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ABG Made Easy


Reading an ABG analysis report may be as cumbersome as drawing blood for ABG analysis. We ll help u diagnose the derangement in ABG in a simplistic way without having to depend on the ICU doctor or Consultant. Let's Get Started..!

The diagnosis is primarily divided into 3 parts.
  1. Acidosis/Alkalosis
  2. Metabolic/Respiratory
  3. Compensatory Changes

First is the pH

          <7.35-Acidosis
          >7.35-Alkalosis

Next is CO2 and HCO3

Quick Fact:

  • CO2 is the Respiratory Component
  • HCO3 is the Metabolic Component

PaCO2:

Normal Value :35-45 mmHg/ 4.7-6kPa

Remember this Golden Rule

CO2 is an acidic gas.
Its conc is increased in acidosis and decreased in alkalosis.
If the above statement is satisfied, then it is Respiratory without any compensation.
For example if pH is 7.2- acidosis, so CO2 should be raised, if it is raised, then it is Respiratory Acidosis.
If pH is 7.5-alkalosis, CO2 should be low, if it is low, then it is Respiratory alkalosis.

HCO3:

Normal value:22-28mmol/L

Here the Golden rule is
HCO3 is alkaline. So Its conc is increased in alkalosis and decreased in acidosis.
If the above statement is satisfied then it is Metabolic without any compensation.
For eg.  pH is 7.2-acidosis, HCO3 should be decreased, if it is decreased, it is Metabolic Acidosis without compensation.

Now comes the confusing part...

Compensation

If the value of CO2 or HCO3 contradicts the  Golden Rules, it implies the presence of compensatory mechanism.

Acidosis(low pH) -But CO2 is down or HCO3 is up or stay normal.
Alkalosis(high pH)-But HCO3 is down or CO2 is up or stay normal.

Examples:

  1.  pH-7.2 CO2-25 HCO3-18
pH is low-Acidosis
CO2(acid) should be high..No its not
HCO3 should be low..Yes it is-Metabolic
So it is a case of Metabolic Acidosis with Respiratory Compensation

  2.   pH-7.5 CO2-25 HCO3-12
pH is high-Alkalosis
CO2 should be low..Yes it is-Respiratory
HCO3 should be high..No its Not
So its a case of Respiratory Alkalosis with Metabolic compensation.

Hope We ve shed some light on basics of ABG..Keep Medizening..!!

Dr.Praful JK, MBBS








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