NU2ICU V3 logoArterial Blood Gases

Understanding of Arterial Blood Gases (ABGs) can take years to become second nature and even then you will come across some that will still stump you.  Understanding ABG results is like everything else in patient care, you have to understand ‘normal’ and then acknowledge that hardly anyone is actually normal…

NOTE   you may use different units of measurement where you work so one millimeter of mercury is equal to 0.133322387415 kilopascals. This means that to convert mmHg to kPa you should multiply your figure by 0.133322387415

Normal Oxygen level

Hypoxia                        Normal                          Hyperoxia

PO2    <90mmHg       90-100mmHg              >100mmHg

SaO2 <94%                 95-100%


Like everything else from the moment you are born things tend to go downhill, including lung function, and to account for that we use this formula;

PO2 = 102mmHg minus 1/3 of patients age = this is the predicted “normal” PaO2 on room air at sea level

So if the patient is 60 years of age the estimated PaO2 on ABG would be 82mmHg (102 – 20).   But remember that’s just for living and doesn’t take into account whether they smoked or their parents smoked around them for the first 18 years of their life or prolonged exposure to dust / pollution / lead and many other complications of existence.


Acids, Hydrogen and pH

Acidic                            Normal                          Alkalotic

pH     <7.35                  7.35-7.45                       >7.45


This is the complicated part, understanding the addition and subtraction of Hydrogen Ions (H+) in the body effect the pH, that is whether the body is acidic, normal or alkalotic.


PCO2, HCO3 and Base Excess (BE)

This is how you gain or give up that Hydrogen, by breathing or waste product loss or overload and the effect it has of buffers (things that try to correct things back to normal.

Respiratory Acidosis  Normal                Respiratory Alkalosis

PCO2 >45mmHg       35-45mmHg       <35mmHg


Metabolic Acid            Normal               Metabolic Alkolosis

HCO3 <22mmol/l     22-26mmol/l     >26mmol/l

BE      >-2mmol/l       -2-+2mmol/l     >+2mmol/l



The best way to interpret ABGs is taking a systemic approach to see what fits from the above information with our result and here is a simple overview (there are many great texts available in greater detail) so starting from the top;

  • is the pH normal?               (7.35 – 7.45)
  • is the pO2?                            (90 – 100mmHg)
  • is the pCO2 normal?          (35 – 45mmHg)
  • is the HCO3 normal?         (22 – 26mmol/l)
  • match the pCO2 or HCO3 with measured pH
  • does the pCO2 or HCO3 go the opposite way to the pH?

Once you see the ABG label the results as to being acid, normal, alkaline just to remind yourself as practice.

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Let’s start;

ABG shows for a nauseous 45 year old male on 4L/PM O2;

pH          7.21                         Acid

PO2        108mmHg             Hyperoxia

PCO2     43mmHg               Normal

HCO3    16mmol/l               Acid

Respiratory system normal, patient acidotic, HCO3 acid and matched with the pH = this is uncompensated metabolic acidosis (if it was compensated the CO2 would be alkalotic) with hyperoxia


ABG reads for a distressed 39 year old female on room air;

pH        7.51                          Alkalotic

PO2      94mmHg                Normal

PCO2   30mmHg                Alkalotic

HCO3   23mmol/l              Normal

Respiratory system alkalotic, patient alkalotic, HCO3 normal and unmatched with the pH = this is uncompensated respiratory alkalosis (if it was compensated the HCO3 would be acid)


ABG reads for a low GCS 71 year old male on 1L/PM O2;

pH        7.31                          Acid

PO2      67mmHg                Hypoxic

PCO2   33mmHg               Alkalotic

HCO3   15mmol/l              Acid

Respiratory system acid (CO2 in opposite direction to pH), patient acidotic, HCO3 acid and matched with the pH = this is partially compensated metabolic acidosis (if it was fully compensated the pH would be normalised) with hypoxia


ABG reads for a vomiting 55 year old female on room air;

pH        7.51                        Alkalotic

PO2      71mmHg               Hypoxic

PCO2   43mmHg              Normal

HCO3  37mmol/l             Alkalotic

Respiratory system normal, patient alkalotic, HCO3 alkalotic and matched with the pH = this is uncompensated metabolic alkalosis (if it was compensated the CO2 would be acidotic) with hypoxia


However for staff starting in ICU your seniors will be happy if you ask them for assistance regarding ABG results and importantly if you let them know that –

  1.   the patient’s condition has changed and you did an ABG to show them
  2.   the patient’s condition has changed and you asking if they want an ABG now

Either way your observation skills based on appearance, SaO2, respiratory rate etc has led you to think “I need to do something” and that is what all treatment starts with – observation!

 Arterial (ABG) versus Venous Blood Gas (VBG)

Why do 2 different gases at the same time?

If asked for an ABG and VBG always do the 2 gas samplings within minutes of each other (however studies have shown serial lactate levels to be just as accurate in determining tissue perfusion)

  • Arterial Oxygen levels indicate external gas exchange  (ability of O2 to get from lungs into arterial blood stream = oxygen delivery)
  • Venous Oxygen levels indicate internal gas exchange (ability of O2 to get from blood stream into cells = oxygen consumption)

In this diagram you can see levels of oxygen in the blood vary by location and if arterial or venous (shown white = left venous, grey = right arterial)

Image –

 Interpretation of ScvO2 (central venous oxygen saturation)

Normal oxygen extraction is 30% corresponding to a ScvO2 > 70%  (table below is for slightly higher SvO2 )

< 70%   =  impaired tissue oxygenation

> 80%  =  too high PaO2

or it could be

  •  cytotoxic dysoxia (ie cyanide poisoning, severe sepsis etc)
  •  microcirculatory shunting (ie severe sepsis, liver failure, hyperthyroidism)
  •  left to right shunt (any blood getting from right side of heart to left side without participating in gas exchange) within heart / lungs

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Oh’s Intensive Care Manual 7th Edition 2014 by Andrew Bersten, Neil Soni

The ICU Book 4th Edition 2015 by Paul L Marino
Pilbeam’s Mechanical Ventilation: Physiological and Clinical Applications 6th Edition by J M Cairo
ACCCN’s Critical Care Nursing 2nd Edition 2012 by Doug Elliott, Leanne Aitken and Wendy Chaboyer


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