Basics of ABG Analysis And Key Parameters
pH shows overall acid–base status- acidemia (<7.35) or alkalemia (>7.45). PaCO₂ reflects the respiratory component (lungs) - High CO₂ leads to respiratory acidosis, low CO₂ causes respiratory alkalosis.
HCO₃⁻ reflects the metabolic component (kidneys). Low bicarbonate indicates metabolic acidosis, high bicarbonate indicates metabolic alkalosis. PaO₂ assess oxygenation.
Test Your Knowledge with Quizzes and Exams
Stepwise ABG Interpretation Method
Use a consistent order to avoid mistakes.
First, Check pH to see if the blood is acidic or basic. Done by checking the pH levels-Normal range (7.35-7.45)
Secondly Decide whether the cause is respiratory using PaCO₂ values or metabolic by analysing HCO₃⁻ by matching which value moves in the direction that explains the pH levels analysed in step 1.
Thirdly Assess compensation where the body tries to restore pH using the opposite system that is the lung compensate for metabolic abnormalities and the Kidney compensates for respiratory abnormalities.
Finally Evaluate PaO₂ for hypoxemia and relate findings to the clinical situation.
Respiratory Disorders and Typical Patterns
Respiratory acidosis
In respiratory acidosis, PaCO₂ rises and pH drops.
Common cases include
- Hypoventilation
- COPD exacerbation
- Airway obstruction
- Oversedation.
Respiratory alkalosis
In respiratory alkalosis, PaCO₂ falls and pH rises.
Common cases include
- hyperventilation
- Anxiety
- Pain
- Early sepsis
- High altitude
Check whether compensation has started by kidneys increasing HCO₃⁻ in chronic respiratory acidosis and decrease HCO₃⁻ in chronic respiratory alkalosis.
Rapid onset problems may show minimal compensation, so symptoms and timing matter.
Metabolic Disorders and Typical Patterns
metabolic acidosis
In metabolic acidosis, HCO₃⁻ drops and pH drops
Common causes include
- DKA
- Diarrhea
- Renal failure
- Lactic acidosis
The body compensates by blowing off CO₂ by faster/deeper breathing, so PaCO₂ decreases.
metabolic alkalosis
In metabolic alkalosis, HCO₃⁻ rises and pH rises
Common causes include
- Vomiting
- NG suction
- Diuretics
Compensation occurs via hypoventilation, so PaCO₂ increases, but this is limited because the body protects oxygenation so compensation may be incomplete.
Compensation, Mixed Disorders, and Oxygenation Checks
Compensation
Compensation means the non-primary system shifts to reduce pH change, but pH may not fully normalize.
If pH is normal but both PaCO₂ and HCO₃⁻ are abnormal, suspect fully compensated disorder.
Mixed Disorder - Acidosis & Alkalosis
If both PaCO₂ and HCO₃⁻ push pH in the same direction (both causing acidosis or both causing alkalosis), suspect a mixed disorder.
Always assess PaO₂ separately: acid–base can look “okay” while the patient is dangerously hypoxemic.
Treat the patient, not just the ABG.
Core Message & Exit Exams For RN and LPN
ABG interpretation becomes easy when you identify pH, match the driver using PaCO₂ vs HCO₃⁻, confirm compensation, and always evaluate oxygenation.
Quick Review Quiz on ABG Interpretation
The following arterial blood gas results would demonstrate to the nurse that the client is experiencing which acid-base imbalance? pH-7.31 PaC02-40, HCO3-18, pa02-92.
a. Metabolic acidosis
b. Metabolic alkalosis
c.Respiratory acidosis
d.Respiratory alkalosis
A nurse is reviewing the arterial blood gas (ABG) results of a client. The client's ABGs are:
pH 7.6
PaC02 40 mm Hg
HC03 32 mEq/L
Which of the following acid-base conditions should the nurse identify the client is experiencing?
A nurse is reviewing the arterial blood gas (ABG) results of a client who the provider suspects has metabolic acidosis. Which of the following results should the nurse expect to see?
PaCO2 above 45 mm Hg
HC03 above 26 mEq/L
Pao, below 70 mm Hg
pH below 7.35
Discussion