Case 35 Endocrinology

Polyuria and Polydipsia

Work it one clue at a time. The diagnosis stays hidden until you reveal it.

Clue 1

18 M presents with polyuria, polydipsia, abdominal pain, and intense vomiting.

RR is 28 breaths per minute. ABG: pH is 7.29, PaCO2 is 26 mm Hg, HCO3− is 12.

What acid/base disturbance(s) is present and what is the underlying cause?

Clue 2

Measurement of the pH of blood (from ABG) is the first diagnostic step in determining the nature of an acid-base disturbance. Acidemia refers to pH < 7.35; alkalemia refers to pH > 7.45.

With a pH of 7.29, our patient is acidemic.

Clue 3

The suffix -emia refers to the acid-base state of blood (either acidic or alkaline), whereas the suffix -osis refers to system-based disorders that affect the acid-base state of blood.

Although there can only be one acid-base state of blood at a given time (either acidemia or alkalemia), multiple acid-base disorders can exist simultaneously.

Serum HCO3− (from the BMP or ABG) and PaCO2 (from the ABG) are necessary to identify the underlying disorder(s) involved in an acid-base disturbance.

Primary metabolic acidosis is characterized by the combination of acidemia and low serum HCO3−, as in this case (recall HCO3− is 12).

Clue 4

So our pt has a primary metabolic acidosis. What’s the next step?

The subtypes of metabolic acidosis are non–anion gap (non-AG) metabolic acidosis and anion gap (AG) metabolic acidosis.

We’ll need some more labs: Na+ 128 mEq/L, Cl− 82 mEq/L, HCO3− 12 mEq/L

To determine the type of metabolic acidosis (non-AG or AG), the AG must be calculated.

In this case, AG = 128 − (82 + 12) = 34 mEq/L, which is elevated (normal is ≤12 mEq/L).

Clue 5

The lungs are responsible for compensating for metabolic processes.

Using Winters’ formula, expected PaCO2 = (1.5 × 12) + 8 ± 2 = 26 ± 2 mm Hg. The PaCO2 in this case is 26 mm Hg, indicating appropriate compensation.

What is the delta anion gap (ΔAG)?

The ΔAG is the difference between the calculated AG and the upper limit of normal for AG (ie, 12 mEq/L). It should be calculated in patients with AG metabolic acidosis to determine whether the AG metabolic acidosis is pure or mixed with other metabolic disorders (ie, concomitant non-AG metabolic acidosis or concomitant metabolic alkalosis).

In this case, the ΔAG = 34 − 12 = 22 mEq/L. In a pure AG metabolic acidosis, serum HCO3− would be expected to decrease by roughly the same amount from 24 to 2 mEq/L. Using the formula (predicted serum HCO3− = 24 − ΔAG ± 5), predicted serum HCO3− = 24 − 22 ± 5 = 2 ± 5.

The serum HCO3− in this case is >7, indicating concomitant metabolic alkalosis.

What finding is present in the video at the start of this thread?

The deep and rapid breathing (Kussmaul’s respirations) is associated with DKA and reflects the compensatory hyperventilation that occurs in response to metabolic acidosis.

Our pt has a serum glucose of 600 mg/dL.

Clue 6

Putting it all together, our pt has primary anion gap metabolic acidosis (from DKA and a new diagnosis of type I DM) with appropriate respiratory compensation and concomitant metabolic alkalosis (from vomiting).

End of case.