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ABG Made Easy

What is the acid-base balance?

The normal ratio of 1 acid for 20 bases in the blood chemistry. The most important acids are hydrogen (H+) and carbonic acid (pCO2) or dissolved CO2. The most important base is bicarbonate (HCO3).

Normal Ranges of ABG

pH 7.35 - 7.45
pCO2 35 - 45
HCO3 22 - 26

What is PaO2?

Partial pressure of O2 in arterial blood, measured in mmHg.
Only able to measure with an arterial blood draw.

What is SPO2?

Oxygen saturation in Hemoglobin expressed as a percentage and measured using a pulse oximeter.

If SPO2 is 90%, what is the approximate PaO2?

60 mmHg or higher

If SPO2 is 85%, what is the patient's condition?

Respiratory Distress

ABG of Acute Respiratory Failure

Also defined as " ventilatory failure, oxygen failure or a combination of both." Whatever the cause, the patient is always hypoxemic.
- PaO2 less than 60 mm Hg
- Oxygen Saturation (SPO2) less than 90%
- Partial pressure of CO2 more than 50 mm Hg with acidemia (pH < 7.30)

Ventilatory Failure

Primarily the inability to move air.
Defined as the inability of the respiratory system to function effectively as a pump normally able to oxygenate arterial blood and eliminate carbon dioxide from the body.
Characterized by a V-Q mismatch - perfusion may be normal but ventilation is inadequate.
Happens with drug overdose and neuromuscular disease.

Oxygen Failure

Primarily the inability to take up oxygen.
Air movement is normal, but lung blood flow is decreased. Happens in CHF, PE, abnormal hemoglobin, hypovolemic shock, cardiac abnormalities.

Combined Ventilatory and Oxygen Failure

Happens in COPD (emphysema, asthma, chronic broncitis)

Location to draw blood gases -

Adult - radial artery
Infant - femoral artery
Get ABG from capillaries in infant unless they must come from an artery.


Blood pH is < 7.35


Blood pH is > 7.45

What does the respiratory system do to promote acid-base balance?

Increased respirations blow off CO2, which increases pH by reducing the amount of Carbonic Acid in the blood. Reduced respirations have the opposite effect.

What does the renal system do to promote acid-base balance?

Kidneys create or reabsorbed bicarbonate (HCO3) which is a buffer and excretes hydrogen ions (H+) via the ammonia in urine.


An attempt by either the respiratory system to resolve a metabolic problem or the renal system to resolve a respiratory problem with the goal of avoiding acid-base imbalance.

How to interpret ABG results

1st determine if pH is normal, if so you are done
If pH is not normal, check pCO2 next.
If pCO2 is not normal, the imbalance is respiratory
otherwise, the imbalance is metabolic.

Respiratory Acidosis

Caused by hypoventilation - Increase in CO2 with build up of Carbonic Acid in the blood
Compensation - kidneys conserve bicarbonate and increase hydrogen concentration in urine.

Respiratory Acidosis Symptoms

Easy fatigue
Shortness of breath

Respiratory Acidosis Nursing Interventions

Oxygen Administration
Promote Respirations
- HOB elevated
- Fan
- Respiratory Treatments
- Administer Medications as per order
- Report Changes in condition to physician

Respiratory Alkalosis

A carbonic acid deficit caused by hyperventilation due to anxiety, CNS disorders, sepsis, mechanical overventilation

Respiratory Alkalosis Symptoms

light headedness
tingling or numbing around the mouth and in the fingers and hands

Respiratory Alkalosis Nursing Interventions

- Paper Bag for Hyperventilation or Oxygen Mask without O2 flowing
- Investigate cause
- Administer meds as ordered to treat problem

Metabolic Acidosis

When an acid other than carbonic acid accumulates in the body or when bicarbonate is lost from body fluids (i.e. diarrhea)
Example of acids that can build up:
ketones - ketoacidosis (DKA)
lactic acid - lactic acidosis from shock

Metabolic Acidosis Symptoms

Kussmaul respirations - increased respirations as compensation to blow off CO2
Fruity or Acetone smelling breath
Urine pH is low since kidneys excrete hydrogen

Metabolic Acidosis Nursing Interventions

- Treat Condition and Cause
- Administer fluids
- Monitor V/S frequently and report changes in condition

Metabolic Alkalosis

Bicarbonate excess or loss of acid from prolonged vomiting or gastric suctioning. May also be caused by ingestion of baking soda or inadvertant administration of sodium bicarbonate.

Metabolic Alkalosis Symptoms

- reduced respiratory rate to retain CO2
- high pH in urine since kidneys will excrete bicarbonate.

Metabolic Alkalosis Nursing Interventions

- Treat Condition and Cause
- Monitor V/S frequently and report changes in condition

When interpreting ABG, what clues would tell you that there is something wrong but the body is compensating thus preventing an imbalance?

If the pH is near but within the margins for normal and the pCO2 and HCO3 are out of range. The parameter with the matching imbalance is the primary problem. The other is due to compensation.

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