Arterial Blood Gas (ABG) Analysis – An overview 

ABG’s are an important diagnostic tool, used to assess the severity of the condition and the impact of any intervention.   

ABG’s provide information concerning: 

1. Oxygenation 
2. Adequacy of Ventilation 
3. Acid-base (pH) balance 

Essentials (normal values in brackets): 

pH (7.35-7.45) – aim is to try to keep pH of the blood in the normal range by changing metabolic and respiratory components as detailed below. 
 – less than 7.35 = acidity of blood 
– more than 7.45 = alkalinity of blood 

PaCO2 (4.5-6.0KPa), partial pressure (concentration of the gas) in the blood; from respiration (‘respiratory component of blood gas’). 

If low, this is referred to as hypocarbia and is usually due to hyperventilation (breathing rapidly for example)
If high, hypercarbia, this is due to CO2 retention (such as in COPD or respiratory failure) 

PaO2 (10.6-13.3KPa), partial pressure of oxygen in arterial blood. 
If low, hypoxemia, poor oxygenation (to organs and tissues) – usually indicates the need for supplementary Oxygen and/or respiratory support (viewed with pCO2 and pH) and supports formal respiratory assessment of the patient (including respiratory rate, work of breathing, GCS etc) 

HCO3- Bicarbonate levels (22-26mmol/l) – Buffer of acids; regulated by the kidneys.  (Metabolic component of the ABG). 

Base Excess (+2 to -2) measures excess or deficit of acid or base in the blood; a negative value is often termed ‘base deficit’. 

Additional values: 

Lactate (0 to 2) – byproduct of anaerobic respiration in the cells; useful measure of tissue perfusion. 
A value >2 usually indicates tissue malperfusion, IV fluids are often used in the first instance to support perfusion, examples of conditions: ischaemia (such as bowel ischaemia), and sepsis.  

Electrolytes – Sodium (Na), Potassium (K), Chloride (Cl), Calcium (Ca2+) 

Haemoglobin and derivates – checks for anaemia, carboxyhaemoglobin, methaemaglobin. 

Blood Glucose level. 

Control of pH 

pH is regulated by the respiratory system and renal system to try to maintain normal values. 

Respiratory system:  

The respiratory system responds almost immediately to try to maintain a normal pH.  This occurs through ventilation by increasing or decreasing carbon dioxide levels by increasing or decreasing respiration. 

Renal system: 

Responds over hours.  Controls pH through the secretion of Hydrogen (H+) ions and reabsorption of bicarbonate.  If both increase, pH rises; if both decrease pH falls. 

Interpretation of ABG:  

1. Determine Oxygenation (PaO2) 
– hypoxaemia PaO2 <10.6KPa 
– Type 1 respiratory failure PaO2 <8KPa 
– Type 2 respiratory failure PaO2 <8KPa in association with a raised PaCO2. 

2. Determine pH (acidosis or alkalosis) 
– Acidosis pH<7.35 
– Alkalosis pH>7.45 

3. Determine respiratory component (PaCO2) 
– Acidosis pH <7.35 with raised PaCO2 (>6KPa) – suggests primary respiratory acidosis; note: if bicarbonate is low suggests mixed respiratory and metabolic acidosis. 
– Alkalosis pH>7.45 with low PaCO2 (<4.5KPa) suggests primary respiratory alkalosis; note if bicarb is high then suggests a mixed alkalosis (respiratory and metabolic component). 

4. Determine Metabolic component (Bicarb) 
– Acidosis (pH<7.35) and Bicarbonate decreased (<22mmol/l) suggests primary metabolic acidosis; note: if PaCO2 also raised then would indicate a mixed acidosis (respiratory and metabolic components). 
– Alkalosis (pH>7.45) and bicarb >26mmol/l suggests primary metabolic alkalosis.  Note: if PaCO2 is also low then this would indicate a mixed alkalosis (respiratory and metabolic components). 

Note: Base excess/deficit can be used in place of bicarbonate in interpretation 
– Low Bicarbonate level correlates with base deficit <-2mmol/l 
– High Bicarbonate level correlates with base excess (>+2 mmol/l) 

5. Determine any compensation 
This refers to any attempt of the respiratory and/or metabolic systems to compensate to restore pH to the normal range. 

Respiratory: 

Acidosis pH<7.35KPa and low PaCO2 <4.5KPa – suggests attempted respiratory compensation to acidosis usually of a metabolic cause 

Alkalosis pH >7.45KPa and high PaCO2 >6KPa – suggests attempted respiratory compensation by increasing PaCO2 usually as a response to metabolic alkalosis. 

Metabolic: 

Acidosis pH<7.35 and bicarbonate >26mmol/l (or base excess >2) – suggests metabolic compensation for respiratory acidosis 

Alkalosis pH>7.45 and bicarb <22mmol/l (base deficit >-2) – suggests metabolic compensation for respiratory alkalosis. 

6. Overall analysis 

– determine the overall contribution of respiratory and metabolic systems and presence of any compensation; with mixed results it is useful to determine whether it is primarily metabolic or respiratory in nature.  In addition, it is important to note whether this is not compensated, partially or fully compensated blood gas analysis. 

7. Detail of other parameters (electrolytes, blood glucose etc) 
Other information to consider: anaemia, electrolyte imbalance, blood glucose. 

For more detailed analysis, please watch our YouTube video: 
https://www.youtube.com/watch?v=fMPHRgf2e3A&t=14s