Cardiovascular System Practice Questions

Q: The AH interval on an ECG represents conduction through which pathway?

The AV node to the bundle of His. ### Explanation The **AH interval** is a measurement obtained during an **Electrophysiology Study (EPS)** using an intracardiac electrogram. It represents the time taken for the electrical impulse to travel from the low right atrium (A) to the Bundle of His (H). **1. Why the correct answer is right:** The AH interval primarily reflects **conduction through the AV node**. It is measured from the onset of the atrial deflection (A) to the first rapid deflection of the His bundle electrogram (H). Because the AV node is the site of physiological delay in the cardiac conduction system, the AH interval is the most significant component of this measurement. It is highly sensitive to autonomic tone and medications (like beta-blockers or calcium channel blockers). **2. Why the incorrect options are wrong:** * **Option A & C:** The **HV interval** (His to Ventricle) represents conduction through the **His-Purkinje system**. It is measured from the His bundle deflection to the onset of ventricular depolarization (V). * **Option D:** Conduction through the **ventricular fibers** is represented by the **QRS complex** on a surface ECG or the ventricular deflection (V) on an intracardiac electrogram. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Normal AH Interval:** 50–120 ms. * **Normal HV Interval:** 35–55 ms. * **Vagal Stimulation:** Increases the AH interval (slows AV conduction) but has no effect on the HV interval. * **Atropine:** Decreases the AH interval. * **Clinical Significance:** A prolonged AH interval indicates **supra-Hisian block** (AV nodal disease), whereas a prolonged HV interval indicates **infra-Hisian block** (distal conduction system disease), which carries a higher risk of progressing to complete heart block.

Q: Peripheral resistance is inversely proportional to which of the following?

Radius. ### Explanation The relationship between peripheral resistance and the physical properties of blood vessels is governed by **Poiseuille’s Law**. The formula for resistance ($R$) is: $$R = \frac{8 \eta L}{\pi r^4}$$ Where: * $\eta$ = Viscosity of blood * $L$ = Length of the vessel * $r$ = Radius of the vessel **Why Radius is Correct:** According to the formula, resistance is **inversely proportional to the fourth power of the radius** ($R \propto 1/r^4$). This means even a small decrease in the radius (vasoconstriction) leads to a massive increase in peripheral resistance. Because it is the only factor in the denominator, it is the correct inverse relationship. **Why Other Options are Incorrect:** * **B. Viscosity:** Resistance is **directly proportional** to viscosity. Conditions like polycythemia increase viscosity, thereby increasing resistance. * **C. Length:** Resistance is **定期 proportional** to the length of the vessel. While vessel length is constant in adults, increased body mass (e.g., obesity) increases total vessel length and resistance. * **D. Elasticity:** While elasticity affects vessel compliance and pulse pressure, it is not a primary variable in Poiseuille’s equation for calculating systemic vascular resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Arterioles** are known as the primary **"Resistance Vessels"** of the body because they have the greatest ability to change their radius. * If the radius of a vessel is halved, the resistance increases by **16 times** ($2^4$). * **Critical Closing Pressure:** The internal pressure at which a small blood vessel collapses and flow ceases; it is heavily influenced by sympathetic tone and radius.

Q: All of the following are true about fetal circulation EXCEPT?

Pulmonary vascular resistance is low. **Explanation:** In fetal circulation, the lungs are non-functional for gas exchange. Consequently, the **pulmonary vascular resistance (PVR) is extremely high** due to hypoxic pulmonary vasoconstriction and the physical compression of vessels by fluid-filled alveoli. This high resistance shunts the majority of right ventricular output away from the lungs through the **ductus arteriosus** into the systemic circulation. Therefore, Option A is the false statement. **Analysis of other options:** * **Option B:** True. The two umbilical arteries carry deoxygenated, nutrient-poor blood from the fetus back to the placenta. * **Option C:** True. In utero, the lungs are collapsed (non-expanded) and the potential air spaces are filled with fetal lung fluid, not air. * **Option D:** True. The placenta serves as the organ of respiration for the fetus, where gas exchange occurs between maternal and fetal blood. **High-Yield NEET-PG Pearls:** 1. **Oxygen Saturation:** The highest $PO_2$ in fetal circulation is found in the **umbilical vein** (approx. 80% saturation), which carries oxygenated blood from the placenta to the fetus. 2. **First Breath Changes:** At birth, lung expansion and increased $O_2$ levels cause a dramatic **drop in PVR** and a rise in systemic vascular resistance (due to loss of low-resistance placental circuit). 3. **Shunts:** There are three anatomical shunts in fetal life: **Ductus venosus** (bypasses liver), **Foramen ovale** (bypasses lungs, RA to LA), and **Ductus arteriosus** (bypasses lungs, Pulmonary Artery to Aorta).

Q: Which of the following statements regarding Laplace's Law is not true?

P = T/w. **Explanation:** Laplace’s Law describes the relationship between the transmural pressure, wall tension, and the radius of a hollow organ (like a blood vessel or cardiac chamber). In cardiovascular physiology, it is used to understand how the heart compensates for changes in pressure and volume. **Why Option D is the Correct (False) Statement:** The formula **P = T/w** is incorrect because pressure (P) is not simply tension divided by wall thickness. According to the modified Laplace Law for thick-walled structures (like the left ventricle), the relationship is expressed as **Wall Stress (σ) = Pr / 2w**. Pressure itself is determined by the tension and radius, not the thickness alone. **Analysis of Other Options:** * **Option B & C (P = 2T/r and P = T/r):** These are the standard forms of Laplace’s Law. **P = 2T/r** applies to spherical structures (like the cardiac ventricles or pulmonary alveoli), while **P = T/r** applies to cylindrical structures (like blood vessels). * **Option A (T = Pr/w):** This represents the formula for **Wall Stress (Tension per unit area)**. It shows that as the radius (r) or pressure (P) increases, the tension on the wall increases, but an increase in wall thickness (w) helps distribute and reduce that stress. **NEET-PG High-Yield Pearls:** 1. **Cardiac Hypertrophy:** In chronic hypertension (increased P), the heart undergoes concentric hypertrophy (increased thickness, w) to keep wall stress (T) constant. 2. **Aneurysms:** As a vessel dilates (increased r), the wall tension (T) required to withstand the same pressure increases, making the vessel more likely to rupture. 3. **Heart Failure:** In a dilated failing heart (increased r), the wall tension is significantly higher, requiring more myocardial oxygen consumption to generate the same systolic pressure.

Q: The Lewis triple response is caused due to:

Histamine. The **Lewis Triple Response** is a classic physiological reaction of the skin to mechanical injury or chemical irritation. It is primarily mediated by the release of **Histamine** from mast cells. ### Why Histamine is the Correct Answer When the skin is firmly stroked or injured, mast cells release histamine. This chemical mediator acts on H1 receptors to produce three distinct components: 1. **Red Reaction (Red Spot):** Localized capillary dilatation occurring within seconds. 2. **Flare:** A spreading redness caused by the **Axon Reflex**, where sensory nerve stimulation leads to the release of Substance P and CGRP, causing arteriolar dilatation. 3. **Wheal:** Localized edema (swelling) due to increased capillary permeability and exudation of fluid. ### Why Other Options are Incorrect * **B. Axon Reflex:** While the axon reflex is the *mechanism* responsible for the "Flare" component, it is not the primary *cause* of the entire triple response. Histamine is the chemical trigger that initiates the reflex. * **C. Injury to Endothelium:** While physical injury triggers the response, the physiological manifestation is a result of chemical mediators (histamine) rather than direct structural damage to the endothelium itself. * **D. Increased Permeability:** This is a *consequence* of histamine release (leading to the Wheal), not the underlying cause of the triple response. ### High-Yield NEET-PG Pearls * **Mediator:** Histamine is the chief mediator. * **Nerve Involvement:** The "Flare" is the only component that requires an intact nerve supply (Axon Reflex). If the sensory nerves are severed and allowed to degenerate, the flare will be absent. * **Antihistamines:** Pre-treatment with H1-receptor antagonists can significantly diminish all three components of the Lewis triple response.

Q: Ventricular muscle receives electrical impulses directly from which structure?

Purkinje system. **Explanation:** The conduction of electrical impulses through the heart follows a specific hierarchical sequence to ensure coordinated ventricular contraction. The correct sequence is: **SA node → Internodal pathways → AV node → Bundle of His → Right and Left Bundle Branches → Purkinje fibers → Ventricular Myocardium.** **Why Purkinje system is correct:** The Purkinje fibers represent the terminal portion of the cardiac conduction system. They are specialized cells located just beneath the endocardium that penetrate the ventricular muscle. Because they are in direct physical contact with the contractile myocytes of the ventricles, they are the immediate structures that deliver the electrical impulse to the ventricular muscle to initiate systole. **Why other options are incorrect:** * **AV node:** Acts as a "gatekeeper," delaying the impulse to allow for atrial emptying. It is located in the interatrial septum, far from the ventricular muscle. * **Bundle of His:** This is the only electrical bridge between the atria and ventricles, but it passes through the fibrous skeleton and does not directly stimulate the myocardium. * **Right and Left bundle branches:** These are subdivisions of the Bundle of His that travel down the interventricular septum. While they carry the impulse toward the apex, they must first branch into the Purkinje network before the impulse can reach the ventricular myocytes. **High-Yield NEET-PG Pearls:** * **Velocity:** Purkinje fibers have the **fastest conduction velocity** in the heart (approx. 1.5–4.0 m/s), ensuring near-simultaneous contraction of the ventricles. * **Slowest Conduction:** Occurs at the **AV node** (approx. 0.01–0.05 m/s), known as the "AV nodal delay." * **Pacemaker Hierarchy:** If the SA node fails, the Purkinje system can act as a tertiary pacemaker with an intrinsic rate of **15–40 bpm**.

Q: Cerebral blood flow auto-regulation occurs within which range of mean arterial pressure?

60-140 mmHg. **Explanation:** **Cerebral Autoregulation** is the physiological process that maintains a constant Cerebral Blood Flow (CBF) despite fluctuations in Mean Arterial Pressure (MAP). This ensures the brain receives a steady supply of oxygen and nutrients. 1. **Why Option A is Correct:** In a healthy normotensive individual, the autoregulatory range is typically **60–140 mmHg** (some texts cite up to 150 or 160 mmHg). Within this window, cerebral arterioles undergo **myogenic adaptation**: they constrict when MAP rises and dilate when MAP falls to keep CBF stable at approximately **50 ml/100g/min**. 2. **Why Other Options are Incorrect:** * **Option B (>200 mmHg):** At pressures above the upper limit (>160 mmHg), autoregulation fails. The high pressure forces the vessels to over-distend, leading to "breakthrough perfusion," which can cause cerebral edema and hypertensive encephalopathy. * **Option C (<50 mmHg):** Below the lower limit (~60 mmHg), maximal vasodilation is reached. Any further drop in MAP leads to a linear decrease in CBF, increasing the risk of cerebral ischemia and syncope. * **Option D (<10 mmHg):** This pressure is incompatible with life and represents profound circulatory collapse. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Hypertension:** The autoregulatory curve **shifts to the right**. These patients require higher MAP to maintain CBF, making them sensitive to rapid blood pressure lowering. * **Most Potent Regulator:** While pressure is important, **Arterial CO₂ (PaCO₂)** is the most potent physiological regulator of CBF. Hypercapnia causes potent vasodilation. * **Cushing’s Reflex:** A triad of hypertension, bradycardia, and irregular respiration, seen when intracranial pressure (ICP) rises to levels approaching MAP.

Q: Which of the following represents the QT interval on an electrocardiogram?

The interval from the beginning of ventricular depolarization to the end of ventricular repolarization.. **Explanation** The **QT interval** represents the total time required for **ventricular depolarization and repolarization**. It is measured from the beginning of the QRS complex (start of ventricular depolarization) to the end of the T wave (completion of ventricular repolarization). Electrophysiologically, it reflects the duration of the ventricular action potential. **Analysis of Options:** * **Option B (Correct):** Accurately defines the QT interval as the entire period of ventricular electrical activity. * **Option A (Incorrect):** The interval from the beginning of atrial depolarization (P wave) to the beginning of ventricular depolarization is the **PR interval**. * **Option C (Incorrect):** Atrial repolarization occurs during the QRS complex and is usually masked; there is no standard named interval for this specific period. * **Option D (Incorrect):** The duration of atrial depolarization is represented by the **P wave** itself. **High-Yield Clinical Pearls for NEET-PG:** 1. **Heart Rate Dependency:** The QT interval varies inversely with heart rate. Therefore, the **Corrected QT (QTc)** is used in clinical practice, most commonly calculated using **Bazett’s Formula**: $QTc = \frac{QT}{\sqrt{RR \text{ interval}}}$. 2. **Normal Values:** A normal QTc is generally $<440$ ms in men and $<460$ ms in women. 3. **Clinical Significance:** * **Prolonged QT:** Increases the risk of **Torsades de Pointes** (a polymorphic ventricular tachycardia). Causes include hypokalemia, hypocalcemia, and drugs (e.g., Macrolides, Quinolones, Antipsychotics). * **Shortened QT:** Often seen in hypercalcemia and digoxin effect.

Question 1

The AH interval on an ECG represents conduction through which pathway?

Accepted Answer

The AV node to the bundle of His

Answer

The His-Purkinje system

Answer

The Purkinje system to ventricular fibers

Answer

The ventricular fibers

Question 2

Peripheral resistance is inversely proportional to which of the following?

Accepted Answer

Radius

Answer

Viscosity

Answer

Length

Answer

Elasticity of blood vessels

Question 3

All of the following are true about fetal circulation EXCEPT?

Accepted Answer

Pulmonary vascular resistance is low

Answer

Umbilical arteries carry deoxygenated blood

Answer

Lungs are collapsed and filled with fluid

Answer

Source of oxygen in fetal circulation is the placenta

Question 4

Which of the following statements regarding Laplace's Law is not true?

Accepted Answer

P = T/w

Answer

T = Pr/W

Answer

P = 2T/r

Answer

P = T/r

Question 5

The Lewis triple response is caused due to:

Accepted Answer

Histamine

Answer

Axon reflex

Answer

Injury to endothelium

Answer

Increased permeability

Question 6

Ventricular muscle receives electrical impulses directly from which structure?

Accepted Answer

Purkinje system

Answer

Bundle of His

Answer

Right and Left bundle branches

Answer

AV node

Question 7

Cerebral blood flow auto-regulation occurs within which range of mean arterial pressure?

Accepted Answer

60-140 mmHg

Answer

>200 mmHg

Answer

<50 mmHg

Answer

<10 mmHg

Question 8

A 25-year-old man has a muscle blood flow of 500 ml/min. His mean arterial pressure is 160 mmHg and mean venous pressure of the muscle is 10 mmHg. Calculate the vascular resistance of the muscle.

Accepted Answer

0.3 mmHg/ml/min

Answer

0.5 mmHg/ml/min

Answer

0.8 mmHg/ml/min

Answer

1 mmHg/ml/min

Question 9

Which of the following represents the QT interval on an electrocardiogram?

Accepted Answer

The interval from the beginning of ventricular depolarization to the end of ventricular repolarization.

Answer

The interval from the beginning of atrial depolarization to the beginning of ventricular repolarization.

Answer

The interval from the beginning of atrial repolarization to the beginning of ventricular depolarization.

Answer

The duration of atrial depolarization.

Question 10

Which of the following statements regarding nitric oxide is FALSE?

Accepted Answer

Acts by increasing cyclic GMP levels

Answer

Derived from endothelium

Answer

Vasodilator

Answer

Derived from arginine

Cardiovascular System — MCQs

Cardiovascular System — MCQs

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