This patient'sarterial blood gas is consistent with metabolic acidosis with appropriate respiratory compensation, likely secondary to diabetic ketoacidosis.
In patients with suspected acid-base disorders, it is important to analyze the arterial blood gas systematically. If the pH is low, the patient has acidemia. The presence of a low pH and low bicarbonate signifies that metabolic acidosis is the primary process. To determine whether or not the decrease in PCO2 represents an appropriate respiratory compensation, one should employ Winters formula: PCO2 = (1.5 x HCO3-) + 8 ± 2. If PCO2 is within the range determined by the formula, it is considered to represent appropriate respiratory compensation. If the observed PCO2 is higher than that determined by the formula, there is mixed metabolic and respiratory acidosis; similarly, if the PCO2 is lower than that calculated by the formula, there is a mixed metabolic acidosis and respiratory alkalosis.
Trachtenbarg reviews the clinical presentation of diabetic ketoacidosis (DKA). The following criteria are required for the diagnosis of DKA: plasma glucose concentration > 250 mg/dL, pH < 7.30, and bicarbonate 18 mEq/L or less. Beta-hydroxybutyrate is a more specific marker of ketoacidosis than serum ketones. Therapy begins with IV insulin and fluid resuscitation. Potassium levels must be carefully monitored, as these patients tend to have depleted potassium stores despite having normal values on laboratory testing, and will become hypokalemic as the acidosis is corrected. The underlying cause of DKA, such as infection or poor medical adherence, should be corrected.
Schiraldi and Guiotto review the diagnosis of acid-base disorders. They advocate using the anion gap and "expected compensation" approach to determining the cause of an acid-base disorder, as these are both accurate and feasible at the bedside.
Illustration A depicts the nomogram of acid-base disorders, which shows the 90% confidence intervals of the normal respiratory and metabolic compensations for primary acid-base disturbances. Illustration B shows how to calculate the appropriate compensation for each of the acid-base disorders.
Answer 2: In primary respiratory acidosis, the PCO2 would be elevated, not decreased.
Answer 3: A mixed acidosis would be diagnosed if the PCO2 was higher than expected based on Winters formula.
Answer 4: Metabolic alkalosis is diagnosed when the pH is higher than 7.4 in the setting of a primary elevation of HCO3-.
Answer 5: Respiratory alkalosis is diagnosed when the pH is higher than 7.4 in the setting of a primary decrease of PCO2.
Trachtenbarg DE. Diabetic ketoacidosis. Am Fam Physician. 2005 May 1;71(9):1705-14.
PMID: 15887449 (Link to Abstract)
Schiraldi F, Guiotto G. Base excess, strong ion difference, and expected compensations: as simple as it is. Eur J Emerg Med. 2014 Feb 12.
PMID: 24526205 (Link to Abstract)