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:

• temperature is 37.8°C
• blood pressure is 118/82 mm Hg
• pulse rate is 104 beats/min
• respiratory rate is 10 breaths/min

Laboratories obtained show the following:

• Sodium 140 mEq/L (mmol/L)
• Potassium 3.4 mEq/L (mmol/L)
• Chloride 99 mEq/L (mmol/L)
• Bicarbonate 20 mEq/L (mmol/L)
• Glucose 243 mg/dL
• BUN 22 mg/dL
• Creatinine 1.6 mg/dL (baseline 1.1)
• Albumin 3.7 g/L
• Lactate 1.0 mmol/L

ABG:

• pH 7.52
• pCO₂ 20 mm Hg

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

Comment:

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.

Reference:

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