Critical Care Medicine-Renal, Electrolyte and Acid Base Disorders>>>>>Acid Base Disorders
Question 3#

An 87-year-old lady is being evaluated in the nursing home after being found to be more lethargic. She had been complaining of decreased appetite due to abdominal pain and ongoing diarrhea for the last 3 days. Her past medical history is significant for hypertension, type II diabetes mellitus, chronic back pain, and diverticulosis. Her medications include metformin, insulin, lisinopril, and naproxen. On physical examination, her:

Laboratories obtained show the following:


Which of the following is the MOST likely diagnosis in this patient?

A. Metabolic acidosis with compensation
B. Respiratory acidosis with metabolic alkalosis
C. Respiratory alkalosis with increased anion gap metabolic acidosis
D. Respiratory alkalosis with acute compensation
E. Mixed disorder

Correct Answer is E


Correct Answer: E

The patient has a mixed acid-base disorder. Her pH is alkalotic. Upon closer look, it appears that she has a respiratory alkalosis (pCO2 20 mm Hg). To determine whether this is an acute or chronic respiratory alkalosis, the following formula can be used: 1-2-4-5 rule.

In acute respiratory alkalosis, for every decrease in PaCO2 by 10, the HCO3 − decreases by 2.

In chronic respiratory alkalosis, for every decrease in PaCO2 by 10, the HCO3 − decreases by 5.

Based on that, her expected HCO3 − is approximately 14 mEq/L. However, in this case, it is 21 mEq/L which is higher than expected. This suggests that there is concurrent metabolic alkalosis present. This is also confirmed by calculating the Δ-Δ ratio—(calculated AG – expected AG)/24 – measured HCO3 −—which aids in determining the presence of any other normal AG metabolic acidosis or if this is a pure high AG metabolic acidosis.

In this case, the Δ-Δ ratio is >1, indicating that there is in fact a metabolic alkalosis present. A Δ-Δ ratio < 1 suggests a normal AG metabolic acidosis.


  1. Berend K, De Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371:1434- 1445.