Learning Compensated And Poorly Compensated Metabolic/ Respiratory Acidosis

While covering the ER, a 62-year-old female you have known for several years presents with her husband. Your patient appears very lethargic and is unable to give a coherent history. Her husband tells you that she began having stomach pain, nausea, and diarrhea 2 days ago. Although she has not been vomiting, she has been unable to drink or eat much due to nausea.
She takes furosemide for edema and albuterol/ipratropium for COPD.
She smokes a pack of cigarettes per day.
On physical examination, her respiratory rate is 30, pulse 104, blood pressure 112/64, and temperature 37.9◦C.
She is lethargic and disoriented.
Oral mucosa is dry.
Her lungs show diminished air movement bilaterally.
Her abdomen is diffusely tender, but there is no rebound.
Rectal exam is negative for occult blood.
The first laboratory test you have available is a room air arterial blood gas:

•  pH 7.12, 
• PaCO2 33 mm Hg,
• PaO2 80 mm Hg,
• HCO3 10 mEq/L, and 
• oxygen saturation 92%. 

This blood gas is most consistent with which of the following processes?

A) Compensated metabolic acidosis.
B) Compensated respiratory acidosis.
C) Poorly compensated metabolic acidosis.
D) Poorly compensated respiratory acidosis.

Answer And Discussion:

The correct answer is “C.”Poorly compensated metabolic acidosis.

 This patient is clearly acidotic, as her pH is well below the normal range of 7.35–7.45. So, obviously, whatever she has, it will be poorly compensated, ruling out “A” and “B.”
 Based on the bicarbonate (HCO3) level and the history of gastrointestinal losses due to diarrhea, you would suspect a metabolic acidosis.
 In order to have appropriate respiratory compensation, the PaCO2 should fall 12 points for every 10 point drop in the HCO3 below the normal level (around 24 mEq/L). In this case, the HCO3 is 10 mEq/L (14 points below normal); therefore, the PaCO2 is expected to drop by about (1.2 × 14 = 16.8) or approximately 17. However, the PaCO2 is not 23 mm Hg; it is 33
mm Hg (close to the normal range of 35–45). The patient’s PaCO2 is too high to appropriately compensate for her metabolic acidosis, and she thus has a poorly compensated metabolic acidosis.

There is another way to do this:
The pH should change by 0.8 for every 10 change in CO2. So, if a patient’s CO2 is 50, the pH should be 7.32 if it is an uncompensated respiratory acidosis. If they are more acidotic (e.g., 7.24), they have a mixed respiratory and metabolic acidosis. If they are less acidotic (e.g., 7.39), they have a compensated respiratory acidosis.