Answers to Questions on Mixed Acid-Base Disorders

from Chapter 8, All You Really Need to Know to Interpret Arterial Blood Gases

Clinical Problem 8-3.

PaCO₂ and HCO₃^- are elevated, but HCO₃^- is elevated more than would be expected from acute respiratory acidosis. Since the patient has been dyspneic for several days it is fair to assume a chronic acid-base disorder. Most likely this patient has a chronic or partially compensated respiratory acidosis. Without electrolyte data and more history, you cannot diagnose an accompanying metabolic disorder.

Clinical Problem 8-4.

Here the answer must be d, "can't be certain without more information." If an acid-base disorder is found (from blood gas, electrolyte data), the next logical step is to determine the clinical causes(s). Elevated PaCO₂, pH and HCO₃^- certainly suggest a metabolic alkalosis, but there are other possibilities. Isolated blood gas values should be viewed as a single point on a plot that can be arrived at from various pathways, and not as diagnostic of any particular acid-base disorder. Making a diagnosis of "metabolic alkalosis" solely on the basis of blood gas values has two potential pitfalls.

PITFALL 1. It suggests a final diagnosis, which is not the case. There are several causes of metabolic alkalosis and the clinical reason has to be found and corrected. Acidosis and alkalosis, with their adjectives metabolic and respiratory, are analogous to "anemia" or "fever." Acidosis and alkalosis should always be viewed as manifestations of underlying clinical problems and never as clinical diagnoses in themselves.

PITFALL 2. The patient may not have metabolic alkalosis or may have metabolic alkalosis plus another serious acid-base disorder. In fact, this patient"s initial blood gas values represent several clinical possibilities: uncomplicated metabolic alkalosis, chronic respiratory acidosis followed by acute hyperventilation (acute respiratory alkalosis), and respiratory acidosis complicated by metabolic alkalosis. For example, suppose the patient's pulmonary function tests and blood gas values were normal one week earlier and in the interval he had taken diuretics; a primary metabolic alkalosis would then be the most likely diagnosis. On the other hand, he could be a patient with chronic CO₂ retention, e.g., PaCO₂ 60 mm Hg and pH 7.41; he then develops pneumonia and hyperventilates, lowering PaCO₂ from 60 to 50 mm Hg and raising pH above normal. This last situation would reflect a state of chronic respiratory acidosis plus an acute increase in ventilation (respiratory alkalosis), not a primary metabolic alkalosis. Thus the patient could have an isolated metabolic problem, an isolated respiratory problem, or a combination. Only by a detailed clinical and laboratory history, including previous blood gas data if available, can the actual cause be determined.

After treatment for congestive heart failure, his baseline arterial blood gas values reflect a state of chronic respiratory acidosis plus a mild metabolic alkalosis. In retrospect, his blood gas values on admission were the result of acute hyperventilation on top of chronic respiratory acidosis.

Clinical Problem 8-5.

The answer is d: metabolic alkalosis plus metabolic acidosis. A patient can have both vomiting (causing metabolic alkalosis) as well as uremia (causing metabolic acidosis) at the same time. This patient has renal failure (BUN 121 mg/dl) with the diagnosis of metabolic acidosis confirmed by the elevated anion gap (25 mEq/L). Despite the AG acidosis, serum CO₂ is elevated at 40 mEq/L (bicarbonate gap is 26 mEq/L) indicating metabolic alkalosis. In this patient alkalosis is the dominant condition, hence the blood is alkalemic (pH 7.51).

From the information provided one cannot rule out a primary respiratory acidosis as an additional problem. (After this patient recovered he showed no evidence of underlying lung disease. Sometimes it requires days or weeks of follow up to fully characterize acid base disorders.)

Clinical Problem 8-6.

This patient's blood gas values suggest a state of chronic respiratory alkalosis: very low PaCO₂, slightly elevated pH. However this assessment does not indicate a specific diagnosis but only suggests possibilities. Accurate diagnosis must be made in conjunction with the clinical picture plus other laboratory studies. Could this patient have a mixed problem respiratory alkalosis plus metabolic acidosis? Her anion gap is

The anion gap is elevated and indicates a metabolic acidosis. However, the acid-base disorder is not just metabolic acidosis since the blood is alkalemic. There is good evidence she has both metabolic acidosis and respiratory alkalosis, the latter disorder from excessive mechanical ventilation. The cause of metabolic acidosis must be looked for since it is not apparent from the information provided. Since the anion gap is elevated, the possibilities include lactic acidosis from hypoperfusion and drug-induced metabolic acidosis.

Clinical Problem 8-7. The patient initially had chronic respiratory alkalosis, resulting from several days of hyperventilation, during which time her kidneys had a chance to excrete bicarbonate and return the pH toward normal. Now her asthmatic condition has worsened; she has acutely hypoventilated. The second set of blood gas values reflects acute respiratory acidosis on top of a chronic respiratory alkalosis. Although her bicarbonate is low, there is no primary metabolic process and treatment must be aimed at her respiratory disorders.

Clinical Problem 8-8. This patient has more than respiratory acidosis because the initial calculated bicarbonate is low (21 mEq/L). There is a concomitant metabolic acidosis, confirmed by an elevated anion gap. He has two causes of metabolic acidosis: shock and severe hypoxemia. After intubation he is ventilated down to a "normal" PaCO₂ of 40 mm Hg, a de facto respiratory alkalosis, yet remains acidemic because his metabolic process (lactic acidosis) has not been corrected. The last set of blood gas values still shows metabolic acidosis and inadequate respiratory compensation, or what some people would call respiratory acidosis.

NOTE: The terms "respiratory alkalosis" for his change in PaCO₂ from 70 mm Hg to 40 mm Hg, and "respiratory acidosis" when his PaCO₂ is 40 mm Hg with metabolic acidosis, are technically correct. However, as long as you understand the changes, and how they came about, it is not important how they are labeled; you could just as well use "hyperventilation" instead of "respiratory alkalosis," and "inadequate respiratory compensation" instead of "respiratory acidosis".

Clinical Problem 8-9.

a) false

b) true

c) true

d) true

e) false

f) true

g) false

h) false