A 43-year-old man is brought to the emergency department after being found unresponsive by his wife. She says he was in his usual state of health when she left home a few hours ago. The patient has a history of depression and type 2 diabetes mellitus. He was recently laid off from work. Blood pressure is 100/60 mm Hg and pulse is 64/min. On physical examination, the patient is responsive to deep sternal rub and has mildly dry mucous membranes. Arterial blood gas analysis reveals the following: Which of the following is the most likely current acid-base status in this patient?

Question

A 43-year-old man is brought to the emergency department after being found unresponsive by his wife.  She says he was in his usual state of health when she left home a few hours ago.  The patient has a history of depression and type 2 diabetes mellitus.  He was recently laid off from work.  Blood pressure is 100/60 mm Hg and pulse is 64/min.  On physical examination, the patient is responsive to deep sternal rub and has mildly dry mucous membranes.  Arterial blood gas analysis reveals the following:  Which of the following is the most likely current acid-base status in this patient?

Quizplus · Accepted Answer

11ec160b_3ce5_62bf_9249_bf690e5dbbba_MD0001_00 The assessment of an arterial blood gas follows a stepwise process: Evaluate the pH (normal: 7.4): A value <7.35 signifies acidosis whereas a value >7.45 indicates alkalosis. This patient has an arterial blood pH of 7.22, which is consistent with acidosis. Identify the primary process: Differentiate respiratory from metabolic processes by determining the PaCO₂ (normal: 40) and HCO₃⁻ (normal: 24). Respiratory acidosis occurs due to the retention of CO₂ in the lungs and presents with a low pH and a high PaCO₂. Metabolic acidosis develops due to depletion of HCO₃⁻ and is characterized by a low pH and a low HCO₃⁻. This patient has a pH of 7.22 (low) and a PaCO₂ of 60 mm Hg (high), which is consistent with respiratory acidosis. Identify compensation: Because increased PaCO₂ leads to an increased hydrogen ion concentration in the body, the kidneys begin to compensate by increasing resorption of HCO₃⁻ to help buffer the excess acid. The renal compensation begins after several hours and takes several days to complete. After approximately 72 hours, the expected compensatory increase in HCO₃⁻ is approximately 4 mEq/L for every 10 mm Hg elevation in PaCO₂. This patient has a HCO₃⁻ of 24 mEq/L, which is normal and indicates respiratory acidosis without compensation. Causes of respiratory acidosis without compensation include any disorder that causes acute hypoventilation (eg, opioid overdose with depression of respiratory drive). (Choices A and E) Metabolic acidosis is indicated by low pH and low serum HCO₃⁻. The respiratory compensation involves hyperventilation to breathe off CO₂ and help increase pH. Respiratory compensation for metabolic disturbances begins right away and is usually maximized within hours; therefore, full (or nearly full) compensation is expected in both the acute and chronic setting unless the patient's breathing is compromised. (Choice B) Metabolic alkalosis is indicated by high pH and high HCO₃⁻. The respiratory compensation involves hypoventilation to retain CO₂ and decrease pH. (Choice C) This patient's low pH and high PaCO₂ are consistent with respiratory acidosis, but the normal HCO₃⁻ indicates that renal compensation has not yet occurred. (Choice D) Respiratory alkalosis is indicated by high pH and low PaCO₂. The metabolic compensation involves increased HCO₃⁻ excretion by the kidneys to help decrease pH and requires approximately 72 hours to complete. Educational objective: Respiratory acidosis presents with low pH and high PaCO₂. HCO₃⁻ initially remains near normal but becomes elevated over several days as renal compensation develops.

A 43-Year-Old Man Is Brought to the Emergency Department After