Mechanics of Breathing — MCQs

10 questions

A newborn presented with chest retractions, dyspnea, and lethargy. The pediatrician diagnosed the baby with respiratory distress syndrome. This occurs due to the deficiency of:

Which of the following statements about lung compliance is false?

Which of the following is most associated with respiratory alkalosis?

Which of the following statements about breathing is incorrect?

Damage to pneumotaxic center along with vagus nerve causes which type of respiration?

Which of the following has prolonged inspiratory spasms that resemble breath holding?

Hysteresis is observed between the deflation and inflation curves in an isolated lung compliance diagram. What is the best description for the same?

Hyaline membrane disease of the lungs is characterized by –

Which of the following areas of nitrogen washout test indicate closing volume?

Q10

Calculate the FEV1/FVC ratio from the spirometry reading shown below.

Mechanics of Breathing Indian Medical PG Practice Questions and MCQs

Question 1: A newborn presented with chest retractions, dyspnea, and lethargy. The pediatrician diagnosed the baby with respiratory distress syndrome. This occurs due to the deficiency of:

A. Dipalmitoyl inositol
B. Dipalmitoylphosphatidylethanolamine
C. Lecithin (Correct Answer)
D. Sphingomyelin

Explanation: ***Lecithin*** - **Respiratory distress syndrome (RDS)** in newborns is primarily caused by a deficiency of pulmonary **surfactant**. - **Lecithin (phosphatidylcholine)**, specifically in its dipalmitoyl form (**dipalmitoylphosphatidylcholine or DPPC**), is the main active component of surfactant, constituting ~40-50% of surfactant lipids. - DPPC is crucial for reducing surface tension in the alveoli and preventing their collapse during expiration. - This is the **primary biochemical deficiency** in neonatal RDS. *Dipalmitoyl inositol* - **Inositol** is a sugar alcohol involved in various cellular processes and is present in surfactant as phosphatidylinositol, but it is not a primary functional component. - Deficiency of this compound does not directly lead to **respiratory distress syndrome**. *Dipalmitoylphosphatidylethanolamine* - **Phosphatidylethanolamine (PE)** is a phospholipid found in cell membranes but is not the primary phospholipid responsible for surfactant function. - Note: This is PE, not PC (phosphatidylcholine). While PE is present in surfactant, its deficiency does not specifically cause **neonatal RDS**. *Sphingomyelin* - **Sphingomyelin** is a sphingolipid found in cell membranes and myelin sheaths, but it is not the critical component of pulmonary surfactant. - The **lecithin-to-sphingomyelin (L/S) ratio** is used to assess fetal lung maturity; an L/S ratio >2 indicates mature lungs capable of producing adequate surfactant.

Question 2: Which of the following statements about lung compliance is false?

A. Decreased in emphysema (Correct Answer)
B. Total compliance is 0.2 L/cm H2O
C. A measure of lung distensibility
D. Change in volume per unit change in pressure

Explanation: ***Decreased in emphysema*** - This statement is **false** because **emphysema** is characterized by the destruction of elastic fibers in the lung parenchyma, which paradoxically leads to an **increase** in lung compliance. - The loss of elastic recoil makes the lungs more distensible and easier to inflate, but also impairs their ability to passively exhale. *Total compliance is 0.2 L/cm H2O* - This value represents the **normal total lung compliance** in a healthy adult (0.17 to 0.25 L/cm H2O), including both lung and chest wall compliance. - Lung compliance alone is typically around 0.2 L/cm H2O for healthy lungs. *A measure of lung distensibility* - **Compliance** is intrinsically defined as a measure of how easily the lungs or chest wall can be stretched or distended. - High compliance means the lungs are easy to inflate, while low compliance means they are stiff and difficult to inflate. *Change in volume per unit change in pressure* - This is the explicit **formula and definition of compliance** (C = ΔV/ΔP). - It quantifies the change in lung volume in response to a given change in transpulmonary pressure.

Question 3: Which of the following is most associated with respiratory alkalosis?

A. SIMV
B. Non invasive ventilation
C. Pressure controlled
D. Assisted control mode ventilation (Correct Answer)

Explanation: ***Assisted control mode ventilation*** - In **assisted control mode**, every patient effort above a set sensitivity triggers a fully supported breath at the set tidal volume or pressure, leading to the potential for **excessive ventilation** and respiratory alkalosis if the patient's respiratory drive is high. - This mode ensures a **minimum number of breaths** per minute, but also delivers full mechanical breaths for any additional patient-initiated breaths, which can result in **hyperventilation**. *SIMV* - **Synchronized intermittent mandatory ventilation (SIMV)** delivers a set number of mandatory breaths, but patient-initiated breaths between these mandatory breaths are either unsupported or supported at a lower level, making it less prone to causing excessive ventilation and alkalosis compared to AC. - SIMV allows for more patient participation in breathing and is often used to **wean patients off ventilation**, whereas AC prioritizes full ventilatory support. *Non invasive ventilation* - While **non-invasive ventilation (NIV)** can cause respiratory alkalosis if settings are too aggressive, it is generally used to avoid intubation and often allows for more patient control over their breathing pattern than AC, especially in modes like BiPAP where inspiratory and expiratory pressures are set. - The goal of NIV is to provide ventilatory support without an artificial airway, and it can be titrated to prevent both hypoventilation and hyperventilation more easily than the full support of AC. *Pressure controlled* - **Pressure-controlled ventilation** delivers breaths until a set inspiratory pressure is reached, with tidal volume varying based on lung compliance and resistance. While it can cause respiratory alkalosis if the set pressure or respiratory rate is too high, it is a *mode* of ventilation rather than a specific *type* of ventilatory support that inherently overventilates. - It focuses on limiting peak inspiratory pressures to protect the lungs, and can be used in either AC or SIMV modes, making its association with alkalosis dependent on specific settings and patient interaction.

Question 4: Which of the following statements about breathing is incorrect?

A. Inspiration is an active process
B. Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O (Correct Answer)
C. Expiration during quiet breathing is passive
D. Compliance is influenced by multiple factors including surfactant.

Explanation: ***Normal breathing occurs when transpulmonary pressure is 5-8 cm H2O*** - This statement is **incorrect** because it misrepresents transpulmonary pressure during normal breathing. - Normal **transpulmonary pressure** during quiet breathing typically ranges from approximately **3-6 cm H2O** during inspiration, with an average of about **5 cm H2O** at functional residual capacity. - The range "5-8 cm H2O" is too high for normal quiet breathing. While transpulmonary pressure can reach 8 cm H2O during deeper inspiration, stating this as the range for "normal breathing" is inaccurate. - Transpulmonary pressure is the difference between alveolar pressure and pleural pressure (P_L = P_alv - P_pl), which drives lung inflation. *Expiration during quiet breathing is passive* - During quiet breathing, **expiration is a passive process** driven by the **elastic recoil of the lungs** and chest wall. - No active muscular contraction is required for air to leave the lungs during unforced expiration. *Inspiration is an active process* - **Inspiration is an active process** requiring muscular contraction, primarily of the **diaphragm and external intercostal muscles**. - These muscles contract to increase the thoracic volume, which decreases intrapleural and alveolar pressures, drawing air into the lungs. *Compliance is influenced by multiple factors including surfactant* - **Lung compliance**, a measure of the lung's distensibility, is significantly influenced by **surfactant**. - Surfactant reduces **surface tension** in the alveoli, preventing their collapse and increasing compliance.

Question 5: Damage to pneumotaxic center along with vagus nerve causes which type of respiration?

A. Cheyne-Stokes breathing
B. Deep and slow breathing
C. Shallow and rapid breathing
D. Apneustic breathing (Correct Answer)

Explanation: ***Apneustic breathing*** - Damage to the **pneumotaxic center** prevents the normal inhibition of inspiration, leading to **prolonged inspiratory gasps**. - **Vagal nerve damage** further removes the inhibitory feedback from the lungs, exacerbating the inspiratory "holds" characteristic of apneustic breathing. *Cheyne-Stokes breathing* - This pattern is characterized by a **crescendo-decrescendo pattern** of breathing, interspersed with periods of **apnea**. - It is often associated with conditions like **heart failure**, stroke, or severe neurological damage, not specifically the pneumotaxic center and vagus nerve. *Deep and slow breathing* - This pattern can be seen in conditions like **Kussmaul breathing** (due to metabolic acidosis) or as a compensatory mechanism. - It does not directly result from the combined damage of the **pneumotaxic center** and the **vagus nerve**. *Shallow and rapid breathing* - This pattern is commonly seen in restrictive lung diseases, anxiety, or pain, where tidal volume is decreased and respiratory rate increased. - It does not reflect the **prolonged inspiration** that would result from a compromised pneumotaxic center and vagal input.

Question 6: Which of the following has prolonged inspiratory spasms that resemble breath holding?

A. Kussmaul breathing
B. Biot breathing
C. Apneustic breathing (Correct Answer)
D. Cheyne-Stokes breathing

Explanation: ***Apneustic breathing*** - This pattern is characterized by **prolonged inspiratory pauses**, resembling breath-holding, followed by a short, insufficient expiratory phase. - It is caused by damage to the **pons** in the brainstem, often due to stroke or trauma, which disrupts the normal rhythm of breathing. *Kussmaul breathing* - Characterized by **deep**, **rapid**, and labored breathing, typically seen in metabolic acidosis like **diabetic ketoacidosis**. - It is a compensatory mechanism to increase CO2 elimination and raise blood pH. *Biot's breathing* - Involves irregular breathing with **periods of apnea** interspersed with shallow breaths. - This pattern is associated with damage to the **medulla oblongata** or severe intracranial pressure. *Cheyne-Stokes breathing* - Characterized by a **crescendo-decrescendo pattern** of respiration, where breathing gradually increases in depth and rate, then decreases, followed by a period of **apnea**. - It is often observed in **heart failure**, stroke, or severe neurological conditions, indicating brainstem or cerebral dysfunction.

Question 7: Hysteresis is observed between the deflation and inflation curves in an isolated lung compliance diagram. What is the best description for the same?

A. Stretching of elastic elements of lung parenchyma
B. Decrease in surface tension in air-water interface at higher lung volumes
C. Variation in surface tension forces at air- liquid interface (Correct Answer)
D. Hering Breuer reflex is operational at higher lung volumes

Explanation: ***Variation in surface tension forces at air-liquid interface*** - The phenomenon of **hysteresis** in lung compliance, particularly the larger loop seen with air-filled lungs compared to saline-filled lungs, is primarily attributable to the **dynamic changes in surface tension** at the air-liquid interface within the alveoli. - During inflation, more energy is required to overcome the opening forces of collapsed alveoli and recruit new ones, leading to a lower volume for a given pressure, while during deflation, previously opened alveoli remain open or close at lower pressures, contributing to the observed difference. *Stretching of elastic elements of lung parenchyma* - While the **elastic elements** of the lung parenchyma contribute to lung compliance, their contribution to hysteresis is relatively minor and would be observed even in saline-filled lungs to a lesser extent. - The difference in hysteresis between air-filled and saline-filled lungs strongly suggests that factors beyond the tissue elasticity are predominantly responsible for the larger hysteresis with air. *Decrease in surface tension in air-water interface at higher lung volumes* - This statement is partially correct regarding surfactant's action. **Surfactant** does reduce surface tension, especially at lower lung volumes, and prevents alveolar collapse. - However, the overall *variation* in surface tension forces throughout the breathing cycle, not just a decrease at higher volumes, is what creates the inspiratory and expiratory limbs of the pressure-volume curve. *Hering Breuer reflex is operational at higher lung volumes* - The **Hering-Breuer reflex** is a protective neurological reflex that terminates inspiration and initiates expiration when the lungs are overinflated. - This reflex is a **neurophysiological control mechanism** for breathing and does not directly explain the physical properties of the lung that contribute to the pressure-volume hysteresis loop.

Question 8: Hyaline membrane disease of the lungs is characterized by –

A. FRC is reduced compared to closing volume (Correct Answer)
B. FRC is increased compared to closing volume
C. FRC is equal to closing volume
D. FRC is not related to closing volume

Explanation: ***FRC is reduced compared to closing volume*** - In **Hyaline Membrane Disease (HMD)**, severe **surfactant deficiency** leads to widespread **atelectasis** and a significant reduction in **functional residual capacity (FRC)**. - Due to the collapse of alveoli and small airways, the **closing volume (CV)**, which is the lung volume at which small airways begin to close, becomes relatively larger than the already reduced FRC. *FRC is increased compared to closing volume* - This statement is incorrect because HMD is characterized by diffuse **atelectasis**, which drastically reduces **FRC**. - An increased FRC relative to closing volume would imply better lung compliance and less small airway closure, contrary to the pathology of HMD. *FRC is equal to closing volume* - This scenario would represent a critical point where extensive airway closure occurs, but in HMD, the **FRC is significantly lower** than the critical closing volume due to severe **surfactant deficiency** and widespread collapse. - While there is considerable airway closure, the FRC is typically *below* the closing volume, leading to shunt and severe hypoxemia. *FRC is not related to closing volume* - This is incorrect because FRC and closing volume are intimately related in lung mechanics, especially in conditions like HMD. - **Closing volume** reflects the point at which airways begin to collapse, and in disease states like HMD, the interplay between a reduced FRC and an elevated closing volume explains the severe gas exchange abnormalities.

Question 9: Which of the following areas of nitrogen washout test indicate closing volume?

A. 1
B. 2
C. 3
D. 4 (Correct Answer)

Explanation: ***4*** - Area 4 represents the **closing volume**. This is the point where the **small airways in the dependent parts of the lungs close**, leading to a sharp increase in the nitrogen concentration in the exhaled gas. - This sharp increase occurs because air from the upper regions of the lungs, which are better ventilated, continues to empty, and this air has a higher concentration of nitrogen as the patient was initially breathing 100% oxygen. *1* - Area 1 is known as the **anatomical dead space**, representing the initial part of exhalation consisting entirely of gas from the conducting airways (which has a near-zero nitrogen concentration). - This phase reflects the emptying of gas that did not participate in gas exchange, thus showing very low nitrogen levels as it's primarily the 100% oxygen inhaled for the test. *2* - Area 2, or the **alveolar washout phase**, shows a rapid increase in nitrogen concentration as exhaled gas now includes a mixture of dead space air and alveolar air. - This phase reflects the emptying of alveoli from different regions of the lung, but not yet the effect of airway closure. *3* - Area 3 is the **alveolar plateau**, where the nitrogen concentration remains relatively stable, indicating uniform emptying from the remaining open alveoli. - This plateau phase shows the mixing of gases from various lung units, with a steady increase in nitrogen as the oxygen washes out.

Question 10: Calculate the FEV1/FVC ratio from the spirometry reading shown below.

A. 60-69 %
B. 70-79 %
C. 80-89 % (Correct Answer)
D. 90-99 %

Explanation: ***80-89 %*** - **Normal FEV1/FVC ratio is >70% in adults, with healthy individuals typically showing 80-90%.** - From the spirometry graph, the total vital capacity (FVC) after full exhalation is approximately **4500 mL**. The volume exhaled in the first second (FEV1) is approximately **4000 mL**. - Therefore, FEV1/FVC = (4000 mL / 4500 mL) × 100% = **88.8%**. This falls within the 80-89% range, indicating **normal lung function**. *60-69 %* - This percentage indicates **severe airflow obstruction**, where the FEV1 is significantly reduced relative to the FVC, which is not supported by the graph's values of **FEV1 ~4000 mL** and **FVC ~4500 mL**. - A ratio of 60-69% is seen in **moderate to severe obstructive lung disease** (COPD, severe asthma). *70-79 %* - This range suggests **mild airflow obstruction**, corresponding to an **FEV1/FVC ratio** that is borderline or slightly reduced (below the normal 80% threshold but above the diagnostic cutoff for obstruction at 70%). - While less severe than 60-69%, it still implies some degree of airway limitation, which is not the case with the calculated ratio of 88.8%. *90-99 %* - This percentage implies an **FEV1/FVC ratio** of 0.9 or higher, meaning that nearly all of the vital capacity is exhaled in the first second. While **88.8%** is close to this range, it does not fall within it. - A ratio this high might be seen in individuals with **excellent lung function** or paradoxically in some cases of **restrictive lung disease** where both FEV1 and FVC are proportionally reduced, but the exact calculated value from the graph is 88.8%, which falls just below 90%.

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