Pulmonary Circulation — MCQs

10 questions

What happens to gas exchange when the Va/Q ratio approaches infinity?

Which condition is primarily responsible for the decrease in arterial PO2 in patients with chronic obstructive pulmonary disease (COPD)?

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

Which of the following statements about high altitude pulmonary edema (HAPE) is true?

During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by

An infant presents with LVH and pulmonary complications. ECG shows left axis deviation. The most likely diagnosis is:

Most specific cardiac anomaly seen in baby born to Diabetic Mother

Which heart chamber has the thickest wall?

Which of the following is true about polyaeritis nodosa?

Q10

5 year old child going for sitting craniotomy, while positioning in O.T. developed end tidal CO2- Zero mmHg, PO2- 80 mm Hg implies that :

Pulmonary Circulation Indian Medical PG Practice Questions and MCQs

Question 1: What happens to gas exchange when the Va/Q ratio approaches infinity?

A. Partial pressure of O2 becomes negligible.
B. No exchange of O2 and CO2 occurs. (Correct Answer)
C. Partial pressure of CO2 becomes negligible.
D. Partial pressures of both CO2 and O2 remain normal.

Explanation: ***No exchange of O2 and CO2 occurs.*** - When the **Va/Q ratio approaches infinity**, it signifies a scenario of **ventilation without perfusion** (Q approaches zero). - This represents **alveolar dead space** - despite adequate ventilation, there is **no blood flow** to participate in gas exchange. - Therefore, **no O2 enters the blood** and **no CO2 leaves the blood**, making this the most accurate description of what happens to gas exchange. *Partial pressure of O2 becomes negligible.* - This statement is incorrect because with **no blood flow** (Q = 0), the alveolar air retains high O2 partial pressure. - O2 is being delivered via ventilation but not removed by blood, so **alveolar PO2** would approach that of **inspired air (~150 mmHg)**, not become negligible. *Partial pressure of CO2 becomes negligible.* - While this statement is technically true (alveolar PCO2 would approach zero/inspired air levels), it doesn't directly answer what happens to **gas exchange**. - With no blood flowing through the alveolus, no **CO2 from venous blood** can reach the alveolus to be excreted. - However, the question asks about **gas exchange** itself, not just partial pressures, making the first option more comprehensive. *Partial pressures of both CO2 and O2 remain normal.* - This statement is incorrect as the **Va/Q mismatch** significantly alters the partial pressures of both gases. - In infinite Va/Q scenario (dead space ventilation), **alveolar PO2 would be high** (approaching inspired air ~150 mmHg) and **alveolar PCO2 would be low** (approaching zero).

Question 2: Which condition is primarily responsible for the decrease in arterial PO2 in patients with chronic obstructive pulmonary disease (COPD)?

A. CO poisoning
B. Shock
C. Cyanide poisoning
D. Ventilation-perfusion mismatch (Correct Answer)

Explanation: ***Ventilation-perfusion mismatch*** - In **COPD**, structural changes in the lungs (emphysema, chronic bronchitis) lead to areas where **ventilation (V)** is poor but **perfusion (Q)** is still present, and vice versa. - This mismatch means that blood flowing through poorly ventilated areas does not pick up enough oxygen, leading to a decreased **arterial PO2**. *Cyanide poisoning* - **Cyanide** inhibits cytochrome c oxidase, blocking **cellular oxygen utilization**, but does not directly cause a decrease in arterial PO2. - Arterial PO2 levels in **cyanide poisoning** are often normal because oxygen is delivered to the tissues but cannot be used. *CO poisoning* - **Carbon monoxide (CO)** binds to **hemoglobin** with a much higher affinity than oxygen, forming **carboxyhemoglobin (COHb)** and reducing the oxygen-carrying capacity of the blood. - While it reduces the oxygen available to tissues, it generally does not significantly decrease the **arterial PO2** itself, as the amount of dissolved oxygen in plasma (which determines PO2) may remain relatively normal initially. *Shock* - **Shock** is a state of inadequate tissue perfusion, which can lead to **hypoxia** at the cellular level. - While systemic issues in shock can impact overall oxygen delivery and utilization, shock itself does not primarily cause a decrease in **arterial PO2** through a direct lung mechanism like ventilation-perfusion mismatch.

Question 3: Which of the following is not true about ventilation-perfusion ratio (V/Q)?

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

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

Question 4: Which of the following statements about high altitude pulmonary edema (HAPE) is true?

A. High altitude pulmonary edema is caused by a combination of factors.
B. High altitude pulmonary edema is associated with raised pulmonary capillary pressure. (Correct Answer)
C. High altitude pulmonary edema involves leakage of proteins and white blood cells into the alveoli.
D. High altitude pulmonary edema is associated with increased permeability of pulmonary capillaries.

Explanation: ***Correct: High altitude pulmonary edema is associated with raised pulmonary capillary pressure.*** **HAPE** is characterized by **exaggerated hypoxic pulmonary vasoconstriction**, leading to increased pressure in the **pulmonary arteries** and **capillaries**, which drives fluid into the alveoli. This elevated **hydrostatic pressure**, rather than inflammation or increased permeability, is the primary mechanism of **fluid transudation** in **HAPE**. The edema fluid is typically a low-protein transudate, reflecting the pressure-driven rather than permeability-driven nature of the condition. *Incorrect: High altitude pulmonary edema is caused by a combination of factors.* While multiple factors contribute to a person's susceptibility to HAPE (including genetic factors, rate of ascent, and individual variability in hypoxic pulmonary vasoconstriction), this statement is too vague and doesn't specify the key physiological mechanism. The *direct cause* of the edema itself is **pulmonary hypertension** due to **hypoxic pulmonary vasoconstriction**. *Incorrect: High altitude pulmonary edema involves leakage of proteins and white blood cells into the alveoli.* **HAPE** primarily involves **transudation** of low-protein fluid due to high **hydrostatic pressure**, not significant exudation of proteins and inflammatory cells. This type of protein-rich leakage with inflammatory cells is more characteristic of **acute respiratory distress syndrome (ARDS)**, which involves significant **capillary damage** and inflammation, not the pressure-driven mechanism of HAPE. *Incorrect: High altitude pulmonary edema is associated with increased permeability of pulmonary capillaries.* Although some minor changes in permeability can occur in HAPE, the dominant mechanism is **pressure-driven fluid transudation** due to **pulmonary hypertension** from hypoxic vasoconstriction, not a primary increase in capillary permeability. Conditions like **ARDS** are primarily characterized by increased **capillary permeability** leading to protein-rich exudative fluid leakage, which is fundamentally different from the hydrostatic mechanism in HAPE.

Question 5: During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by

A. Large amount of smooth muscle in pulmonary arterioles
B. Increase in the number of open capillaries (Correct Answer)
C. Sympathetically mediated greater distensibility of pulmonary vessels
D. Smaller surface area of pulmonary circulation

Explanation: ***Increase in the number of open capillaries*** - During heavy exercise, the significant increase in cardiac output is accommodated by the **recruitment of previously closed pulmonary capillaries**. - This recruitment, along with **distension of existing capillaries**, reduces overall pulmonary vascular resistance, allowing blood flow to increase without a substantial rise in pulmonary arterial pressure. *Large amount of smooth muscle in pulmonary arterioles* - While pulmonary arterioles do contain smooth muscle, their primary role is in **regulating regional blood flow** and response to hypoxia, not facilitating large increases in overall blood flow during exercise. - The pulmonary circulation is characterized by **low resistance** and high capacitance compared to the systemic circulation, meaning it has less smooth muscle tone at baseline. *Sympathetically mediated greater distensibility of pulmonary vessels* - The pulmonary vasculature has **limited sympathetic innervation** compared to systemic vessels, and sympathetic activity plays a minor role in its distensibility during exercise. - Changes in pulmonary vascular resistance during exercise are primarily due to **mechanical factors** (recruitment and distension) rather than neurogenic control. *Smaller surface area of pulmonary circulation* - The pulmonary circulation actually has a **vast capillary surface area** crucial for efficient gas exchange. - A smaller surface area would lead to **higher resistance** and a greater pressure increase for a given flow, which contradicts the observation during exercise.

Question 6: An infant presents with LVH and pulmonary complications. ECG shows left axis deviation. The most likely diagnosis is:

A. Tricuspid atresia (Correct Answer)
B. TAPVC
C. VSD
D. TGA

Explanation: ***Tricuspid atresia*** - **Left ventricular hypertrophy (LVH)** is common because the left ventricle must pump blood to both the systemic and pulmonary circulations through a **ventricular septal defect (VSD)** and/or **patent ductus arteriosus (PDA)**. - **Left axis deviation** on ECG is characteristic of tricuspid atresia due to the hypoplasia or absence of the right ventricle and the dominance of the left ventricle. *TAPVC* - **Total anomalous pulmonary venous connection (TAPVC)** typically presents with right ventricular hypertrophy and right axis deviation on ECG, as the right ventricle handles the entire systemic venous return. - Pulmonary complications are common, but the cardiac structural changes and ECG findings differentiate it from tricuspid atresia. *VSD* - A **ventricular septal defect (VSD)** alone would typically cause **right ventricular hypertrophy** or biventricular hypertrophy depending on the size and shunt direction. - While a large VSD can cause **pulmonary hypertension** and complications, it usually does not present with isolated LVH and left axis deviation without other associated anomalies. *TGA* - **Transposition of the great arteries (TGA)** typically presents with **right ventricular hypertrophy** and right axis deviation on ECG, as the RV functions as the systemic ventricle. - While cyanosis and pulmonary complications occur, the ECG pattern shows RVH, not LVH with left axis deviation.

Question 7: Most specific cardiac anomaly seen in baby born to Diabetic Mother

A. Ventricular septal defect
B. Heart blocks
C. Tetralogy of Fallot
D. Transposition of Great arteries (Correct Answer)

Explanation: ***Transposition of Great arteries*** - **Transposition of the great arteries (TGA)** is the most specific congenital heart defect associated with infants born to mothers with **pre-gestational diabetes**. - Poor glycemic control in the first trimester of pregnancy is a significant risk factor for the development of TGA. *Ventricular septal defect* - **Ventricular septal defect (VSD)** is the most common congenital heart defect overall, but it is not specific to diabetic mothers, as its occurrence is common in the general population. - While VSDs can occur in infants of diabetic mothers, they are less characteristic of this population compared to TGA. *Heart blocks* - **Congenital heart blocks** are most commonly associated with **maternal autoimmune diseases**, such as Systemic Lupus Erythematosus (SLE), due to the transplacental transfer of anti-Ro/SSA and anti-La/SSB antibodies. - They are not a specific cardiac anomaly linked to maternal diabetes. *Tetralogy of Fallot* - **Tetralogy of Fallot** is a complex congenital heart defect involving four anomalies, but it is not specifically or disproportionately linked to maternal diabetes compared to other congenital heart defects. - Its etiology is multifactorial, with genetic and environmental factors playing roles.

Question 8: Which heart chamber has the thickest wall?

A. Right atrium
B. Left atrium
C. Left ventricle (Correct Answer)
D. Right ventricle

Explanation: ***Left ventricle*** - The left ventricle is responsible for pumping **oxygenated blood** to the entire systemic circulation, requiring significant force. - Its muscular wall is the **thickest** to generate the high pressures needed to overcome systemic vascular resistance [1]. *Right atrium* - The right atrium receives deoxygenated blood from the body and pumps it to the right ventricle, which is a **low-pressure circuit** [2]. - Its walls are relatively thin compared to the ventricles, as it only needs to provide a small "kick" to fill the right ventricle. *Left atrium* - The left atrium receives oxygenated blood from the lungs and pumps it to the left ventricle, operating under **low pressure**. - Its walls are thin, similar to the right atrium, as it does not need to generate high pressures. *Right ventricle* - The right ventricle pumps deoxygenated blood to the **pulmonary circulation**, which is a **low-pressure system** [1]. - While thicker than the atria, its wall is thinner than the left ventricle because it faces less resistance and pumps against lower pressures to the lungs.

Question 9: Which of the following is true about polyaeritis nodosa?

A. It shows fibrinoid necrosis in large blood vessels
B. HBsAg is positive in 30% patients (Correct Answer)
C. It has ANCA positivity
D. Affected individuals have involvement of pulmonary circulation.

Explanation: ### HBsAg is positive in 30% patients - **Polyarteritis nodosa (PAN)** is strongly associated with **hepatitis B virus (HBV)** infection; about 30% of patients with PAN have evidence of current or past HBV infection, particularly **HBsAg positivity**. - This association suggests that HBV infection can trigger the immune complex vasculitis characteristic of PAN. ### It shows fibrinoid necrosis in large blood vessels - PAN primarily affects **medium-sized muscular arteries**, not typically large blood vessels [1]. - The inflammation causes **fibrinoid necrosis** and aneurysmal dilations in these medium-sized arteries [1]. ### It has ANCA positivity - **Polyarteritis nodosa (PAN)** is generally considered an **ANCA-negative vasculitis**. - **ANCA positivity** (especially c-ANCA/PR3-ANCA or p-ANCA/MPO-ANCA) is characteristic of other small-vessel vasculitides like **Granulomatosis with polyangiitis** or **Microscopic polyangiitis**. ### Affected individuals have involvement of pulmonary circulation. - A defining characteristic of **Polyarteritis nodosa (PAN)** is that it generally **spares the pulmonary circulation** [1]. - Pulmonary involvement is more commonly seen in other vasculitides, such as **Granulomatosis with polyangiitis (Wegener's)** or **Eosinophilic granulomatosis with polyangiitis (Churg-Strauss)**.

Question 10: 5 year old child going for sitting craniotomy, while positioning in O.T. developed end tidal CO2- Zero mmHg, PO2- 80 mm Hg implies that :

A. Left lung collapse
B. Endotracheal tube blocked with secretions
C. Endotracheal tube in Oesophagus
D. Venous air embolism (Correct Answer)

Explanation: ***Venous air embolism*** - A sudden drop in **end-tidal CO2 (ETCO2)** to zero, coupled with a normal or mildly decreased PO2, in a patient undergoing a sitting craniotomy strongly suggests a venous air embolism. The sitting position and open venous sinuses in neurosurgery create a pressure gradient favoring air entrainment. - While pulmonary issues can cause some ETCO2 changes, a **sudden complete loss** (zero mmHg) is characteristic of a circulatory collapse or severe obstruction to pulmonary blood flow, as seen with significant air emboli. *Left lung collapse* - A left lung collapse (atelectasis) would primarily cause a **decrease in PO2** due to ventilation-perfusion mismatch, and a subsequent **increase in ETCO2** due to increased dead space and shunting, rather than a sudden drop to zero. - While lung collapse can lead to some ETCO2 variations, it would not typically result in a **complete absence** of exhaled CO2. *Endotracheal tube blocked with secretions* - An endotracheal tube blocked with secretions would lead to a significant **increase in peak inspiratory pressures**, **decreased tidal volumes**, and a gradual rather than sudden drop in ETCO2, often to a measurable but low level, not zero. - The patient would also likely exhibit signs of **respiratory distress** and difficulty ventilating, which is not directly indicated by the isolated ETCO2 reading. *Endotracheal tube in Oesophagus* - If the endotracheal tube were in the esophagus, there would be no CO2 entering the breathing circuit from the lungs, leading to an **ETCO2 of zero**. - However, this would also be immediately accompanied by **very low or unmeasurable PO2** because no oxygen would be entering the lungs. The given PO2 of 80 mmHg makes this less likely.

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