for measuring CO. $CO = \text{Oxygen Consumption} / (\text{Arterial } O_2 \text{ content} - \text{Mixed Venous } O_2 \text{ content})$. * **Preload:** According to the **Frank-Starling Law**, increased preload increases SV and thus CO, up to a physiological limit.

Question 330: What is the main role of angiotensin II?

A. Increased TPR (Correct Answer)
B. Constriction of afferent renal arteriole
C. Decreased release of aldosterone
D. Diuresis

Explanation: **Explanation:** Angiotensin II (AT-II) is a potent octapeptide and a central component of the Renin-Angiotensin-Aldosterone System (RAAS). Its primary physiological role is to maintain blood pressure and fluid balance. **1. Why "Increased TPR" is correct:** Angiotensin II is one of the most powerful direct **vasoconstrictors** known. It acts on **AT1 receptors** located on vascular smooth muscle cells throughout the systemic circulation. By causing widespread arteriolar constriction, it significantly increases **Total Peripheral Resistance (TPR)**, which directly raises the Mean Arterial Pressure (MAP = CO × TPR). **2. Analysis of Incorrect Options:** * **B. Constriction of afferent renal arteriole:** AT-II preferentially constricts the **efferent arteriole** more than the afferent. This increases glomerular capillary hydrostatic pressure, thereby maintaining the Glomerular Filtration Rate (GFR) even when renal blood flow is low. * **C. Decreased release of aldosterone:** AT-II actually **stimulates** the zona glomerulosa of the adrenal cortex to release aldosterone, which promotes sodium and water retention. * **D. Diuresis:** AT-II is **anti-diuretic** and **anti-natriuretic**. It promotes water and salt retention both directly (by stimulating Na+/H+ exchange in the proximal tubule) and indirectly (via aldosterone and ADH release). **High-Yield Clinical Pearls for NEET-PG:** * **Receptor Specificity:** Most known cardiovascular effects (vasoconstriction, thirst, aldosterone release) are mediated via **AT1 receptors**. AT2 receptors generally mediate vasodilation and anti-proliferation. * **ACE Inhibitors/ARBs:** These are first-line antihypertensives because they block the production or action of AT-II, leading to decreased TPR. * **Thirst Center:** AT-II acts on the subfornical organ in the brain to stimulate the thirst mechanism.

Practice by Chapter

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Cardiovascular Responses to Exercise and Stress

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