Respiratory Acidosis and Alkalosis — MCQs

10 questions

Why does hyperventilation cause paresthesia?

The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L

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 patient presents with the following arterial blood gas (ABG) and electrolyte values: pH: 7.34, Na: 135 mEq/L, Cl: 93 mEq/L, HCO3: 20 mEq/L, Random Blood Sugar (RBS): 420 mg/dl. What is the most likely acid-base disturbance?

A male patient presents to the emergency department. The arterial blood gas report is as follows: pH, 7.2; pCO2, 81 mmHg; and HCO3, 40 meq/L. Which of the following is the most likely diagnosis?

The primary respiratory compensation for metabolic acidosis is?

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?

All of the following statements about acid-base disorders are true, EXCEPT:

Q10

In metabolic acidosis, what compensatory mechanism is activated first?

Respiratory Acidosis and Alkalosis Indian Medical PG Practice Questions and MCQs

Question 1: Why does hyperventilation cause paresthesia?

A. Increased O2
B. Decreased CO2 (Correct Answer)
C. Decreased pH
D. Increased CO2

Explanation: ***Decreased CO2*** - Hyperventilation leads to an excessive loss of **carbon dioxide (CO2)** from the body, causing **respiratory alkalosis**. - The resulting alkalosis decreases the concentration of **ionized calcium** in the blood, leading to neuronal excitability and thus paresthesia. *Increased O2* - While hyperventilation increases the amount of **oxygen (O2)** breathed in, it is not the direct cause of paresthesia. - The key physiological change leading to paresthesia is related to changes in **blood gas chemistry**, specifically CO2 and pH. *Decreased pH* - Hyperventilation causes a **decrease in CO2**, which subsequently leads to an **increase in pH** (respiratory alkalosis), not a decrease in pH. - A decrease in pH (acidosis) generally leads to different symptoms, and is not the cause of paresthesia in this context. *Increased CO2* - Hyperventilation by definition involves **expelling more CO2** than normal, leading to a decrease in CO2 levels, not an increase. - An underlying increase in CO2 would lead to **respiratory acidosis**, which has a different clinical presentation.

Question 2: The interpretation of the following ABG value is: pH = 7.5, pCO2 = 50 mm Hg, HCO3 = 30 mEq/L

A. Respiratory acidosis
B. Metabolic acidosis
C. Metabolic alkalosis (Correct Answer)
D. Normal acid-base balance

Explanation: ***Metabolic alkalosis (partially compensated)*** - The **pH of 7.5** indicates **alkalosis**, and the elevated **bicarbonate (HCO3) of 30 mEq/L** is the primary driver of this high pH. - The elevated **pCO2 of 50 mm Hg** represents **partial respiratory compensation**, where the body retains CO2 to lower the pH toward normal. - Since the pH remains elevated (not normalized to 7.35-7.45), this is **partially compensated** rather than fully compensated. *Respiratory acidosis* - This would be characterized by a **low pH** and an **elevated pCO2**, which is not seen here as the pH is high. - Although pCO2 is elevated, the **high pH** and **high bicarbonate** rule out primary respiratory acidosis. *Metabolic acidosis* - This would present with a **low pH** and a **low bicarbonate** concentration. - The given values show a **high pH** and **high bicarbonate**, which is the opposite of metabolic acidosis. *Normal acid-base balance* - A normal acid-base balance would have a **pH between 7.35-7.45**, a **pCO2 between 35-45 mm Hg**, and an **HCO3 between 22-26 mEq/L**. - All three values are outside of their normal ranges, indicating an acid-base disturbance.

Question 3: 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 4: A patient presents with the following arterial blood gas (ABG) and electrolyte values: pH: 7.34, Na: 135 mEq/L, Cl: 93 mEq/L, HCO3: 20 mEq/L, Random Blood Sugar (RBS): 420 mg/dl. What is the most likely acid-base disturbance?

A. Normal Anion Gap Metabolic Acidosis (NAGMA)
B. Respiratory Acidosis
C. High Anion Gap Metabolic Acidosis (HAGMA) (Correct Answer)
D. Metabolic Alkalosis

Explanation: ### High Anion Gap Metabolic Acidosis (HAGMA) - The **pH (7.34)** indicates **acidemia**, and the **low bicarbonate (20 mEq/L)** suggests a metabolic acidosis [1], [2]. - Calculation of the anion gap: Na - (Cl + HCO3) = 135 - (93 + 20) = 22 mEq/L. An anion gap > 12 mEq/L is considered high, confirming **High Anion Gap Metabolic Acidosis (HAGMA)** [4]. The **RBS of 420 mg/dl** also points towards a likely cause such as **diabetic ketoacidosis** [3]. *Normal Anion Gap Metabolic Acidosis (NAGMA)* - This would be present if the calculated anion gap were within the normal range (typically 8-12 mEq/L). - Causes of NAGMA (e.g., hyperchloremic acidosis) are typically associated with increased chloride levels to compensate for the bicarbonate loss, which is not the primary finding here [4]. *Respiratory Acidosis* - This condition is characterized by a **low pH** and an **elevated PaCO2**, which is not provided but implied by the **low bicarbonate** not fitting a respiratory picture [2]. - While the pH is low, the primary disturbance given the other values (especially the low bicarbonate) is metabolic, not respiratory. *Metabolic Alkalosis* - Metabolic alkalosis is characterized by an **elevated pH** and an **elevated bicarbonate level**, which contradicts the presented values of low pH and low bicarbonate [2]. - This condition would involve a net gain of bicarbonate or a loss of acids, which is the opposite of the findings in this patient.

Question 5: A male patient presents to the emergency department. The arterial blood gas report is as follows: pH, 7.2; pCO2, 81 mmHg; and HCO3, 40 meq/L. Which of the following is the most likely diagnosis?

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

Explanation: ***Respiratory acidosis*** - The **pH of 7.2** indicates **acidemia**, while the **elevated pCO2 (81 mmHg)** points to a primary respiratory problem [2]. - The elevated **HCO3 (40 meq/L)** suggests **renal compensation** attempting to buffer the increased carbonic acid [1]. *Respiratory alkalosis* - This condition presents with an **elevated pH (alkalemia)** and a **decreased pCO2**, which is opposite to the given ABG values [2]. - While there might be metabolic compensation with a decreased HCO3, the primary disturbance is an increase in respiratory rate leading to excessive CO2 exhalation. *Metabolic acidosis* - Metabolic acidosis is characterized by a **low pH** and a **low HCO3**, with a compensatory decrease in pCO2 [1]. - The given ABG shows a high HCO3, which rules out primary metabolic acidosis. *Metabolic alkalosis* - This condition would typically show an **elevated pH** and an **elevated HCO3**, with a compensatory increase in pCO2. - While both HCO3 and pCO2 are high in the given ABG, the low pH points to a primary acidosis, not alkalosis.

Question 6: The primary respiratory compensation for metabolic acidosis is?

A. HCO3 loss
B. Cl- loss
C. Hyperventilation (Correct Answer)
D. Ammonia excretion in kidney

Explanation: ***Hyperventilation*** - In **metabolic acidosis**, the body attempts to raise the pH by decreasing the **partial pressure of carbon dioxide (PCO2)**. - **Hyperventilation** increases the excretion of CO2, a volatile acid, which directly reduces the amount of carbonic acid in the blood and helps to buffer the excess acid. *HCO3 loss* - **Bicarbonate (HCO3-) loss** is a cause or consequence of metabolic acidosis, not a compensatory mechanism. - The kidneys generally try to *retain* or regenerate bicarbonate during acidosis, rather than losing it. *Cl- loss* - **Chloride ion (Cl-) loss** is not a primary respiratory compensatory mechanism for metabolic acidosis. - While shifts in chloride can occur in acid-base imbalances, they are typically related to renal handling or fluid shifts, not direct respiratory compensation. *Ammonia excretion in kidney* - **Ammonia excretion** by the kidneys is a renal (kidney) compensatory mechanism, not a respiratory one. - The kidneys excrete ammonia to excrete hydrogen ions (H+), thereby regenerating bicarbonate and helping to correct the acidosis over a longer period.

Question 7: 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 8: 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 9: All of the following statements about acid-base disorders are true, EXCEPT:

A. Metabolic acidosis is compensated by increasing Pco2 (Correct Answer)
B. Buffering may be intra & extra cellular
C. pH determined by Pco2 and HCO3
D. Respiratory acidosis is compensated by HCO3

Explanation: ***Metabolic acidosis is compensated by increasing Pco2*** - In **metabolic acidosis**, the primary problem is a decrease in **bicarbonate (HCO3-)**. - The compensatory response is **respiratory**, involving an increase in **respiratory rate** and depth to **decrease Pco2**, thereby *raising* the pH back towards normal. Increasing Pco2 would worsen the acidosis. *Buffering may be intra & extra cellular* - **Buffering systems** operate both **intracellularly** (e.g., proteins, phosphates) and **extracellularly** (e.g., bicarbonate-carbonic acid system, hemoglobin). - This dual buffering ensures a rapid and widespread response to changes in acid-base balance throughout the body. *pH determined by Pco2 and HCO3* - According to the **Henderson-Hasselbalch equation**, pH is directly proportional to the ratio of **bicarbonate (HCO3-)** to **Pco2**. - This means that changes in either Pco2 (respiratory component) or HCO3- (metabolic component) will directly influence the overall pH of the blood. *Respiratory acidosis is compensated by HCO3* - In **respiratory acidosis**, the primary problem is an increase in **Pco2** due to hypoventilation. - The compensatory response is **renal**, involving increased reabsorption of **bicarbonate (HCO3-)** and increased excretion of H+ ions to buffer the excess acid.

Question 10: In metabolic acidosis, what compensatory mechanism is activated first?

A. Decreased CO2 excretion
B. Increased respiratory rate (Correct Answer)
C. Increased renal HCO3- excretion
D. Increased renal H+ secretion

Explanation: ***Increased respiratory rate*** - In metabolic acidosis, the body attempts to **decrease PCO2** through increasing ventilation, thus reducing the **acid load** by expelling more CO2. - This **respiratory compensation** is rapid and begins within minutes to hours of the onset of acidosis. *Decreased CO2 excretion* - This option is incorrect because the body's compensatory mechanism for acidosis involves **increasing CO2 excretion** through hyperventilation, not decreasing it. - Decreased CO2 excretion would lead to **respiratory acidosis**, further worsening the metabolic acidosis. *Increased renal HCO3- excretion* - In metabolic acidosis, the kidneys aim to **conserve bicarbonate**, not excrete it, to buffer the excess acid. - Increased renal HCO3- excretion would exacerbate acidosis by reducing the body's primary **buffer system**. *Increased renal H+ secretion* - This is a renal compensatory mechanism that occurs in metabolic acidosis, but it is **slower to activate** (hours to days) compared to respiratory compensation. - While important for long-term acid-base balance, it is **not the first mechanism** to be activated.

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