Arterial Blood Gases (ABG's)

Aterial Blood Gases provide a rapid and accurate method for the analysis of acid-base disturbances.

Normal arterial pH is 7.40. pH is one of the most tightly regulated values in the body - homeostaic mechanisms are in place that maintain the pH to within very tight tolerances. Any variation of more than 0.02 in either direction is a sign of poor control. Blood is considered acidic at values below 7.38 and alkaline at values above 7.42.

The primary processes involved are acidosis and alkalosis - meaning that something is happening to cause the condition in the blood of acidemia or alkalemia. They love to pimp you on the difference between acidosis and acidemia - remember that the first is a process and the second is the resulting condition.

Acid-base disturbances are due to either problems with the respiratory system or with the metabolism (usually the kidneys in particular). When a disturbance exists with one system (for example, the lungs), the other system attempts to compensate by driving the pH back towards 7.40.



The three most important values in a blood gas are written as three numbers separated by slashes, in the following format:
pH / pCO2 / pO2
The normal values for these entities are:
7.40 / ~40 / 80-100
You'll also need to know the Bicarbonate (found in the electrolytes), and possibly the Sodium and Chloride in case you need to calculate an anion gap.

First, a review:
H20 + CO2 H2CO3 H+ + HCO3-
water + carbon dioxide carbonic acid hydrogen ion + bicarbonate

People tend to get confused on the difference between CO2 and HCO3-. Remember that the job of the lungs is to expel the gas carbon dioxide (CO2) and bring in O2, and that bicarbonate (HCO3-) is an alkaline agent (sodium bicarbonate is regularly used in the hospital to raise patients' pH).

The trick seems to be to think of CO2 as an acid. In looking at the above equation, you can see that if you were to increase CO2, (by hypoventilating, for example), you would drive the equation towards carbonic acid. Obviously, this increases the acidity of the blood (lowers the pH).

The carbonic acid dissociates into a strong acid (H+) and a weak base (HCO3-).

Similarly, if you were to hyperventilate and blow off lots of CO2, you would drive the equation to the left, using up acidic molecules, and thereby raising the pH.

So the lungs can control pH by controlling CO2 - they retain it to lower pH and make blood more acidic, and they expel CO2 to raise pH and make the blood alkaline. Those are the fundamental points to remember about respiratory disorders.

The kidneys have different mechanisms. They control the excretion of the ions on the RIGHT side of the equation - H+ and HCO3-.

Obviously, excreting H+ raises the pH (less acid - more alkaline blood) and retaining H+ lowers the pH (more acid = more acidid blood).

Similarly, excreting HCO3- lowers the pH (less base = more acidic blood), and retaining HCO3- raises the pH (more base = more alkaline blood).

That's it for the basics of metabolic disorders - the kidney modifies the concentrations of H+ and HCO3-. There are many other metabolic reasons why the blood becomes alkaline or acidic, but always keep these basics in mind.

The final thing to consider is that when one system (say the lungs) tips the balance, the other system (say the kidneys) tries to compensate. A good rule of thumb is that the second system can NEVER overcompensate. In simple acid-base disorders, you can assume that if the blood is acidic, the primary disorder is an acidosis (DON'T assume, therefore, that the primary disorder is an alkalosis that has overcompensated).

The equations listed below are designed to help determine if the compensation is adequate, and many times will help you to uncover additional acid-base disturbances that you might have missed by interpeting a blood gas too quickly.



The following calculator will help you to interpret any individual ABG. Simply enter the values into the spaces at the top - click the "Calculate" button when you're done. The algorithms used are explained below.

    pH: / pCO2: / pO2:    
    HCO3: Na+: Cl-:    

 

Six steps of ABG interpretation Your results
  1. The first step is to determine if the blood is acidemic or alkalemic. If the pH is < 7.38, then the primary process is acidosis. If the pH is > 7.42, the primary process is alkalosis.
  1. Next, we need to look at the pCO2 and the HCO3 in order to determine if the primary cause is metabolic or respiratory.
    Acidosis: pCO2 = respiratory
    HCO3 = metabolic
    Alkalosis: pCO2 = respiratory
    HCO3 = metabolic
  1. This step is only applied if the primary cause is respiratory - you will determine if the condition is acute or chronic. For each 10mm Hg that the pCO2 changes, the pH should move in the opposite direction. If the pCO2 drops, the pH should rise, and vice versa.
    The amount that the pH changes tells us whether the change is acute or chronic.
    * A change in the pH of 0.08 for each 10mm Hg indicates an ACUTE condition.
    * A change in the pH of 0.03 for each 10mm Hg indicates a CHRONIC condition.
  1. This step is only applied in the presence of metabolic acidosis - an anion gap is calculated.
    Anion Gap = Na+ - (Cl- + HCO3-)
    * A normal anion gap is < 12. If the anion gap is >12, consider one of the following eight causes:
    Methanol 
    Uremia (renal failure) 
    DKA (or other ketones) 
    Paraldehyde / PhenforminAssociated
    Iron, INH, Isopropyl alcoholwith
    Lactic acidincreased
    EtOH, Ethylene glycolosmolar
    Salicylatesgap!
  1. This step is only applied if there is an increased anion gap metabolic acidosis (if you had to do the math in step 4) - you then calculate the "Corrected HCO3-".
    Corrected HCO3- = Measured HCO3- + (anion gap - 12)

    • If corrected HCO3- < 24, a second acidosis is present in addition to the already diagnosed metabolic acidosis.
    • If corrected HCO3- > 24, there is a co-existing metabolic alkalosis in addition to the already diagnosed metabolic acidosis.
    • If corrected HCO3- = 24, then the already diagnosed metabolic acidosis is the sole problem.
  1. This final step is only applied if the primary cause is metabolic (acidosis or alkalosis) - you then calculate the "Expected pCO"2.
    Expected pCO2 = (1.5 * serum HCO3-) + 8 [+/- 2]

    • If metabolic acidodis: If measured pCO2 > expected pCO2, a respiratory acidosis is present in addition to the already diagnosed metabolic acidosis.
    • If metabolic alkalosis: If measured pCO2 > 55, a respiratory acidosis is present in addition to the already diagnosed metabolic alkalosis.
  1. This step is not normally considered part of ABG interpretation, and is only applied in the case of respiratory disorders. It can be useful to determine the "A-a Gradient" to determine the cause of a hypoxia.
    The A-a gradient (alveolar oxygen:arterial oxygen gradient) is calculated using the following formula:
    A-a Gradient = (150 - [1.25 * pCO2] ) - pO2

    A normal gradient is 10-20mm Hg, with the higher end of that range beoming more normal with increasing age.
    • An increased A-a gradient suggests an underlying pulmonary disorder that interferes with gas transfer, such as a ventilation-perfusion mismatch as seen on VQ scan. It basically means that there is significantly more oxygen in the alveoli than in the arteries, and that the lungs are not doing their job of transporting that oxygen into the blood.
    • A normal A-a gradient in the setting of hypoxemia suggests that the hypoxemia is due to hypoventilation, and can be corrected with increased ventilation.
SUMMARY OF RESULTS:



The Short Version:

More ventilation = less pCO2, alkalosis.
Hyperventilating corrects an acidosis. Hypoventilation causes one.
Think of CO2 as an acid when it's in the lungs, and as a base when it's in the blood.
For each change in pCO2 of 10, the pH should change by 0.8.
For each change in pO2 of 10, the pH should change by 0.8.

The three most important values in a blood gas are written as three numbers separated by slashes, in the following format:
pH / pCO2 / pO2
The normal values for these entities are:
7.40 / ~40 / 80-100
When they deviate from normal, use this chart to determine what the primary acid-base abnormality is likely to be. Obviously, up arrows indicate an increase in a value. Thick arrows represent the primary areas of concern, or the areas of greatest deviation.
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis