Respiratory System — MCQs
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What does a pulse oximeter primarily measure?
All are accessory muscles of inspiration except
Which condition is primarily responsible for the decrease in arterial PO2 in patients with chronic obstructive pulmonary disease (COPD)?
What is the definition of Muller's maneuver?
Surfactant acts to maintain lung compliance by decreasing which factor?
The effort during normal respiration is primarily due to?
Which of the following is measured by the Bellows spirometer?
Hyaline membrane disease of the lungs is characterized by –
Which lung volume cannot be measured by spirometry?
What physiological mechanism is primarily responsible for apnea in patients undergoing mechanical ventilation?
Respiratory System Indian Medical PG Practice Questions and MCQs
Question 881: What does a pulse oximeter primarily measure?
- A. Oxygen content of blood
- B. Oxygen saturation (Correct Answer)
- C. Partial pressure of oxygen
- D. Carbon dioxide levels
Explanation: ***Oxygen saturation*** - A pulse oximeter primarily measures the **percentage of hemoglobin** in arterial blood that is saturated with oxygen. - This is often reported as **SpO2** (peripheral oxygen saturation), an estimate of SaO2 (arterial oxygen saturation). *Oxygen content of blood* - The **total amount of oxygen** in the blood includes dissolved oxygen and oxygen bound to hemoglobin. - Pulse oximeters only measure the proportion of hemoglobin bound to oxygen, not the absolute amount of oxygen. *Partial pressure of oxygen* - This refers to the **amount of oxygen dissolved in the plasma** and is denoted as PaO2. - Measurement of PaO2 requires an **arterial blood gas (ABG)** analysis, which is an invasive procedure. *Carbon dioxide levels* - Pulse oximeters do **not measure CO2**; they use light absorption at specific wavelengths to differentiate oxyhemoglobin from deoxyhemoglobin. - Measurement of carbon dioxide requires **capnography** or arterial blood gas analysis.
Question 882: All are accessory muscles of inspiration except
- A. Serratus anterior
- B. Serratus posterior superior
- C. Scaleni
- D. Latissimus dorsi (Correct Answer)
Explanation: ***Latissimus dorsi*** - The **latissimus dorsi** is primarily an **accessory muscle of forced EXPIRATION**, not inspiration. - When it contracts with the arms fixed, it **depresses the lower ribs** and **compresses the thorax**, aiding in forceful exhalation. - Its primary actions involve **extension, adduction, and internal rotation of the humerus**, and it has **no role in elevating ribs or expanding the thoracic cavity** during inspiration. *Serratus anterior* - The **serratus anterior** muscle helps in **protracting the scapula** and also plays a role in respiration by **elevating the ribs** when the shoulder girdle is fixed, thus aiding inspiration. - Its action helps to increase the **anteroposterior and transverse diameters of the thoracic cavity**. *Serratus posterior superior* - The **serratus posterior superior** muscles are directly attached to the ribs (2-5) and are considered **accessory muscles of inspiration**. - They **elevate the upper ribs**, thereby increasing the volume of the thoracic cavity during inhalation. *Scaleni* - The **scalene muscles** (anterior, middle, and posterior) elevate the first two ribs, significantly contributing to the **expansion of the thoracic cage** during inspiration. - They are considered important **accessory muscles of inspiration**, especially during forceful breathing.
Question 883: 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 884: What is the definition of Muller's maneuver?
- A. Forceful expiration against closed glottis
- B. Forceful expiration against open glottis
- C. Normal inspiration against closed glottis
- D. Forceful inspiration against closed glottis (Correct Answer)
Explanation: ***Forceful inspiration against closed glottis*** - **Muller's maneuver** involves attempting to inhale deeply while keeping the mouth and nose closed, creating a significant **negative intrathoracic pressure**. - This maneuver is used to assess conditions like **tracheomalacia** or **obstructive sleep apnea**, where the decreased pressure can cause airway collapse. *Forceful expiration against closed glottis* - This describes the **Valsalva maneuver**, which increases **intrathoracic pressure**, often used to test autonomic function. - Unlike **Muller's maneuver**, it involves pushing air out rather than drawing it in. *Forceful expiration against open glottis* - This action is a normal **forced exhalation**, often used in spirometry measurements to assess lung function. - It does not involve a closed glottis and therefore does not create the same pressure changes as the Muller or Valsalva maneuvers. *Normal inspiration against closed glottis* - While it involves inspiration against a closed glottis, the key distinction is "normal" inspiration, which would not generate the significant negative intrathoracic pressures characteristic of a **Muller's maneuver**. - **Muller's maneuver** specifically implies a **forceful** attempt to inhale.
Question 885: Surfactant acts to maintain lung compliance by decreasing which factor?
- A. Surface tension (Correct Answer)
- B. Pleural fluid secretion
- C. Intrathoracic pressure
- D. Pleural pressure
Explanation: ***Surface tension*** - **Surfactant** directly reduces the **surface tension** at the air-liquid interface within the alveoli. - By lowering surface tension, surfactant prevents alveolar collapse, particularly at low lung volumes, and increases **lung compliance**. *Intrathoracic pressure* - **Intrathoracic pressure** (also known as pleural pressure) is the pressure within the chest cavity, which fluctuates with breathing. - While surfactant affects lung mechanics, it doesn't directly influence the overall intrathoracic pressure. *Pleural fluid secretion* - **Pleural fluid** lubricates the pleural surfaces and is secreted by the pleural membranes. - Surfactant's primary role is in the alveoli to reduce surface tension, not to regulate **pleural fluid secretion**. *Pleural pressure* - **Pleural pressure** is the pressure in the space between the parietal and visceral pleura. - Surfactant improves lung compliance, which indirectly affects how pressure changes during breathing, but it doesn't directly control the **pleural pressure** itself.
Question 886: The effort during normal respiration is primarily due to?
- A. Respiratory air passages
- B. Alveolar air spaces
- C. Lung elasticity
- D. Creating negative pleural pressure (Correct Answer)
Explanation: ***Creating negative pleural pressure*** - **Inspiration** (the active phase of normal respiration) occurs when the diaphragm contracts and the external intercostal muscles lift the rib cage, increasing the volume of the thoracic cavity. - This increase in volume creates a **negative pressure in the pleural space**, which pulls the lungs outward and causes air to rush in. *Lung elasticity* - Lung elasticity is crucial for **expiration**, as the elastic recoil of the lungs helps to push air out passively. - It does not actively contribute to the effort of **inspiration**; rather, it creates the opposing force that needs to be overcome. *Respiratory air passages* - The respiratory air passages (trachea, bronchi, bronchioles) serve as conduits for air flow but do not directly create the "effort" of breathing. - Their primary role is to **conduct, warm, humidify, and filter** the air. *Alveolar air spaces* - Alveolar air spaces are where **gas exchange** takes place between the air and the blood. - They are the destination of the inspired air and do not generate the mechanical effort required for breathing.
Question 887: Which of the following is measured by the Bellows spirometer?
- A. TLC
- B. RV
- C. Closing volume
- D. ERV (Correct Answer)
Explanation: ***ERV*** - The **Bellow's spirometer**, like other spirometers, measures **expiratory reserve volume (ERV)** directly. - Spirometry measures volumes that can be exhaled or inhaled, but not those that remain in the lungs after complete exhalation. *TLC* - **Total lung capacity (TLC)** cannot be measured directly by a spirometer because it includes the **residual volume (RV)**. - TLC is typically calculated using techniques like **helium dilution** or **body plethysmography**. *RV* - **Residual volume (RV)** is the volume of air remaining in the lungs after a maximal exhalation and cannot be expelled. - Since RV cannot be exhaled, it cannot be measured directly by a spirometer; it requires indirect methods. *Closing volume* - **Closing volume** is the lung volume at which small airways begin to close during exhalation. - It is measured using **specialized techniques** involving tracer gases, not standard spirometry.
Question 888: 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 889: Which lung volume cannot be measured by spirometry?
- A. RV (Correct Answer)
- B. TV
- C. IRV
- D. ERV
Explanation: ***RV (Residual Volume)*** - **Residual volume (RV)** is the volume of air remaining in the lungs after a maximal exhalation and cannot be expelled. - Since it cannot be exhaled, it cannot be directly measured by a spirometer, which relies on the movement of air in and out of the lungs. *TV (Tidal Volume)* - **Tidal volume (TV)** is the volume of air inspired or expired with a normal breath. - It is easily measured by a spirometer during normal breathing. *IRV (Inspiratory Reserve Volume)* - **Inspiratory reserve volume (IRV)** is the additional volume of air that can be forcibly inhaled after a normal inspiration. - This volume can be measured by spirometry as it represents a change in lung air volume achievable by the patient. *ERV (Expiratory Reserve Volume)* - **Expiratory reserve volume (ERV)** is the additional volume of air that can be forcibly exhaled after a normal expiration. - This volume can be directly measured by a spirometer during a forced exhalation.
Question 890: What physiological mechanism is primarily responsible for apnea in patients undergoing mechanical ventilation?
- A. Increased sensitivity of central chemoreceptors to elevated PCO2 causes hyperventilation.
- B. Elevated arterial oxygen tension (PaO2) reduces central chemoreceptor activity.
- C. Decreased carbon dioxide tension (PCO2) leads to reduced respiratory drive. (Correct Answer)
- D. Elevated arterial oxygen tension (PaO2) reduces peripheral chemoreceptor activity.
Explanation: ***Decreased carbon dioxide tension (PCO2) leads to reduced respiratory drive.*** - **Mechanical ventilation** often 'blows off' CO2, leading to a decrease in **arterial PCO2** below the patient's apneic threshold. - This reduction in **PCO2** directly diminishes the stimulation of **central chemoreceptors**, which are the primary drivers of ventilation, thus causing **apnea**. - When PCO2 falls below the apneic threshold (typically around 30-35 mmHg), the respiratory drive ceases entirely. *Increased sensitivity of central chemoreceptors to elevated PCO2 causes hyperventilation.* - This describes the opposite scenario - **hyperventilation** occurs when chemoreceptors are overly stimulated by high CO2, not apnea. - In mechanically ventilated patients, the problem is **low CO2**, not high CO2 or increased sensitivity. *Elevated arterial oxygen tension (PaO2) reduces central chemoreceptor activity.* - **Central chemoreceptors** are primarily sensitive to changes in **PCO2** and pH in the cerebrospinal fluid, not directly to changes in **PaO2**. - While high **PaO2** can slightly suppress ventilation by reducing peripheral chemoreceptor activity, it does not directly affect central chemoreceptors. *Elevated arterial oxygen tension (PaO2) reduces peripheral chemoreceptor activity.* - **Peripheral chemoreceptors** (in the carotid and aortic bodies) are mainly stimulated by **hypoxemia** (low **PaO2**). - While elevated **PaO2** does reduce their activity, these receptors play a secondary role in regulating normal breathing compared to the central chemoreceptors' response to **CO2**. - Their suppression alone is usually insufficient to cause apnea in mechanically ventilated patients.
Practice by Chapter
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Control of Breathing
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