Acid-Base Balance Practice Questions

Q: All are useful for the treatment of metabolic alkalosis except?

Ammonium hydroxide. **Explanation:** Metabolic alkalosis is characterized by an increase in plasma bicarbonate ($HCO_3^-$) and a rise in arterial pH. Treatment focuses on reversing the underlying cause and providing chloride ions to facilitate bicarbonate excretion. **Why Ammonium Hydroxide is the Correct Answer:** Ammonium hydroxide ($NH_4OH$) is a **strong base**. Administering a base to a patient already in a state of alkalosis would worsen the condition by further increasing the pH. In contrast, **Ammonium Chloride** ($NH_4Cl$) is used to treat severe metabolic alkalosis because the liver converts it into urea and **Hydrochloric acid (HCl)**, which provides the necessary $H^+$ ions to neutralize excess bicarbonate. **Analysis of Incorrect Options:** * **Sodium Chloride (NaCl):** Most cases of metabolic alkalosis are "chloride-responsive" (e.g., due to vomiting or NG suction). Saline restores ECF volume and provides $Cl^-$, allowing the kidneys to excrete $HCO_3^-$. * **Potassium Chloride (KCl):** Alkalosis often causes hypokalemia (due to $H^+/K^+$ exchange). Replacing potassium is crucial because hypokalemia maintains alkalosis by promoting $H^+$ secretion in the distal tubule. * **Hydrochloric Acid (HCl):** In severe, life-threatening alkalosis (pH > 7.55) where saline/potassium replacement is insufficient or contraindicated, dilute HCl can be infused via a central line to directly lower the pH. **High-Yield Clinical Pearls for NEET-PG:** * **Chloride-Responsive Alkalosis:** Urinary $Cl^-$ 20 mmol/L (e.g., Conn’s syndrome, Cushing’s). Does not respond to NaCl; requires treating the mineralocorticoid excess. * **Acetazolamide:** A carbonic anhydrase inhibitor that can be used to treat metabolic alkalosis by inducing bicarbonate diuresis.

Q: Aerial blood gases obtained while the patient is receiving oxygen reveal the following values: pH 7.30; PCO2 39; bicarbonate 18 mEq/L; oxygen saturation 85%. What is the acid-base status of the patient?

Metabolic acidosis. ### Explanation To solve acid-base problems, follow a systematic three-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. A pH of **7.30** indicates **acidemia**. **2. Identify the Primary Process:** * **Metabolic:** Look at Bicarbonate ($HCO_3^-$). Normal is 22–26 mEq/L. Here, it is **18 mEq/L (Low)**. Low bicarbonate matches the acidic pH. * **Respiratory:** Look at $PCO_2$. Normal is 35–45 mmHg. Here, it is **39 mmHg (Normal)**. Since the $HCO_3^-$ is the parameter deviating in the direction of the pH, the primary diagnosis is **Metabolic Acidosis**. **3. Evaluate Compensation:** In metabolic acidosis, the body compensates by hyperventilating to lower $PCO_2$ (Winters' Formula). Here, the $PCO_2$ is 39 mmHg (within the normal range), meaning respiratory compensation has not yet occurred or is insufficient. Therefore, it is uncompensated metabolic acidosis. --- ### Why the other options are incorrect: * **Option A (Respiratory Acidosis):** This would require a high $PCO_2$ (>45 mmHg) and a low pH. * **Option B (Combined Acidosis):** This would require both a low $HCO_3^-$ and a high $PCO_2$. Here, the $PCO_2$ is normal. * **Option D (Compensated Metabolic Acidosis):** For "compensation" to be the label, the pH must return toward the normal range (7.35–7.45), and the $PCO_2$ must be significantly lower than 40 mmHg. --- ### NEET-PG High-Yield Pearls: * **Winters’ Formula:** Expected $PCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. If the measured $PCO_2$ is higher than expected, a concurrent respiratory acidosis exists. * **Anion Gap (AG):** Always calculate AG in metabolic acidosis ($Na^+ - [Cl^- + HCO_3^-]$). Normal is 12 ± 2. * **Golden Rule:** The lungs compensate for metabolic issues quickly (minutes to hours), while the kidneys compensate for respiratory issues slowly (2–3 days).

Q: Interpret the following ABG values: PaCO2-40 mmHg, HCO3-55 mEq/L, and pH-7.7?

Uncompensated metabolic alkalosis. To interpret any ABG, follow a systematic three-step approach: **1. Analyze the pH:** The normal pH range is 7.35–7.45. A pH of **7.7** indicates a profound **alkalemia**. **2. Identify the Primary Cause:** * The **HCO₃⁻ is 55 mEq/L** (Normal: 22–26 mEq/L). An elevated bicarbonate level is consistent with metabolic alkalosis. * The **PaCO₂ is 40 mmHg** (Normal: 35–45 mmHg). Since the PaCO₂ is within the normal range, there is no respiratory contribution to the alkalemia. **3. Determine Compensation:** In metabolic alkalosis, the body should compensate by hypoventilating to retain CO₂ (increasing PaCO₂). Since the PaCO₂ remains at a perfect 40 mmHg despite a high pH, **no compensation** has occurred yet. Therefore, the diagnosis is **Uncompensated Metabolic Alkalosis.** **Analysis of Incorrect Options:** * **B & C (Acidosis):** These are incorrect because the pH (7.7) is alkaline, not acidic. * **D (Compensated Respiratory Acidosis):** In respiratory acidosis, the primary change is an *increase* in PaCO₂ (>45 mmHg) with a *low* pH (<7.35). Compensation would involve the kidneys retaining HCO₃⁻ to bring the pH back toward normal. Here, the pH is high and PaCO₂ is normal. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Thumb":** For every 1 mEq/L rise in HCO₃⁻, the PaCO₂ should rise by approximately **0.7 mmHg**. * **Common Causes:** Vomiting (loss of HCl), nasogastric suction, and diuretic use (contraction alkalosis). * **Limits of Compensation:** Respiratory compensation for metabolic alkalosis is limited because hypoxia (due to hypoventilation) eventually triggers the drive to breathe, rarely allowing PaCO₂ to rise above 55 mmHg.

Q: In chronic respiratory alkalosis, HCO3- falls by how many mmol/L for every 10 mm Hg decrease in PCO2?

4. ### Explanation **1. Understanding the Concept** In respiratory alkalosis, the primary disturbance is a decrease in $PCO_2$ (hypocapnia). To maintain a normal pH, the body compensates by decreasing the concentration of $HCO_3^-$ through renal excretion. The degree of compensation differs significantly between acute and chronic stages: * **Acute Phase:** Compensation relies on immediate chemical buffering (minimal). * **Chronic Phase (2–5 days):** Compensation relies on the kidneys decreasing $H^+$ secretion and $HCO_3^-$ reabsorption. This is much more effective. For every **10 mmHg decrease in $PCO_2$**, the $HCO_3^-$ levels drop by: * **Acute:** 2 mmol/L * **Chronic:** 4 mmol/L (The "Rule of 2 and 4") **2. Analysis of Options** * **Option A (1):** Incorrect. This value is not part of the standard compensation rules for respiratory alkalosis. * **Option B (2):** Incorrect. This represents the $HCO_3^-$ drop in **acute** respiratory alkalosis. * **Option C (3):** Incorrect. While some texts suggest a range (3–5), the standard "High-Yield" rule for exams is 4 mmol/L. * **Option D (4):** **Correct.** This is the established physiological compensation rate for chronic respiratory alkalosis. **3. High-Yield Clinical Pearls for NEET-PG** * **The 1-2-4-5 Rule (for every 10 mmHg change in $PCO_2$):** * **Acute Resp. Acidosis:** $HCO_3^-$ increases by **1** * **Chronic Resp. Acidosis:** $HCO_3^-$ increases by **3.5 to 4** (often simplified to **4**) * **Acute Resp. Alkalosis:** $HCO_3^-$ decreases by **2** * **Chronic Resp. Alkalosis:** $HCO_3^-$ decreases by **4 to 5** (standard answer is **4**) * **Common Cause:** High altitude (chronic) or hyperventilation/anxiety (acute). * **Limit of Compensation:** In chronic respiratory alkalosis, $HCO_3^-$ can drop to as low as 12–15 mmol/L.

Q: All of the following are seen in persistent vomiting EXCEPT?

Decreased K+ in urine. **Explanation:** Persistent vomiting leads to a complex acid-base and electrolyte disturbance known as **Metabolic Alkalosis**. **Why "Decreased K+ in urine" is the correct answer (The Exception):** In persistent vomiting, despite the body being in a state of total body potassium depletion (hypokalemia), there is actually **increased K+ excretion in the urine** (Paradoxical Kaliuresis). This occurs due to two main reasons: 1. **Secondary Hyperaldosteronism:** Loss of fluid and Cl- in vomitus leads to volume depletion, activating the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone acts on the distal tubule to reabsorb Na+ at the expense of secreting K+ into the urine. 2. **Bicarbonaturia:** The high blood pH leads to increased filtration of HCO3-. To maintain electrical neutrality, the kidney excretes K+ along with the negatively charged bicarbonate. **Why the other options are incorrect:** * **Elevated pH & Metabolic Alkalosis:** Loss of gastric HCl (H+ ions) directly increases blood pH, leading to primary metabolic alkalosis. * **Hypokalemia:** Potassium is lost directly in the gastric juice and, more significantly, through the kidneys due to the RAAS activation mentioned above. **High-Yield Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** In severe cases, the kidney prioritizes Na+ reabsorption over H+ excretion to maintain volume. This results in acidic urine despite systemic alkalosis. * **The "Classic" Picture:** Metabolic alkalosis, Hypochloremia, Hypokalemia, and Paradoxical Aciduria. * **Treatment:** The mainstay of treatment is **Normal Saline (0.9% NaCl)** to restore volume and chloride levels, which shuts down the RAAS drive.

Q: A patient's acid-base status reveals a pH of 7.46 and pCO2 of 30 mm Hg. What is the underlying acid-base disorder?

Respiratory alkalosis. ### Explanation To solve any acid-base question, follow a systematic three-step approach: **1. Determine the pH status:** The normal arterial pH range is **7.35 – 7.45**. In this patient, the pH is **7.46**, which is >7.45. This indicates **Alkalemia** (Alkalosis). **2. Identify the primary cause (Respiratory vs. Metabolic):** * **Respiratory:** Look at $pCO_2$ (Normal: 35–45 mmHg). $CO_2$ acts as an acid. A decrease in $pCO_2$ (hypocapnia) causes alkalosis. * **Metabolic:** Look at $HCO_3^-$ (Normal: 22–26 mEq/L). In this case, the $pCO_2$ is **30 mmHg** (Low). Since a low $pCO_2$ matches the alkaline pH, the primary disorder is **Respiratory Alkalosis**. --- #### Why the other options are incorrect: * **Metabolic Acidosis:** Would present with a low pH ( 7.45) and a high $HCO_3^-$. * **Respiratory Acidosis:** Would present with a low pH ( 45 mmHg) due to hypoventilation. --- #### High-Yield Clinical Pearls for NEET-PG: * **Common Causes:** Hyperventilation (anxiety, pain), high altitude (hypoxia-induced), pulmonary embolism, and early salicylate poisoning. * **Compensation:** In acute respiratory alkalosis, for every 10 mmHg drop in $pCO_2$, the $HCO_3^-$ drops by **2 mEq/L**. In chronic cases, it drops by **4–5 mEq/L**. * **Electrolyte Shift:** Alkalosis leads to a decrease in ionized calcium (as calcium binds more to albumin), which can cause **tetany** or paresthesia despite normal total serum calcium levels.

Question 1

A 2-day-old neonate presents with lethargy and decreased interest in breastfeeding. Physical examination shows tachypnea with a normal heart rate and breath sounds. Initial blood chemistry values include normal glucose, sodium, potassium, chloride, and bicarbonate levels. Initial blood gas values reveal a pH of 7.53, partial pressure of oxygen (PO2) of 103 mmHg, and partial pressure of carbon dioxide (PCO2) of 27 mmHg. Which of the following treatment strategies is indicated?

Accepted Answer

Decrease the respiratory rate to treat respiratory alkalosis

Answer

Administer alkali to treat metabolic acidosis

Answer

Administer alkali to treat respiratory acidosis

Answer

Decrease the respiratory rate to treat metabolic acidosis

Question 2

All are useful for the treatment of metabolic alkalosis except?

Accepted Answer

Ammonium hydroxide

Answer

Sodium chloride

Answer

Potassium chloride

Answer

Hydrochloric acid

Question 3

Aerial blood gases obtained while the patient is receiving oxygen reveal the following values: pH 7.30; PCO2 39; bicarbonate 18 mEq/L; oxygen saturation 85%. What is the acid-base status of the patient?

Accepted Answer

Metabolic acidosis

Answer

Respiratory acidosis

Answer

Combined metabolic and respiratory acidosis

Answer

Compensated metabolic acidosis

Question 4

Interpret the following ABG values: PaCO2-40 mmHg, HCO3-55 mEq/L, and pH-7.7?

Accepted Answer

Uncompensated metabolic alkalosis

Answer

Mixed respiratory and metabolic acidosis

Answer

Combined respiratory and metabolic acidosis

Answer

Compensated respiratory acidosis

Question 5

In chronic respiratory alkalosis, HCO3- falls by how many mmol/L for every 10 mm Hg decrease in PCO2?

Accepted Answer

4

Answer

1

Answer

2

Answer

3

Question 6

In renal failure, metabolic acidosis is primarily due to which of the following mechanisms?

Accepted Answer

Decreased ammonia synthesis

Answer

Increased hydrogen ion production

Answer

Loss of bicarbonate

Answer

Use of diuretics

Question 7

In renal failure, metabolic acidosis is primarily due to which of the following mechanisms?

Accepted Answer

Decreased excretion of ammonia

Answer

Decreased chloride loss

Answer

Loss of bicarbonate

Answer

Use of diuretics

Question 8

All of the following are seen in persistent vomiting EXCEPT?

Accepted Answer

Decreased K+ in urine

Answer

Hypokalemia

Answer

Elevated pH of blood

Answer

Metabolic alkalosis

Question 9

A patient has a pH of 7.2, PCO2 of 12, and PO2 of 55. What is the most likely underlying cause?

Accepted Answer

Metabolic acidosis and respiratory alkalosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Question 10

A patient's acid-base status reveals a pH of 7.46 and pCO2 of 30 mm Hg. What is the underlying acid-base disorder?

Accepted Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory acidosis

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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