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Acid-Base Disorders and Interpretation

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Acid-Base Disorders and Interpretation Acid-Base Disorders 2013 Stephen D. Sisson MD Objectives At the completion of this module, learners will rate their knowledge as "good" or better on the following topics in acid-base disorders: 1. The general approach to the patient with an acid-base disorder 2. Arterial versus venous blood gas interpretation 3. The differential diagnosis of metabolic acidosis a. The role of the anion gap in evaluating the patient with a metabolic acidosis b. The role of the urine anion gap in the patient with a normal anion gap metabolic acidosis 4. The role of the osmolar gap in a patient with delirium 5. The differential diagnosis of metabolic alkalosis a. The role of urine chloride in evaluating the patient with a metabolic alkalosis 6. The differential diagnosis of respiratory acidosis 7. The differential diagnosis of respiratory alkalosis Case 1: Approach to the patient with a potential acid/base disorder John Chase is a 47-year-old man with a history of diabetes and gastroparesis. He presents to the Emergency Department with an 8-hour history of nausea and vomiting typical for a flare of his gastroparesis. Past medical history is otherwise unremarkable. On physical exam, Mr. Chase looks quite uncomfortable, and vomits several times during the examination. He is afebrile, and vital signs are notable for mild orthostasis. Despite his discomfort, physical exam is only notable for the complete absence of bowel sounds. Laboratory evaluation was begun in triage, and results are now available to you: ABG: pH 7.40/pCO2 40/pO2 96 BMP: Na 128/K 3.1/Cl 83/HCO3 22/BUN 24/Cr 1.9/Glucose 556 Which ONE of the following statements is correct? A. Mr. Chase has a single acid/base disorder: metabolic acidosis. B. Mr. Chase has a single acid/base disorder: metabolic alkalosis. C. Mr. Chase has a combined metabolic acidosis and metabolic alkalosis. D. Mr. Chase has a combined metabolic and respiratory disorder. E. Serum pH, pCO2 and HCO3 are normal, thus no acid/base disorder is present. Pop Up answers A. Incorrect. While Mr. Chase has a metabolic acidosis, he has another acid/base disorder as well. B. Incorrect. While Mr. Chase has a metabolic alkalosis, he has another acid/base disorder as well. C. Correct! As we shall learn later, Mr. Chase has a metabolic acidosis and metabolic alkalosis, which counterbalance each other to the extent that pH, pCO2 and serum bicarbonate are normalized. A tip off is the presence of an elevated anion gap, which is calculated at 23. D. Incorrect. There is no evidence of a respiratory disorder. E. Incorrect. Normal pH, pCO2 and HCO3 do not exclude acid/base disorders. Summary answer The correct answer is C: Mr. Chase has a combined metabolic acidosis and metabolic alkalosis. Introduction This module presents a step-wise approach to the patient with a potential acid/base disorder. Following these steps will help to generate a differential diagnosis on a patient with a suspected acid/base disorder, setting the stage for the next steps in evaluation and management of the patient. Certain diagnoses (e.g., lactic acidosis) are potentially disastrous if missed, thus making comprehension of acid/base disorders among the essential skills of a physician. An important aspect of evaluating a patient with an acid-base disorder is to evaluate the entire clinical picture, and balance the clinical presentation with the laboratory results. In other words, the differential diagnosis generated should be interpreted in light of the clinical presentation. While this module reviews the majority of causes of acid/base disorders, rare and unusual clinical disorders should not be excluded if the presentation does not match the differential. When interpreting acid/base disorders, it is useful to try to explain the disorder in the simplest terms possible. Single acid/base disorders are more common than double acid/base disorders, which in turn are more common than triple acid/base disorders (which are rare). Also be aware that an acidemic disorder may counterbalance an alkalemic disorder, so that serum pH may be normal. A normal pH does not exclude an acid base disorder. Among the most common clinical examples in which two acid/base disorders are present yet the pH remains normal is in the presence of a coexistent metabolic acidosis and metabolic alkalosis. A tip off to this situation is an elevated serum anion gap (discussed below). Mr. Chase, the patient discussed above, likely has a metabolic alkalosis from vomiting and dehydration, and also has a metabolic acidosis, probably from diabetic ketoacidosis. Compensatory mechanisms The body will attempt to normalize acid/base equilibrium in response to an acid/base disorder. If a primary disorder results in acidosis, the body will compensate by creating an alkalosis (and vice-verse if the primary disorder is an alkalosis). If the primary disorder is respiratory, the body will compensate with a metabolic process (and vice-verse if the primary disorder is metabolic). For example, if the primary process is a respiratory acidosis, the body will attempt to compensate by creating a metabolic alkalosis. Step 1 in evaluating acid/base disorders The first step in determining whether or not an acid/base disorder is present is to look at the serum pH. Normal serum pH is 7.40 (7.35-7.45). Values lower than normal represent an acidosis; values higher than normal represent an alkalosis. This first step is very important, because the body does not overcorrect for a primary disorder, unless a second disorder is present. For example, if the primary disorder is an acidosis (respiratory or metabolic), the serum pH will be low. At this point it is useful to note that when the primary disorder is a respiratory alkalosis, the body can occasionally correct the pH to normal. Once you have determined whether the patient has an acidosis or an alkalosis, look at the pCO2 and the HCO3 to see which process (i.e. respiratory vs. metabolic) will explain the primary disorder. Normal serum bicarbonate is 24mEq/dl (plus or minus 2); normal serum pCO2 is 40 (plus or minus 5). Here is a sample case: A 41-year-old man presents with the following blood tests; define his acid/base disorder. ABG: pH 7.26/pCO2 34/pO2 96 BMP: Na 130/K 3.5/Cl 100/HCO3 18/BUN 18/Creatinine 1.9/Glucose 88 Starting with the serum pH, we see that it is lower than normal. Therefore, he has an acidosis. The next step is to determine if the acidosis is metabolic or respiratory. If the acidosis were metabolic, serum bicarbonate would be low. If the acidosis were respiratory, pCO2 would be elevated. In this patient, we see that serum bicarbonate is low, suggesting the presence of a metabolic acidosis as the explanation of the low serum pH. Venous blood gas monitoring Because arterial blood gas monitoring is painful to the patient and often difficult to obtain, some will use the venous blood gas to trend results after an initial arterial blood gas has been obtained. When initially evaluating a patient for an acid/base disorder, arterial blood gas results should be used, and should the patient change clinically, repeat arterial blood gas determinations should be obtained. Central venous blood gas samples have stronger correlations to arterial blood gas results than do peripheral blood gas samples, and therefore are preferred. 1 Venous blood has already had oxygen extracted for use by tissues, and is returning carbon dioxide in peripheral tissues to the lungs. As a result, venous blood pH is lower than arterial blood, and venous pCO2 is higher than arterial blood. Therefore, when estimating the arterial pH from a central venous blood gas, we add 0.05 to estimate the arterial pH (with peripheral venous blood samples, we add 0.03). Since venous pCO2 is higher than arterial pCO2, we subtract 5 from the venous pCO2 to estimate arterial pCO2. With peripheral venous blood samples, results are much more variable, and we subtract 3-8 from venous pCO2 to estimate arterial pCO2.1, 2 These results are summarized in Table 1. Table 1: Venous blood gas results conversion to estimate arterial blood gas results *Less reliable than central venous blood sampling Case 2: Metabolic acidosis Your next patient is Eric Murphy, a 67-year-old male brought in by ambulance after a witnessed loss of consciousness. He is accompanied by his girlfriend, who provides most of the history. Mr. Murphy has a history of an enlarged prostate, and has been complaining of worsening dysuria for the past 10 days, with fevers and malaise. Urine has been foul-smelling and cloudy during this time period. Yesterday, Mr. Murphy seemed confused and disoriented, and this morning he collapsed after standing up to get out of bed. A preliminary exam finds Mr. Murphy febrile and hypotensive, but no localizing signs are noted on exam. Laboratory examination is as follows: ABG: pH 7.20/pCO2 20/pO2 88 BMP: Na 136/K 5.0/Cl 108/HCO3 8/BUN 47/Creatinine 2.2/Glucose 70 Which ONE of the following statements is correct? A. Mr. Murphy's most significant acid/base disorder is a respiratory alkalosis. B. The differential diagnosis in this patient includes pulmonary embolus, cirrhosis, and restrictive lung disease. C. This patient has a double acid/base disorder: respiratory acidosis and metabolic acidosis. D. Mr. Murphy has a metabolic acidosis with respiratory compensation; serum lactate is likely to be elevated. E. Mr. Murphy has a metabolic alkalosis; the differential includes over-diuresis, vomiting, and renal artery stenosis Pop Up Answers A. Incorrect. Serum pH is low, demonstrating an acidosis.
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