Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 591: What is true about primary pulmonary hypoventilation?

A. It does not respond to chemical stimuli. (Correct Answer)
B. It is characterized by hypocapnoea and normal PaO2.
C. It is common in children.
D. Respiratory alkalosis is diagnostic.

Explanation: **Explanation:** **Primary Alveolar Hypoventilation (PAH)**, also known as **Ondine’s Curse**, is a rare neurological disorder characterized by the failure of autonomic control of breathing. 1. **Why Option A is Correct:** The hallmark of PAH is a **decreased or absent sensitivity of the central chemoreceptors** to hypercapnia (high $CO_2$) and hypoxia (low $O_2$). While voluntary breathing (controlled by the motor cortex) remains intact, the brainstem fails to trigger involuntary breaths in response to chemical stimuli, especially during sleep. 2. **Why the other options are incorrect:** * **Option B:** Since the patient is hypoventilating, they retain $CO_2$ and fail to take in enough $O_2$. Therefore, it is characterized by **hypercapnia** (high $PaCO_2$) and **hypoxemia** (low $PaO_2$), not hypocapnia. * **Option C:** While a congenital form exists (CCHS), primary pulmonary hypoventilation is classically described as a rare condition that typically presents in **males in their third or fourth decades** of life. * **Option D:** Hypoventilation leads to $CO_2$ retention, which results in **respiratory acidosis**, not alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Ondine’s Curse:** Named after a myth where a nymph cursed a mortal to forget to breathe if he fell asleep. * **PFT Findings:** Lung function tests (spirometry) and respiratory muscle strength are typically **normal**; the defect is purely in the central respiratory drive. * **Diagnosis:** Confirmed by demonstrating hypercapnia during sleep that disappears or improves during wakefulness/voluntary hyperventilation. * **Association:** Congenital cases are often linked to mutations in the **PHOX2B gene**.

Question 592: Fowler's method is employed for the measurement of:

A. Residual volume
B. Alveolar PO2
C. Anatomic dead space (Correct Answer)
D. Physiologic dead space

Explanation: **Explanation:** **Fowler’s Method** (Single-breath Nitrogen Washout) is the gold standard for measuring **Anatomic Dead Space**. **Mechanism:** The patient takes a single breath of 100% oxygen, which fills the entire respiratory tract. During expiration, the concentration of nitrogen in the exhaled air is measured. 1. Initially, the exhaled air comes from the conducting zones (anatomic dead space), which contains **0% nitrogen** (only the inhaled O2). 2. As expiration continues, nitrogen levels rise as alveolar air (which contains nitrogen) mixes with the dead space air. 3. The volume of air expired until the nitrogen concentration reaches a plateau represents the anatomic dead space. **Analysis of Incorrect Options:** * **A. Residual Volume:** Measured using **Helium Dilution** or **Body Plethysmography**, as it cannot be measured by simple spirometry or single-breath tests. * **B. Alveolar PO2:** Calculated using the **Alveolar Gas Equation** [$PAO2 = FiO2(Pb – PH2O) – (PaCO2/R)$]. * **D. Physiologic Dead Space:** Measured using **Bohr’s Method**, which utilizes arterial and expired CO2 levels. Note: In healthy individuals, anatomic and physiologic dead space are nearly equal. **High-Yield Clinical Pearls for NEET-PG:** * **Bohr’s Method = CO2** (Physiologic Dead Space). * **Fowler’s Method = N2** (Anatomic Dead Space). * **Anatomic Dead Space** is roughly **2 ml/kg** of body weight (approx. 150 ml in a 70kg adult). * Dead space increases with upright posture, large inspirations, and drugs like atropine (bronchodilation).

Question 593: During heavy exercise, at what dyspneic index value does dyspnea typically appear?

A. 85%
B. 70% (Correct Answer)
C. 60%
D. 40%

Explanation: ### Explanation **Concept Overview** The **Dyspneic Index (DI)**, also known as the Breathing Reserve, is a physiological parameter used to quantify the relationship between a person's ventilatory capacity and their actual ventilatory requirement. It is calculated using the formula: $$DI = \frac{MVV - V_E}{MVV} \times 100$$ *(Where **MVV** = Maximum Voluntary Ventilation and **$V_E$** = Minute Ventilation)* **Why 70% is Correct** In a healthy individual at rest, the DI is approximately **90%**. During physical exertion, the minute ventilation ($V_E$) increases to meet metabolic demands, causing the DI to fall. Clinical studies and physiological observations indicate that the subjective sensation of breathlessness (dyspnea) typically manifests when the **Dyspneic Index falls to approximately 70-75%**. At this threshold, the breathing reserve is sufficiently reduced that the effort of breathing becomes consciously apparent. **Analysis of Incorrect Options** * **Option A (85%):** This value represents a mild reduction from the resting state. At 85%, the breathing reserve is still high enough that a healthy individual does not experience significant respiratory distress. * **Option C (60%) & Option D (40%):** These values represent severe respiratory strain. While a person will certainly feel dyspneic at these levels, the *onset* of dyspnea occurs much earlier (at 70%). A DI of 60% or lower is often seen in patients with chronic obstructive pulmonary disease (COPD) even during minimal exertion. **High-Yield Clinical Pearls for NEET-PG** * **MVV Calculation:** MVV is roughly equal to $FEV_1 \times 35$ (or 40). * **Normal MVV:** Approximately 150–170 L/min in healthy young males. * **Dyspnea in Disease:** In restrictive or obstructive lung diseases, the MVV is significantly reduced, causing the DI to hit the 70% threshold even during light activities or at rest. * **Key Formula:** Remember that $DI + \text{Breathing Reserve \%} = 100\%$. Dyspnea appears when the breathing reserve is reduced by about 30%.

Question 594: The presence of hemoglobin in normal arterial blood increases its oxygen-carrying capacity approximately how many times?

A. 10
B. 30
C. 50
D. 70 (Correct Answer)

Explanation: **Explanation:** The oxygen-carrying capacity of blood is determined by two factors: oxygen dissolved in plasma and oxygen bound to hemoglobin (Hb). 1. **Dissolved Oxygen:** According to Henry’s Law, the amount of dissolved $O_2$ is proportional to the partial pressure ($PaO_2$). In normal arterial blood ($PaO_2 \approx 100\text{ mmHg}$), only **$0.3\text{ mL}$ of $O_2$** is dissolved in every $100\text{ mL}$ of blood. 2. **Hemoglobin-Bound Oxygen:** Each gram of Hb can carry $1.34\text{ mL}$ of $O_2$. In a normal individual ($Hb \approx 15\text{ g/dL}$), the oxygen bound to Hb is approximately $15 \times 1.34 = \mathbf{20.1\text{ mL}}$ **of $O_2$** per $100\text{ mL}$ of blood. **The Calculation:** To find the increase in capacity, we divide the total oxygen content by the dissolved oxygen: $$\text{Ratio} = \frac{20.1\text{ mL (Bound)}}{0.3\text{ mL (Dissolved)}} \approx \mathbf{67\text{ times}}$$ Rounding to the nearest clinical estimate provided in standard textbooks (like Guyton), the presence of hemoglobin increases the $O_2$ carrying capacity by approximately **70 times**. **Analysis of Incorrect Options:** * **A (10) & B (30):** These values significantly underestimate the efficiency of hemoglobin. Without Hb, the heart would have to pump blood at an impossible rate to meet tissue demands. * **C (50):** While closer, it does not account for the full physiological capacity of $15\text{ g/dL}$ of hemoglobin. **High-Yield Clinical Pearls for NEET-PG:** * **Hüfner's Constant:** $1.34\text{ mL}$ (the amount of $O_2$ bound per gram of Hb). * **Solubility Coefficient of $O_2$:** $0.003\text{ mL/dL/mmHg}$. * **Clinical Significance:** In cases of severe anemia, the dissolved $O_2$ remains the same ($0.3\text{ mL}$), but the total $O_2$ content drops drastically, leading to hemic hypoxia. * **CO2 Comparison:** $CO_2$ is about **20 times** more soluble in plasma than $O_2$.

Question 595: In which of the following conditions does a reduction in aerial oxygen tension occur?

A. Anemia
B. Carbon monoxide poisoning
C. Moderate exercise
D. Hypoventilation (Correct Answer)

Explanation: ### Explanation The question asks for a condition characterized by a reduction in **aerial oxygen tension**, which refers to the partial pressure of oxygen within the alveoli ($PAO_2$). **Why Hypoventilation is Correct:** According to the **Alveolar Gas Equation**: $$PAO_2 = FiO_2(P_{atm} - PH_2O) - \frac{PaCO_2}{R}$$ In **hypoventilation**, there is a failure to adequately exchange air, leading to the retention of carbon dioxide ($CO_2$). As the partial pressure of arterial $CO_2$ ($PaCO_2$) rises, it displaces oxygen within the limited space of the alveoli. Consequently, the alveolar oxygen tension ($PAO_2$) decreases, leading to hypoxemia. This is a classic cause of a "normal A-a gradient" hypoxia. **Analysis of Incorrect Options:** * **Anemia (A):** In anemia, the $PAO_2$ and the amount of oxygen dissolved in plasma ($PaO_2$) are normal. The pathology lies in reduced hemoglobin concentration, leading to decreased total **oxygen content**, not tension. * **Carbon Monoxide (CO) Poisoning (B):** CO competes with $O_2$ for hemoglobin binding sites. While it severely reduces oxygen saturation ($SaO_2$) and content, the $PAO_2$ and $PaO_2$ remain normal because the lungs' ability to transfer gas is unaffected. * **Moderate Exercise (C):** During moderate exercise, increased ventilation and cardiac output typically maintain or even slightly increase $PAO_2$ to meet metabolic demands. **Clinical Pearls for NEET-PG:** * **A-a Gradient:** Hypoventilation and High Altitude are the only two causes of hypoxia where the **A-a gradient remains normal**. * **Oxygen Tension vs. Content:** Tension refers to partial pressure (dissolved gas), while content includes oxygen bound to hemoglobin. * **High-Yield Rule:** If $PaCO_2$ is elevated, think hypoventilation; if $PaCO_2$ is normal/low but $PaO_2$ is low, think V/Q mismatch or diffusion defect.

Question 596: All of the following are true about obstructive lung disease except?

A. Decreased FEV1
B. Decreased MEFR
C. Increased RV
D. Increased diffusion capacity (Correct Answer)

Explanation: **Explanation:** In obstructive lung diseases (e.g., Asthma, COPD, Bronchiectasis), the primary pathology is **increased airway resistance**, making it difficult to exhale air rapidly. **Why Option D is the Correct Answer:** Diffusion capacity (DLCO) measures the ability of gases to transfer from the alveoli to the pulmonary capillaries. In obstructive diseases, DLCO is either **decreased** (as in Emphysema, due to destruction of the alveolar-capillary membrane) or **normal** (as in Asthma). It is **never increased** as a hallmark of obstructive pathology. Therefore, "Increased diffusion capacity" is the false statement. **Analysis of Incorrect Options:** * **A. Decreased FEV1:** This is the hallmark of obstruction. Narrowed airways increase resistance, significantly reducing the volume of air exhaled in the first second. * **B. Decreased MEFR:** Maximum Expiratory Flow Rate (MEFR) represents the flow during the middle portion of expiration. It is highly sensitive to airway obstruction and is characteristically reduced. * **C. Increased RV:** Due to premature airway closure (air trapping), air remains stuck in the lungs at the end of expiration, leading to an increase in Residual Volume (RV) and Total Lung Capacity (TLC). **High-Yield Clinical Pearls for NEET-PG:** * **FEV1/FVC Ratio:** The most important diagnostic parameter for obstruction is a **decreased FEV1/FVC ratio (<0.7)**. In restrictive disease, this ratio is normal or increased. * **Flow-Volume Loop:** Obstructive disease shows a characteristic **"scooped-out"** appearance on the expiratory limb. * **DLCO Differentiation:** DLCO is the key to distinguishing types of COPD; it is **decreased in Emphysema** but typically **normal in Chronic Bronchitis**.

Question 597: Peripheral chemoreceptors respond to hypoxia by activating oxygen-sensitive channels. Which type of channel is primarily involved in this response?

A. Na+ channel
B. Ca+2 channel
C. K+ channel (Correct Answer)
D. Cl- channel

Explanation: **Explanation:** The peripheral chemoreceptors (located in the **Carotid and Aortic bodies**) are the primary sensors for arterial hypoxia. The mechanism of signal transduction in the **Type I (Glomus) cells** is a high-yield physiological process: 1. **Mechanism of Hypoxia Sensing:** Under normal conditions, oxygen-sensitive **K+ channels** remain open, allowing potassium efflux and maintaining a resting membrane potential. 2. **The Response:** When arterial $PO_2$ falls (hypoxia), these specific K+ channels **close**. This reduction in K+ conductance leads to **depolarization** of the Glomus cell. 3. **Downstream Effects:** Depolarization triggers the opening of voltage-gated $Ca^{2+}$ channels, leading to an influx of calcium and subsequent exocytosis of neurotransmitters (primarily **ATP** and Dopamine). These stimulate the glossopharyngeal nerve (from carotid bodies) to increase the firing rate to the respiratory centers. **Analysis of Incorrect Options:** * **A. Na+ channel:** While sodium influx causes depolarization in many excitable tissues, it is not the primary "sensor" or initiator of the hypoxic response in glomus cells. * **B. Ca+2 channel:** Calcium channels are involved *later* in the process (synaptic release), but they are opened as a result of the depolarization caused by K+ channel closure. * **D. Cl- channel:** Chloride channels do not play a significant role in the acute transduction of the hypoxic stimulus in chemoreceptors. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Carotid bodies (at the bifurcation of common carotid) are more important for respiratory control than aortic bodies. * **Nerve Supply:** Carotid body → **Hering’s Nerve** (branch of Glossopharyngeal/CN IX); Aortic body → **Vagus Nerve** (CN X). * **Threshold:** Peripheral chemoreceptors significantly increase firing only when arterial $PO_2$ drops below **60 mmHg**. * **Central vs. Peripheral:** Central chemoreceptors respond to $\uparrow PCO_2$ and $\downarrow pH$ (via $H^+$ in CSF), but **not** to hypoxia. Hypoxia is sensed *only* by peripheral chemoreceptors.

Question 598: Which of the following physiological changes occurs during pregnancy?

A. Tidal volume increases (Correct Answer)
B. Arterial pO2 decreases
C. Cardiac output decreases
D. Respiratory acidosis

Explanation: **Explanation:** During pregnancy, the respiratory system undergoes significant physiological adaptations to meet the increased metabolic demands of the fetus and the mother. **1. Why Tidal Volume (TV) Increases:** Progesterone acts as a direct respiratory stimulant, increasing the sensitivity of the central respiratory center to CO2. This leads to an increase in **Tidal Volume (by ~40%)**, while the respiratory rate remains relatively constant. This increase in TV enhances minute ventilation, ensuring efficient gas exchange. **2. Analysis of Incorrect Options:** * **Arterial pO2 decreases:** Incorrect. Due to hyperventilation (increased TV), the arterial pO2 actually **increases** slightly (typically 100–105 mmHg) to facilitate oxygen transfer across the placenta. * **Cardiac output decreases:** Incorrect. Cardiac output **increases** significantly (by 30–50%) during pregnancy to support fetal growth and increased maternal blood volume. * **Respiratory acidosis:** Incorrect. The progesterone-driven hyperventilation causes a "washout" of CO2, leading to a decrease in arterial pCO2 (hypocapnia). This results in a state of **compensated respiratory alkalosis**, not acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common change:** Increase in Tidal Volume. * **Lung Volumes:** Functional Residual Capacity (FRC), Residual Volume (RV), and Expiratory Reserve Volume (ERV) all **decrease** due to the upward displacement of the diaphragm by the gravid uterus. * **Vital Capacity (VC):** Remains **unchanged** because the decrease in RV is compensated by the increase in TV. * **Oxygen Dissociation Curve:** Shifts to the **right** (due to increased 2,3-DPG), favoring oxygen unloading to the fetus.

Question 599: The affinity of hemoglobin for oxygen increases with a fall in pH. What is this phenomenon called?

A. Bain-Bridge effect
B. Bohr effect (Correct Answer)
C. Haldane effect
D. Herring effect

Explanation: ### Explanation **1. The Correct Answer: Bohr Effect** The **Bohr effect** describes the phenomenon where hemoglobin’s affinity for oxygen is inversely related to acidity (H⁺ concentration) and the concentration of carbon dioxide (PCO₂). * **Mechanism:** When pH falls (becomes more acidic) or PCO₂ rises, H⁺ ions bind to specific amino acid residues on hemoglobin. This stabilizes the **T-state (Tense state)** or deoxygenated form of hemoglobin, causing the Oxygen-Dissociation Curve (ODC) to **shift to the right**. * **Physiological Significance:** This occurs primarily in metabolically active tissues, facilitating the unloading of oxygen where it is needed most. **2. Analysis of Incorrect Options** * **A. Bainbridge Effect:** This is a cardiovascular reflex. An increase in venous return stretches the right atrium, leading to an increase in heart rate to prevent blood pooling in the veins. * **C. Haldane Effect:** Often confused with the Bohr effect, this occurs in the **lungs**. It describes how the binding of oxygen to hemoglobin promotes the release of CO₂. Deoxygenated hemoglobin has a higher affinity for CO₂ than oxygenated hemoglobin. * **D. Hering-Breuer Reflex:** This is a pulmonary stretch reflex. It prevents over-inflation of the lungs; when stretch receptors in the bronchi are activated, they send signals via the Vagus nerve to inhibit the inspiratory center. **3. High-Yield Clinical Pearls for NEET-PG** * **Right Shift of ODC (Decreased Affinity):** Remember the mnemonic **"CADET, face Right!"** (CO₂, Acidosis, DPG [2,3-BPG], Exercise, Temperature). * **Left Shift of ODC (Increased Affinity):** Occurs with Alkalosis, decreased 2,3-BPG, Hypothermia, and **Fetal Hemoglobin (HbF)**. * **Key Distinction:** Bohr effect = CO₂/H⁺ affecting O₂ binding (Tissues). Haldane effect = O₂ affecting CO₂ binding (Lungs).

Question 600: Increased alveolar-arterial difference in PaO2 is seen in all except?

A. COPD
B. Pulmonary Edema
C. GBS (Correct Answer)
D. Interstitial Lung Disease

Explanation: The **Alveolar-arterial (A-a) gradient** measures the difference between the oxygen concentration in the alveoli ($P_AO_2$) and the arterial blood ($PaO_2$). It is a crucial tool for differentiating causes of hypoxemia. ### **Why GBS is the Correct Answer** **Guillain-Barré Syndrome (GBS)** causes hypoxemia through **Alveolar Hypoventilation** due to respiratory muscle weakness. In pure hypoventilation, the lungs themselves are healthy; therefore, oxygen transfers normally across the alveolar-capillary membrane. Since both alveolar and arterial oxygen levels decrease proportionately, the **A-a gradient remains normal**. * *Note:* Other causes of hypoxemia with a normal A-a gradient include high altitude (low $FiO_2$) and CNS depression (opioid overdose). ### **Why the Other Options are Incorrect** In these conditions, the A-a gradient is **increased** because there is a primary defect in gas exchange: * **COPD:** Causes **Ventilation-Perfusion (V/Q) mismatch** due to airway obstruction and alveolar destruction. * **Pulmonary Edema:** Increases the diffusion distance and causes V/Q mismatch/shunting as fluid fills the alveoli. * **Interstitial Lung Disease (ILD):** Causes **Diffusion impairment** due to thickening and fibrosis of the alveolar-capillary membrane. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Formula:** $A-a\text{ Gradient} = P_AO_2 - PaO_2$. 2. **Normal Range:** Approximately 5–15 mmHg (increases with age: $\text{Age}/4 + 4$). 3. **Rule of Thumb:** * **Normal A-a Gradient:** Extrapulmonary causes (Hypoventilation, Low $FiO_2$). * **Increased A-a Gradient:** Intrapulmonary causes (V/Q mismatch, Shunt, Diffusion defect). 4. Hypoventilation is always associated with **increased $PaCO_2$** (Hypercapnia).

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