Respiratory System Practice Questions

Q: What is the estimated PaO2 after giving FiO2 at 0.5 in a normal person?

> 200 mmHg. ***> 200 mmHg*** - In a **normal healthy person** breathing FiO2 of 0.5 (50% oxygen), the expected **PaO2** is typically **250-300 mmHg**. - Using the **alveolar gas equation**: PAO2 = FiO2(PB - PH2O) - PaCO2/RQ = 0.5(760 - 47) - 40/0.8 ≈ **306 mmHg** - The normal **A-a gradient** is 5-15 mmHg, so PaO2 = 306 - 10 ≈ **296 mmHg** - **Clinical rule of thumb**: PaO2 ≈ 5 × FiO2% = 5 × 50 = **250 mmHg** (approximation accounting for physiological shunt) - Therefore, the expected range is clearly **> 200 mmHg** in a normal individual *150–200 mmHg* - This range would indicate **mild oxygenation impairment** or increased shunt fraction - While adequate for tissue oxygenation, this is **lower than expected** for a normal person on 50% oxygen - May suggest underlying **mild V/Q mismatch** or early pulmonary dysfunction *100–150 mmHg* - This represents **moderate impairment** in oxygen transfer - Indicates significant **pulmonary pathology** such as pneumonia, ARDS, or substantial shunt - Not consistent with normal lung function on FiO2 0.5 *< 100 mmHg* - This represents **severe hypoxemia** despite supplemental oxygen - Indicates **critical pulmonary dysfunction** with large shunt or severe V/Q mismatch - Requires immediate intervention and is never expected in a healthy individual on 50% oxygen

Q: The chloride shift occurs rapidly and is essentially complete within:

1 second. ***1 second*** - The **chloride shift**, an exchange of bicarbonate and chloride ions across the red blood cell membrane, is a very rapid process. - This rapid kinetics ensures efficient **CO2 transport** from tissues to the lungs. *2 seconds* - While seemingly a short duration, **2 seconds** is generally considered longer than the actual time frame for the completion of the chloride shift. - The physiological need for immediate CO2 buffering necessitates a faster mechanism. *5 seconds* - A duration of **5 seconds** would imply a slower rate of gas exchange than physiologically required for efficient CO2 transport. - Such a delay could lead to transient but significant alterations in **blood pH**. *60 seconds* - **60 seconds** (1 minute) is far too long for a process critical to immediate blood gas regulation. - If the chloride shift took this long, it would severely impair the body's ability to excrete **CO2** and maintain acid-base balance.

Q: What is the physiological effect of very high positive end-expiratory pressure in a patient with respiratory distress?

Decreased blood pressure. ***Decreased blood pressure*** - Very high **positive end-expiratory pressure (PEEP)** increases intrathoracic pressure, which in turn reduces **venous return** to the heart. - This decreased preload leads to a **reduction in cardiac output**, ultimately causing **hypotension** (decreased blood pressure). - This is a well-recognized hemodynamic complication of excessive PEEP in mechanical ventilation. *Increased blood pressure* - High PEEP typically lowers, rather than increases, blood pressure due to its effects on **venous return** and **cardiac output**. - The elevated intrathoracic pressure acts as a barrier to venous return, reducing preload and thus blood pressure. *Decreased body temperature* - **PEEP** primarily affects cardiovascular and respiratory physiology, not **thermoregulation**. - Body temperature changes are usually related to systemic inflammation, infection, or environmental factors, not directly to PEEP settings. *Increased body temperature* - Similar to decreased body temperature, **PEEP** does not directly regulate body temperature. - An elevated body temperature (fever) would suggest an underlying **infection** or **inflammatory process**, which might be present in a patient with respiratory distress but is not a direct physiological effect of high PEEP.

Q: All of the following factors influence the hemoglobin dissociation curve, except.

Plasma sodium concentration. ***Plasma sodium concentration*** - While essential for **osmolality** and **electrolyte balance**, plasma sodium concentration does not directly influence the binding affinity of hemoglobin for oxygen. - Changes in sodium concentration primarily affect fluid shifts and neural function, not the **hemoglobin dissociation curve**. *CO2 tension* - An increase in **PCO2** (hypercapnia) leads to a **rightward shift** of the hemoglobin dissociation curve, indicating decreased oxygen affinity. - This effect, known as the **Bohr effect**, facilitates oxygen release in tissues with high metabolic activity. *Temperature* - An increase in **body temperature** causes a **rightward shift** in the hemoglobin dissociation curve, leading to reduced oxygen affinity. - This is beneficial during exercise or fever, as it promotes oxygen unloading to active tissues. *2–3 DPG levels* - **2,3-bisphosphoglycerate (2,3-BPG)** binds to deoxygenated hemoglobin, stabilizing its T-state and reducing its affinity for oxygen, thus shifting the curve to the **right**. - During chronic hypoxia or anemia, 2,3-BPG levels increase to enhance oxygen delivery to tissues.

Q: Which test is primarily used to assess gas diffusion capacity in the lungs?

DLCO. ***DLCO*** - **DLCO (Diffusing Capacity of the Lungs for Carbon Monoxide)** specifically measures the transfer of gas from the alveoli to the red blood cells, directly assessing the **gas diffusion capacity** of the lungs. - It is crucial for identifying interstitial lung diseases, emphysema, or other conditions affecting the **alveolar-capillary membrane**. *Spirometry* - **Spirometry** primarily assesses **lung volumes and airflow rates**, such as FEV1 and FVC, to diagnose obstructive or restrictive ventilatory defects. - It does not directly measure the efficiency of **gas exchange** across the alveolar-capillary membrane. *Both DLCO and Spirometry* - While both are important in pulmonary function testing, they measure different aspects of **lung function**. DLCO specifically measures **diffusion capacity**, while spirometry measures **airflow and lung volumes**. - Therefore, they are not primarily used for the *same* assessment. *None of the options* - DLCO is indeed the primary test for assessing **gas diffusion capacity** in the lungs. - This option is incorrect because a correct answer is provided.

Q: The factor responsible for the left shift of the HbO2 dissociation curve is

Fall in temperature. ***Fall in temperature*** - A **decrease in temperature** causes the hemoglobin to bind more tightly to oxygen, reducing its release into the tissues. - This increased affinity of hemoglobin for oxygen shifts the **HbO2 dissociation curve to the left**. *Increase in 2,3 DPG in RBC* - An **increase in 2,3-Bisphosphoglycerate (2,3-DPG)** in red blood cells decreases hemoglobin's affinity for oxygen. - This effect promotes oxygen release to tissues and causes a **right shift** of the HbO2 dissociation curve. *Fall in pH* - A **decrease in pH** (increased acidity), known as the **Bohr effect**, reduces hemoglobin's affinity for oxygen. - This facilitates oxygen unloading in tissues and results in a **right shift** of the HbO2 dissociation curve. *Increase level of CO2 in blood* - An **increase in CO2 levels** in the blood also contributes to the Bohr effect by lowering pH and forming carbaminohemoglobin. - This reduces hemoglobin's affinity for oxygen, promoting oxygen release and causing a **right shift** of the HbO2 dissociation curve.

Q: Which of the following is not a normal stimulus for resting ventilation?

J receptors. ***J receptors*** - **J receptors** (juxtacapillary receptors) are located in the alveolar walls and are primarily stimulated by **pulmonary edema**, inflammation, or vascular congestion. - Their stimulation typically causes rapid, shallow breathing, but they are **completely inactive during normal, resting ventilation**. - These receptors only become active under pathological conditions, making them **not a normal stimulus for resting breathing**. *Stretch receptors* - **Pulmonary stretch receptors** in the airways respond to lung distension, mediating the **Hering-Breuer reflex** which helps regulate breathing depth and rate. - These receptors are **active during normal tidal breathing** and contribute to the rhythmic pattern of respiration at rest. *PO2* - **Peripheral chemoreceptors** (carotid and aortic bodies) monitor **arterial PO2** and do have **tonic baseline activity** at normal PO2 levels (~95-100 mmHg). - While their contribution is **minimal at normal oxygen levels**, they are present and functioning, making them technically a (weak) normal stimulus. - They become a **major stimulus only when PO2 drops below 60 mmHg** (hypoxemia). *PCO2* - **PCO2** (specifically, the H+ concentration in the cerebrospinal fluid derived from PCO2) is the **most potent and immediate normal stimulus** for resting ventilation. - **Central chemoreceptors** in the medulla are extremely sensitive to changes in CSF pH, directly regulating breathing to maintain arterial PCO2 within a narrow range (~40 mmHg).

Q: The rhythm for inspiration starts in:

Pre-Botzinger complex. ***Pre-Botzinger complex*** - The **Pre-Botzinger complex** is a cluster of neurons in the medulla oblongata recognized as the primary site for generating the **respiratory rhythm**. - It establishes the basic pattern for **inspiratory efforts**, acting as the central pattern generator for breathing. *Pneumotaxic centre* - The **pneumotaxic center** (located in the pons) fine-tunes the respiratory rhythm by **inhibiting inspiration**, thus regulating the rate and depth of breathing. - While it modulates respiration, it does not originate the basic inspiratory rhythm. *Apneustic centre* - The **apneustic center** (located in the pons) prolongs inspiration by stimulating the inspiratory neurons in the medulla. - Its main role is to promote deep, prolonged inspiratory gasps, but not to initiate the rhythm. *DRG* - The **Dorsal Respiratory Group (DRG)** in the medulla contains inspiratory neurons that primarily control the **diaphragm** and **external intercostals**. - While essential for inspiration, the DRG receives its rhythmicity from the Pre-Botzinger complex and acts as an integration center for various inputs.

Q: Which of the following factors does not directly influence oxygen delivery to tissues?

Type of fluid administered. ***Type of fluid administered*** - While fluid administration can indirectly affect oxygen delivery by altering blood volume and cardiac output, the **type of fluid itself (e.g., crystalloid vs. colloid)** does not directly influence the oxygen-carrying capacity of the blood or its release to tissues. - The direct effect of fluid resuscitation is on **hemodynamic parameters**, which then influence delivery. *Oxygen saturation* - **Oxygen saturation** directly reflects the percentage of hemoglobin binding sites occupied by oxygen, thus determining the amount of oxygen carried by each unit of blood. - A decrease in oxygen saturation significantly reduces the **total oxygen content** available for tissue delivery. *Cardiac output* - **Cardiac output** (heart rate × stroke volume) is a primary determinant of blood flow to tissues, and therefore directly influences the rate at which oxygenated blood is delivered throughout the body. - A lower cardiac output leads to **reduced oxygen delivery** despite adequate oxygen content in the blood. *Hemoglobin concentration* - **Hemoglobin concentration** directly dictates the blood's capacity to carry oxygen, as hemoglobin is the main oxygen-carrying molecule in red blood cells. - A low hemoglobin concentration (anemia) results in **decreased oxygen-carrying capacity** and thus impaired oxygen delivery to tissues.

Question 1

True about normal expiration is:

Accepted Answer

Chest wall has a tendency to move outward which is balanced by inward recoil of alveoli

Answer

In expiration pleural pressure is equal to alveolar pressure

Answer

Muscles that elevate the chest cage are classified as muscles of expiration

Answer

At the end of normal expiration, the air in the lung is FRC

Question 2

What is the estimated PaO2 after giving FiO2 at 0.5 in a normal person?

Accepted Answer

> 200 mmHg

Answer

< 100 mmHg

Answer

150–200 mmHg

Answer

100–150 mmHg

Question 3

The chloride shift occurs rapidly and is essentially complete within:

Accepted Answer

1 second

Answer

2 seconds

Answer

5 seconds

Answer

60 seconds

Question 4

What is the physiological effect of very high positive end-expiratory pressure in a patient with respiratory distress?

Accepted Answer

Decreased blood pressure

Answer

Increased blood pressure

Answer

Decreased body temperature

Answer

Increased body temperature

Question 5

All of the following factors influence the hemoglobin dissociation curve, except.

Accepted Answer

Plasma sodium concentration

Answer

Temperature

Answer

2–3 DPG levels

Answer

CO2 tension

Question 6

Which test is primarily used to assess gas diffusion capacity in the lungs?

Accepted Answer

DLCO

Answer

Spirometry

Answer

Both DLCO and Spirometry

Answer

None of the options

Question 7

The factor responsible for the left shift of the HbO2 dissociation curve is

Accepted Answer

Fall in temperature

Answer

Increase in 2,3 DPG in RBC

Answer

Fall in pH

Answer

Increase level of CO2 in blood

Question 8

Which of the following is not a normal stimulus for resting ventilation?

Accepted Answer

J receptors

Answer

Stretch receptors

Answer

PCO2

Answer

PO2

Question 9

The rhythm for inspiration starts in:

Accepted Answer

Pre-Botzinger complex

Answer

Pneumotaxic centre

Answer

Apneustic centre

Answer

DRG

Question 10

Which of the following factors does not directly influence oxygen delivery to tissues?

Accepted Answer

Type of fluid administered

Answer

Cardiac output

Answer

Oxygen saturation

Answer

Hemoglobin concentration

Respiratory System — MCQs

Respiratory System — MCQs

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