ABGs

Normal values

  • pH: 7.35 to 7.45
  • pCO2: 35 to 45 mmHg
  • pO2: 80-100 mmHg
  • HCO3: 22 to 26 mEq/L
  • Base Excess: -2 to 2

Quick Acid-base status

Below is a quick and simple way to look at ABGs. This is a simplified, two-step approach to figure out whether or not an acidosis or alkalosis is related to a metabolic or a respiratory cause. Note that this method does not account for any type of compensation.

  1. First look at the pH and determine whether it is acidotic (less than 7.45), normal (7.35 to 7.45), or alkalotic (greater than 7.45).
  2. Look at the CO2 and the HCO3. For respiratory causes of the derangement, the CO2 will move in the opposite direction of the pH. For a metabolic cause, the HCO3 will be moving in the same direction as the pH.
  • Metabolic acidosis: Decreased pH, decreased HCO3-
  • Metabolic alkalosis: Increased pH, increased HCO3-
  • Respiratory acidosis: Decreased pH, increased pCO2
  • Respiratory alkalosis: Increased pH, decreased pCO2
  • ROME = Respiratory Opposite, Metabolic Equal (pH and pCO2)

Acid-base compensation

Below is a chart showing an expanded look at the above method for determining acid-base status. Using this method, you look at the pH, pCO2 and the HCO3 for each ABG analysis. Using these three values, we can determine whether or not there is some form of compensation along with the primary acid-base derangement.

Compensation comes in the form of respiratory or metabolic, depending on the primary cause. Metabolic compensation of a primarily respiratory acid-base derangement occurs with the renal system raising or lowering bicarbonate concentration. Respiratory compensation of a primarily metabolic derangement occurs with the altering of minute ventilation to adjust pCO2.

Note that the chart below does allow further interpretation of an ABG compared to the two-step method above, but this is still not all inclusive for completely interpreting an ABG.

(Work in progress!)

ConditionpHpCO2HCO3
Respiratory acidosis without compensationless than 7.35greater than 45normal
Respiratory acidosis with partial compensationless than 7.35greater than 45greater than 26
Respiratory acidosis with full compensationnormalgreater than 45greater than 26
Respiratory alkalosis without compensationgreater than 7.45less than 35normal
Respiratory alkalosis with partial compensationgreater than 7.45less than 35less than 22
Respiratory alkalosis with full compensationnormalless than 35less than 22
Metabolic acidosis without compensationless than 7.35normalless than 22
Metabolic acidosis with partial compensationless than 7.35less than 35less than 22
Metabolic acidosis with full compensationnormalless than 35less than 22
Metabolic alkalosis without compensationgreater than 7.45normalgreater than 26
Metabolic alkalosis with partial compensationgreater than 7.45greater than 45greater than 26
Metabolic alkalosis with full compensationnormalgreater than 45greater than 26