ABG Arterial Blood Gases

Arterial Blood Gas analysis typically measures:

• pH Acidity

• pCO2 Partial pressure of carbon dioxide

• pO2 Partial pressure of oxygen

• CO2 Carbon dioxide content

• BE (base excess) The loss of buffer base to neutralize acid

And may include:

• SO2 Oxygen saturation

These measurements are often used to evaluate oxygenation of the tissues and pulmonary function.(1)

pCO2  
Partial Pressure of Carbon Dioxide 

pCO2 reflects the the amount of carbon dioxide gas dissolved in the blood. Indirectly, the pCO2 reflects the exchange of this gas through the lungs to the outside air. 

Two factors each have a significant impact on the pCO2: 

1. How rapidly and deeply the individual is breathing

• Someone who is hyperventilating will "blow off" more CO2, leading to lower pCO2 levels

• Someone who is holding their breath will retain CO2, leading to increased pCO2 levels

2. The lungs capacity for freely exchanging CO2 across the alveolar membrane

• With pulmonary edema, there is an extra layer of fluid in the alveoli that interferes with the lungs' ability to get rid of CO2. This leads to a rise in pCO2.

• With an acute asthmatic attack, even though the alveoli are functioning normally, there may be enough upper and middle airway obstruction to block alveolar ventilation, leading to CO2 retention.(1)

* This leads to hyperventilation, a more important consideration than the embolized/infarcted areas of the lung that do not function properly. In cases of massive pulmonary embolism, the infarcted or non-functioning areas of the lung assume greater significance and the pCO2 may increase.

pO2
Partial Pressure of Oxygen 

pO2 reflects the amount of oxygen gas dissolved in the blood. It primarily measures the effectiveness of the lungs in pulling oxygen into the blood stream from the atmosphere.(1)

CO2
Carbon dioxide

CO2 is a measurement of all the CO2 in the blood. 

Most of this is in the form of bicarbonate (HCO3), controlled by the kidney. A small amount (5%) of the CO2 is dissolved in the blood, and in the form of soluble carbonic acid (H2CO3).

For this reason, changes in CO2 content generally reflect such metabolic issues as renal function and unusual losses (diarrhea). Respiratory disease can ultimately effect CO2 content, but only slightly and only if prolonged.(1)

BE
Base Excess or Base Deficit

Whenever there is an accumulation of metabolically-produced acids, the body attempts to neutralize those acids to maintain a constant acid-base balance.  This neutralizing is achieved by using up various "buffering" compounds in the blood stream, to bind the acids, disallowing them from contributing to more acidity.  About half of these buffering compounds come from HCO3, and the other half from plasma and red blood cell proteins and phosphates. The words "base deficit" and "base excess" are equivalent and are generally used interchangeably. The only difference is that base deficit is expressed as a positive number and base excess is expressed as a negative number. A "Base Deficit" of 10 means that 10 mEqu/L of buffer has been used up to neutralize metabolic acids (like lactic acid). Another way to say the same thing would be the "Base Excess is minus 10." (1)

SO2  
Oxygen saturation 

SO2 measures the percent of hemoglobin which is fully combined with oxygen. 

While this measurement can be obtained from an arterial or venous blood sample, it's major attractive feature is that it can be obtained non-invasively and continuously through the use of a "pulseoximeter."

Normally, oxygen saturation on room air is in excess of 95%. With deep or rapid breathing, this can be increased to 98-99%. While breathing oxygen-enriched air (40% - 100%), the oxygen saturation can be pushed to 100%.(1)