Respiratory Regulation of Acid-Base Balance — MCQs

10 questions

What is the best immediate management strategy for a patient experiencing respiratory alkalosis due to anxiety-induced hyperventilation?

A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?

Which of the following is the common cause of respiratory failure type 2 ?

Renal tubular acidosis with ABG value pH = 7.24 PO2=80; PaCO2= 36 Na = 131; HCO3 = 14 Cl= 90; BE = -13 Glucose = 135 the above ABG picture suggests –

A hyperventilating patient has the following ABG values: pH=7.53, pCO2=20 mmHg, HCO3= 26 mEq/L. What is the most likely diagnosis?

A person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?

In plasma, if pH is 5, what is the fraction of base to acid?

Which one of the following biochemical abnormalities can be produced by repeated vomiting?

Which drug is given to prevent acute mountain sickness?

Q10

Carbonic anhydrase activity is found in all of the following except?

Respiratory Regulation of Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 1: What is the best immediate management strategy for a patient experiencing respiratory alkalosis due to anxiety-induced hyperventilation?

A. Rebreathing in paper bag (Correct Answer)
B. IPPV
C. Normal saline
D. Acetazolamide

Explanation: ***Rebreathing in paper bag*** - This helps to **increase the inspired CO2 concentration**, thereby correcting the hypocapnia (low CO2) caused by hyperventilation. - It's a simple, non-invasive method to raise arterial PCO2 and normalize blood pH in acute respiratory alkalosis. *IPPV* - **Intermittent positive pressure ventilation (IPPV)** would further reduce CO2 by assisting ventilation and is typically used for respiratory *acidosis* or failure [1]. - This intervention would worsen the patient's respiratory alkalosis rather than alleviating it. *Normal saline* - **Normal saline** administration is primarily used for volume expansion or to correct electrolyte imbalances; it does not directly address respiratory alkalosis. - It would not correct the underlying issue of excessive CO2 exhalation. *Acetazolamide* - **Acetazolamide** is a carbonic anhydrase inhibitor that reduces bicarbonate reabsorption and is used to treat metabolic alkalosis or as a diuretic. - It would not be an immediate or appropriate solution for acute respiratory alkalosis and might even worsen the acid-base balance if used improperly.

Question 2: A patient with pH of 7, pCO2 of 30 mmHg and Bicarbonate levels of 10 meq. What is the acid base abnormality?

A. Respiratory alkalosis
B. Metabolic alkalosis
C. Respiratory Acidosis
D. Metabolic Acidosis (Correct Answer)

Explanation: ***Metabolic Acidosis*** - The pH is 7, which is severely **acidotic** (normal range 7.35-7.45). This indicates an acid-base disorder where the body is too acidic. - The **bicarbonate level is 10 mEq/L** (normal range 22-26 mEq/L), which is significantly low, directly contributing to the acidosis and pointing towards a metabolic origin. *Respiratory alkalosis* - This condition involves an **elevated pH** (alkalosis) due to a primary decrease in pCO2. - In this case, the pH is acidic, not alkaline. *Metabolic alkalosis* - This condition involves an **elevated pH** (alkalosis) due to a primary increase in bicarbonate levels. - Here, the pH is acidic and bicarbonate is low, directly contradicting metabolic alkalosis. *Respiratory Acidosis* - This condition involves a **decreased pH** (acidosis) due to a primary increase in pCO2. - Although the pH is acidotic, the pCO2 is 30 mmHg (normal range 35-45 mmHg), which is low, indicating a respiratory compensation rather than the primary cause.

Question 3: Which of the following is the common cause of respiratory failure type 2 ?

A. Chronic bronchitis exacerbation (Correct Answer)
B. Acute attack asthma
C. ARDS
D. Pneumonia

Explanation: ***Chronic bronchitis exacerbation*** - **Chronic bronchitis** is a common cause of **Type 2 respiratory failure**, characterized by **hypercapnia** (elevated CO2) due to impaired alveolar ventilation [1]. - An exacerbation worsens **airflow obstruction** and leads to increased work of breathing and CO2 retention [1]. *Acute attack asthma* - While severe asthma can cause respiratory failure, it typically presents initially as **Type 1 (hypoxemic)**, with severe bronchospasm and V/Q mismatch [2]. - **Hypercapnia** in asthma is a sign of **severe, impending respiratory collapse** rather than the primary cause of respiratory failure. *ARDS* - **Acute Respiratory Distress Syndrome (ARDS)** is a classic cause of **Type 1 (hypoxemic) respiratory failure**, characterized by widespread inflammation and fluid accumulation in the lungs [2]. - ARDS primarily involves impaired oxygenation rather than CO2 elimination issues, unless it progresses to severe stages with significant muscle fatigue. *Pneumonia* - **Pneumonia** predominantly causes **Type 1 (hypoxemic) respiratory failure** due to consolidation and V/Q mismatch in affected lung areas, leading to impaired oxygen diffusion [2]. - While severe, widespread pneumonia can eventually lead to ventilatory failure, its initial and primary impact is on oxygenation.

Question 4: Renal tubular acidosis with ABG value pH = 7.24 PO2=80; PaCO2= 36 Na = 131; HCO3 = 14 Cl= 90; BE = -13 Glucose = 135 the above ABG picture suggests –

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

Explanation: The ABG shows a pH of 7.24, indicating **acidemia** [1]. The HCO3 is 14 mEq/L, which is significantly **low**, and the base excess (BE) is -13 [1]. The PaCO2 of 36 mmHg is within the normal range, indicating no significant primary respiratory derangement [2]. The overall picture is consistent with an uncompensated or partially compensated **metabolic acidosis** [1][2]. ***Metabolic acidosis*** - The **low pH (acidemia)**, **low bicarbonate (HCO3)**, and **negative base excess (BE)** are direct indicators of metabolic acidosis [1]. - The **PaCO2 within normal limits** or slightly decreased suggests either no respiratory compensation or insufficient compensation for the metabolic derangement [1][2]. *Respiratory acidosis* - This would present with a **low pH** and an **elevated PaCO2** as the primary defect, which is not seen here (PaCO2 is normal) [1]. - Bicarbonate would typically be normal or elevated if compensated, not significantly decreased. *Respiratory alkalosis* - This would be characterized by an **elevated pH** and a **low PaCO2**, which is the opposite of the findings in this ABG [1]. - HCO3 would be normal or low if compensated. *Metabolic alkalosis* - This would present with an **elevated pH** and an **elevated HCO3**, which contradicts the given ABG values (low pH and low HCO3) [2].

Question 5: A hyperventilating patient has the following ABG values: pH=7.53, pCO2=20 mmHg, HCO3= 26 mEq/L. What is the most likely diagnosis?

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

Explanation: ***Respiratory alkalosis*** - The pH of 7.53 indicates **alkalemia**, and the low pCO2 (20 mmHg) is the primary driver, signifying **respiratory alkalosis** - A hyperventilating patient exhales more CO2, leading to a decrease in its partial pressure in the blood and a subsequent rise in pH - The HCO3 is within normal range (26 mEq/L), indicating **uncompensated respiratory alkalosis** *Metabolic alkalosis* - This would be characterized by a high pH and an elevated **HCO3**, but the HCO3 is within the normal range (26 mEq/L) - While it causes alkalemia, the primary disturbance here is respiratory, not metabolic *Metabolic acidosis* - This would present with a **low pH** and a low **HCO3**, which is contrary to the given ABG values - The patient's pH is elevated, indicating an alkalotic state, not acidotic *Respiratory acidosis* - This would be defined by a **low pH** and an elevated **pCO2**, which is the exact opposite of the provided ABG results - The patient's high pH and low pCO2 rule out respiratory acidosis

Question 6: A person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?

A. H2O ⇌ H+ + OH-
B. H+ + NH3 ⇌ NH4+
C. CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+
D. CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (Correct Answer)

Explanation: ***CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-*** - This reaction represents the **bicarbonate buffer system**, which is central to maintaining **pH balance** in the body. - In response to **metabolic acidosis**, the body hyperventilates to **decrease CO2** levels, shifting the equilibrium to the left and reducing H+ which compensates for the increased acidity. *H2O ⇌ H+ + OH-* - This reaction describes the **autoionization of water**, which is fundamental but does not directly explain the body's respiratory compensation mechanism for metabolic acidosis. - While it shows the presence of H+ ions, it doesn't illustrate how the respiratory system manipulates CO2 to influence pH. *H+ + NH3 ⇌ NH4+* - This reaction represents the **ammonia buffer system** primarily active in the **kidneys** for acid excretion. - It plays a role in renal compensation for pH imbalances, but it is not the mechanism for respiratory compensation. *CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+* - This represents the **dissociation of beta-hydroxybutyric acid**, a **ketone body** produced in diabetic ketoacidosis (DKA). - While DKA is the cause of the metabolic acidosis in this patient, this specific reaction describes the *production* of H+ ions, not the *respiratory compensatory mechanism* to address it.

Question 7: In plasma, if pH is 5, what is the fraction of base to acid?

A. 0.01
B. 0.1 (Correct Answer)
C. 1
D. 10

Explanation: ***0.1*** - This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1. - Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$ - Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$ - Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$ - Among the given options, **0.079 is closest to 0.1**, making this the correct answer. - Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation. *0.01* - This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$. - Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5. - The calculated ratio of 0.079 is much closer to 0.1 than to 0.01. *1* - A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa. - This would give pH = 6.1, not the given pH of 5. - This represents a **neutral buffer condition**, not the acidic state described. *10* - This ratio indicates an **alkaline** solution with 10 times more base than acid. - Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis). - This contradicts the given acidic pH of 5.

Question 8: Which one of the following biochemical abnormalities can be produced by repeated vomiting?

A. Metabolic acidosis
B. Metabolic alkalosis (Correct Answer)
C. Ketosis
D. Uraemia

Explanation: ***Metabolic alkalosis*** - Repeated vomiting leads to the loss of **hydrochloric acid (HCl)** from the stomach, causing **hypochloremic metabolic alkalosis** with an increase in serum **bicarbonate (HCO3-)** and a rise in blood pH. - The loss of H+ and Cl- ions results in **compensatory hypokalemia** as the kidneys exchange K+ for H+ to maintain electroneutrality. - **Volume depletion** from vomiting triggers aldosterone secretion, which further promotes K+ loss and H+ excretion, perpetuating the alkalosis (contraction alkalosis). - This is one of the most common causes of metabolic alkalosis in clinical practice. *Metabolic acidosis* - This condition is characterized by a decrease in **serum pH** and **bicarbonate levels**, typically due to excess acid production or bicarbonate loss from diarrhea or renal tubular acidosis. - Vomiting does not directly cause metabolic acidosis; rather, it leads to the opposite effect by removing acidic gastric contents. *Ketosis* - **Ketosis** occurs when the body metabolizes fat for energy, producing **ketone bodies**, common in conditions like uncontrolled diabetes or prolonged starvation. - While severe, prolonged vomiting with reduced oral intake can indirectly lead to starvation ketosis, the primary and most characteristic biochemical abnormality of repeated vomiting is metabolic alkalosis, not ketosis. *Uraemia* - **Uraemia** is a syndrome caused by the accumulation of **nitrogenous waste products** (urea, creatinine) in the blood, primarily due to kidney failure. - Vomiting may be a *symptom* of uraemia, but it does not *cause* uraemia. Kidney function is the primary determinant of urea levels.

Question 9: Which drug is given to prevent acute mountain sickness?

A. Acetazolamide (Correct Answer)
B. Dexamethasone
C. Digoxin
D. Diltiazem

Explanation: ***Acetazolamide*** - **Acetazolamide** is a **carbonic anhydrase inhibitor** that acidifies the blood and increases respiratory drive, thereby aiding acclimatization to high altitude. - It is the **drug of choice** for the prevention and treatment of acute mountain sickness (AMS) and high altitude cerebral edema (HACE). *Dexamethasone* - **Dexamethasone**, a corticosteroid, is primarily used for the **treatment** of severe AMS and HACE, rather than prevention. - It works by reducing swelling and inflammation in the brain but does not facilitate physiological acclimatization. *Digoxin* - **Digoxin** is a cardiac glycoside used to treat **heart failure** and **atrial fibrillation**. - It has no role in the prevention or treatment of acute mountain sickness. *Diltiazem* - **Diltiazem** is a **calcium channel blocker** used to treat hypertension, angina, and certain arrhythmias. - It is not indicated for the prevention or treatment of acute mountain sickness.

Question 10: Carbonic anhydrase activity is found in all of the following except?

A. Brain
B. Kidney
C. RBC
D. Plasma (Correct Answer)

Explanation: ***Plasma*** - **Carbonic anhydrase** is an intracellular enzyme that catalyzes the rapid interconversion of carbon dioxide and water to carbonic acid, **bicarbonate**, and protons. - It is notably **absent in plasma** in healthy individuals, as it is primarily found within cells where its function is crucial for pH regulation and CO2 transport. *Brain* - Carbonic anhydrase is found in various brain cells, including **neurons**, **oligodendrocytes**, and **astrocytes**. - It plays a vital role in pH regulation, fluid balance, and the production of cerebrospinal fluid (CSF) within the **central nervous system**. *Kidney* - The kidney is rich in carbonic anhydrase, particularly in the **proximal tubules** and collecting ducts. - It is critical for **bicarbonate reabsorption** and proton excretion, essential processes for maintaining acid-base balance. *RBC* - **Red blood cells (RBCs)** contain a high concentration of carbonic anhydrase (specifically CA-I and CA-II isoforms). - This enzyme facilitates the rapid conversion of CO2 to bicarbonate for transport to the lungs and the reverse reaction for **CO2 exhalation**.

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