Chapter 9. Fluid, Electrolyte, & Acid–Base Disorders
A 28-year-old man with a history of depression is found down and brought to the emergency department. He responds to voice, moves his extremities spontaneously, and opens his eyes to pain only. Initial vital signs: temperature 98.0°F, pulse 72 beats/min, blood pressure 118/65 mm Hg, respiratory rate 28 breaths/min, and O2 saturation 99% on a non-rebreather mask. Primary survey is within normal limits. Secondary survey reveals only superficial abrasions. Initial laboratory data include ABG: pH 7.36, pCO2 38 mm Hg, paO2 173 mm Hg, and HCO3− 20 mmol/L.
Serum salicylate 824 mg/L (normal 30-300 mg/L)
Based on this patient’s history, exam findings, and laboratory data, what is this patient’s acid–base derangement?
B. Respiratory acidosis with appropriate renal compensation.
C. Anion gap metabolic acidosis with appropriate respiratory compensation.
D. Non-anion gap metabolic acidosis with appropriate respiratory compensation.
E. Mixed metabolic acidosis and metabolic alkalosis.
The correct answer is C. Anion gap metabolic acidosis with appropriate respiratory compensation. As above, values from an arterial blood gas are typically reported in this order: pH, pCO2, paO2, and HCO3–.
This patient’s pH is normal, but near-acidotic. HCO3– and pCO2 are both low. Decreased HCO3– will cause acidosis, while decreased pCO2 will cause alkalosis, thus the patient’s primary disorder is metabolic. His calculated anion gap is 22. The pCO2 is within the 36-40 mm Hg range of appropriate compensation predicted by Winter’s formula, thus no concurrent respiratory disorder exists. Therefore, this patient has an anion gap metabolic acidosis with appropriate respiratory compensation. Respiratory compensation for metabolic disorders is rapid (note his hyperventilation), hence the normal pH.
If respiratory compensation were inappropriate, one would need to consider another, simultaneous process, in addition to alkalosis, for example, the respiratory acidosis that occurs in the later stages of salicylate poisoning, when central respiratory drive has been suppressed.
Note the patient’s ΔΔ = 2.5. Considering this finding in isolation, one would conclude that the patient has a mixed anion-gap metabolic acidosis and metabolic alkalosis. This underscores the need to take into account all of the patient’s data when making determinations of acid-base status. There is no additional evidence of a metabolic alkalosis, thus this additional disturbance is unlikely.
A 54-year-old alcoholic man is discovered to have a new gastric cancer. He reports 15-lb weight loss over the past month. Laboratory data include an albumin of 2.4 ...