Metabolic Acidosis

The modifier, "Metabolic," just means this type of acidosis has to do with metabolism...or, the collection of the chemical changes that sustain life.

Every day carbon dioxide (CO2 mainly) and nonvolatile acids (mostly sulfuric acid from metabolizing amino acids) are produced via normal metabolism.

From CO2 comes carbonic acid and its buffer, bicarbonate (HCO3).

Oxygen combines with glucose to create energy, with carbon dioxide being the waste product of this process. CO2 is then carried from the blood to the alveoli to the lungs for expiration. We blow off this respiratory acid when ventilation is sufficient.

The body produces a buffer known as bicarbonate (HCO3) that is alkaline; HCO3 binds up with excess acids to neutralize them. When it can't bind up the excess hydrogen ions through buffering, the acids build up until the environment becomes acidic.

 

Metabolic Acidosis--Definition and Causes

Definition: Metabolic acidosis is a pathologic process in which the concentration of hydrogen ions  increases (pH <7.35) and the HCO3 concentration decreases (< 22 mEq/L).

Normal pH range is 7.35-7.45.

Normal HCO3 is 22-28 mEq/L.

Causes: 

1. Increased acid generation.
2. Lactic acidosis, ketoacidosis (diabetes, starvation, alcoholism), ingestion (poisoning from methanol, ethylene glycol, aspirin, tolene, dethylene glycol, propylene glycol.)
3. Loss of bicarbonate (diarrhea, carbonic anhydrase inhibiters).
4. Diminished renal acid excretion (chronic kidney disease, renal tubular acidosis).

 

 

The Plot Thickens

In the appropriate clinical setting, the diagnosis of metabolic acidosis may be readily apparent without ABGs (i,e., lactic acidosis with shock, hyperchloremic acidosis with diarrhea, ketoacidosis with uncontrolled type 1 diabetes, etc.). However...

Metabolic acidosis can coexist with overriding respiratory acidosis or a compensatory respiratory alkalosis, leading to a normal or even elevated HCO3.

The development of metabolic acidosis will normally generate a compensatory respiratory response.

An arterial blood gas is necessary to determine the presence of a primary respiratory disorder or to accurately determine the degree of respiratory compensation for metabolic disorders.

In all simple acid-base disorders, the primary abnormality will cause a compensatory response such that the HCO3 concentration and pCO2 will move in the same direction (either both will increase or both will decrease). These directional changes will always act to return the pH toward the normal range.

Several mathematical rules are acceptable for clinical use. They include:

• pCO2 = 1.5 x HCO3 + 8 ± 2.
• This equation was derived in children: pCO2 = HCO3 + 15.

The pCO2 should approximate the decimal digits (hundreds place) of the arterial pH.

As an example, if the pH is 7.25, then the pCO2 should be approximately 25 mmHg.

 

Signs and Symptoms of Metabolic Acidosis

• Tachycardia
• Pulmonary edema
• Tachypnea
• Confusion, and/or
• Coma

 

Management of Metabolic Acidosis

Treatment: Increase rate and depth of ventilations to attempt to blow off CO2. Consider Bicarbonate administration to bind the acid and neutralize it.

In the Field, it is not important to know anything other than a general appreciation of simple acid-base disorders. Instead of equations, ABGs, and calculations, attention should be on support for the primary cause(s) of the metabolic acidosis, such as DKA (diabetic ketoacidosis), renal failure, heart attack, diarrhea, aspirin overdose, or poisoning (such as antifreeze); that is, maintenance of ABC (airway, breathing, and circulation), IV access, and pain relief during transport to an appropriate facility.