Shunt physiology (low V/Q) US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Shunt physiology (low V/Q). These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Shunt physiology (low V/Q) US Medical PG Question 1: 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
Shunt physiology (low V/Q) 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.
Shunt physiology (low V/Q) US Medical PG Question 2: 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
Shunt physiology (low V/Q) 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.
Shunt physiology (low V/Q) US Medical PG Question 3: Four days after undergoing an elective total hip replacement, a 65-year-old woman develops a DVT that embolizes to the lung. Along with tachypnea, tachycardia, and cough, the patient would most likely present with a PaO2 of what?
- A. 120 mmHg
- B. 100 mmHg
- C. 85 mmHg (Correct Answer)
- D. 110 mmHg
- E. 60 mmHg
Shunt physiology (low V/Q) Explanation: ***85 mmHg***
- A pulmonary embolism (PE) causes a **ventilation-perfusion (V/Q) mismatch**, leading to **hypoxemia** and a reduced PaO2.
- While exact values vary, a PaO2 of 85 mmHg indicates **mild to moderate hypoxemia**, which is common in PE, especially with accompanying symptoms like tachypnea and tachycardia.
*120 mmHg*
- This value is significantly **higher than normal (75-100 mmHg)** and would indicate **hyperoxia**, which is inconsistent with acute pulmonary embolism causing respiratory distress.
- A patient with PE would typically have **reduced oxygenation**, not supernormal levels, unless receiving high-flow supplemental oxygen.
*100 mmHg*
- A PaO2 of 100 mmHg is at the **upper end of the normal range** (75-100 mmHg) and would imply **no significant hypoxemia**.
- Given the patient's symptoms of tachypnea, tachycardia, and cough following a DVT with embolization, a normal or high-normal PaO2 is unlikely without aggressive oxygen therapy (which is not stated).
*110 mmHg*
- This value is **above the normal range** and suggests **hyperoxia**, which is contrary to the pathophysiology of a pulmonary embolism.
- A PE impairs gas exchange, leading to a decrease in PaO2, not an increase.
*60 mmHg*
- A PaO2 of 60 mmHg indicates **significant hypoxemia**, which might occur in a severe, large pulmonary embolism or in a patient with underlying lung disease.
- While possible, 85 mmHg represents a more common, moderate hypoxemia seen in PE, especially given the prompt presentation of symptoms.
Shunt physiology (low V/Q) US Medical PG Question 4: A 36-year-old woman is admitted to the hospital for the evaluation of progressive breathlessness. She has no history of major medical illness. Her temperature is 37°C (98.6°F), pulse is 110/min, and respirations are 22/min. Pulse oximetry on room air shows an oxygen saturation of 99%. Cardiac examination shows a loud S1 and S2. There is a grade 2/6 early systolic murmur best heard in the 2nd right intercostal space. Cardiac catheterization shows a mixed venous oxygen saturation of 55% (N= 65–70%). Which of the following is the most likely cause of this patient's breathlessness?
- A. Increased peripheral shunting
- B. Decreased hemoglobin concentration
- C. Increased carbon dioxide retention
- D. Increased pulmonary vascular resistance
- E. Decreased left ventricular ejection fraction (Correct Answer)
Shunt physiology (low V/Q) Explanation: ***Decreased left ventricular ejection fraction***
- The key finding is a **mixed venous oxygen saturation of 55% (normal 65-70%)** with **normal arterial oxygen saturation (99%)**, which indicates **increased tissue oxygen extraction**
- Increased oxygen extraction occurs when **cardiac output is reduced** → tissues must extract more oxygen from each pass of blood to meet metabolic demands
- This is the classic physiologic compensation in **heart failure with reduced ejection fraction**
- The cardiac findings (loud heart sounds, systolic murmur) suggest underlying cardiac pathology causing reduced cardiac output and progressive breathlessness
*Increased peripheral shunting*
- Peripheral shunting (e.g., arteriovenous malformations) would cause **venous blood to bypass capillary beds**, resulting in **decreased oxygen extraction** and **higher mixed venous O2 saturation**, not lower
- Would typically cause **hypoxemia** with reduced pulse oximetry, but this patient has 99% oxygen saturation
*Decreased hemoglobin concentration*
- Anemia reduces oxygen-carrying capacity but would not explain the **low mixed venous oxygen saturation** to this degree
- The **pulse oximetry of 99%** indicates adequate oxygen saturation of available hemoglobin
- Anemia typically causes **high cardiac output** (compensatory) rather than the low cardiac output state indicated by the low mixed venous O2 saturation
*Increased carbon dioxide retention*
- **Hypercapnia** results from **hypoventilation** and impaired gas exchange, typically causing **respiratory acidosis**
- Would present with altered mental status, drowsiness, or signs of respiratory failure
- Does not explain the **low mixed venous oxygen saturation** with normal arterial oxygen saturation
- The cardiac findings point to a primary cardiac rather than respiratory problem
*Increased pulmonary vascular resistance*
- **Pulmonary hypertension** causes **right ventricular dysfunction** and can present with breathlessness and a loud P2 component of S2
- However, isolated pulmonary hypertension would not cause the same degree of **systemic oxygen extraction** increase
- The low mixed venous O2 saturation indicates **reduced systemic cardiac output**, which primarily reflects **left ventricular dysfunction** rather than isolated right-sided pathology
Shunt physiology (low V/Q) US Medical PG Question 5: A 2-year-old boy is brought to a pediatrician because his parents have noticed that he seems to be getting tired very easily at home. Specifically, they have noticed that he is often panting for breath after walking around the house for a few minutes and that he needs to take naps fairly often throughout the day. He has otherwise been well, and his parents do not recall any recent infections. He was born at home, and his mom did not receive any prenatal care prior to birth. Physical exam reveals a high-pitched, harsh, holosystolic murmur that is best heard at the lower left sternal border. No cyanosis is observed. Which of the following oxygen tension profiles would most likely be seen in this patient? (LV = left ventricle, RV = right ventricle, and SC = systemic circulation).
- A. LV: normal, RV: normal, SC: normal
- B. LV: normal, RV: increased, SC: normal (Correct Answer)
- C. LV: decreased, RV: increased, SC: decreased
- D. LV: decreased, RV: normal, SC: decreased
- E. LV: normal, RV: normal, SC: decreased
Shunt physiology (low V/Q) Explanation: ***LV: normal, RV: increased, SC: normal***
- The patient's presentation with easy fatigability, dyspnea on exertion, and a **holosystolic murmur** at the **lower left sternal border** strongly suggests a **ventricular septal defect (VSD)**. These symptoms result from a **left-to-right shunt**, leading to increased blood flow and pressure in the **right ventricle (RV)** and pulmonary circulation.
- In a VSD, highly oxygenated blood from the **left ventricle (LV)** shunts into the RV. This increases the **oxygen tension** in the RV, while the LV and systemic circulation (SC) typically maintain normal oxygen tension if the shunt is not so large that it causes **pulmonary hypertension** with **Eisenmenger syndrome**.
*LV: normal, RV: normal, SC: normal*
- This profile would indicate a **normal cardiovascular system** without any significant shunting or cardiac anomaly.
- It does not align with the patient's symptoms of easy fatigability, dyspnea, and the presence of a pathological murmur.
*LV: decreased, RV: increased, SC: decreased*
- A **decreased oxygen tension in the left ventricle** and **systemic circulation** typically indicates a **right-to-left shunt** or severe **pulmonary disease**, often associated with **cyanosis**, which is noted as absent in this patient.
- While RV oxygen tension *could* be increased in some complex congenital heart diseases with right-to-left shunting (e.g., mixing lesions), the overall profile does not fit the characteristic presentation of a VSD without cyanosis.
*LV: decreased, RV: normal, SC: decreased*
- This profile with **decreased oxygen tension in the left ventricle** and **systemic circulation** suggests a condition where oxygenated blood supply to the systemic circulation is compromised, such as severe **left ventricular dysfunction** or a **right-to-left shunt**.
- A **normal RV oxygen tension** without **cyanosis** makes this unlikely in the context of the patient's symptoms.
*LV: normal, RV: normal, SC: decreased*
- A **decreased oxygen tension in the systemic circulation** with **normal LV and RV oxygen tension** is inconsistent with a **VSD**.
- This profile might be observed in conditions like severe **anemia** or **hypoxia** without a primary cardiac shunt.
Shunt physiology (low V/Q) US Medical PG Question 6: A person is exercising strenuously on a treadmill for 1 hour. An arterial blood gas measurement is then taken. Which of the following are the most likely values?
- A. pH 7.56, PaO2 100, PCO2 44, HCO3 38
- B. pH 7.32, PaO2 42, PCO2 50, HCO3 27
- C. pH 7.57 PaO2 100, PCO2 23, HCO3 21 (Correct Answer)
- D. pH 7.38, PaO2 100, PCO2 69 HCO3 42
- E. pH 7.36, PaO2 100, PCO2 40, HCO3 23
Shunt physiology (low V/Q) Explanation: ***pH 7.57, PaO2 100, PCO2 23, HCO3 21***
- After 1 hour of strenuous exercise, this represents **respiratory alkalosis with mild metabolic compensation**, which is the expected finding in a healthy individual during sustained vigorous exercise.
- The **low PCO2 (23 mmHg)** reflects appropriate **hyperventilation** in response to increased metabolic demands and lactic acid production. During intense exercise, minute ventilation increases dramatically, often exceeding the rate of CO2 production.
- The **slightly elevated pH (7.57)** and **mildly decreased HCO3 (21 mEq/L)** indicate that respiratory compensation has slightly overshot, creating mild alkalosis, while the bicarbonate is consumed both in buffering lactate and through renal compensation.
- **Normal PaO2 (100 mmHg)** confirms adequate oxygenation maintained by increased ventilation.
*pH 7.36, PaO2 100, PCO2 40, HCO3 23*
- These are **completely normal arterial blood gas values** with no evidence of any physiological stress or compensation.
- After 1 hour of strenuous exercise, we would expect **hyperventilation with decreased PCO2**, not a normal PCO2 of 40 mmHg. This profile would be consistent with rest, not vigorous exercise.
- The absence of any respiratory or metabolic changes makes this inconsistent with the clinical scenario.
*pH 7.56, PaO2 100, PCO2 44, HCO3 38*
- This profile suggests **metabolic alkalosis** (high pH, high HCO3) with inadequate respiratory compensation (normal to slightly elevated PCO2).
- This is **not consistent with strenuous exercise**, which produces metabolic acid (lactate), not metabolic base. The elevated HCO3 suggests vomiting, diuretic use, or other causes of metabolic alkalosis.
*pH 7.32, PaO2 42, PCO2 50, HCO3 27*
- This indicates **respiratory acidosis** (low pH, high PCO2) with **severe hypoxemia** (PaO2 42 mmHg).
- During strenuous exercise, healthy individuals **increase ventilation** to enhance O2 delivery and remove CO2, so both hypoxemia and hypercapnia are unexpected and would suggest severe cardiopulmonary disease or hypoventilation.
*pH 7.38, PaO2 100, PCO2 69, HCO3 42*
- This demonstrates **compensated respiratory acidosis** (normal pH, markedly elevated PCO2 and HCO3).
- The **very high PCO2 (69 mmHg)** indicates severe **hypoventilation**, which is the opposite of what occurs during exercise. This profile suggests chronic respiratory failure with metabolic compensation, such as in severe COPD.
Shunt physiology (low V/Q) US Medical PG Question 7: A 2-year-old boy is brought to the physician because of coughing and difficulty breathing that started shortly after his mother found him in the living room playing with his older brother's toys. He appears anxious. Respirations are 33/min and pulse oximetry on room air shows an oxygen saturation of 88%. Physical examination shows nasal flaring and intercostal retractions. Auscultation of the lungs shows a high-pitched inspiratory wheeze and absent breath sounds on the right side. There is no improvement in his oxygen saturation after applying a non-rebreather mask with 100% FiO2. Which of the following terms best describes the most likely underlying mechanism of the right lung's impaired ventilation?
- A. Alveolar hyperventilation
- B. Alveolar dead space
- C. Diffusion limitation
- D. Alveolar hypoventilation (Correct Answer)
- E. Right-to-left shunt
Shunt physiology (low V/Q) Explanation: ***Alveolar hypoventilation***
- The clinical presentation strongly suggests **foreign body aspiration** causing complete obstruction of the right main bronchus, leading to **alveolar hypoventilation** of the entire right lung.
- **Alveolar hypoventilation** means reduced or absent air movement into the alveoli. In this case, the mechanical obstruction prevents ventilation (V/Q = 0), while perfusion continues normally, creating severe V/Q mismatch.
- The **hypoxemia unresponsive to 100% FiO2** occurs because blood perfusing the unventilated right lung remains deoxygenated (shunt-like physiology), but the underlying mechanism is **ventilation failure** (hypoventilation), not an anatomical shunt.
- **Absent breath sounds** on the right confirm no air entry to that lung, which is the definition of regional hypoventilation.
*Right-to-left shunt*
- A **true anatomical right-to-left shunt** refers to blood bypassing the lungs entirely through intracardiac defects (VSD, ASD, PDA with Eisenmenger syndrome) or intrapulmonary arteriovenous malformations.
- While the obstructed lung creates **shunt-like physiology** (blood passes unventilated alveoli), the mechanism is **hypoventilation due to airway obstruction**, not an anatomical shunt.
- The distinction is important: shunt describes the physiological effect (V/Q = 0), but hypoventilation describes the mechanism (airway obstruction preventing ventilation).
*Alveolar hyperventilation*
- This refers to **increased alveolar ventilation** beyond metabolic needs, leading to increased CO2 elimination and respiratory alkalosis.
- The patient shows tachypnea (33/min), which represents compensatory effort, but the right lung has **decreased ventilation** (hypoventilation), not hyperventilation.
*Alveolar dead space*
- **Alveolar dead space** occurs when alveoli are **ventilated but not perfused** (V/Q approaching infinity), as seen in pulmonary embolism.
- This scenario shows the opposite: the right lung is **perfused but not ventilated** due to airway obstruction.
*Diffusion limitation*
- **Diffusion limitation** occurs when gas exchange across the alveolar-capillary membrane is impaired (pulmonary fibrosis, interstitial lung disease).
- This patient has **mechanical obstruction preventing air from reaching the alveoli**, not a problem with diffusion across intact membranes.
- Diffusion limitation typically responds partially to supplemental oxygen, unlike complete obstruction.
Shunt physiology (low V/Q) US Medical PG Question 8: A 32-year-old female with Crohn's disease diagnosed in her early 20s comes to your office for a follow-up appointment. She is complaining of headaches and fatigue. Which of the following arterial blood gas findings might you expect?
- A. High PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
- B. Low PaO2, low O2 saturation (SaO2), low O2 content (CaO2)
- C. Normal PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
- D. Normal PaO2, normal O2 saturation (SaO2), low O2 content (CaO2) (Correct Answer)
- E. Low PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
Shunt physiology (low V/Q) Explanation: ***Normal PaO2, normal O2 saturation (SaO2), low O2 content (CaO2)***
- Patients with **Crohn's disease** are prone to developing **iron deficiency anemia** due to chronic inflammation, malabsorption, and blood loss, leading to reduced hemoglobin levels.
- While PaO2 and SaO2 measure oxygen *tension* and *percentage saturation* of available hemoglobin, respectively, **O2 content (CaO2)** directly reflects the *total amount* of oxygen delivered to tissues, which is primarily dependent on hemoglobin concentration. Therefore, with anemia, CaO2 will be low despite normal PaO2 and SaO2 because there is less hemoglobin to carry oxygen.
*High PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- High PaO2 would indicate **hyperoxygenation**, which is not an expected complication of Crohn's disease or its associated anemia.
- Normal O2 content is inconsistent with the presence of anemia, which significantly reduces the body's total oxygen-carrying capacity.
*Low PaO2, low O2 saturation (SaO2), low O2 content (CaO2)*
- Low PaO2 and SaO2 suggest a primary **respiratory problem** or severe hypoxemia, which is not directly linked to Crohn's disease or the typical presentation of iron deficiency anemia.
- While low O2 content is correct for anemia, the accompanying low PaO2 and SaO2 indicate a different underlying pathology for oxygen transport issues.
*Normal PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- This finding would indicate **normal oxygenation** and oxygen-carrying capacity, which is contrary to the clinical scenario of a patient with Crohn's likely complicated by anemia.
- The patient's symptoms of headaches and fatigue are consistent with poor tissue oxygenation, which would not occur if all these parameters were normal.
*Low PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- A low PaO2 with a normal SaO2 is physiologically unlikely unless there is a **left shift of the oxygen dissociation curve** with adequate hemoglobin, which doesn't fit the expected anemic state.
- Normal O2 content would rule out the presence of anemia as a cause for the symptoms, which is a common complication in Crohn's disease.
Shunt physiology (low V/Q) US Medical PG Question 9: A 52-year-old woman presents to the emergency department with breathlessness for the past 6 hours. She denies cough, nasal congestion or discharge, sneezing, blood in sputum, or palpitation. There is no past history of chronic respiratory or cardiovascular medical conditions, but she mentions that she has been experiencing frequent cramps in her left leg for the past 5 days. She is post-menopausal and has been on hormone replacement therapy for a year now. Her temperature is 38.3°C (100.9°F), the pulse is 116/min, the blood pressure is 136/84 mm Hg, and the respiratory rate is 24/min. Edema and tenderness are present in her left calf region. Auscultation of the chest reveals rales over the left infrascapular and scapular region. The heart sounds are normal and there are no murmurs. Which of the following mechanisms most likely contributed to the pathophysiology of this patient’s condition?
- A. Secretion of vasodilating neurohumoral substances in pulmonary vascular bed
- B. Increased right ventricular preload (Correct Answer)
- C. Decreased physiologic dead space
- D. Alveolar hyperventilation
- E. Decreased alveolar-arterial oxygen tension gradient
Shunt physiology (low V/Q) Explanation: ***Increased right ventricular preload***
- The patient's presentation (acute breathlessness, unilateral leg cramps, calf tenderness and edema, rales) combined with risk factors (post-menopausal, hormone replacement therapy) strongly suggests **pulmonary embolism (PE)** from deep vein thrombosis (DVT).
- In PE, thrombus occludes pulmonary vasculature causing **increased pulmonary vascular resistance**, which increases **right ventricular afterload** (the resistance the RV must overcome to eject blood).
- **Note:** While this option states "preload," the primary mechanism is actually increased RV **afterload**. However, this is the most appropriate answer among the given options, as the increased resistance does lead to RV strain and potential backup of blood that can secondarily affect preload.
*Secretion of vasodilating neurohumoral substances in pulmonary vascular bed*
- The primary vascular response in PE is **vasoconstriction**, not vasodilation.
- Hypoxia and mediator release cause **pulmonary vasoconstriction** distal to the embolus, further increasing pulmonary vascular resistance.
*Decreased physiologic dead space*
- In PE, there is **ventilation-perfusion (V/Q) mismatch** where lung regions are ventilated but not perfused due to embolic obstruction.
- This actually **increases physiologic dead space** because these areas are ventilated but cannot participate in gas exchange.
*Alveolar hyperventilation*
- Patients with PE often develop **tachypnea and hyperventilation** due to hypoxia, anxiety, and chest discomfort.
- However, this is a **compensatory response** to hypoxemia, not the primary pathophysiological mechanism causing the condition.
*Decreased alveolar-arterial oxygen tension gradient*
- The **A-a gradient is increased in PE** due to V/Q mismatch and shunting, reflecting impaired gas exchange.
- A decreased A-a gradient would indicate efficient gas exchange, which contradicts the hypoxia and breathlessness seen in PE.
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