V/Q mismatch

V/Q mismatch US Medical PG Question 1: 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 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 US Medical PG Question 2: A 75-year-old female presents to your office with her daughter. The patient states that she feels perfectly well and that she does not know why she is present. The daughter states that over the last several years, the patient has become forgetful and recently forgot her grandchild's name, along with the groceries she was supposed to buy. She was also found lost 10 miles away from her house last week. The daughter also states that the patient has had urinary incontinence over the last few months and has been seeing little children in the morning that are not present. The patient denies any recent falls. Her vitals are normal and her physical exam does not reveal any focal neurological deficits. Her mini-mental status exam is scored 22/30. What is the most accurate test for this patient?

• A. CT angiography of head
• B. CT scan of head
• C. Lumbar puncture
• D. MRI scan of head (Correct Answer)
• E. PET scan of head

V/Q mismatch Explanation: ***MRI scan of head*** - An MRI scan of the head is the **most accurate initial test** to evaluate cognitive decline and rule out structural/reversible causes of dementia. - This patient's presentation includes **progressive memory loss, disorientation, urinary incontinence, and visual hallucinations** - suggestive of **Lewy Body Dementia (LBD)** or potentially **Normal Pressure Hydrocephalus (NPH)**, though gait disturbance (a key NPH feature) is notably absent. - MRI provides detailed visualization of **brain atrophy patterns**, **ventricular enlargement** (for NPH), **white matter lesions** (vascular dementia), **hippocampal atrophy** (Alzheimer's), and excludes other reversible causes like **subdural hematoma, tumor, or stroke**. - **Must be performed first** before any invasive procedures like lumbar puncture. *CT scan of head* - A CT scan is useful for acute conditions like **hemorrhage, stroke, or mass lesions**, but it is **significantly less sensitive** than MRI for detecting subtle changes critical for dementia diagnosis. - Cannot adequately visualize **cortical atrophy, hippocampal volume loss, or subtle white matter changes** that help differentiate dementia subtypes. - While faster and more accessible, it is not the "most accurate" test for cognitive decline evaluation. *CT angiography of head* - CT angiography specifically visualizes **blood vessels** to detect **aneurysms, stenoses, or vascular malformations**. - While vascular disease can contribute to dementia, this test does not evaluate the **brain parenchyma** or structural changes necessary for diagnosing neurodegenerative conditions. - Not indicated as the initial test for cognitive impairment without focal vascular symptoms. *Lumbar puncture* - Lumbar puncture analyzes **cerebrospinal fluid (CSF)** for biomarkers (**amyloid-beta, tau, alpha-synuclein**), infection, or inflammation. - It is an **invasive procedure** that should only be performed **after neuroimaging** to rule out increased intracranial pressure, hydrocephalus, or mass lesions. - While useful for confirming specific dementia diagnoses (e.g., Alzheimer's or LBD biomarkers), it is a **second-line test**, not the initial most accurate diagnostic study. *PET scan of head* - PET imaging (FDG-PET or amyloid-PET) measures **metabolic activity** or **specific protein deposits** and is highly specific for certain dementias like **Alzheimer's disease** or **Frontotemporal dementia**. - It is typically a **specialized second-line test** used after structural imaging when the diagnosis remains unclear. - **More expensive and less available** than MRI, and not necessary as the initial most accurate test for broad cognitive impairment evaluation.

V/Q mismatch US Medical PG Question 3: Which mechanism is primarily responsible for the increase in pulmonary diffusing capacity during exercise?

• A. Decreased airway resistance
• B. Reduced membrane thickness
• C. Increased alveolar ventilation
• D. Pulmonary capillary recruitment (Correct Answer)

V/Q mismatch Explanation: ***Pulmonary capillary recruitment*** - During exercise, more **pulmonary capillaries** that were previously unperfused or poorly perfused open up, increasing the **surface area available for gas exchange**. - This **recruitment** directly enhances the pulmonary diffusing capacity by providing more sites for oxygen to cross from the alveoli into the blood. *Decreased airway resistance* - While airway resistance can decrease during exercise due to **bronchodilation**, this primarily affects **airflow** and ventilation, not the efficiency of gas diffusion across the alveolar-capillary membrane. - Reduced airway resistance facilitates getting air into and out of the lungs but does not expand the surface area for diffusion or thin the membrane. *Reduced membrane thickness* - The thickness of the **alveolar-capillary membrane** is a structural characteristic that does not significantly change acutely during exercise. - While a thinner membrane would improve diffusion, this is not the primary mechanism behind the exercise-induced increase in diffusing capacity. *Increased alveolar ventilation* - Increased alveolar ventilation ensures a higher **partial pressure of oxygen** in the alveoli. - While essential for delivering oxygen, it primarily affects the **driving pressure for diffusion** rather than the physical capacity of the diffusion barrier itself.

V/Q mismatch US Medical PG Question 4: A 78-year-old man is brought in to the emergency department by ambulance after his wife noticed that he began slurring his speech and had developed facial asymmetry during dinner approximately 30 minutes ago. His past medical history is remarkable for hypertension and diabetes. His temperature is 99.1°F (37.3°C), blood pressure is 154/99 mmHg, pulse is 89/min, respirations are 12/min, and oxygen saturation is 98% on room air. Neurologic exam reveals right upper and lower extremity weakness and an asymmetric smile. Which of the following is the next best step in management?

• A. Alteplase
• B. MRI brain
• C. CT head (Correct Answer)
• D. Aspirin
• E. CTA head

V/Q mismatch Explanation: ***CT head*** - A **non-contrast CT head** is the immediate priority to differentiate between ischemic and hemorrhagic stroke, which is critical for guiding subsequent treatment decisions. - Given the patient's acute neurological deficits (slurred speech, facial asymmetry, weakness) and vascular risk factors (hypertension, diabetes), **stroke is highly suspected**, and identifying intracerebral hemorrhage is crucial before considering thrombolytic therapy. *Alteplase* - **Alteplase** (tPA) is a thrombolytic agent used for acute ischemic stroke, but its administration is **contraindicated in hemorrhagic stroke**. - Initiating alteplase without first ruling out hemorrhage with a CT scan could lead to catastrophic bleeding. *MRI brain* - While an **MRI brain** can provide more detailed imaging of stroke, it is typically **not the initial imaging modality** in the emergency setting due to longer acquisition times and limited availability, especially when emergent differentiation between ischemic and hemorrhagic stroke is needed. - Its use is usually reserved for cases where the CT is inconclusive or for later evaluation. *Aspirin* - **Aspirin** is an antiplatelet agent used in the management of ischemic stroke, but it should **not be given until a hemorrhagic stroke has been ruled out** via CT head. - Administering aspirin in the context of an intracerebral hemorrhage could worsen bleeding. *CTA head* - A **CT angiography (CTA) head** is used to visualize the cerebral vasculature and identify large vessel occlusions, which can guide thrombectomy decisions in ischemic stroke. - However, performing a **non-contrast CT head is a prerequisite** to rule out hemorrhage before proceeding with CTA or any other advanced imaging or therapeutic interventions.

V/Q mismatch US Medical PG Question 5: A 72-year-old man presents to the emergency department after a fall. The patient was found lying down on the floor in his room in his retirement community. The patient has a past medical history of Alzheimer dementia and a prosthetic valve. His current medications include donepezil and warfarin. His temperature is 97.7°F (36.5°C), blood pressure is 85/50 mmHg, pulse is 160/min, respirations are 13/min, and oxygen saturation is 97% on room air. The patient is started on IV fluids and a type and screen is performed. Laboratory values are ordered as seen below. Hemoglobin: 13 g/dL Hematocrit: 39% Leukocyte count: 5,500 cells/mm^3 with normal differential Platelet count: 225,000/mm^3 INR: 2.5 AST: 10 U/L ALT: 12 U/L A chest radiograph and EKG are performed and are within normal limits. A full physical exam is within normal limits. The patient's vitals are repeated. His temperature is 99.5°F (37.5°C), blood pressure is 110/70 mmHg, pulse is 90/min, respirations are 10/min, and oxygen saturation is 98% on room air. Which of the following is the best next step in management?

• A. CT scan (Correct Answer)
• B. Urgent blood transfusion
• C. Fresh frozen plasma
• D. Exploratory laparoscopy
• E. Exploratory laparotomy

V/Q mismatch Explanation: ***CT scan*** - A patient with a **prosthetic valve** on **warfarin** and a fall is at high risk for **intracranial hemorrhage**, even without focal neurological deficits. - While initial vitals improved after IV fluids, the mechanism of injury (fall) and medication profile warrant a **CT scan** of the head to rule out serious internal injury, especially given the history of dementia which might mask symptoms. *Urgent blood transfusion* - The patient's **hemoglobin (13 g/dL)** and **hematocrit (39%)** are within normal limits, indicating no acute need for blood transfusion due to hemorrhage. - Transfusions are typically reserved for patients with significant blood loss or severe symptomatic anemia. *Fresh frozen plasma* - The patient's **INR of 2.5** is within the therapeutic range for a patient with a prosthetic valve on warfarin. - There is no evidence of active bleeding or supratherapeutic anticoagulation that would necessitate the administration of **fresh frozen plasma (FFP)** to reverse anticoagulation. *Exploratory laparoscopy* - There are no clinical signs or symptoms, such as abdominal pain, distension, or evidence of intra-abdominal bleeding (e.g., declining hemoglobin, peritoneal signs), to suggest an indication for an **exploratory laparoscopy**. - The patient's physical exam was described as normal. *Exploratory laparotomy* - Similar to laparoscopy, there is no clinical evidence of acute abdominal injury or hemorrhage, which would necessitate an **exploratory laparotomy**. - This invasive procedure is reserved for cases with strong suspicion of significant intra-abdominal pathology or trauma.

V/Q mismatch US Medical PG Question 6: A 24-year-old male is brought in by ambulance to the emergency department after he was found unresponsive at home for an unknown length of time. Upon arrival, he is found to be severely altered and unable to answer questions about his medical history. Based on clinical suspicion, a panel of basic blood tests are obtained including an arterial blood gas, which shows a pH of 7.32, a pCO2 of 70, and a bicarbonate level of 30 mEq/L. Which of the following is most likely the primary disturbance leading to the values found in the ABG?

• A. Respiratory acidosis (Correct Answer)
• B. Metabolic alkalosis
• C. Respiratory alkalosis
• D. Metabolic acidosis
• E. Mixed alkalosis

V/Q mismatch Explanation: ***Respiratory acidosis*** - The **pH (7.32)** is acidic (normal 7.35-7.45), and the **pCO2 (70 mmHg)** is significantly elevated (normal 35-45 mmHg), indicating **primary respiratory acidosis** due to hypoventilation. - The **bicarbonate (30 mEq/L)** is elevated above normal (22-26 mEq/L), indicating **partial metabolic compensation** by the kidneys retaining bicarbonate to buffer the acidosis. - This pattern suggests **chronic respiratory acidosis** (e.g., from COPD, CNS depression, neuromuscular disease) with renal compensation. *Metabolic alkalosis* - This would present with **elevated pH** (>7.45) and **elevated bicarbonate** as the primary disturbance, often with compensatory elevation in pCO2. - The patient's **pH is acidic (7.32)**, not alkalotic, ruling out metabolic alkalosis as the primary process. *Respiratory alkalosis* - This would present with **elevated pH** (>7.45) and **decreased pCO2** (<35 mmHg) due to hyperventilation. - The patient has the opposite: **acidic pH and elevated pCO2**, ruling out respiratory alkalosis. *Metabolic acidosis* - This would present with **decreased pH** and **decreased bicarbonate** (<22 mEq/L) as the primary disturbance. - While the pH is low, the **bicarbonate is elevated (30 mEq/L)**, not decreased, ruling out metabolic acidosis as the primary disorder. *Mixed alkalosis* - A mixed alkalosis would involve simultaneous respiratory and metabolic processes causing **elevated pH**. - The patient's **pH is acidic (7.32)**, making any form of alkalosis impossible as the primary disturbance.

V/Q mismatch US Medical PG Question 7: During heavy exercise, what is the primary mechanism for maintaining arterial pH despite increased lactic acid production?

• A. Increased bicarbonate reabsorption
• B. Phosphate buffering
• C. Increased hydrogen secretion
• D. Hyperventilation (Correct Answer)

V/Q mismatch Explanation: ***Hyperventilation*** - **Hyperventilation** during heavy exercise increases the expulsion of **carbon dioxide (CO2)**, shifting the **bicarbonate buffer system** equilibrium to the left. - This reduction in **CO2** effectively removes **hydrogen ions (H+)**, thereby helping to maintain **arterial pH** despite rising **lactic acid** levels. *Increased bicarbonate reabsorption* - While the kidneys adapt by increasing **bicarbonate reabsorption** to compensate for acidosis, this is a **slower renal mechanism** for pH regulation, taking hours to days, rather than an immediate response during acute exercise. - The rapid pH regulation during exercise primarily relies on respiratory and chemical buffer systems, not renal function. *Phosphate buffering* - The **phosphate buffer system** is indeed important for intracellular and renal tubular fluid buffering. - However, its buffering capacity in the extracellular fluid and plasma is relatively limited compared to the **bicarbonate system** due to its lower concentration. *Increased hydrogen secretion* - **Increased hydrogen secretion** by the renal tubules is a long-term mechanism for compensating for acidosis, which helps excrete excess **acid** and regenerate **bicarbonate**. - This is a slow, renal regulatory process and not the primary rapid mechanism for maintaining pH during the immediate demands of heavy exercise.

V/Q mismatch US Medical PG Question 8: A 35-year-old man presents to pulmonary function clinic for preoperative evaluation for a right pneumonectomy. His arterial blood gas at room air is as follows: pH: 7.34 PaCO2: 68 mmHg PaO2: 56 mmHg Base excess: +1 O2 saturation: 89% What underlying condition most likely explains these findings?

• A. Cystic fibrosis
• B. Bronchiectasis
• C. Chronic obstructive pulmonary disease (Correct Answer)
• D. Obesity
• E. Acute respiratory distress syndrome

V/Q mismatch Explanation: ***Chronic obstructive pulmonary disease*** - This patient exhibits **compensated respiratory acidosis** (low pH, high PaCO2, slightly elevated base excess) and **hypoxemia** (low PaO2), which are characteristic findings in chronic obstructive pulmonary disease (COPD) with underlying respiratory failure. - The history of a planned **pneumonectomy** also suggests a significant pre-existing lung pathology often seen in patients with severe COPD. *Cystic fibrosis* - While cystic fibrosis can lead to chronic lung disease, it typically presents at a younger age and is associated with a history of recurrent infections and exocrine gland dysfunction. - While it can manifest similarly in ABG, the age and the planned pneumonectomy make COPD a more likely primary cause in this context. *Bronchiectasis* - Bronchiectasis involves permanent dilation of the bronchi, often leading to chronic cough, sputum production, and recurrent infections. - While it can cause respiratory compromise, the ABG findings are more classically associated with the widespread air trapping and V/Q mismatch seen in COPD. *Obesity* - Severe obesity can lead to **obesity hypoventilation syndrome**, presenting with hypercapnia and hypoxemia. - However, the patient's age and the context of a planned pneumonectomy make an underlying primary lung disease like COPD a more focused explanation for the ABG pattern. *Acute respiratory distress syndrome* - Acute respiratory distress syndrome (ARDS) is an **acute** and severe form of respiratory failure characterized by severe hypoxemia and bilateral opacities on chest imaging. - The ABG findings in ARDS typically show **severe hypoxemia** with **respiratory alkalosis** early on, evolving to acidosis, and it is an acute process, not a chronic pre-existing condition suitable for elective surgery.

V/Q mismatch US Medical PG Question 9: In which of the following pathological states would the oxygen content of the trachea resemble the oxygen content in the affected alveoli?

• A. Emphysema
• B. Exercise
• C. Pulmonary embolism (Correct Answer)
• D. Pulmonary fibrosis
• E. Foreign body obstruction distal to the trachea

V/Q mismatch Explanation: ***Pulmonary embolism*** - A pulmonary embolism blocks **blood flow** to a portion of the lung, creating **dead space ventilation** (high V/Q ratio). - In the affected alveoli, **no blood perfusion** means no oxygen extraction occurs, so the alveolar oxygen content remains **high and similar to tracheal/inspired air**. - This is the classic physiological state where ventilation continues but perfusion is absent, preventing gas exchange. *Foreign body obstruction distal to the trachea* - A complete obstruction **prevents fresh air** from reaching the affected alveoli. - The trapped gas undergoes **resorption atelectasis**: oxygen is absorbed into capillary blood, CO2 diffuses in, and alveolar gas equilibrates with **venous blood** composition. - Alveolar oxygen content becomes **very low**, not similar to tracheal air. *Emphysema* - Emphysema involves destruction of **alveolar walls** and enlargement of airspaces with impaired gas exchange. - While V/Q mismatch occurs, oxygen is still extracted by perfusing blood. - Alveolar oxygen content is **lower than tracheal air** due to ongoing (though inefficient) gas exchange. *Exercise* - During exercise, **oxygen consumption increases** dramatically with enhanced cardiac output and oxygen extraction. - Alveolar oxygen content is **significantly lower** than tracheal air due to increased oxygen uptake by blood. *Pulmonary fibrosis* - Pulmonary fibrosis causes **thickening of the alveolar-capillary membrane**, impairing oxygen diffusion. - Despite diffusion limitation, blood still perfuses the alveoli and extracts oxygen. - Alveolar oxygen content is **lower than tracheal air**, though the A-a gradient is increased.

V/Q mismatch US Medical PG Question 10: 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 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 Flashcard 1 of 10

Question: What is the ideal V/Q ratio for adequate gas exchange? _____

Answer: 1 (ventilation matches perfusion)

V/Q mismatch Flashcard 2 of 10

Question: Does 100% O2 improve PaO2 in V/Q mismatch due to shunt? _____

Answer: No

V/Q mismatch Flashcard 3 of 10

Question: The _____ ratio is the ratio of alveolar ventilation to pulmonary blood flow

Answer: ventilation/perfusion (V/Q)

V/Q mismatch Flashcard 4 of 10

Question: _____ is perfusion of lung regions that are not ventilated (V/Q = 0)

Answer: Shunt

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V/Q mismatch Flashcard 5 of 10

Question: The V/Q ratio at the base of the lung is normally _____, indicating wasted perfusion

Answer: 0.6

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V/Q mismatch Flashcard 6 of 10

Question: In zone _____ of the lung, high alveolar pressure may compress the capillaries and reduce blood flow in this zone

Answer: 1

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V/Q mismatch Flashcard 7 of 10

Question: In the lungs, hypoxia causes vaso-_____

Answer: constriction

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V/Q mismatch Flashcard 8 of 10

Question: Certain organisms that thrive in high O2 (e.g. TB) flourish in the _____ of the lung

Answer: apex

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V/Q mismatch Flashcard 9 of 10

Question: Is the A - a gradient normal, increased, or decreased, in response to V/Q mismatch? _____

Answer: Increased

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V/Q mismatch Flashcard 10 of 10

Question: With exercise (increased cardiac output), there is vasodilation of apical capillaries in the lung, which causes the V/Q ratio to approach a value of _____

Answer: 1

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