Acid-Base Regulation Part 2

A 65-year-old man had a heart attack and experiences cardiopulmonary arrest while being transported to the emergency department. Use the following laboratory values obtained from arterial blood to answer the question

Plasma pH = 7.12

Plasma P_CO2 = 60 mm Hg

Plasma HCO₃^- concentration = 19 mEq/L

Question: Which of the following options best describes his acid-base disorder?

• Respiratory acidosis with partial renal compensation

• Metabolic acidosis with partial respiratory compensation

• Mixed acidosis: combined metabolic and respiratory acidosis

• Mixed alkalosis: combined respiratory and metabolic alkalosis

Answer explanation

Rationale: C) Because the patient has a low plasma pH (normal = 7.4), he has acidosis. The fact that his plasma bicarbonate concentration is also low (normal = 24 mEq/l) indicates that he has metabolic acidosis. However, he also appears to have respiratory acidosis because his plasma is high (normal = 40 mm Hg). The rise in is due to his impaired breathing as a result of cardiopulmonary arrest. Therefore, the patient has a mixed acidosis with combined metabolic and respiratory acidosis

A 65-year-old man had a heart attack and experiences cardiopulmonary arrest while being transported to the emergency department. Use the following laboratory values obtained from arterial blood to answer the question

Plasma pH = 7.12

Plasma PCO2 = 60 mm Hg

Plasma HCO3- concentration = 19 mEq/L

In this patient, which of the following laboratory results would be expected, compared with normal?

• Increased renal excretion of bicarbonate

• Decreased urinary titratable acid

• Increased urine pH

• Increased renal excretion of ammonia (NH4 +)

Answer explanation

Rationale: D) An important compensation for respiratory acidosis is increased renal production of NH4 + and increased NH4 + excretion. In acidosis, urinary excretion of would be reduced, as would urine pH, and urinary titratable acid would be slightly increased as a compensatory response to the acidosis.

What is the most likely cause of his acidosis if the patient has the following laboratory values:

arterial pH = 7.13

plasma HC03 = 15mEq/L

plasma chloride concentration = 118 mEq/l

arterial pCO2= 28 mm Hg

plasma Na+ concentration = 141 mEq/l.

• Salicylic acid poisoning

• Diabetes mellitus

• Diarrhea

• Emphysema

Answer explanation

Rationale: C) The patient has a lower than normal pH and is therefore acidotic. Because the plasma bicarbonate concentration is also lower than normal, the patient has metabolic acidosis with respiratory compensation (i.e., is lower than normal). The plasma anion gap is in the normal range, suggesting that the metabolic acidosis is not caused by excess nonvolatile acids such as salicylic acid or ketoacids caused by diabetes mellitus. Therefore, the most likely cause of the metabolic acidosis is diarrhea, which would cause a loss of in the feces and would be associated with a normal anion gap and a hyperchloremic (increased chloride concentration) metabolic acidosis.

Using the laboratory values calculate the plasma anion gap and what it corresponds to.

Use this formula to calculate: Anion Gap = Na⁺ - (Cl⁻ + HCO₃⁻)

arterial pH = 7.13

plasma HC03 = 15mEq/L

plasma chloride concentration = 118 mEq/l

arterial pCO2= 28 mm Hg

plasma Na+ concentration = 141 mEq/l.

• 7mEq/L, normal

• 8 mEq/L, suggesting diarrhea

• 8 mEq/L, acidosis due to hypochloremia

Answer explanation

Rationale: B.) Anion Gap: 141- (118 + 15) = 8. Normal anion gap (8 to 12 mEq/L) in the presence of acidosis suggests causes like diarrhea, renal tubular acidosis, or acetazolamide use. The patient has hyperchloremia with 118 Cl concentration.

Which of the following is a cause of metabolic alkalosis?

• Diarrhea

• Chronic renal failure

• Ethylene glycol ingestion

• Treatment with acetazolamide

• Hyperaldosteronism

Answer explanation

Rationale:  A cause of metabolic alkalosis is hyperaldosteronism; increased aldosterone levels cause increased H+ secretion by the distal tubule and increased reabsorption of “new” HCO3−. Diarrhea causes loss of HCO3− from the gastrointestinal (GI) tract and acetazolamide causes loss of HCO3− in the urine, both resulting in hyperchloremic metabolic acidosis with normal anion gap. Ingestion of ethylene glycol and salicylate poisoning leads to metabolic acidosis with increased anion gap

Which set of arterial blood values describes a patient with chronic renal failure (eating a normal protein diet) and decreased urinary excretion of NH4+?

• a

• b

• c

• d

• e

Answer explanation

Rationale: E. In patients who have chronic renal failure and ingest normal amounts of protein, fixed acids will be produced from the catabolism of protein. Because the failing kidney does not produce enough NH4+ to excrete all of the fixed acid, metabolic acidosis (with respiratory compensation) results.

Which set of arterial blood values describes a patient with untreated diabetes mellitus and increased urinary excretion of NH4+?

• a

• b

• c

• d

• e

Answer explanation

Rationale:  E. Untreated diabetes mellitus results in the production of keto acids, which are fixed acids that cause metabolic acidosis. Urinary excretion of NH4+ is increased in this patient because an adaptive increase in renal NH3 synthesis has occurred in response to the metabolic acidosis.

In acidosis, kidneys respond by:

• Increasing HCO₃⁻ excretion

• Decreasing H⁺ secretion

• Reabsorbing all HCO₃⁻

• Retaining CO₂

Answer explanation

Rationale: (C) In acidosis, the kidneys help restore pH by reabsorbing all bicarbonate, increasing hydrogen ion secretion, and producing new bicarbonate. These actions counter the acid overload. Other options like excreting bicarbonate, retaining CO₂, or decreasing H⁺ secretion would worsen acidosis or involve non-renal mechanisms.

Increasing alveolar ventilation increases the blood pH because

•   it activates neural mechanisms that remove acid from the blood

• it makes hemoglobin a stronger acid.

• it increases the PO2 of the blood.

• it decreases the PCO2 in the alveoli.

• the increased muscle work of increased breathing generates more CO2.

Answer explanation

Rationale: (D) Increasing alveolar ventilation lowers alveolar PCO₂, which pulls CO₂ out of the blood. Because CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, reducing CO₂ drives the reaction left, cutting H⁺ concentration and raising pH.