Cardiovascular System Practice Questions

Q: Rapid depolarization (phase 0) of the action potential of ventricular muscle results from the opening of which type of channels?

Voltage-gated Na+ channels. **Explanation:** The ventricular muscle action potential is a **"fast response"** action potential consisting of five distinct phases (0–4). **1. Why the Correct Answer is Right:** * **Phase 0 (Rapid Depolarization):** When the cell membrane reaches its threshold potential (approx. -70mV), **fast voltage-gated Na+ channels** open. This leads to a massive, rapid influx of sodium ions into the cell, causing the membrane potential to shoot up from -90mV to approximately +20 to +30mV. This phase is characterized by a high $dV/dt$ (slope), representing rapid conduction velocity. **2. Why the Incorrect Options are Wrong:** * **Option A (Voltage-gated Ca2+ channels):** These are responsible for the **Phase 2 (Plateau phase)** of the ventricular action potential. While they do cause depolarization in the SA/AV nodes (slow response potentials), they are not responsible for Phase 0 in ventricular muscle. * **Option C (ACh-activated K+ channels):** These are found primarily in the SA node and atria. Activation by the vagus nerve (parasympathetic) causes hyperpolarization and slows the heart rate; they do not contribute to ventricular depolarization. * **Option D (Inward rectifying K+ channels):** These channels (specifically $I_{K1}$) are primarily responsible for maintaining the **Resting Membrane Potential (Phase 4)** and contributing to late repolarization. **High-Yield Clinical Pearls for NEET-PG:** * **Class I Antiarrhythmics** (e.g., Lidocaine, Flecainide) work by blocking these Phase 0 voltage-gated Na+ channels. * **Tetrodotoxin (Pufferfish toxin)** specifically blocks these fast Na+ channels, inhibited Phase 0. * **Contrast with Pacemaker Cells:** In the SA node, Phase 0 is caused by **Ca2+ influx** (L-type channels), not Na+ influx. This is a frequent point of confusion in exams.

Q: A 50-year-old woman is brought to the emergency department unconscious following a motor vehicle accident. She is placed on telemetry to monitor her vitals continuously, and she remains hypotensive despite receiving a bolus of fluids and being placed on IV fluids. She regains consciousness and her pulse is 180/mm. Her ECG shows regularly spaced narrow, QRS complexes. No P wave is visible in most leads, but there is a small downward deflection immediately following QRS complexes in lead II. Carotid massage is performed, and her pulse is reduced to 80/min. Stimulation of the afferent fibres in which of the following nerves most likely resulted in the favourable response observed?

Glossopharyngeal nerve. **Explanation:** The clinical presentation describes a patient in **Supraventricular Tachycardia (SVT)**, specifically AV Nodal Reentrant Tachycardia (AVNRT), characterized by a rapid heart rate (180/min), narrow QRS complexes, and retrograde P waves (the "downward deflection" following the QRS). The successful resolution of the tachycardia via carotid massage is due to the **Baroreceptor Reflex**. 1. **Why Glossopharyngeal Nerve is Correct:** The **Carotid Sinus**, located at the bifurcation of the common carotid artery, contains baroreceptors sensitive to stretch. Carotid massage mimics high blood pressure, stimulating these receptors. The **afferent (sensory) limb** of this reflex is carried by the **Hering’s nerve**, which is a branch of the **Glossopharyngeal nerve (CN IX)**. These signals reach the nucleus tractus solitarius in the medulla, leading to increased parasympathetic (vagal) outflow and decreased sympathetic tone, which slows conduction through the AV node and terminates the SVT. 2. **Why Other Options are Incorrect:** * **Cardiac/Carotid Sympathetic Nerves:** These are involved in the efferent limb of sympathetic responses (increasing heart rate/contractility). Stimulating these would worsen tachycardia, not resolve it. * **Hypoglossal Nerve (CN XII):** This is a purely motor nerve responsible for tongue movements and has no role in the baroreceptor reflex arc. **High-Yield Pearls for NEET-PG:** * **Baroreceptor Reflex Arc:** Afferent = CN IX (Carotid sinus) & CN X (Aortic arch); Efferent = CN X (Parasympathetic) & Sympathetic chains. * **Vagal Maneuvers:** First-line treatment for stable SVT (Carotid massage, Valsalva). * **Drug of Choice for SVT:** Adenosine (6mg rapid IV bolus). * **Contraindication:** Never perform bilateral carotid massage simultaneously, as it can cause profound bradycardia or cerebral ischemia.

Q: Stimulation of the sympathetic nerves to the normal heart:

Decreases the duration of the QT interval. **Explanation:** Sympathetic stimulation of the heart is mediated by **Norepinephrine** acting on **$\beta_1$ receptors**. This results in positive chronotropy (increased heart rate) and positive dromotropy (increased conduction velocity). **Why Option C is Correct:** The **QT interval** represents the total time for ventricular depolarization and repolarization (ventricular systole). When the heart rate increases due to sympathetic stimulation, the cardiac cycle shortens. To maintain efficiency at high rates, the action potential duration must decrease. Sympathetic activity accelerates repolarization by increasing the activity of potassium channels (delayed rectifiers), thereby **shortening the QT interval**. **Analysis of Incorrect Options:** * **Option A:** The **TP interval** represents the period of ventricular diastole. As heart rate increases, the diastolic period (TP interval) is the most significantly **shortened** component of the cardiac cycle. * **Option B:** Sympathetic stimulation increases conduction velocity through the AV node (positive dromotropy). This results in a **shorter PR interval**, not a longer one. * **Option D:** In a normal heart, every P wave is followed by a QRS complex (1:1 conduction). Sympathetic stimulation does not cause AV block; rather, it enhances conduction. **NEET-PG High-Yield Pearls:** * **Bazett’s Formula:** Used to calculate the corrected QT ($QTc = QT / \sqrt{RR}$). It adjusts for the fact that QT interval naturally varies with heart rate. * **Vagal Stimulation:** Opposite to sympathetic effects, it decreases heart rate, prolongs the PR interval, and increases the TP interval. * **Propranolol:** A $\beta$-blocker that would increase the PR interval and potentially lengthen the QT interval by antagonizing sympathetic effects.

Q: A cardiologist asks for the measurement of electromechanical systole (QS2), left ventricular ejection time (LVET), and pre-ejection period (PEP). The cardiologist is informed by the technician that the carotid transducer is not functioning. Which of the following readings could not be obtained?

LVET and PEP. To understand this question, we must look at how **Systolic Time Intervals (STI)** are measured using non-invasive tools: the Electrocardiogram (ECG), Phonocardiogram (PCG), and the Carotid Pulse Tracing (CPT). ### 1. Why Option D is Correct * **QS2 (Total Electromechanical Systole):** This is measured from the onset of the QRS complex on the ECG to the first high-frequency component of the second heart sound (S2) on the PCG. It does **not** require a carotid tracing. * **LVET (Left Ventricular Ejection Time):** This is the interval from the beginning of the carotid pressure rise to the dicrotic notch (incisura). It represents the time the aortic valve is open. **Without a carotid transducer, LVET cannot be measured.** * **PEP (Pre-Ejection Period):** This is calculated using the formula: **PEP = QS2 – LVET**. Since PEP is a derived value dependent on LVET, it also **cannot be obtained** if the carotid transducer is non-functional. ### 2. Why Other Options are Incorrect * **Option A & C:** These are incorrect because **QS2** can still be measured using only the ECG and PCG. * **Option B:** While LVET cannot be obtained, QS2 can be, making this combination incorrect. ### 3. Clinical Pearls for NEET-PG * **PEP/LVET Ratio:** This is a sensitive indicator of ventricular function. A **rise** in the ratio (increased PEP, decreased LVET) indicates **Left Ventricular Failure**. * **Effect of Drugs:** Positive inotropic agents (like Digitalis) **shorten** both PEP and LVET. * **Aortic Stenosis:** Characteristically shows a **prolonged LVET** due to the increased time required to eject blood through a narrowed orifice. * **Heart Failure:** Typically shows a **prolonged PEP** (due to delayed pressure rise) and a **shortened LVET** (due to reduced stroke volume).

Question 1

What are the characteristics of blood flow in capillaries?

Accepted Answer

Low velocity and high shear stress

Answer

High velocity, high stress

Answer

High velocity, low stress

Answer

Low velocity and pulsatile

Question 2

Venodilation in most tissues is primarily caused by which of the following factors?

Accepted Answer

Decreased oxygen tension

Answer

Decreased potassium concentration

Answer

Increased hydrogen ion concentration (acidosis)

Answer

Increased carbon dioxide concentration

Question 3

Rapid depolarization (phase 0) of the action potential of ventricular muscle results from the opening of which type of channels?

Accepted Answer

Voltage-gated Na+ channels

Answer

Voltage-gated Ca2+ channels

Answer

Acetylcholine-activated K+ channels

Answer

Inward rectifying K+ channels

Question 4

Coronary blood flow to the left ventricle increases during all of the following EXCEPT?

Accepted Answer

Early systole

Answer

Myocardial hypoxia

Answer

Stimulation of sympathetic nerves of the heart

Answer

Arterial hypertension

Question 5

Normally, the rate of the heart beat in a human is determined by:

Accepted Answer

The sinoatrial node

Answer

The bundle of His

Answer

All cardiac muscle

Answer

The cervical ganglion

Question 6

A 50-year-old woman is brought to the emergency department unconscious following a motor vehicle accident. She is placed on telemetry to monitor her vitals continuously, and she remains hypotensive despite receiving a bolus of fluids and being placed on IV fluids. She regains consciousness and her pulse is 180/mm. Her ECG shows regularly spaced narrow, QRS complexes. No P wave is visible in most leads, but there is a small downward deflection immediately following QRS complexes in lead II. Carotid massage is performed, and her pulse is reduced to 80/min. Stimulation of the afferent fibres in which of the following nerves most likely resulted in the favourable response observed?

Accepted Answer

Glossopharyngeal nerve

Answer

Cardiac sympathetic nerves

Answer

Carotid sympathetic nerves

Answer

Hypoglossal nerve

Question 7

Which of the following is NOT true about the sympathetic vasodilator system?

Accepted Answer

It has a strong basal tone.

Answer

It originates from the spinal cord.

Answer

The fibers to skeletal muscles are cholinergic.

Answer

After sympathectomy, the blood vessels dilate.

Question 8

Stimulation of the sympathetic nerves to the normal heart:

Accepted Answer

Decreases the duration of the QT interval

Answer

Increases the duration of the TP interval

Answer

Increases the duration of the PR interval

Answer

Leads to fewer P waves than QRS complexes

Question 9

Which of the following is NOT true about U waves?

Accepted Answer

The U wave normally goes in the same direction as the T wave.

Answer

It is due to delayed repolarization of Purkinje fibers.

Answer

The maximum normal amplitude of the U wave is 1-2 mm.

Answer

U waves generally become visible when the heart rate falls below 65 bpm.

Question 10

A cardiologist asks for the measurement of electromechanical systole (QS2), left ventricular ejection time (LVET), and pre-ejection period (PEP). The cardiologist is informed by the technician that the carotid transducer is not functioning. Which of the following readings could not be obtained?

Accepted Answer

LVET and PEP

Answer

QS2

Answer

QS2 and LVET

Answer

QS2 and PEP

Cardiovascular System — MCQs

Cardiovascular System — MCQs

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