Arterial Blood Gas Analysis: ABG Interpretation Made Easy

We often encounter patients who are critically sick and need monitoring more than vitals like BP and Pulse rate. Patients are often required to be checked for ABG or Arterial Blood Gas Analysis to find what exactly is happening in the body of the patient’s system. But it is often encountered that many students don’t know how to read ABG, and that’s why we are providing Arterial Blood Gas: ABG Interpretation Made Easy.

Normal Values to check in ABG

Before actually starting to interpret the ABG reports, one needs to remember the following normal values, without which, it is impossible to analyze anything. You can check all the values seen in ABG on Wikipedia.

The basic components of Arterial Blood Gas are divided as follows:

  1. pH
  2. HCO3
  3. PaCo2

pH: 7.35-7.45

More than 7.45: Alkalosis

Less than 7.35: Acidosis

PaCO2: Arterial Pressure of CO2: 35-45mm Hg

Any disturbance means Respiratory cause.

  • More than 45mm Hg: Respiratory Acidosis (could be compensatory)
  • Less than 35mm Hg: Respiratory Alkalosis (could be compensatory)

HCO3: Bicarbonate: 22-26 mmol/L

Any disturbance means Metabolic cause.

  • More than 26: Metabolic Alkalosis (could be compensatory)
  • Less than 22: Metabolic Acidosis (could be compensatory)

Approach To Interpretation of ABG

Following are the steps one should follow when reading the Arterial Blood Gas and interpreting the values simultaneously:

1. See whether pH is within the normal range or not. If not, then:

a. If Normal, the blood gas is compensated or not

b. If it is outside the range, then it is uncompensated or partially compensated

2. See whether PaCO2 is normal and:

(i) If PaCO2 is normal, then:

a. pH: Normal, then blood gas is normal

b. pH decreased, then uncompensated metabolic acidosis

c. pH increased, then uncompensated metabolic acidosis

(ii) If the PaCO2 is higher than normal, then:

a. pH decreased and HCO3 is normal, then it is uncompensated respiratory acidosis

b. pH decreased and HCO3 above normal, then it is partially compensated respiratory acidosis

c. pH is between 7.35-7.40 and the HCO3 are elevated, then it is compensated respiratory acidosis

d. pH increased and HCO3 is elevated, then it is partially compensated metabolic acidosis

We hope this was pretty easy to remember, even if you cannot remember the steps, just remember to go approach like pH>PaCO2>HCO3. It will be easy once you start practising. Once you have established the imbalance, you will be able to find out the cause behind Acidosis/Alkalosis.

Like high ABG Acidosis refers to four causes: Ketoacidosis, Lactic acid acidosis, Renal failure and Toxins. Once you have established the cause of an imbalance, you will be able to correct it in the patients.

Winter’s Formula for analyzing Metabolic Acidosis:

Calculate predicted PaCO2 using Winters Formula as below:

Predicted PaCO2 = 1.5 x [HCO3-] +/- 8

  • If the actual measured PaCO2 is lower than predicted, there is an associated respiratory alkalosis
  • If the actual measured PaCO2 is higher than predicted, there is a concomitant respiratory acidosis.

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