Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 751: Why does hyperventilation cause paresthesia?

A. Increased O2
B. Decreased CO2 (Correct Answer)
C. Decreased pH
D. Increased CO2

Explanation: ***Decreased CO2*** - Hyperventilation leads to an excessive loss of **carbon dioxide (CO2)** from the body, causing **respiratory alkalosis**. - The resulting alkalosis decreases the concentration of **ionized calcium** in the blood, leading to neuronal excitability and thus paresthesia. *Increased O2* - While hyperventilation increases the amount of **oxygen (O2)** breathed in, it is not the direct cause of paresthesia. - The key physiological change leading to paresthesia is related to changes in **blood gas chemistry**, specifically CO2 and pH. *Decreased pH* - Hyperventilation causes a **decrease in CO2**, which subsequently leads to an **increase in pH** (respiratory alkalosis), not a decrease in pH. - A decrease in pH (acidosis) generally leads to different symptoms, and is not the cause of paresthesia in this context. *Increased CO2* - Hyperventilation by definition involves **expelling more CO2** than normal, leading to a decrease in CO2 levels, not an increase. - An underlying increase in CO2 would lead to **respiratory acidosis**, which has a different clinical presentation.

Question 752: Which of the following is not true about ventilation-perfusion ratio (V/Q)?

A. Low V/Q in shunt
B. High V/Q in dead space
C. V/Q is highest at lung base (Correct Answer)
D. Normal V/Q is approximately 0.8

Explanation: ***V/Q is highest at lung base*** - This statement is **incorrect** because the **V/Q ratio is actually lowest at the lung base** and highest at the apex due to gravity's differential effects on ventilation and perfusion. - At the lung base, both ventilation and perfusion are highest, but **perfusion increases more significantly than ventilation**, leading to a lower V/Q ratio. *Low V/Q in shunt* - A **shunt** represents an extreme form of low V/Q, where there is **perfusion without ventilation (V/Q = 0)**. - This results in **unoxygenated blood** returning to the systemic circulation. *High V/Q in dead space* - **Dead space ventilation** occurs when there is **ventilation without perfusion (V/Q = infinity)**. - This means that air enters the alveoli but **no gas exchange** can occur because there is no blood flow. *Normal V/Q is approximately 0.8* - The **overall average V/Q ratio** for healthy lungs is indeed approximately **0.8**. - This value reflects the balance between **total alveolar ventilation** (around 4 L/min) and **total pulmonary blood flow** (around 5 L/min).

Question 753: Which of the following best represents 'anatomic dead space'?

A. The space between the lungs and chest wall
B. The tiny air sacs in the lungs
C. Trachea
D. The conducting airways from nose to terminal bronchioles (Correct Answer)

Explanation: ***The conducting airways from nose to terminal bronchioles*** - This definition accurately describes **anatomic dead space**, which includes all the parts of the respiratory system that conduct air but do not participate in **gas exchange** - These structures include the **nose**, **pharynx**, **larynx**, **trachea**, **bronchi**, and **bronchioles** up to the terminal bronchioles *The space between the lungs and chest wall* - This describes the **pleural space**, which contains a thin layer of fluid that lubricates the lungs and allows them to move smoothly against the chest wall during breathing - It does not represent an area where air is held and not exchanged, but rather a potential space crucial for lung mechanics *The tiny air sacs in the lungs* - These are the **alveoli**, which are the primary sites of **gas exchange** in the lungs - The air within the alveoli represents the functional or **respiratory zone**, where oxygen enters the bloodstream and carbon dioxide is expelled, and thus is not dead space *Trachea* - While the **trachea** is indeed part of the conducting airways and contributes to **anatomic dead space**, it is only one component of it - The term "anatomic dead space" refers to the entire volume of these non-exchanging airways, not just a single structure

Question 754: Which of the following best represents 'anatomic dead space' in the respiratory system?

A. Alveoli
B. Trachea (Correct Answer)
C. Bronchi
D. Pleural cavity

Explanation: ***Trachea*** - The **trachea** is the classic textbook example of **anatomic dead space** as it is the largest single component of the conducting zone. - Anatomic dead space refers to the **conducting airways** (nose, pharynx, larynx, trachea, bronchi, bronchioles) that transport air but do not participate in gas exchange. - The trachea alone contributes approximately **half of the total anatomic dead space** (~75 mL out of ~150 mL in adults), making it the most significant individual structure. - Air in the trachea never participates in gas exchange and is "wasted" ventilation. *Alveoli* - **Alveoli** are the primary sites of gas exchange in the lungs, where oxygen diffuses into the blood and carbon dioxide diffuses out. - Air filling the alveoli participates in **effective respiration**, not dead space. - However, poorly perfused alveoli can contribute to **physiologic dead space** (not anatomic). *Bronchi* - The **bronchi** are also part of the conducting airways and do contribute to anatomic dead space. - However, when asking what "best represents" anatomic dead space, the **trachea** is the more appropriate answer as it is the single largest contributor and the standard teaching example. - The bronchi collectively contribute less volume than the trachea alone. *Pleural cavity* - The **pleural cavity** is the space between the parietal and visceral pleura, containing lubricating fluid. - It is not part of the respiratory airways and does not contain air that participates in ventilation. - Therefore, it is not considered part of anatomic dead space.

Question 755: Which of the following laboratory findings is most consistent with hypoxia due to acute respiratory distress syndrome (ARDS)?

A. Increased PaCO2 with decreased pH
B. Increased A-a gradient (Correct Answer)
C. Decreased PaO2 with normal PaCO2
D. Normal A-a gradient

Explanation: ***Increased A-a gradient*** - In ARDS, the **lung pathology** (e.g., alveolar edema, collapse, or consolidation) impairs gas exchange, leading to a significant **mismatch between ventilation and perfusion**. - This mismatch results in a larger-than-normal difference between the alveolar oxygen partial pressure (PAO2) and the arterial oxygen partial pressure (PaO2), which is measured as an **increased A-a gradient**. *Increased PaCO2 with decreased pH* - This finding describes **respiratory acidosis**, which would typically occur in severe **hypoventilation** or end-stage ARDS with respiratory failure. - In initial or moderate ARDS, patients often compensate with **hyperventilation** due to hypoxia, leading to decreased or normal PaCO2. *Decreased PaO2 with normal PaCO2* - While a decreased PaO2 is characteristic of hypoxia in ARDS, a **normal PaCO2** in the presence of significant hypoxemia still implies an impairment in gas exchange that would manifest as an increased A-a gradient. - This specific combination (decreased PaO2, normal PaCO2) is not as specific as the A-a gradient for identifying the underlying cause of hypoxia due to shunt or V/Q mismatch. *Normal A-a gradient* - A normal A-a gradient suggests that **gas exchange in the lungs is efficient**, and any hypoxia is likely due to **hypoventilation** or **low inspired oxygen**. - This finding would rule out significant intrinsic lung disease, such as ARDS, as the primary cause of hypoxia.

Question 756: A patient with acute pulmonary embolism is found to have hypoxia. What is the most likely mechanism causing hypoxia in this condition?

A. Hypoventilation
B. Diffusion impairment
C. Ventilation-perfusion mismatch (Correct Answer)
D. Shunt

Explanation: ***Ventilation-perfusion mismatch*** - A pulmonary embolism blocks blood flow to a portion of the lung, creating areas that are **ventilated but not perfused** (increased dead space with high V/Q ratio). - Blood is redirected to the remaining perfused lung areas, which then become relatively **overperfused** (low V/Q ratio), impairing efficient oxygen uptake. - This V/Q mismatch—with both high V/Q (dead space) and low V/Q (relative shunt) areas—leads to **hypoxemia**, making it the **most common mechanism** of hypoxia in acute PE. *Hypoventilation* - This condition involves a generalized decrease in alveolar ventilation, leading to **hypercapnia** (increased CO2) and hypoxemia. - While PE can cause shortness of breath and tachypnea, the primary mechanism of hypoxia is not due to overall reduced ventilation, but rather disrupted matching of ventilation to perfusion. *Diffusion impairment* - Diffusion impairment occurs when the alveolar-capillary membrane is compromised, preventing proper oxygen transfer, as seen in conditions like **pulmonary fibrosis** or **interstitial lung disease**. - Pulmonary embolism primarily affects **blood flow distribution**, not the structural integrity or diffusion capacity of the alveolar-capillary membrane. *Shunt* - A true shunt occurs when deoxygenated blood bypasses ventilated alveoli entirely and enters systemic circulation, as seen in **intracardiac defects** or severe **ARDS**. - While massive PE can rarely lead to right-to-left shunting through a patent foramen ovale (due to increased right heart pressure), the **primary and most common mechanism** of hypoxia in typical acute PE is V/Q mismatch, not shunt.

Question 757: What is the most common underlying cause of hypoxemia in patients with pneumonia?

A. Shunting
B. Reduced lung volume
C. Impaired gas exchange
D. Ventilation-perfusion mismatch (Correct Answer)

Explanation: ***Ventilation-perfusion mismatch*** - This occurs when areas of the lung are either **well-perfused but poorly ventilated** (e.g., due to alveolar filling or collapse in pneumonia), or **well-ventilated but poorly perfused**. - In pneumonia, inflammatory exudates and consolidation fill alveoli, impairing ventilation while perfusion to these areas continues, creating a **low V/Q ratio** and leading to hypoxemia. *Shunting* - **True shunting** (blood bypassing ventilated lung entirely) is a severe form of V/Q mismatch where the V/Q ratio is zero. - While shunting can occur in severe pneumonia, it represents an extreme, non-correctable form of V/Q mismatch and is not the *most common* or primary mechanism for hypoxemia in the broader spectrum of pneumonia. *Reduced lung volume* - **Reduced lung volume** can contribute to hypoxemia by limiting the overall surface area for gas exchange, but it is not the primary or most direct mechanism caused by the pathological changes in pneumonia. - It often results from conditions like atelectasis or pleural effusions, which may coexist with pneumonia but are distinct from the primary parenchymal inflammation. *Impaired gas exchange* - **Impaired gas exchange** is a general term describing the inability to adequately oxygenate blood and/or remove carbon dioxide. - While V/Q mismatch is a specific mechanism of impaired gas exchange, "impaired gas exchange" itself is too broad and does not pinpoint the underlying physiological process most commonly responsible in pneumonia.

Question 758: Which of the following conditions is a common cause of hypoxia with a normal A-a gradient?

A. Pulmonary fibrosis
B. Pulmonary embolism
C. Pneumonia
D. Hypoventilation (Correct Answer)

Explanation: ***Hypoventilation*** - **Hypoventilation** reduces the partial pressure of oxygen in the alveoli (PAO2) due to inadequate ventilation, leading to decreased arterial oxygen tension (PaO2). - The **A-a gradient** remains normal because both PAO2 and PaO2 decrease proportionally, maintaining their normal difference. *Pulmonary fibrosis* - **Pulmonary fibrosis** causes hypoxia primarily due to impaired diffusion and V/Q mismatch. - This leads to a **widened A-a gradient** as oxygen transfer from alveoli to blood is compromised. *Pulmonary embolism* - A **pulmonary embolism** causes hypoxia due to V/Q mismatch, specifically creating dead space (ventilated but not perfused alveoli). - This results in an **increased A-a gradient** because the inefficiency of gas exchange elevates the difference between alveolar and arterial oxygen. *Pneumonia* - **Pneumonia** causes hypoxia due to accumulation of fluid and inflammatory cells in the alveoli, leading to V/Q mismatch and sometimes shunting. - This pathology results in a **widened A-a gradient** because the effective diffusion of oxygen from affected alveoli into the capillaries is impaired.

Question 759: A 56-year-old male with COPD presents with worsening shortness of breath. ABG analysis shows PaO2 of 55 mmHg. Which physiological mechanism primarily contributes to his hypoxemia?

A. Right-to-left shunt
B. Hypoventilation
C. Ventilation-perfusion mismatch (Correct Answer)
D. Reduced inspired oxygen tension

Explanation: ***Ventilation-perfusion mismatch*** - **COPD** causes destruction of alveolar walls and trapping of air, leading to areas of the lung that are poorly ventilated but still perfused, creating a **low V/Q ratio**. - Conversely, other areas may have good ventilation but reduced perfusion due to vascular changes, creating a **high V/Q ratio**, both contributing significantly to **hypoxemia**. *Right-to-left shunt* - A right-to-left shunt involves the bypass of pulmonary circulation by venous blood, which is a common cause of hypoxemia when the shunt fraction is large. - While shunting can occur in severe COPD (e.g., due to atelectasis or significant pulmonary hypertension with right-sided heart failure), it is not the primary or most common mechanism for hypoxemia in typical COPD exacerbations. *Hypoventilation* - While chronic hypoventilation can occur in severe COPD due to respiratory muscle fatigue or CO2 retention, it primarily leads to **hypercapnia (elevated PaCO2)**. - Although it can contribute to hypoxemia, **V/Q mismatch** is the predominant mechanism for the low PaO2 observed in most COPD patients. *Reduced inspired oxygen tension* - This mechanism is relevant in scenarios like high altitude or rebreathing expired air, where the **fraction of inspired oxygen (FiO2)** is low. - It does not apply to a patient presenting with COPD in a typical clinical setting where ambient air is breathed.

Question 760: Which physiological mechanism is primarily involved in the sensation of shortness of breath during an asthma attack?

A. Increased airway resistance (Correct Answer)
B. Increased carbon dioxide retention
C. Decreased lung compliance in restrictive lung diseases
D. Reduced arterial oxygen saturation

Explanation: ***Increased airway resistance*** - During an asthma attack, smooth muscles in the airways constrict and the *airway lining swells*, leading to a significant **narrowing of the bronchi and bronchioles**. - This **increased resistance** to airflow makes it harder to breathe out, resulting in the sensation of *shortness of breath* and *wheezing*. *Decreased lung compliance in restrictive lung diseases* - This mechanism is characteristic of **restrictive lung diseases** like *pulmonary fibrosis*, where the lungs become stiffer and harder to inflate. - While it causes shortness of breath, it is *not the primary mechanism* in *obstructive diseases* like asthma. *Reduced arterial oxygen saturation* - Although *hypoxemia* can occur in severe asthma attacks, it is often a *secondary consequence* of impaired gas exchange due to airway obstruction, not the initial cause of the sensation of breathlessness. - The sensation of dyspnea often precedes significant drops in *oxygen saturation*. *Increased carbon dioxide retention* - Like hypoxemia, *hypercapnia* (increased CO2 retention) can happen in *severe asthma* when ventilation is severely compromised. - However, the feeling of shortness of breath is primarily triggered by the effort needed to overcome *airway resistance*, rather than directly by CO2 levels, especially in the early stages of an attack.

Practice by Chapter

Mechanics of Breathing

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Pulmonary Ventilation

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Pulmonary Circulation

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Gas Exchange in the Lungs

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Oxygen and Carbon Dioxide Transport

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Control of Breathing

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Respiratory Adjustments in Health and Disease

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High Altitude Physiology

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Diving Physiology

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Respiratory Function Tests

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