V/Q mismatch in ARDS US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for V/Q mismatch in ARDS. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
V/Q mismatch in ARDS US Medical PG Question 1: A 71-year-old man is admitted to the ICU with a history of severe pancreatitis and new onset difficulty breathing. His vital signs are a blood pressure of 100/60 mm Hg, heart rate of 100/min, respirations of 27/min, temperature of 36.7°C (98.1°F), and oxygen saturation of 85% on room air. Physical examination shows a cachectic male in severe respiratory distress. Rales are heard at the base of each lung. The patient is intubated and a Swan-Ganz catheter is inserted. Pulmonary capillary wedge pressure is 8 mm Hg. An arterial blood gas study reveals a PaO2: FiO2 ratio of 180. The patient is diagnosed with acute respiratory distress syndrome. In which of the following segments of the respiratory tract are the cells responsible for the symptoms observed in this patient found?
- A. Alveolar sacs (Correct Answer)
- B. Terminal bronchioles
- C. Bronchi
- D. Respiratory bronchioles
- E. Bronchioles
V/Q mismatch in ARDS Explanation: ***Alveolar sacs***
- **Acute respiratory distress syndrome (ARDS)** is characterized by widespread inflammatory injury to the **alveolar-capillary membrane**, leading to increased permeability and fluid accumulation in the alveolar sacs.
- The symptoms, including **severe hypoxemia** (PaO2:FiO2 ratio < 300), **non-cardiogenic pulmonary edema** (PCWP ≤ 18 mmHg), and **bilateral lung infiltrates**, directly result from damage to the **Type I and Type II pneumocytes** and endothelial cells within the alveolar units.
*Terminal bronchioles*
- These are the last airways that **do not contain alveoli**, primarily involved in air conduction rather than gas exchange.
- While inflammation can extend to these structures in severe lung injury, the primary site of impaired gas exchange and fluid accumulation in ARDS occurs distal to them, in the respiratory zone.
*Bronchi*
- The bronchi are primarily involved in **air conduction** and consist of cartilage, smooth muscle, and ciliated epithelium, but they do not participate in gas exchange.
- Injury to the bronchi would manifest as airway obstruction or mucus hypersecretion rather than the diffuse alveolar damage seen in ARDS.
*Respiratory bronchioles*
- These are the first airways that contain a **small number of alveoli** and participate in gas exchange, but their primary role is still more conductive than the alveolar sacs.
- Although they can be affected in ARDS, the most critical damage and symptoms arise from the more extensive gas exchange surface of the alveolar sacs.
*Bronchioles*
- Bronchioles are small airways lacking cartilage, primarily responsible for **airflow regulation** and conduction.
- While they can be affected by inflammation, the extensive impairment of gas exchange and the characteristic pathology of ARDS specifically involves the **alveolar units**, not primarily the bronchioles.
V/Q mismatch in ARDS US Medical PG Question 2: A 21-year-old man presents to his physician because he has been feeling increasingly tired and short of breath at work. He has previously had these symptoms but cannot recall the diagnosis he was given. Chart review reveals the following results:
Oxygen tension in inspired air = 150 mmHg
Alveolar carbon dioxide tension = 50 mmHg
Arterial oxygen tension = 71 mmHg
Respiratory exchange ratio = 0.80
Diffusion studies reveal normal diffusion distance. The patient is administered 100% oxygen but the patient's blood oxygen concentration does not improve. Which of the following conditions would best explain this patient's findings?
- A. Septal defect since birth (Correct Answer)
- B. Use of opioid medications
- C. Pulmonary fibrosis
- D. Pulmonary embolism
- E. Vacation at the top of a mountain
V/Q mismatch in ARDS Explanation: ***Septal defect since birth***
- A congenital heart disease like a **septal defect** causes a right-to-left **shunt**, meaning deoxygenated blood bypasses the lungs and mixes with oxygenated blood.
- This type of shunt leads to **hypoxemia that is refractory to 100% oxygen** because the shunted blood will never pick up oxygen from the lungs.
*Use of opioid medications*
- Opioid use causes **respiratory depression**, leading to **hypoventilation** and increased arterial CO2 with decreased arterial O2.
- However, the hypoxemia from hypoventilation would typically improve significantly with **100% oxygen administration**, unlike in this case.
*Pulmonary fibrosis*
- **Pulmonary fibrosis** causes thickening of the alveolar-capillary membrane, leading to impaired gas exchange and **diffusion limitation**.
- While it causes hypoxemia, the diffusion studies are stated to be **normal**, and hypoxemia due to diffusion limitation often improves with supplemental oxygen.
*Pulmonary embolism*
- A **pulmonary embolism** leads to V/Q mismatch by blocking blood flow to a portion of the lung, causing ventilation with no perfusion.
- Hypoxemia from V/Q mismatch generally **responds well to supplemental oxygen**, as the non-affected lung areas can compensate, unlike the scenario described.
*Vacation at the top of a mountain*
- Being at a high altitude causes **hypobaric hypoxia**, meaning there is a reduced partial pressure of oxygen in the inspired air.
- This type of hypoxemia typically **improves with supplemental oxygen** as it increases the inspired oxygen tension, which is contrary to the patient's findings.
V/Q mismatch in ARDS US Medical PG Question 3: A neonate suffering from neonatal respiratory distress syndrome is given supplemental oxygen. Which of the following is a possible consequence of oxygen therapy in this patient?
- A. Anosmia
- B. Atelectasis
- C. Atopy
- D. Blindness (Correct Answer)
- E. Cardiac anomalies
V/Q mismatch in ARDS Explanation: ***Blindness***
- High concentrations of supplemental oxygen in neonates, particularly premature infants, can lead to **retinopathy of prematurity (ROP)**.
- ROP involves abnormal growth of blood vessels in the retina, which can detach the retina and result in **permanent blindness**.
*Anosmia*
- **Anosmia** is the loss of the sense of smell, typically caused by nasal polyps, head trauma, or certain viral infections.
- It is **not a recognized complication** of oxygen therapy in neonates.
*Atelectasis*
- **Atelectasis** refers to the collapse of lung tissue, which can be caused by bronchial obstruction or hypoventilation.
- While underlying respiratory distress syndrome can predispose to atelectasis, oxygen therapy itself typically aims to improve ventilation and **does not directly cause atelectasis**.
*Atopy*
- **Atopy** is a genetic predisposition to developing allergic diseases such as asthma, eczema, and allergic rhinitis.
- It is **unrelated to oxygen therapy** and is determined by genetic factors and environmental exposures.
*Cardiac anomalies*
- **Cardiac anomalies** (congenital heart defects) are structural problems in the heart present at birth, resulting from abnormal fetal development.
- They are **not a consequence of oxygen therapy** given postpartum; oxygen therapy may be used to manage their symptoms.
V/Q mismatch in ARDS US Medical PG Question 4: Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?
- A. Inhaling the inspiratory reserve volume (IRV)
- B. Exhaling the entire vital capacity (VC)
- C. Exhaling the expiratory reserve volume (ERV)
- D. Breath holding maneuver at functional residual capacity (FRC)
- E. Inhaling the entire vital capacity (VC) (Correct Answer)
V/Q mismatch in ARDS Explanation: ***Inhaling the entire vital capacity (VC)***
- As lung volume increases from FRC to TLC (which includes inhaling the entire VC), alveolar vessels are **stretched open**, and extra-alveolar vessels are **pulled open** by the increased radial traction, leading to a decrease in PVR.
- This **maximizes the cross-sectional area** of the pulmonary vascular bed, lowering resistance.
*Inhaling the inspiratory reserve volume (IRV)*
- While inhaling IRV increases lung volume, it's not the maximal inspiration of the entire VC where **PVR is typically at its lowest**.
- PVR continues to decrease as lung volume approaches total lung capacity (TLC).
*Exhaling the entire vital capacity (VC)*
- Exhaling the entire vital capacity leads to very low lung volumes, where PVR significantly **increases**.
- At low lung volumes, **alveolar vessels become compressed** and extra-alveolar vessels **narrow**, increasing resistance.
*Exhaling the expiratory reserve volume (ERV)*
- Exhaling the ERV results in a lung volume below FRC, which causes a **marked increase in PVR**.
- This is due to the **compression of alveolar vessels** and decreased radial traction on extra-alveolar vessels.
*Breath holding maneuver at functional residual capacity (FRC)*
- At FRC, the PVR is at an **intermediate level**, not its lowest.
- This is the point where the opposing forces affecting alveolar and extra-alveolar vessels are somewhat balanced, but not optimized for minimal resistance.
V/Q mismatch in ARDS US Medical PG Question 5: A baby is born after the 32nd gestational week by cesarean delivery. The mother suffered from gestational diabetes; however, she had no other pregnancy-related diseases and was otherwise healthy. The baby has a blood pressure of 100/58 mm Hg, heart rate of 104/min, and oxygen saturation of 88%. The child has tachypnea, subcostal and intercostal retractions, nasal flaring, and cyanosis. The cyanosis is responding well to initial administration of oxygen. The nasogastric tube was positioned without problems. Which of the following is the most likely diagnosis?
- A. Tracheoesophageal fistula
- B. Pneumonia
- C. Neonatal respiratory distress syndrome (NRDS) (Correct Answer)
- D. Sepsis
- E. Congenital heart anomaly with right-to-left shunt
V/Q mismatch in ARDS Explanation: ***Neonatal respiratory distress syndrome (NRDS)***
- The premature birth (32nd week), presence of **tachypnea**, **retractions**, **nasal flaring**, **cyanosis** responding to oxygen, and maternal **gestational diabetes** are all highly suggestive of NRDS.
- Maternal gestational diabetes can delay fetal lung maturity, increasing the risk of **surfactant deficiency**, which is the primary cause of NRDS.
*Tracheoesophageal fistula*
- This condition typically presents with **choking**, **coughing**, and **regurgitation** during feeding, often with inability to pass a nasogastric tube into the stomach.
- The successful positioning of the **nasogastric tube** makes this diagnosis less likely.
*Pneumonia*
- While pneumonia can cause respiratory distress, the **early onset** in a premature infant with maternal gestational diabetes points more strongly towards **NRDS**.
- Pneumonia would typically have signs of **infection** such as fever, though early neonatal pneumonia can be atypical.
*Sepsis*
- Sepsis can cause respiratory distress, but it's usually accompanied by other signs of systemic infection, such as **fever or hypothermia**, **lethargy**, and poor feeding and often signs of **circulatory compromise**.
- The clinical picture provided primarily points towards a respiratory rather than a systemic infectious cause primarily.
*Congenital heart anomaly with right-to-left shunt*
- While this can cause **cyanosis** and respiratory distress, the prompt response to oxygen management makes a significant right-to-left shunt less likely.
- A significant right-to-left shunt would typically cause **cyanosis** that is refractory to oxygen administration.
V/Q mismatch in ARDS US Medical PG Question 6: A 72-year-old obese man presents as a new patient to his primary care physician because he has been feeling tired and short of breath after recently moving to Denver. He is a former 50 pack-year smoker and has previously had deep venous thrombosis. Furthermore, he previously had a lobe of the lung removed due to lung cancer. Finally, he has a family history of a progressive restrictive lung disease. Laboratory values are obtained as follows:
Oxygen tension in inspired air = 130 mmHg
Alveolar carbon dioxide tension = 48 mmHg
Arterial oxygen tension = 58 mmHg
Respiratory exchange ratio = 0.80
Respiratory rate = 20/min
Tidal volume = 500 mL
Which of the following mechanisms is consistent with these values?
- A. Shunt physiology
- B. High altitude
- C. V/Q mismatch
- D. Pulmonary fibrosis
- E. Hypoventilation (Correct Answer)
V/Q mismatch in ARDS Explanation: ***Hypoventilation***
- The arterial oxygen tension (PaO2) of 58 mmHg is consistent with hypoxemia, and the alveolar carbon dioxide tension (PACO2) of 48 mmHg (normal 35-45 mmHg) indicates **hypercapnia**, a hallmark of hypoventilation.
- The **alveolar-arterial (A-a) gradient** can be calculated using the alveolar gas equation: PAO2 = PiO2 - PACO2/R. Here, PAO2 = 130 mmHg - 48 mmHg/0.8 = 130 - 60 = 70 mmHg. The A-a gradient is PAO2 - PaO2 = 70 - 58 = 12 mmHg, which is within the normal range (5-15 mmHg), indicating that the hypoxemia is primarily due to **decreased alveolar ventilation**.
*Shunt physiology*
- A shunt would cause a significant reduction in PaO2 and a **widened A-a gradient** (typically >15 mmHg) due to deoxygenated blood bypassing ventilated areas.
- While shunts do not typically cause hypercapnia unless very severe, the normal A-a gradient here rules out a significant shunt as the primary mechanism for hypoxemia.
*High altitude*
- Moving to a high altitude (like Denver) causes a decrease in **inspired oxygen tension (PiO2)**, leading to hypoxemia.
- However, the provided inspired oxygen tension (130 mmHg) is above what would be expected for significant high-altitude hypoxemia at sea level equivalent, and the hypoxemia here is associated with hypercapnia, which is not a direct result of high altitude itself.
*V/Q mismatch*
- A V/Q mismatch leads to hypoxemia and a **widened A-a gradient**, as some areas of the lung are either underventilated or underperfused.
- While it can cause hypoxemia, a V/Q mismatch is typically associated with **normal or low PaCO2** due to compensatory hyperventilation, not hypercapnia, and the A-a gradient would be elevated.
*Pulmonary fibrosis*
- Pulmonary fibrosis is a restrictive lung disease that leads to impaired gas exchange, causing hypoxemia primarily due to **V/Q mismatch** and **diffusion limitation**.
- This would result in a **widened A-a gradient** and often a **low PaCO2** due to compensatory hyperventilation, rather than the elevated PaCO2 observed in this patient.
V/Q mismatch in ARDS US Medical PG Question 7: Two days after undergoing left hemicolectomy for a colonic mass, a 62-year-old man develops shortness of breath. His temperature is 38.1°C (100.6°F), pulse is 80/min, respirations are 22/min, and blood pressure is 120/78 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 88%. Cardiopulmonary examination shows decreased breath sounds and decreased fremitus at both lung bases. Arterial blood gas analysis on room air shows:
pH 7.35
PaO2 70 mm Hg
PCO2 40 mm Hg
An x-ray of the chest shows a collapse of the bases of both lungs. Which of the following is the most likely underlying mechanism of this patient's hypoxemia?
- A. Increased anatomic dead space
- B. Decreased hemoglobin oxygen-binding capacity
- C. Decreased chest wall compliance
- D. Increased tidal volume
- E. Decreased ratio of ventilated alveoli (Correct Answer)
V/Q mismatch in ARDS Explanation: ***Decreased ratio of ventilated alveoli***
- The patient's presentation with **shortness of breath**, **decreased breath sounds and fremitus at both lung bases**, and **collapsed lung bases on chest x-ray** points to **atelectasis**.
- **Atelectasis** is a common cause of hypoxemia post-surgery. It occurs when alveoli collapse, leading to areas of the lung that are perfused but not ventilated, resulting in a **ventilation-perfusion (V/Q) mismatch** with a decreased ratio of ventilated alveoli.
*Increased anatomic dead space*
- **Anatomic dead space** refers to the conducting airways where gas exchange does not occur. This value is relatively constant and would not increase significantly to cause such profound hypoxemia in this context.
- Conditions like chronic obstructive pulmonary disease (COPD) can increase dead space, but the patient's acute postoperative presentation and chest X-ray findings do not support this as the primary cause.
*Decreased hemoglobin oxygen-binding capacity*
- This would involve issues like **carbon monoxide poisoning** or specific hemoglobinopathies, which are not indicated by the clinical picture or ABG results (normal pH, PaO2 70 mmHg, PCO2 40 mmHg).
- The PaO2 and SaO2 values indicate a problem with oxygen uptake, not oxygen transport by hemoglobin once bound.
*Decreased chest wall compliance*
- While surgery can cause **pain leading to splinting** and reduced chest wall expansion, which impacts compliance, the primary mechanism of hypoxemia in atelectasis is the **collapse of alveoli**, not solely reduced chest wall movement.
- The **collapsed lung bases** on X-ray directly point to alveolar collapse rather than a general decrease in chest wall compliance as the primary problem.
*Increased tidal volume*
- **Increased tidal volume** would typically improve ventilation and oxygenation, not lead to hypoxemia.
- The patient's **hypoxemia (SaO2 88%, PaO2 70 mmHg)** clearly indicates a problem with oxygen uptake, not an enhancement of respiratory function.
V/Q mismatch in ARDS US Medical PG Question 8: A 68-year-old man comes to the emergency room with difficulty in breathing. He was diagnosed with severe obstructive lung disease a few years back. He uses his medication but often has to come to the emergency room for intravenous therapy to help him breathe. He was a smoker for 40 years smoking two packs of cigarettes every day. Which of the following best represents the expected changes in his ventilation, perfusion and V/Q ratio?
- A. Normal ventilation, low or nonexistent perfusion and infinite V/Q ratio
- B. Medium ventilation and perfusion, V/Q that equals 0.8
- C. Higher ventilation and perfusion with lower V/Q ratio
- D. Low ventilation, normal perfusion and low V/Q ratio (Correct Answer)
- E. Lower ventilation and perfusion, but higher V/Q ratio
V/Q mismatch in ARDS Explanation: ***Low ventilation, normal perfusion and low V/Q ratio***
- In severe **obstructive lung disease** (like COPD), there is airflow limitation, leading to areas of **hypoventilation** in the lungs.
- While ventilation is compromised, blood flow (perfusion) to these areas can remain relatively normal, resulting in a **decreased V/Q ratio**.
*Normal ventilation, low or nonexistent perfusion and infinite V/Q ratio*
- This scenario describes a lung unit with **dead space ventilation**, where there is ventilation but no blood flow (e.g., in a pulmonary embolism).
- The patient's history of **obstructive lung disease** primarily indicates impaired airflow, not a lack of perfusion.
*Medium ventilation and perfusion, V/Q that equals 0.8*
- A **V/Q ratio of 0.8** represents the **ideal normal** ventilation-perfusion matching in a healthy lung.
- The patient has severe obstructive lung disease, which by definition means there is significant mismatch, not normal physiology.
*Higher ventilation and perfusion with lower V/Q ratio*
- While hyperventilation can occur in attempts to compensate, the primary issue in obstructive disease is **impaired ventilation**, not increased ventilation, leading to decreased gas exchange.
- A lower V/Q ratio is expected, but it is driven by **low ventilation**, not higher ventilation and perfusion.
*Lower ventilation and perfusion, but higher V/Q ratio*
- Although both ventilation and perfusion can be affected in severe disease, a **higher V/Q ratio** typically implies areas of increased dead space (more ventilation than perfusion).
- In obstructive disease, the predominant problem is **impaired air entry**, leading to underventilated units with relatively preserved perfusion, thus a **low V/Q ratio**.
V/Q mismatch in ARDS US Medical PG Question 9: A 62-year-old man is brought to the emergency department with a 2-day history of cough productive of yellowish sputum. He has had fever, chills, and worsening shortness of breath over this time. He has a 10-year history of hypertension and hyperlipidemia. He does not drink alcohol or smoke cigarettes. His current medications include atorvastatin, amlodipine, and metoprolol. His temperature is 38.9°C (102.0°F), pulse is 105/min, respirations are 27/min, and blood pressure is 110/70 mm Hg. He appears in mild distress. He has rales over the left lower lung field. The remainder of the examination shows no abnormalities. Leukocyte count is 15,000/mm3 (87% segmented neutrophils). Arterial blood gas analysis on room air shows:
pH 7.44
pO2 68 mm Hg
pCO2 28 mm Hg
HCO3- 24 mEq/L
O2 saturation 91%
An x-ray of the chest shows a consolidation in the left lower lobe. Asking the patient to lie down in the left lateral decubitus position would most likely result in which of the following?
- A. Decreased ventilation of the left lung
- B. Worsen the hypocapnia
- C. Increase in A-a gradient (Correct Answer)
- D. Increased perfusion of right lung
- E. Improve the hypoxemia
V/Q mismatch in ARDS Explanation: ***Increase in A-a gradient***
- Placing the patient in the **left lateral decubitus position** would worsen V/Q mismatch because the **diseased left lung** (with consolidation) would receive increased perfusion due to gravity.
- This increased perfusion to a poorly ventilated area would further impair gas exchange, leading to a larger **alveolar-arterial (A-a) gradient**.
*Decreased ventilation of the left lung*
- While lying on the left side might slightly restrict the expansion of the left lung, the primary issue is the **consolidation** itself, which already severely impairs ventilation.
- The main problem with positioning is not a further decrease in ventilation but rather the **redistribution of blood flow** to an already compromised lung.
*Worsen the hypocapnia*
- The patient has **hypocapnia (pCO2 28 mm Hg)** due to tachypnea as compensation for hypoxemia, indicating increased minute ventilation.
- While worsening the V/Q mismatch will worsen hypoxemia, it's unlikely to directly worsen hypocapnia further; the body would still try to compensate through increased respiratory drive unless the respiratory muscles become fatigued.
*Increased perfusion of right lung*
- In the left lateral decubitus position, **perfusion due to gravity** would increase in the dependent (left) lung, not the non-dependent (right) lung.
- The right lung would experience relatively decreased perfusion compared to the left lung in this position.
*Improve the hypoxemia*
- Lying on the side of the **diseased lung** (left) typically **worsens hypoxemia** because gravity directs more blood flow to the poorly ventilated, consolidated lung.
- To improve hypoxemia, the patient should be positioned with the **healthy lung dependent** (e.g., right lateral decubitus or semi-Fowler's with the right lung lower) to optimize V/Q matching.
V/Q mismatch in ARDS US Medical PG Question 10: During a clinical study examining the diffusion of gas between the alveolar compartment and the pulmonary capillary blood, men between the ages of 20 and 50 years are evaluated while they hold a sitting position. After inhaling a water-soluble gas that rapidly combines with hemoglobin, the concentration of the gas in the participant's exhaled air is measured and the diffusion capacity is calculated. Assuming that the concentration of the inhaled gas remains the same, which of the following is most likely to increase the flow of the gas across the alveolar membrane?
- A. Deep exhalation
- B. Entering a cold chamber
- C. Treadmill exercise (Correct Answer)
- D. Standing straight
- E. Assuming a hunched position
V/Q mismatch in ARDS Explanation: ***Correct: Treadmill exercise***
- **Treadmill exercise** increases cardiac output and pulmonary blood flow, which in turn recruits and distends more **pulmonary capillaries**. This increases the **surface area** available for gas exchange and reduces the diffusion distance, thereby enhancing the flow of gas across the alveolar membrane.
- Exercise also typically leads to deeper and more frequent breaths, increasing the **ventilation-perfusion matching** and overall efficiency of gas exchange.
- According to Fick's law of diffusion (Vgas = A/T × D × ΔP), increasing the surface area (A) directly increases gas flow.
*Incorrect: Deep exhalation*
- **Deep exhalation** would empty the lungs more completely, potentially leading to alveolar collapse in some regions and thus **decreasing the alveolar surface area** available for gas exchange.
- This would also reduce the **driving pressure** for gas diffusion by lowering the alveolar concentration of the inhaled gas.
*Incorrect: Entering a cold chamber*
- Exposure to a **cold chamber** can cause **bronchoconstriction** in some individuals, particularly those with reactive airways, which would increase airway resistance and potentially reduce alveolar ventilation.
- While metabolic rate may slightly increase in the cold, the primary effect on the lungs is unlikely to promote increased gas diffusion in a healthy individual.
*Incorrect: Standing straight*
- **Standing straight** is a normal physiological posture and does not significantly alter the **pulmonary capillary recruitment** or the alveolar surface area in a way that would dramatically increase gas flow compared to a seated position.
- There might be minor gravitational effects on blood flow distribution, but these are generally less impactful than dynamic changes like exercise.
*Incorrect: Assuming a hunched position*
- **Assuming a hunched position** can restrict chest wall expansion and diaphragm movement, leading to **reduced tidal volume** and overall alveolar ventilation.
- This posture, by reducing lung volumes and potentially compressing the lungs, would likely **decrease the effective surface area** for gas exchange and therefore reduce gas flow.
More V/Q mismatch in ARDS US Medical PG questions available in the OnCourse app. Practice MCQs, flashcards, and get detailed explanations.
