A 58-year old man was admitted because of pontine hemorrahge. He developed repeated vomiting due to paralytic ileus because of sepsis from pneumonia. He was clinically dehydrated. Serum pH was 7.512, BE 7.7, Na 149, K 2.7, Ur 13.2, Cr 113, urine pH was 6.0. Which of the following is correct?
A. Loss of gastric acid results in loss of choloride and potassium, and this scenario is likely a hypochloremic, hypokalemic metabolic alkalosis with hypovolemia.
B. There must be a pre-existing undiagnosed renal problem because normally the body should not produce an acidic urine in face of metabolic alkalosis.
C. The renin-angiotensin-aldosterone axis is activated, and the Na/K pump is favored over the Na/H pump when Na is reabsorbed.
D. Replacement of potassium alone will rectify the whole situation and correct the alkalosis
Answer: A
This situation arises from the primary loss of gastric acid from repeated vomiting or drainage resulting in loss of chloride and potassium ions, as well as volume. Despite the metabolic alkalosis, the body still produces an acidic urine. The specific situation in this scenario is called "paradoxical aciduria".
The mechanisms of paradoxical aciduria are:
1. Nasogastric suction or refractory vomiting results in loss of gastric acid.
2. Physiologic stress and hypovolemia promote renal retention of sodium and water.
3. To retain sodium, the renin-angiotensin-aldosterone is activated, the kidney must release other cations (potassium and hydrogen) for exchange with Na
4. The body tries to maintain adequate potassium level as the priority, so instead of using Na/K pump in the distal tubule which would result in further loss of K, the Na/H pump is favored. Sodium is then reabsorbed as hydrogen ions are excreted, making the urine acidic. Therefore this situation aggravates the state of metabolic alkalosis.
To rectify the whole situation, the patient must be volume-resuscitated by isotonic saline to decrease the activity of the renin-angiotensin-aldosterone axis. Supplemental KCl is administered to restore the potassium concentration to normal. An indication that a patient has been fully resuscitated is that the urine pH becomes appropriately alkaline. In rare situations of very severe alkalemia, carbonic anhydrase inhibitor can be given to reduce the renal tubule’s capacity to generate protons for transport to urine. Surgical correction is required if there is surgical obstruction of the GI tract.