There are two kind of blood gas tests: one is done using blood from an artery (ABG) and one is done using blood from a vein (VBG). The ABG is used to measure oxygen. Both the ABG and VBG tests can measure carbon dioxide and acidity (pH) in the blood.
There are various ways to help remember how to look at blood gases and people will have different methods of how to interpret them. This method worked for me, but if you find a way that works for you, stick with it!
In the ED the most common blood gas we use is the VBG. Most of you will routinely take a blood sample, test it, and then take it a doctor to sign and interpret it. In this post we are looking at the basics of a VBG and the basic steps to interpreting them:
Know the normal and abnormal values when you review an VBG. They’re fairly easy to remember but if you can’t remember they are on the print out:
• For pH, the normal value is 7.35 to 7.45
• For pCO2 (Carbon Dioxide) it’s 35-45
• For HCO3 (Bicarbonate) it’s 22-26
I used to write out the values on piece of paper like in the diagram below and fill out the value to help me interpret the results. Note that the paCO2 is intentionally inverted for the purpose of this method.
Determine if the pH is under acidosis or alkalosis
The next thing to do is to determine the acidity or alkalinity of the blood through the value of pH. The pH level of a healthy human should be between 7.35 to 7.45. The human body is constantly striving to keep pH in balance.
• A pH level below 7.35 is acidosis
• A pH level above 7.45 is alkalosis
Determine if the acid base is respiratory or metabolic
After determining whether the pH is acidotic or alkelitic you need to determine is whether the acid-base is Respiratory or Metabolic.
• paCO2 = Respiratory
• HCO3 = Metabolic
I found this to be a useful way of remembering the difference between the two; the mnemonic RO-ME. Use
• When pH is up, PaCO2 is down = Alkalosis
• When pH is down, PaCO2 is up = Acidosis
• When pH is up, HCO3 is up = Alkalosis
• When pH is down, HCO3 is down = Acidosis
Determine the Compensation
The last step is to determine if the ABG is Compensated, Partially Compensated, or Uncompensated. Here’s the trick:
• If pH is NORMAL, PaCO2 and HCO3 are both ABNORMAL = Compensated
• If pH is ABNORMAL, PaCO2 and HCO3 are both ABNORMAL = Partially Compensated / mixed acidosis **
• If pH is ABNORMAL, PaCO2 or HCO3 is ABNORMAL = Uncompensated
** In some very unwell patients you can have a clinical picture of mixed acidosis or alkalosis (i.e. respiratory and metabolic acidosis / respiratory and metabolic alkalosis)
This can seem very daunting when you are looking at blood gases for the first time. The only way to get your head around it is by looking at blood gas results and using you interpretation method. The more you do, the more it will start to make sense and then you’ll have a Eureka! moment and you’ll just get it! Don’t forget, if you have a blood gas you want explaining, please ask!