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Arterial Blood Gas

Understanding Arterial Blood Gases or “ABG’s”

What is an ABG?

The abbreviation for an Arterial Blood Gas is ABG. This is a measurement of certain components of arterial blood.

The pH is the first measurement. The pH determines the acid-base balance/reading of arterial blood. Ideally, the pH of blood would be 7.40. Multiple variables can affect the blood pH. If one of these variables causes a change in the pH that’s too far from 7.45, the cells of the body malfunction. Ultimately, the body has two main buffering systems: the renal system and the respiratory system. These two body systems typically balance each other to manage an optimum pH environment within the body. They can be visualized of as being on either end of a seesaw. When one end moves in one direction, the other moves in the opposite direction, maintaining the pH balance.

The balancing element for the respiratory system is carbon dioxide (CO2) that is formed at the cellular level and removed/exhaled by the respiratory system/lungs. The balancing characteristic of the renal system is the dissolved bicarbonate (HCO3), manufactured by the kidneys. In addition, the kidneys also assist in leveling the pH by eliminating ions of hydrogen (H+). The functioning of the two systems interacting is through the development of carbonic acid (H2CO3). Movement is fluid and constant through the carbonic acid system. The translation, is that water (H2O) combines with CO2 and forms carbonic acid. If needed, carbonic acid (H2CO3) then breaks up to form bicarbonate (HCO3) and hydrogen ions (H+). This balance functions in both directions. By this balancing of back and forth, pH balance is attained. The respiratory system/lungs balance pH by manipulating and controlling the CO2 level. An increase or decrease in the respiratory rate controls this. Deeper and faster breathing “blows off” more CO2. Conversely, shallow and slower breathing “retains” more Carbon Dioxide. The renal system balances pH by eliminating hydrogen ions (H+) and producing HCO3.

Metabolic activity in the body is reflected by the renal system. For instance, a patient who becomes hypoxic (low oxygen) will undergo anaerobic metabolism, which causes an increase in lactic acid production. The increased lactic acid will use or bind available HCO3 and be demonstrated by a lower HCO3 level. Hence, the HCO3 level on the ABG results is an indicator of metabolic (renal) acid-base balance.

Balance always must be attained by the opposite system. If a child were on one side of a seesaw and an adult on the other, we would expect the adult’s side of the seesaw to go down and the child’s side to go up. We cannot make the child go down by adding another adult to the adult’s side. Similarly, our body coordinates pH by utilizing the opposite system to balance pH. Therefore, if the pH is not in balance secondary to a respiratory disorder, the renal system then manipulates the pH of the body back to normal. Vice versa, if the renal system is malfunctioning and causing the pH disorder, the respiratory system compensates. This development is called compensation. Complete compensation returns the pH balance to normal. Complete compensation is not always possible. There are occasions when the imbalance is too great for respiratory and renal compensation to return the pH back to normal. This is termed incomplete compensation.

System causing imbalance of pH/ Compensation system
Metabolic (HCO3) Respiratory (pCO2)
Respiratory (pCO2) Metabolic (HCO3)

Let’s review:
• Arterial Blood Gases (ABG’s) measure acid-base balance in the blood
• The respiratory component is Carbon dioxide (CO2) in the acid-base balance
• The renal component is Bicarbonate (HCO3) in the acid-base balance
• CO2 and HCO3 function with carbonic acid to balance the blood pH
• The opposite system compensates for the pH imbalance and compensation attempts to bring blood pH back to normal.

An ABG can provide two sets of information. The first is the acid-base balance of the blood, and the second is blood oxygen level. The measurements of oxygen in the blood is as follows: the oxygen is called “pO2″ and the oxygen saturation is called “O2 sat”. The oxygen in the blood is measured in millimeters of mercury (mmHg). The second measurement is the O2 sat (oxygen saturation), which signifies the number of hemoglobin sites with attached oxygen. The O2 sat is indicated as a total number percentage of the total sites that have hemoglobin with oxygen attached. The oxygen saturation can be non-invasively, continually monitored with pulse oximetry.

An ABG can identify four main states of the body, other than normal. These are: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.

Six Steps to ABG Analysis

1. Step one in analyzing ABGs is to examine the pH. Normal blood pH is 7.4, plus or minus 0.05, forming the range 7.35 to 7.45. Your patient is acidic if blood pH falls below 7.35. If blood pH raises above 7.45, your patient is alkalotic.
2. Step two is to look at the pCO2. A normal pCO2 level is a 35-45mmHg. Below 35, your patient is alkalotic, above 45, your patient is acidic.
3. Step three is to examine the HCO3 level, normal HCO3 is 22-26 mEq/L. Hence, if the HCO3 is below 22, your patient is acidotic or they are alkalotic if their HCO3 if greater than 26.
4. Step four we match the pCO2 or the HCO3 with the pH to ascertain the acid-base disorder. For instance, if the pH is acidotic, and the CO2 is acidotic, then the patient has respiratory acidosis (caused by the respiratory system). Thus, if the pH is alkalotic and the HCO3 is alkalotic, your patient is in metabolic alkalosis.
5. Step five: does the CO2 or HCO3 go in the opposite direction of the pH? If it does, that system is compensating. For instance, if the pH is acidotic, the CO2 is acidotic, and the HCO3 is alkalotic, this translates into respiratory acidosis with a metabolic (renal) compensation.
6. In step six, we evaluate the PaO2 and O2 sat. If they are below normal values, this is evidence of hypoxemia in your patient.

Normal Arterial Values Range
pH 7.35-7.45
pCO2 (respiratory) 35-45
pO2(respiratory) 80-100
O2 Sat.(respiratory) 95-100%
HCO3(renal) 22-26
BE(renal) +or-2

Now let’s try an example:
pH 7.29 acidotic
pCO2 55 acidotic
pO2 55 low
O2 Sat. 85% low
HCO3 25 normal

Step 1. The pH is acidotic (normal 7.35-7.45)
Step 2. The CO2 is acidotic (normal 35-45)
Step 3. The HCO3 is normal
Step 4. The CO2 coincides with the pH, thus the imbalance is termed respiratory acidosis
Step 5. The HCO3 is normal, thus there is no compensation
Step 6. The PaO2 and O2 sat are decreased, indicating hypoxemia

The exact diagnosis for this Arterial Blood Gas is: Uncompensated respiratory acidosis. The renal system is not yet compensated for the acidosis or is unable to.