Acid-Base Balance Practice Questions

Q: Good measure of systemic perfusion on ABG is:

Lactate and / or the base deficit. ***Lactate and / or the base deficit*** - **Lactate** is a direct indicator of **anaerobic metabolism**, which occurs when tissue oxygen supply is insufficient to meet demand, reflecting poor systemic perfusion. - **Base deficit** (or base excess) quantifies the overall metabolic acid-base status and is sensitive to changes in unmeasured anions like lactate, making it a good marker of **tissue hypoperfusion** and metabolic acidosis. *Bicarbonate* - While bicarbonate reflects the body's primary **buffer system**, changes in bicarbonate can be influenced by both respiratory and metabolic processes and thus are not as specific a marker for systemic perfusion as lactate or base deficit. - A low bicarbonate often indicates **metabolic acidosis**, but it doesn't pinpoint the cause as precisely as lactate, which directly reflects anaerobic metabolism. *PCO2* - **PCO2** primarily reflects the **ventilatory status** and respiratory component of acid-base balance. - While extreme changes can indirectly affect perfusion (e.g., hypercapnia leading to vasodilation), it is not a direct or reliable measure of **systemic tissue perfusion**. *pH* - **pH** indicates the overall acid-base balance but is a **composite measure** influenced by both respiratory (PCO2) and metabolic (bicarbonate, lactate) factors. - It does not specifically isolate **perfusion deficits** as clearly as lactate or base deficit, which directly reflect metabolic responses to tissue hypoxia.

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

Respiratory alkalosis. ***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

Q: A young patient presents with muscle spasms, numbness in the hands and feet, seizures, and difficulty in breathing due to laryngospasm. His blood work reveals an electrolyte imbalance. What is the most likely cause of these manifestations?

Respiratory Alkalosis. ***Respiratory Alkalosis*** - **Hyperventilation** (the likely underlying cause) leads to decreased partial pressure of carbon dioxide (**PCO2**), causing an increase in pH and **respiratory alkalosis**. - This **alkalosis** decreases **ionized calcium** levels by increasing calcium binding to albumin, leading to **hypocalcemia**. - **Hypocalcemia** causes increased neuromuscular excitability, resulting in **muscle spasms, numbness** (paresthesias), **seizures**, and **laryngospasm** (difficulty breathing). - This is the classic presentation of **hypocalcemic tetany** secondary to respiratory alkalosis. *Metabolic Alkalosis* - This imbalance is primarily characterized by an increase in **bicarbonate (HCO3-)** concentration, often due to **vomiting** or diuretic use. - While it can also cause alkalosis leading to **hypocalcemia** and similar neurological symptoms, the acute and severe presentation with prominent tetany and laryngospasm is more characteristic of **respiratory alkalosis**. - Metabolic alkalosis typically has a more gradual onset. *Respiratory Acidosis* - Caused by **hypoventilation**, leading to an increase in **PCO2** and a decrease in pH (acidosis). - **Acidosis increases ionized calcium**, so this would not cause hypocalcemic symptoms. - This condition typically manifests as **somnolence, confusion**, or CNS depression, not the neuromuscular excitability seen in this patient. *Metabolic Acidosis* - Characterized by a decrease in **bicarbonate (HCO3-)** and a decrease in pH, often due to conditions like **diabetic ketoacidosis** or **renal failure**. - **Acidosis increases ionized calcium**, making hypocalcemic tetany unlikely. - Symptoms usually include **Kussmaul breathing** (compensatory hyperventilation) and potential cardiac arrhythmias, which do not match this patient's presentation of tetany and laryngospasm.

Q: Acid-base imbalance is suspected in a patient. Which of the following parameters would you use for initial determination of acid-base status?

pH, PaCO2, and Bicarbonate. ***pH, PaCO2, and Bicarbonate*** - The **pH** provides immediate assessment of overall acid-base status (acidemia if 7.45) - The **PaCO2** reflects the respiratory component - elevated in respiratory acidosis or compensated metabolic alkalosis; decreased in respiratory alkalosis or compensated metabolic acidosis - The **HCO3- (bicarbonate)** reflects the metabolic component - essential for determining whether the primary disorder is metabolic or respiratory - This triad forms the **standard approach** to arterial blood gas (ABG) interpretation taught in all major medical textbooks - Together, these three parameters allow complete initial classification of acid-base disorders using the Henderson-Hasselbalch relationship *pH and PaCO2* - While pH and PaCO2 are critical measurements, **without bicarbonate**, you cannot differentiate between metabolic and respiratory disorders or assess metabolic compensation - For example, a low pH with normal PaCO2 could indicate metabolic acidosis, but you need HCO3- to confirm this diagnosis - Incomplete for initial acid-base determination *pH, PaCO2, and Base excess* - Base excess is a **calculated parameter** used to quantify the metabolic component of acid-base disturbances - While useful, it is considered a **secondary parameter** for more detailed metabolic analysis rather than essential for initial determination - Standard ABG interpretation uses bicarbonate, not base excess, as the primary metabolic parameter *pH, PaCO2, Bicarbonate, and Base excess* - While this includes all relevant parameters, **base excess is redundant** for initial determination - Base excess adds quantitative information about metabolic component but is not required for the initial classification of acid-base status - The essential triad for initial assessment is pH, PaCO2, and HCO3-

Q: During heavy exercise, what is the primary mechanism for maintaining arterial pH despite increased lactic acid production?

Hyperventilation. ***Hyperventilation*** - **Hyperventilation** during heavy exercise increases the expulsion of **carbon dioxide (CO2)**, shifting the **bicarbonate buffer system** equilibrium to the left. - This reduction in **CO2** effectively removes **hydrogen ions (H+)**, thereby helping to maintain **arterial pH** despite rising **lactic acid** levels. *Increased bicarbonate reabsorption* - While the kidneys adapt by increasing **bicarbonate reabsorption** to compensate for acidosis, this is a **slower renal mechanism** for pH regulation, taking hours to days, rather than an immediate response during acute exercise. - The rapid pH regulation during exercise primarily relies on respiratory and chemical buffer systems, not renal function. *Phosphate buffering* - The **phosphate buffer system** is indeed important for intracellular and renal tubular fluid buffering. - However, its buffering capacity in the extracellular fluid and plasma is relatively limited compared to the **bicarbonate system** due to its lower concentration. *Increased hydrogen secretion* - **Increased hydrogen secretion** by the renal tubules is a long-term mechanism for compensating for acidosis, which helps excrete excess **acid** and regenerate **bicarbonate**. - This is a slow, renal regulatory process and not the primary rapid mechanism for maintaining pH during the immediate demands of heavy exercise.

Q: What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?

Increased bicarbonate excretion. ***Increased bicarbonate excretion*** - Prolonged vomiting leads to the loss of **gastric acid (HCl)**, causing **metabolic alkalosis**. The kidneys compensate by increasing the excretion of **bicarbonate (HCO3-)** to restore acid-base balance. - This renal compensation is the primary mechanism to eliminate the excess alkali from the body. *Increased chloride reabsorption* - In **metabolic alkalosis** due to vomiting, the body tends to reabsorb less chloride, not more, in an attempt to excrete bicarbonate. - **Chloride depletion** can actually hinder bicarbonate excretion by promoting sodium reabsorption with bicarbonate. *Increased potassium excretion* - **Hypokalemia** can occur with prolonged vomiting due to increased aldosterone activity and direct renal loss associated with metabolic alkalosis. - However, increased potassium excretion itself is not the primary mechanism for correcting the acid-base disorder; rather, it is a consequence or a contributing factor to the imbalance. *Decreased hydrogen secretion* - In response to alkalosis, the kidneys would typically decrease, not increase, **hydrogen ion (H+) secretion** in an effort to retain H+ and normalize pH. - Decreased H+ secretion is a compensatory mechanism, but the direct excretion of bicarbonate is more crucial for correcting the metabolic alkalosis.

Q: Normal anion gap is___ mmol/L?

8-16. ***8-16*** - The normal range for the **anion gap** is generally considered to be 8-16 mmol/L, reflecting the unmeasured anions in the plasma. - This range can vary slightly between laboratories, but **8-16 mmol/L** is the most commonly accepted range in clinical practice. *30-34* - This range is significantly **higher than normal** and would indicate a **high anion gap metabolic acidosis**, rather than a normal anion gap. - A high anion gap suggests an accumulation of **unmeasured acids** in the body, such as in lactic acidosis or ketoacidosis. *20-24* - This value is also **elevated** compared to the normal range, suggesting a high anion gap. - An anion gap in this range would prompt investigation into causes of **metabolic acidosis** with an increased anion gap. *0-4* - This range is significantly **lower than normal** and could indicate a **low or negative anion gap**, which is a rare finding. - A low anion gap is often associated with conditions like **hypoalbuminemia**, multiple myeloma (due to paraproteins), or severe hypernatremia.

Q: A child with diarrhea has deep & rapid respiration. Most likely diagnosis is:

Metabolic acidosis. ***Metabolic acidosis*** - Diarrhea leads to a loss of **bicarbonate** from the gastrointestinal tract, causing a decrease in blood pH [1]. - **Deep and rapid respirations** (Kussmaul breathing) are a compensatory mechanism to blow off CO2 and raise the blood pH [1], [2]. *Respiratory acidosis* - This condition results from **hypoventilation**, leading to CO2 retention and a reduced pH [2]. - Deep and rapid breathing would actually improve respiratory acidosis by expelling CO2. *Metabolic alkalosis* - Characterized by an **excess of bicarbonate** and an elevated blood pH [3]. - Compensatory mechanisms would involve decreased respiration to retain CO2, not increased [3]. *Respiratory alkalosis* - This is caused by **hyperventilation**, which leads to excessive CO2 expulsion and an elevated pH [2]. - While hyperventilation causes deep and rapid breathing, it leads to alkalosis, not acidosis (which is indicated by compensatory breathing in this scenario).

Q: Hydrogen ions are directly eliminated from the body primarily by:

Kidney. ***Kidney*** - The kidneys play a crucial role in **long-term acid-base balance** by excreting excess hydrogen ions (H+) and reabsorbing bicarbonate. - This process involves the secretion of H+ into the renal tubules, primarily by **proximal tubule cells** and **intercalated cells** of the collecting ducts. *Lungs* - The lungs eliminate carbon dioxide (CO2), which is in equilibrium with carbonic acid (H2CO3) and hydrogen ions in the blood. This provides **short-term acid-base regulation**. - While essential for pH balance, the lungs primarily control volatile acids, not directly eliminating hydrogen ions as a waste product in the same way the kidneys do. *Liver* - The liver is involved in various metabolic processes, including the metabolism of proteins and some organic acids, but it does **not directly eliminate hydrogen ions** as a primary function of acid-base regulation. - Its role in acid-base balance is indirect, such as producing urea from ammonia, which helps remove nitrogenous waste products. *Stomach* - The stomach secretes **hydrochloric acid (HCl)**, contributing to an acidic environment for digestion, but it does **not eliminate hydrogen ions** from the body to maintain systemic acid-base balance. - The secreted hydrogen ions are primarily involved in the digestive process within the gastrointestinal tract.

Question 1

Persistent vomiting causes which of the following electrolyte disturbances?

Accepted Answer

Hyponatremic hyperchloremia and paradoxical aciduria

Answer

Hypernatremia without decreased chloride and hypokalemic metabolic alkalosis

Answer

Hypokalemic metabolic alkalosis and paradoxical aciduria

Answer

Hyponatremic hyperchloremia and hypokalemic metabolic alkalosis

Question 2

Good measure of systemic perfusion on ABG is:

Accepted Answer

Lactate and / or the base deficit

Answer

Bicarbonate

Answer

PCO2

Answer

pH

Question 3

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

Accepted Answer

Respiratory alkalosis

Answer

Metabolic alkalosis

Answer

Metabolic acidosis

Answer

Respiratory acidosis

Question 4

A young patient presents with muscle spasms, numbness in the hands and feet, seizures, and difficulty in breathing due to laryngospasm. His blood work reveals an electrolyte imbalance. What is the most likely cause of these manifestations?

Accepted Answer

Respiratory Alkalosis

Answer

Metabolic Alkalosis

Answer

Respiratory Acidosis

Answer

Metabolic Acidosis

Question 5

Acid-base imbalance is suspected in a patient. Which of the following parameters would you use for initial determination of acid-base status?

Accepted Answer

pH, PaCO2, and Bicarbonate

Answer

pH, PaCO2, and Base excess

Answer

pH and PaCO2

Answer

pH, PaCO2, Bicarbonate, and Base excess

Question 6

During heavy exercise, what is the primary mechanism for maintaining arterial pH despite increased lactic acid production?

Accepted Answer

Hyperventilation

Answer

Increased bicarbonate reabsorption

Answer

Phosphate buffering

Answer

Increased hydrogen secretion

Question 7

What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?

Accepted Answer

Increased bicarbonate excretion

Answer

Increased chloride reabsorption

Answer

Increased potassium excretion

Answer

Decreased hydrogen secretion

Question 8

Normal anion gap is___ mmol/L?

Accepted Answer

8-16

Answer

30-34

Answer

20-24

Answer

0-4

Question 9

A child with diarrhea has deep & rapid respiration. Most likely diagnosis is:

Accepted Answer

Metabolic acidosis

Answer

Respiratory acidosis

Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Question 10

Hydrogen ions are directly eliminated from the body primarily by:

Accepted Answer

Kidney

Answer

Lungs

Answer

Liver

Answer

Stomach

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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