An arterial blood gas (ABG) analysis assesses a patient's acid-base status, oxygenation, and ventilation. It measures pH, partial pressures of oxygen and carbon dioxide, and calculates bicarbonate and oxygen saturation. ABG interpretation first identifies acidemia or alkalemia and then uses carbon dioxide and bicarbonate levels to determine if the primary disturbance is respiratory or metabolic, and whether compensation has occurred. Abnormal values are then evaluated in context of the patient's history and physical exam.

1. ARTERIAL BLOOD GAS
ANALYSIS (ABGS)
DR. SHILULI MATE

2. INTRODUCTION
• Arterial blood gas analysis assesses a patient’s partial pressure of oxygen
PaO2 - oxygenation status, partial pressure of PaCO2 - ventilation status
(chronic or acute respiratory failure, and is changed by hyperventilation (rapid
or deep breathing) and hypoventilation (slow or shallow breathing); and acid-
base status. Although oxygenation and ventilation can be assessed non-
invasively via pulse oximetry and end-tidal carbon dioxide monitoring,
respectively, blood gas analysis is the standard.

3. INTRODUCTION
• There are many diseases that are evaluated using an ABG which include
acute respiratory distress syndrome (ARDS), severe sepsis, septic shock,
hypovolemic shock, diabetic ketoacidosis, renal tubular acidosis, acute
respiratory failure, heart failure, cardiac arrest, asthma and inborn errors of
metabolism.

4. ABG Components:
• pH = measured acid-base balance of the blood
• PaO2 = measured the partial pressure of oxygen in arterial blood
• PaCO2 = measured the partial pressure of carbon dioxide in arterial blood
• HCO3 = calculated concentration of bicarbonate in arterial blood
• Base excess/deficit = calculated relative excess or deficit of base in arterial blood
• SaO2 = calculated arterial oxygen saturation unless a co-oximetry is obtained, in which
case it is measured

5. NORMAL RANGES
• pH (7.35-7.45)
• PaO2 (75-100 mmHg)
• PaCO2 (35-45 mmHg)
• HCO3 (22-26 meq/L)
• Base excess/deficit (-4 to +2)
• SaO2 (95-100%)

6. INTERPRETATION
i. The first step is to look at the pH and assess for the presence of acidemia (pH<7.35) or
alkalemia (pH>7.45). If the pH is in the normal range (7.35-7.45), use a pH of 7.40 as
a cutoff point.
ii. Next, evaluate the respiratory and metabolic components of the ABG results, the
PaCO2 and HCO3, respectively. The PaCO2 indicates whether the acidosis or
alkalemia is primarily from a respiratory or metabolic acidosis/alkalosis. Paco2>40
with a pH<7.4, indicates a respiratory acidosis, and <40 and pH<7.4 indicates a
respiratory alkalosis (but is often from hyperventilation from anxiety or compensation
for a metabolic acidosis).

7. CONT.…
• Next, assess for evidence of compensation for the primary acidosis or alkalosis by looking
for the value (PaCO2 or HCO3) that is not consistent with the pH.
• Lastly, assess the PaO2 for any abnormalities in oxygenation.

8. CONT.…
• Example 1]: ABG : pH = 7.39, PaCO2 = 51 mm Hg, PaO2 = 59 mm Hg, HCO3 = 30 mEq/L and SaO2 =
90%, on room air.
1. pH is in the normal range, so use 7.40 as a cutoff point, in which case it is <7.40 so acidosis is present.
2. The PaCO2 is elevated indicating a respiratory acidosis, and the HCO3 is elevated indicating a metabolic
alkalosis.
3. The value consistent with the pH is the PaCO2. Therefore, this is a primary respiratory acidosis. The
acid-base that is inconsistent with the pH is the HCO3, as it is elevated indicating a metabolic alkalosis,
so there is compensation signifying a primary disorder that is not acute because it takes days for metabolic
compensation to be effective.
4. Last, the PaO2 is decreased indicating an abnormality with oxygenation. However, a history and physical
will help delineate the severity and urgency of required interventions, if any.

9. CONT.…
• Example 2[6]: ABG : pH = 7.45, PaCO2 = 32 mm Hg, PaO2 = 138 mm Hg, HCO3 = 23
mEq/L, the base deficit = 1 mEq/L, and SaO2 is 92%, on room air.
1. pH is in the normal range. Using 7.40 as a cutoff point, it is >7.40 so alkalemia is present.
2. The PaCO2 is decreased indicating a respiratory alkalosis and the HCO3 is normal but on the
low end of normal.
3. The value consistent with the pH is the PaCO2. Therefore, this is a primary respiratory
alkalosis. The HCO3 is in the range of normal and, thus, not inconsistent with the pH, so there
is a lack of compensation.
4. Last, the PaO2 is within the normal range, so there is no abnormality in oxygenation