Was your answer D to Fridays CHEST Challenge question? Nice work! A 63-year-old man with a history of hypertension and peripheral vascular disease is admitted after a several day history of nausea, vomiting, and abdominal pain. An acute abdominal series reveals an ileus, which is improving on serial films following the administration of IV fluids and nasogastric suctioning. You are asked to see the patient because his abdominal pain, which had initially improved, has now worsened significantly. His temperature is 38.7_C and his BP is 87/55 mm Hg. His abdomen is tender to palpation and no bowel sounds are present. Repeat flat plate film does not show any free air or dilated loops of bowel. Immediate laboratory values are the following: sodium of 140 mEq/L (140 mmol/L), potassium of 4.5 mEq/L (4.5 mmol/L), chloride of 80 mEq/L (80 mmol/L), and bicarbonate of 25 mEq/L (25 mmol/L). Analysis of a simultaneously obtained arterial blood gas reveals a pH of 7.40, Pco2 of 40 mm Hg, and a Po2 of 100 mm Hg. What is the acid-base disorder? A. The results are internally inconsistent. B. There is no disorder present. C. There is a metabolic acidosis and a respiratory alkalosis. D. There is a metabolic acidosis and a metabolic alkalosis. This patient has a complex acid-base disorder consisting of a metabolic acidosis and a metabolic alkalosis (choice D is correct; choices A, B, and C are incorrect). Clues to a complex disorder include a normal pH, a Paco2 and HCO3 that deviate in opposite directions, or a pH change in the opposite direction to a known primary disorder. It is important to approach complex acid-base disorders in a very systematic fashion. The first step is to ensure that the data are internally accurate using the Henderson equation [H_ _ 24 Å~ Paco2/HCO3_]. The H_ concentration can be estimated from the pH. A pH of 7.40 corresponds to a H_ concentration of 40, and between 7.2 and 7.5, H_ changes by 1 mmol/L for each 0.01 change in pH. In this case, 40 _ 24 Å~ 40/25 (_ 40) and, thus, the data are internally consistent (choice A is incorrect). Once the data have been verified, the next step is to determine the primary disorder by examining the pH. On first glance at the laboratory values, there is no obvious acid-base disorder. If an acid-base disorder was present, the third step is to determine the appropriateness of any compensation in order to look for a second disorder. This is not necessary in this case because there is no obvious primary disorder, and both the bicarbonate and Paco2 levels are normal. The final step is to determine the anion gap. The anion gap in this patient is 35 (140 _ [25 _ 80] _ 35). Therefore, a severe and most likely lactic acidosis is present. An anion gap _ 30 almost always signifies lactic acidosis. Given the clinical history, this is most likely from bowel ischemia. If this were the only disorder, one would expect either a low pH or a reduced Pco2 secondary to a compensatory respiratory alkalosis, which is not seen here (choice C is incorrect). As acids are buffered by bicarbonate, each mEq/L increase in the anion gap should lead to roughly 1 mEq/L decrease in the bicarbonate. The normal bicarbonate level in this patient suggests a concomitant metabolic alkalosis masking the acidosis. This can be confirmed by comparing the change in the anion gap with the change in the bicarbonate. This is done by calculating the “delta gap” [(anion gap _ 12) _ (24 _ bicarbonate)]. A normal delta gap should be zero but can be up to 6 to allow for measurement error. If the delta gap is _ 6, there is a metabolic alkalosis present; if the delta gap reveals a negative number, there is a non-gap metabolic acidosis present. In this patient, the delta gap is (35 _ 12) _ (24 _ 24), or 23, so a metabolic alkalosis is present, most likely from the vomiting and nasogastric suctioning. Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore). 1980;59(3):161-187. Wrenn K. The delta gap: an approach to mixed acid-base disorders. Ann Emerg Med. 1990;19(11):1310-1313.
Posted on: Mon, 03 Nov 2014 14:00:30 +0000
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