A 66-year-old man is hospitalized for elective surgical repair of an abdominal aortic aneurysm. The patient has had midabdominal and low back pain due to an enlarging infrarenal aortic aneurysm. Medical history also includes hypertension and prolonged tobacco use. Preoperative cardiopulmonary examination is normal, and blood cell counts and serum chemistry studies are within normal limits. During the surgery, prolonged clamping of the infrarenal aorta and iliac arteries is required due to complex aneurysm anatomy, but there are no other intraoperative complications. The patient's postoperative laboratory results are as follows: Over the next several hours, which of the following changes is most likely to occur in this patient in response to the acid-base imbalance?

Question

A 66-year-old man is hospitalized for elective surgical repair of an abdominal aortic aneurysm.  The patient has had midabdominal and low back pain due to an enlarging infrarenal aortic aneurysm.  Medical history also includes hypertension and prolonged tobacco use.  Preoperative cardiopulmonary examination is normal, and blood cell counts and serum chemistry studies are within normal limits.  During the surgery, prolonged clamping of the infrarenal aorta and iliac arteries is required due to complex aneurysm anatomy, but there are no other intraoperative complications.  The patient's postoperative laboratory results are as follows:  Over the next several hours, which of the following changes is most likely to occur in this patient in response to the acid-base imbalance?

Quizplus · Accepted Answer

Explanation: This patient has developed lactic acidosis (ie, metabolic acidosis) likely due to prolonged clamping of the infrarenal aorta and consequent lower body tissue ischemia. Although respiratory compensation with increased ventilation and removal of CO₂ constitutes the major compensation for metabolic acidosis, the kidneys also compensate via increased bicarbonate (HCO₃⁻) reabsorption and increased H⁺ excretion. This renal compensation is expected with any etiology of acidosis other than those driven by the kidneys (eg, renal tubular acidosis, renal failure) and involves the following: ₂ ₃ ⁻ ⁺ The majority of renal HCO₃⁻ reabsorption takes place in the proximal tubule, and some reabsorption takes place in the distal tubules. To maintain total body electronegative balance, the increased HCO₃⁻ reabsorption facilitates increased urinary chloride (Cl⁻) excretion via beta-intercalated cells in the collecting duct (Choice E). The kidneys can excrete only a limited amount of acid as free H⁺; therefore, most H⁺ is excreted in the form of ammonium (NH₄⁺) or the titratable acid, dihydrogen phosphate (H₂PO₄⁻). These compounds are part of the 2 most important urinary buffer systems (ie, NH₃/NH₄⁺, HPO₄²⁻/H₂PO₄⁻). Increased excretion of ammonium and the titratable aciddihydrogen phosphate (Choice C) allows for a net increase in serum HCO₃⁻. These renal mechanisms provide a relatively small compensatory correction for primary metabolic acidosis and, over several days, would constitute the majority of compensation for primary respiratory acidosis. (Choice A) Acidosis stimulates central respiratory drive to increase ventilation and facilitate CO₂removal. ₂ (Choice B) Carbonic anhydrase is necessary for HCO₃⁻ reabsorption in the renal proximal tubule; therefore, activity is increased in response to acidosis. ₃ ⁻ Educational objective: The kidneys play a role in the compensation for primary metabolic acidosis via increased HCO₃⁻ reabsorption and H⁺ excretion. The increased HCO₃⁻ reabsorption leads to increased urinary chloride (Cl⁻) excretion. Most of the excreted H⁺ is in the form of ammonium (NH₄⁺) or the titratable acid, dihydrogen phosphate (H₂PO₄⁻). ₃ ⁻ ⁺ ₃ ⁻ ⁻ ⁺ ₄ ⁺ ₂ ₄ ⁻ References: Roles and mechanisms of urinary buffer excretion. (http://www.ncbi.nlm.nih.gov/pubmed/3310662)

A 66-Year-Old Man Is Hospitalized for Elective Surgical Repair of an Abdominal