Arterial Blood Gas (ABG) Analysis and Acid-Base Balance

What does an Arterial Blood Gas measure?

Checks balance of acids and bases

Measures the level of oxygen and carbon dioxide in the blood

What values are measured in an ABG?

pH, PaCO2, PaO2, SaO2, HCO3-

Why is PH checked in an ABG?

- Hydrogen levels affect enzyme activity

- Most pathological conditions disturb acid-base balance

How does the body maintain pH range of 7.35-7.45?

- Buffers help regulate pH, keeping it within normal limits, until acids can be excreted

- Respiratory & Renal system compensation

What does PaO2 measure?

- Measurement of the amount of oxygen dissolved in the blood

- A Pao2 level less than 60mmHg results in tissue hypoxia

What does SaO2 measure?

Oxygen saturation - refers to the number of hemoglobin binding sites that have oxygen attached to them

What does PaCO2 measure?

- Partial pressure of CO2 in the blood as a byproduct of cellular metabolism

- Know as the respiratory component

- For ABGs, CO2 is regarded as an acid

What does HCO3 measure?

- Measurement of the bicarbonate content of the blood

- Concentration is regulated by the kidneys and the renal production of bicarbonate

What do acid-base disturbances result from?

Abnormality of the metabolic or respiratory systems

If the respiratory system is responsible for a acid-base disturbance what will happen?

Carbon dioxide level will change

- Respiratory Acidosis (build-up of CO2)

- Respiratory Alkalosis (loss of too much CO2)

If the metabolic system is responsible for a acid-base disturbance what will happen?

The bicarbonate level will change

- Metabolic Acidosis (build-up if non-volatile acids)

- Metabolic Alkalosis (reduced amounts of non-volatile acids)

How does the body compensate acid-base disturbances?

1. Buffer systems

2. Renal compensation (slower acting - up to 1 day) - will change bicarb level

3. Respiratory compensation (quick acting - minutes) - will change CO2 level

What are the 6 steps for analysing ABGs?

1. Evaluate oxygenation - PaO2 and SaO2

2. Evaluate pH - is it normal, acidotic or alkalotic

3. Evaluate the PaCO2 - is it normal, low (alkalosis) or high (acidosis)

4. Evaluate the HCO3 - it is normal, acidosis (low), alkalosis (high)

5. Match the PaCo2 or HCO3 with the pH

6. Assess for compensation (has the other values changed? Has the pH returned to normal?) - complete, partial or uncompensated

If the patient has a high PaCO2 what is occuring?

Respiratory Acidosis

If the patient has a low PaCO2 what is occuring?

Respiratory Alkalosis

If the patient has a low HCO3 what is occuring?

Metabolic acidosis

If the patient has a high HCO3 what is occuring?

Metabolic Alkalosis