Respiratory System Practice Questions

Q: Why does hyperventilation cause paresthesia?

Decreased CO2. ***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.

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

V/Q is highest at lung base. ***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).

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

The conducting airways from nose to terminal bronchioles. ***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

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

Trachea. ***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.

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

Increased A-a gradient. ***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.

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

Ventilation-perfusion mismatch. ***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.

Q: Which of the following laboratory findings is most consistent with a diagnosis of carbon monoxide poisoning?

Normal PaO2 with decreased oxygen saturation. ***Normal PaO2 with decreased oxygen saturation*** - Carbon monoxide (CO) binds to hemoglobin with an affinity 200-250 times greater than oxygen, forming **carboxyhemoglobin (COHb)** [2]. This reduces the **oxygen-carrying capacity** of the blood and shifts the oxygen dissociation curve to the left, but it does **not affect the partial pressure of oxygen (PaO2)** dissolved in the plasma [1]. - The pulse oximeter, which typically measures oxygen saturation, will show a falsely high reading because it cannot differentiate between oxyhemoglobin and carboxyhemoglobin, but actual **oxygen saturation is decreased**. *Increased PaCO2 and decreased pH* - This pattern suggests **respiratory acidosis**, which is not a direct or primary finding of carbon monoxide poisoning. - While severe CO poisoning can lead to lactic acidosis, an increase in PaCO2 points to impaired ventilation, not specifically CO toxicity [3]. *Decreased PaO2 with normal oxygen saturation* - A decreased PaO2 with normal oxygen saturation is a contradictory finding and not physiologically consistent, as oxygen saturation is directly dependent on PaO2. - This pattern would indicate a measurement error or a highly unusual physiological state, neither of which is characteristic of CO poisoning. *Decreased PaCO2 with normal PaO2* - This suggests **respiratory alkalosis**, often due to hyperventilation. - While patients with CO poisoning may hyperventilate due to hypoxia, this ABG pattern is not the defining laboratory finding for CO poisoning, and **PaO2 would remain normal** until very late stages.

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

Ventilation-perfusion mismatch. ***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.

Q: Which of the following is a common cause of hypoxia due to ventilation-perfusion mismatch?

Pulmonary embolism. ***Pulmonary embolism*** - A **pulmonary embolism** blocks blood flow to a portion of the lung, but ventilation to that area may remain intact, creating a high V/Q ratio as **perfusion is reduced** relative to ventilation [1]. - This **V/Q mismatch** means that ventilated alveoli are not adequately perfused, preventing efficient gas exchange and leading to hypoxemia [1]. *Methemoglobinemia* - This condition involves an altered form of hemoglobin that cannot bind oxygen or releases it abnormally, leading to **functional anemia** and tissue hypoxia [2]. - While it causes hypoxia, it primarily affects the **oxygen-carrying capacity of blood** rather than causing a ventilation-perfusion mismatch within the lungs [2]. *Anemia* - **Anemia** is a reduction in the number of red blood cells or the amount of hemoglobin, leading to a decreased **oxygen-carrying capacity** of the blood [2]. - It results in **hypoxic hypoxia** due to insufficient oxygen delivery to tissues, but it does not primarily cause a V/Q mismatch in the lungs [2]. *Asthma* - **Asthma** causes **airway obstruction** (bronchoconstriction, mucus plugging, inflammation), leading to areas of reduced ventilation. - While asthma can cause V/Q mismatch (low V/Q areas), it's typically due to **impaired ventilation**, whereas pulmonary embolism primarily causes mismatch by impairing perfusion [1].

Question 1

Why does hyperventilation cause paresthesia?

Accepted Answer

Decreased CO2

Answer

Increased O2

Answer

Decreased pH

Answer

Increased CO2

Question 2

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

Accepted Answer

V/Q is highest at lung base

Answer

Low V/Q in shunt

Answer

High V/Q in dead space

Answer

Normal V/Q is approximately 0.8

Question 3

Which of the following best represents 'anatomic dead space'?

Accepted Answer

The conducting airways from nose to terminal bronchioles

Answer

The space between the lungs and chest wall

Answer

The tiny air sacs in the lungs

Answer

Trachea

Question 4

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

Accepted Answer

Trachea

Answer

Alveoli

Answer

Bronchi

Answer

Pleural cavity

Question 5

Which factor predominantly influences the rightward shift of the oxygen dissociation curve?

Accepted Answer

2,3-Bisphosphoglycerate (2,3-BPG)

Answer

pH (Bohr effect)

Answer

Temperature increase

Answer

Carbon monoxide levels

Question 6

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

Accepted Answer

Increased A-a gradient

Answer

Increased PaCO2 with decreased pH

Answer

Decreased PaO2 with normal PaCO2

Answer

Normal A-a gradient

Question 7

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

Accepted Answer

Ventilation-perfusion mismatch

Answer

Hypoventilation

Answer

Diffusion impairment

Answer

Shunt

Question 8

Which of the following laboratory findings is most consistent with a diagnosis of carbon monoxide poisoning?

Accepted Answer

Normal PaO2 with decreased oxygen saturation

Answer

Increased PaCO2 and decreased pH

Answer

Decreased PaO2 with normal oxygen saturation

Answer

Decreased PaCO2 with normal PaO2

Question 9

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

Accepted Answer

Ventilation-perfusion mismatch

Answer

Shunting

Answer

Reduced lung volume

Answer

Impaired gas exchange

Question 10

Which of the following is a common cause of hypoxia due to ventilation-perfusion mismatch?

Accepted Answer

Pulmonary embolism

Answer

Methemoglobinemia

Answer

Anemia

Answer

Asthma

Respiratory System — MCQs

Respiratory System — MCQs

On this page

Practice by Chapter

Want unlimited practice?