The arterial blood gas (ABG) measures the acid-base balance (pH) and oxygenation of an arterial blood sample. An ABG can be used to assess respiratory compromise, status peri- or post-cardiopulmonary arrest, and medical conditions that cause metabolic abnormalities (such as sepsis, diabetic ketoacidosis, renal failure, toxic substance ingestion, drug overdose, trauma or burns). An ABG can also be used to evaluate the effectiveness of oxygen therapy, ventilatory support, fluid and electrolyte replacement.

CLINICAL SIGNIFICANCE 
pH
7.35-7.45
The pH tells you if your patient is acidotic or alkalotic. It is a measurement of the acid content or hydrogen ions [H+] in the blood.
Low pH indicates a higher concentration of hydrogen ions (acidosis) while a high pH indicates a lower concentration of hydrogen ions (alkalosis).

PaCO2
35-45 mm Hg
The PaCO2 level is the respiratory component of the ABG. It is a measurement of carbon dioxide (CO2) in the blood and is affected by CO2 removal in the lungs. A higher PaCO2 level indicates acidosis while a lower PaCO2 level indicates alkalosis.

HCO3-
22-26 mEq/L
The HCO3- level is the metabolic component of the ABG.
It is a measurement of the bicarbonate content of the blood and is affected by renal production of bicarbonate. A lower HCO3- level indicates acidosis while a higher HCO3- level indicates alkalosis.

PaO2
80-100 mm Hg
The PaO2 level is a measurement of the amount of oxygen dissolved in the blood. A PaO2 level less than 60% results in tissue hypoxia.

SaO2
95-100%
SaO2, or oxygen saturation, refers to the number of hemoglobin binding sites that have oxygen attached to them. How easily oxygen attaches to hemoglobin can be affected by body temperature, pH, 2,3- diphosphoglycerate levels, and CO2 levels.

ABG ANALYSIS
Steps
Clinical significance
Step 1: Analyze the pH
pH < 7.35 = acidosis pH > 7.45 = alkalosis
Determine if the pH is within the normal range, or reflects acidosis or alkalosis.

Step 2: Analyze the PaCO2
PaCO2 > 45 = acidosis PaCO2 < 35 = alkalosis
Carbon dioxide is produced in the tissues of the body and eliminated in the lungs. Changes in the PaCO2 level reflect lung function.

Step 3: Analyze the HCO3-
HCO3- < 22 = acidosis HCO3- > 26 = alkalosis
Bicarbonate is produced by the kidneys. Changes in the HCO3- level reflect metabolic function of the kidneys.

Step 4: Match the PaCO2 or HCO3- with pH
If pH < 7.35 and PaCO2 > 45 and HCO3- level is normal, the patient has
respiratory acidosis.
If pH < 7.35 and HCO3- < 22 and PaCO2 level is normal, the patient has
metabolic acidosis.
If pH > 7.45 and PaCO2 < 35 and the HCO3- level is normal, the patient has respiratory alkalosis.
If pH is > 7.45 and HCO3- > 26 and the PaCO2 level is normal, the patient has metabolic alkalosis.

Step 5: Assess for compensation by determining whether the PaCO2 or the HCO3- go in the opposite direction of the pH
When a patient has an acid-base imbalance, the respiratory and metabolic systems try to correct the imbalances the other system has produced.

If pH 7.35-7.40, PaCO2 > 45, and HCO3- > 26, the patient has
compensated respiratory acidosis.
If pH 7.35-7.40, PaCO2 <35, and
HCO3- <22, the patient has compensated metabolic acidosis.
If pH 7.40-7.45, PaCO2 <35, and
HCO3- < 22, the patient has compensated respiratory alkalosis.
If pH 7.40-7.45, PaCO2 > 45, and
HCO3- > 26, the patient has compensated metabolic alkalosis.

To compensate for respiratory acidosis, the kidneys excrete more hydrogen ions and elevate serum HCO3-, in an effort to normalize the pH.

To compensate for metabolic acidosis, the patient's respiratory center is stimulated and the patient hyperventilates to blow off more CO2, raising the pH.

To compensate for respiratory alkalosis, the metabolic system is activated to retain hydrogen ions and lower serum HCO3-, in an effort to raise the pH.

To compensate for metabolic alkalosis, the patient’s respiratory center is suppressed; decreased rate and depth of respiration causes CO2 to be retained, lowering the pH.

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