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Introduction to Arterial Blood Gases An arterial blood gas (ABG) test measures the acidity (pH), levels of oxygen, and carbon dioxide in arterial blood. ABG’s are often used in conjunction with other tests to check how well the lungs are able to move oxygen into the blood and remove carbon dioxide from the blood. Lets look at ABG values, and what the ABG measures: • pH. This value measures the hydrogen ions (H+) in the blood. The normal pH of blood is between 7.35 and 7.45. A pH of less than 7.35 is called acidotic, and a pH of greater than 7.45 is considered alkalotic. • Partial pressure of carbon dioxide (PaCO2). A measure of the pressure of carbon dioxide in the blood and how well carbon dioxide is able to move out of the body. Remember, CO2 is an acid, so the higher the pCO2, the more acidic the value is. • Bicarbonate (HCO3). Bicarbonate serves as a buffer that keeps the pH of blood from becoming too acidic. The higher the bicarbonate (HCO3) the more alkaline the value. • Oxygen saturation (O2Sat). O2 saturation measures how much of the hemoglobin in the red blood cells is carrying oxygen. • Partial pressure of oxygen (PaO2). The pressure of oxygen in the blood and how well oxygen is able to move from the airspace of the lungs into the blood. Although it is included in arterial blood gas values, oxygen values have no role in diagnosing an ABG. If oxygenation is not normal, consider supplementing with oxygen, per clinical evaluation. So, when looking at the bullets above, only three (3) items influence the blood gas diagnosis: the pH, pCO2, and HCO3. Implications of ABG An arterial blood gas (ABG) test is done to: • Assess presence of respiratory diseases. • Assess how treatment for lung disease is working • Determine if oxygen or mechanical ventilation is needed. • Determine if the right amount of oxygen is being adminisered. • Measure the acid-base level in patients with heart failure, kidney failure, uncontrolled diabetes, sleep disorders, severe infections, or after a drug overdose. Results To calculate ABG’s we use the normal parameters in yellow… Parameter Range Partial pressure of carbon dioxide (PaCO2): 35-45 mm Hg (4.6-5.9 kPa) pH: 7.35-7.45 Bicarbonate (HCO3): 22-26 mEq/L (22-26 mmol/L) Oxygen content (O2CT): 15-22 mL per 100 mL of blood (6.6-9.7 mmol/L) Oxygen saturation (O2Sat): 95%-100% (0.95-1.00) The normal values listed here are called a reference range. The normal values for children may differ from the adult values listed here. See Attachment Uncompensated or Compensated ABGs Many times, a condition that leads to an ABG diagnosis is battled by the body’s effort to help correct, or compensate. It is the body’s way to assist, to try to return the pH toward normal. Compensation rarely returns the pH to exactly normal, but it does sometimes get it closer. Compensation is either Respiratory or Metabolic, or not present (Uncompensated). Look back to the Tic Tac Toe grid. If a value is off by itself in another column, it is the compensation! You document compensation by putting the first name of the system represented. If pCO2 is in the column by itself, then it is Respiratory Compensation. If the HCO3 is in a column off by itself, then it is Metabolic Compensation. Remember, compensation takes time. Metabolic compensation is not usually seen for 6-12 hours, with maximum effect in 2-4 days. Respiratory compensation can start increased ventilation within minutes to reduce a bodily (metabolic) acidotic problem, and maximum effect can take 12-24 hours. If compensation is present but the pH is abnormal, it is considered partially compensated. If the compensation has driven the ph back to almost normal, it is considered compensated. But it is not necessary to label which type compensation when you interpret the ABG. So, for a respiratory acidosis that has evidence of compensation, your interpretation would be documented as Compensated Respiratory Acidosis. If both respiratory and metabolic systems are contributing to the same acidosis or alkalosis, it is referred to as Combined . Variables Affecting Results Variables that may affect readings include: fever or hypothermia, anemia or polycythemia, smoking prior to the test, secondhand smoke inhalation, carbon monoxide, or certain paint or varnish removers in closed or poorly ventilated areas. So now you can identify the source of the problem, the presence of compensation, and the type. Bravo!