Components of the Arterial Blood Gas The arterial blood gas provides the following values: pH Measurement of acidity or alka...
Components of the Arterial Blood Gas
The arterial blood gas provides the following values:
pH
Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present.
The normal range is 7.35 to 7.45
PaO2
The partial pressure of oxygen that is dissolved in arterial blood.
The normal range is 80 to 100 mm Hg.
SaO2
The arterial oxygen saturation.
The normal range is 95% to 100%.
PaCO2
The amount of carbon dioxide dissolved in arterial blood.
The normal range is 35 to 45 mm Hg.
HCO3
The calculated value of the amount of bicarbonate in the bloodstream.
The normal range is 22 to 26 mEq/liter
B.E.
The base excess indicates the amount of excess or insufficient level of bicarbonate in the system.
The normal range is –2 to +2 mEq/liter.
(A negative base excess indicates a base deficit in the blood.)
Respiratory Acidosis
Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg.
Acidosis is caused by an accumulation of CO2 which combines with water in the body to produce carbonic acid, thus, lowering the pH of the blood. Any condition that results in hypoventilation can cause respiratory acidosis.
These conditions include:
• Central nervous system depression related to head injury
• Central nervous system depression related to medications such as narcotics, sedatives, or anesthesia
• Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs
• Pulmonary disorders such as atelectasis, pneumonia,pneumothorax, pulmonary edema, or bronchial obstruction
• Massive pulmonary embolus
• Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension
Respiratory Alkalosis
Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg. Any condition that causes hyperventilation can result in respiratory alkalosis.
These conditionsinclude:
• Psychological responses, such as anxiety or fear
• Pain
• Increased metabolic demands, such as fever, sepsis, pregnancy,or thyrotoxicosis
• Medications, such as respiratory stimulants.
• Central nervous system lesions
Metabolic Acidosis
Metabolic acidosis is defined as a bicarbonate level of less than 22 mEq/L with a pH of less than 7.35. Metabolic acidosis is caused by either a deficit of base in the bloodstream or an excess of acids, other than CO2. Diarrhea and intestinal fistulas may cause decreased levels of base.
Causes of increased acids include:
• Renal failure
• Diabetic ketoacidosis
• Anaerobic metabolism
• Starvation
• Salicylate intoxication
Metabolic Alkalosis
Metabolic alkalosis is defined as a bicarbonate level greater than 26 mEq/liter with a pH greater than 7.45. Either an excess of base or a loss of acid within the body can cause metabolic alkalosis. Excess base occurs from ingestion of antacids, excess use of bicarbonate, or use of lactate in dialysis. Loss of acids can occur secondary to protracted vomiting, gastric suction, hypochloremia, excess administration of diuretics, or high levels of aldosterone.
Steps to an Arterial Blood Gas Interpretation
Step One
Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. If it is above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic.
Step Two
If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a respiratory or metabolic problem. To do this, assess the PaCO2 level. Remember that with a respiratory problem, as the pH decreases below 7.35, the PaCO2 should rise. If the pH rises above 7.45, the PaCO2 should fall. Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions, then the problem is primarily respiratory in nature.
Step Three
Finally, assess the HCO3 value. Recall that with a metabolic problem, normally as the pH increases, the HCO3 should also increase. Likewise, as the pH decreases, so should the HCO3.
Compare the two values. If they are moving in the same direction, then the problem is primarily metabolic in nature. The following chart summarizes the relationships between pH, PaCO2 and HCO3.
