Mixed Acid-Base Disorders — MCQs

10 questions

A patient of CKD has presented with protracted vomiting. ABG shows pH = 7.40, pCO2 = 40 mm Hg, HCO3 = 24 mEq/L, Na = 145 mEq/L, Chloride = 100 mEq/L. What is the observation?

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?

The lab reports of a patient given below: pH = 7.2, HCO3 = 10 mEq/L, PCO2 = 30 mmHg. This exemplifies which of the following disorders?

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:

When resuscitating a patient in shock which of the following is not an adequate parameter to predict end point of resuscitation?

What is the most common underlying cause of hypoxemia in patients with pneumonia?

When VA/Q is infinity, it means

Q10

A 60-year-old man with type 2 diabetes on metformin and insulin presents with 3 days of nausea, vomiting, and diffuse abdominal pain. He appears ill and confused. Vital signs: BP 95/60 mmHg, HR 115/min, RR 28/min, T 37.2°C. Labs show glucose 380 mg/dL, pH 7.28, HCO3 18 mEq/L, anion gap 24, serum osmolality 310 mOsm/kg, negative urine ketones, creatinine 2.8 mg/dL (baseline 1.1), lactate 8.2 mmol/L. Apply physiological principles to determine the primary acid-base and metabolic disturbance.

Mixed Acid-Base Disorders Indian Medical PG Practice Questions and MCQs

Question 1: A patient of CKD has presented with protracted vomiting. ABG shows pH = 7.40, pCO2 = 40 mm Hg, HCO3 = 24 mEq/L, Na = 145 mEq/L, Chloride = 100 mEq/L. What is the observation?

A. Normal anion gap metabolic acidosis
B. No acid base abnormality
C. High anion gap metabolic acidosis and metabolic alkalosis (Correct Answer)
D. High anion gap metabolic acidosis

Explanation: ***High anion gap metabolic acidosis and metabolic alkalosis*** - The **calculated anion gap** is 145 - (100 + 24) = 21, which is elevated (normal 8-12), indicating a **high anion gap metabolic acidosis**. [1] - The **ΔΔ ratio (ΔAG / ΔHCO3)** is (21-12) / (24-24) = 9/0, which is indeterminate but given the **normal pH and Bicarbonate**, a co-existing metabolic alkalosis that is compensating for the acidosis is likely. [1] *Normal anion gap metabolic acidosis* - This would be characterized by a **normal anion gap** (8-12 mEq/L), which is not the case here (elevated to 21 mEq/L). [1] - Normal anion gap acidosis usually involves **loss of bicarbonate** or **addition of chloride**, leading to hyperchloremia. *No acid base abnormality* - While the **pH and HCO3** are within the normal range, the elevated anion gap indicates an underlying acid-base disturbance. [1] - A comprehensive assessment, including anion gap calculation, reveals an abnormality **despite normal pH**. [1] *High anion gap metabolic acidosis* - Although there is a **high anion gap metabolic acidosis**, the **normal pH and bicarbonate** suggest a second primary acid-base disorder. [1] - In an isolated high anion gap metabolic acidosis, the pH and bicarbonate would typically be **lower than normal**.

Question 2: 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 3: 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 4: The lab reports of a patient given below: pH = 7.2, HCO3 = 10 mEq/L, PCO2 = 30 mmHg. This exemplifies which of the following disorders?

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

Explanation: ***Metabolic acidosis*** - The pH of 7.2 is acidic, and the **bicarbonate (HCO3) of 10 mEq/L** is significantly low (normal: 22-28 mEq/L), indicating a primary metabolic disturbance causing acidosis. - The **PCO2 of 30 mmHg** is also low (normal: 35-45 mmHg), which represents **partial respiratory compensation** through hyperventilation to blow off CO2 and raise pH. - This is a classic example of **metabolic acidosis with respiratory compensation**. *Metabolic alkalosis* - This condition would be characterized by a **high pH** and a **high bicarbonate (HCO3)** level, which is the opposite of the given values. - The body would attempt to compensate by increasing PCO2 through hypoventilation. *Respiratory acidosis* - This would present with a **low pH** and a **high PCO2** (>45 mmHg), indicating a primary respiratory problem leading to CO2 retention and acid accumulation. - Metabolic compensation would show elevated HCO3, not the low HCO3 (10 mEq/L) seen here. *Respiratory alkalosis* - This condition is characterized by a **high pH** (>7.45) and a **low PCO2**, due to excessive ventilation causing CO2 elimination. - While PCO2 is low in the given scenario, the pH is acidic (7.2), not alkalotic, ruling out this diagnosis.

Question 5: 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 6: 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 7: When resuscitating a patient in shock which of the following is not an adequate parameter to predict end point of resuscitation?

A. Mixed venous oxygen saturation
B. Base deficit
C. Lactate
D. Blood pressure (Correct Answer)

Explanation: ***Blood pressure*** - While essential for initial assessment and guiding treatment, **blood pressure** can be maintained within normal limits even in significant shock states due to compensatory mechanisms [1]. - Blood pressure alone does not reflect **tissue perfusion** or cellular oxygenation, which are the true endpoints of resuscitation [1]. *Mixed venous oxygen saturation* - **Mixed venous oxygen saturation (SvO2)** reflects the balance between oxygen delivery and consumption, providing insight into global tissue oxygenation. - A low SvO2 indicates inadequate oxygen delivery relative to demand, making it a valuable target for guiding resuscitation. *Base deficit* - **Base deficit** is a measure of metabolic acidosis and reflects the severity of tissue hypoperfusion and anaerobic metabolism. - Normalization of base deficit indicates correction of metabolic derangements and improved tissue perfusion. *Lactate* - **Lactate** is a product of anaerobic metabolism, which occurs when tissues are not adequately perfused or oxygenated. - Elevated lactate levels indicate tissue hypoperfusion, and serial measurements are crucial for monitoring the effectiveness of resuscitation and predicting outcomes.

Question 8: What is the most common underlying cause of hypoxemia in patients with pneumonia?

A. Shunting
B. Reduced lung volume
C. Impaired gas exchange
D. Ventilation-perfusion mismatch (Correct Answer)

Explanation: ***Ventilation-perfusion mismatch*** - This occurs when areas of the lung are either **well-perfused but poorly ventilated** (e.g., due to alveolar filling or collapse in pneumonia), or **well-ventilated but poorly perfused**. - In pneumonia, inflammatory exudates and consolidation fill alveoli, impairing ventilation while perfusion to these areas continues, creating a **low V/Q ratio** and leading to hypoxemia. *Shunting* - **True shunting** (blood bypassing ventilated lung entirely) is a severe form of V/Q mismatch where the V/Q ratio is zero. - While shunting can occur in severe pneumonia, it represents an extreme, non-correctable form of V/Q mismatch and is not the *most common* or primary mechanism for hypoxemia in the broader spectrum of pneumonia. *Reduced lung volume* - **Reduced lung volume** can contribute to hypoxemia by limiting the overall surface area for gas exchange, but it is not the primary or most direct mechanism caused by the pathological changes in pneumonia. - It often results from conditions like atelectasis or pleural effusions, which may coexist with pneumonia but are distinct from the primary parenchymal inflammation. *Impaired gas exchange* - **Impaired gas exchange** is a general term describing the inability to adequately oxygenate blood and/or remove carbon dioxide. - While V/Q mismatch is a specific mechanism of impaired gas exchange, "impaired gas exchange" itself is too broad and does not pinpoint the underlying physiological process most commonly responsible in pneumonia.

Question 9: When VA/Q is infinity, it means

A. Dead space (Correct Answer)
B. Unrelated to VA/Q ratio
C. The PO2 of alveolar air is 159 mmHg and PCO2 is 0 mmHg
D. Atelectasis

Explanation: ***Dead space*** - An infinite V/Q ratio implies that **ventilation (V)** is occurring, but **perfusion (Q)** is zero. - This scenario defines **dead space**, where air enters the alveoli but no blood flow is available for gas exchange. - This is the **most accurate and complete answer** to describe the physiological meaning of VA/Q = ∞. *Unrelated to VA/Q ratio* - This statement is incorrect because VA/Q being infinity is a specific and highly significant state within the **ventilation-perfusion relationship**. - An infinite ratio directly indicates a complete decoupling of ventilation and perfusion, with profound physiological consequences. *The PO2 of alveolar air is 159 mmHg and PCO2 is 0 mmHg* - While this describes the **gas composition** in dead space (VA/Q = ∞), it is not the **physiological term** for the condition. - With no perfusion, alveolar air remains essentially **unchanged from inspired air**: PO2 ≈ 150-159 mmHg (atmospheric level) and PCO2 ≈ 0 mmHg. - No oxygen is extracted and no CO2 is added because there is **no blood flow**. - However, "dead space" is the more precise physiological answer. *Atelectasis* - **Atelectasis** refers to the collapse of lung tissue, which typically leads to an absence of **ventilation (V)** in that region. - This condition would result in a **VA/Q ratio of zero** (V=0, Q present), the opposite of infinity.

Question 10: A 60-year-old man with type 2 diabetes on metformin and insulin presents with 3 days of nausea, vomiting, and diffuse abdominal pain. He appears ill and confused. Vital signs: BP 95/60 mmHg, HR 115/min, RR 28/min, T 37.2°C. Labs show glucose 380 mg/dL, pH 7.28, HCO3 18 mEq/L, anion gap 24, serum osmolality 310 mOsm/kg, negative urine ketones, creatinine 2.8 mg/dL (baseline 1.1), lactate 8.2 mmol/L. Apply physiological principles to determine the primary acid-base and metabolic disturbance.

A. Hyperosmolar hyperglycemic state complicated by lactic acidosis from metformin (Correct Answer)
B. Diabetic ketoacidosis with renal failure from volume depletion
C. Sepsis-induced lactic acidosis with stress hyperglycemia
D. Alcoholic ketoacidosis with concurrent diabetic emergency
E. Mixed metabolic acidosis from uremia and starvation ketosis

Explanation: ***Hyperosmolar hyperglycemic state complicated by lactic acidosis from metformin*** - The patient exhibits severe hyperglycemia and high serum osmolality without significant ketonemia, typical of **Hyperosmolar Hyperglycemic State (HHS)** in Type 2 Diabetes. - The high **anion gap metabolic acidosis** is primarily explained by a markedly elevated **serum lactate (8.2 mmol/L)**, likely due to **Metformin-Associated Lactic Acidosis (MALA)** precipitated by acute kidney injury. *Diabetic ketoacidosis with renal failure from volume depletion* - **Negative urine ketones** and a relative lack of severe metabolic acidosis solely from ketones rule out classic Diabetic Ketoacidosis (DKA). - While volume depletion and renal failure are present, the absence of **ketonemia/ketonuria** points away from DKA toward an HHS-dominant pattern. *Sepsis-induced lactic acidosis with stress hyperglycemia* - Although sepsis can cause lactic acidosis, the glucose of 380 mg/dL and signs of severe dehydration are more characteristic of a **primary diabetic emergency** rather than simple stress hyperglycemia. - The patient lacks definitive localized infection signs or a classic **febrile response**, making MALA secondary to renal failure a more specific explanation for the high lactate. *Alcoholic ketoacidosis with concurrent diabetic emergency* - **Alcoholic ketoacidosis** typically presents with positive ketones and a history of chronic alcohol abuse followed by starvation, which is not indicated here. - The serum glucose in alcoholic ketoacidosis is often low or normal, unlike the **hyperglycemia** seen in this patient. *Mixed metabolic acidosis from uremia and starvation ketosis* - While the creatinine is elevated (2.8 mg/dL), the **anion gap of 24** and lactate of 8.2 suggest lactic acidosis is the dominant driver rather than **uremic toxins** alone. - **Starvation ketosis** would result in positive ketones, which are explicitly documented as negative in this case.

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