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  • Dr Stella E. Okereke-Nwokeji

How to interpret acid blood gases (ABGs)

An arterial blood gas (ABG) test measures the acid and carbon dioxide in the blood from an artery. It checks how well your lungs are able to take in oxygen as blood passes through the lungs and remove carbon dioxide from the blood.

Before oxygen and carbon dioxide enter the body tissue, an ABG test is drawn from an artery.  

What does an ABG measure?

  • PH (7.36 -7.45): This is a numeric scale use to show how acidity or alkalotic the blood is. It measures how carbon dioxide is able to move out of the body. A PH of 7 is neural (neither acidic nor basic). A pH of less than 7.0 is acid and a pH higher than 7.0 is basic.

  • Partial Pressure O2 (80%-100%): This measures the oxygen dissolved in the blood and how well oxygen is able to move from the lungs into the blood.

  • Partial Pressure CO2 (35 – 45mEq/L): This measures the carbon dioxide dissolved in the blood and how well carbon dioxide is able to move out of the body.

  • Bicarbonate ((HCO3 (24-25mEq/L)): Bicarbonate is a chemical buffer that keeps the pH of blood from becoming too acidic or too basic.

  • Oxygen Saturation ((O2Sat (<95%)): This is measured as a percentage of the amount of hemoglobin which are oxygenated in the blood.

Four steps in ABGs interpretation

STEP 1

Is it acidosis or Alkalosis?

Use the PH to determine this The PH determines if the blood is Acidic (Acidosis) or basic (Alkalosis).

<7.35 (Acidosis)

7.35-7.45 (Normal or compensated)

<7.45 (Alkalosis)

STEP 2

What effect does it have on respiration?

Use the PaCo2 to determination the respiratory effect. PaCo2 measures how well carbon dioxide is able to move out of the body.

<35 (Lends toward alkalosis; Causes high PH and neutralizes low PH)

35-45 (Normal or compensated)

>35 (Lends toward acidosis; Causes low PH and neutralizes high PH)

STEP 3

If it not respiratory (ruled out), could it be metabolic?

Alkalosis (High PH) – (High PaCo2 – Metabolic) (Low PaCo2 – Respiratory)

Acidosis (Low PH) – (High) PaCo2 – Respiratory) (Low PaCo2 – Metabolic)

PaCo2 is abnormal; PH is normal - Compensation

PH > 7.4 - Compensated alkalosis

PH < 7.4 - Compensated acidosis

STEP 4

Are you able to verify metabolic effect using HCO3?

Normal HCO3 (24-25mEq/L): Use this value to match what you have and verify metabolic effect.


Credit:npclinicians.com