Arterial Blood Gas

Understanding how to interpret an arterial blood gas (ABG) is crucial but can be tricky when starting out so let’s break it up into easier chunks.

Components of the ABG

Normal values and reference ranges can vary, so always check the reference ranges provided on the report.

• pH: 7.35-7.45
• pO2: 10-14 kPa
• pCO2: 4.5-6 kPa
• Base Excess: -2 – 2 mmol/L
• HCO3-: 22 – 26 mmol/L
• Anion Gap: 4-12 mEq/L

How to Interpret an ABG

1. Check the Oxygen
   • This is the first thing to check, as it is the most dangerous to leave untreated
2. pH
   • Is the pH normal? If it isn’t, is there an acidosis or an alkalosis?
   • If there is an acidosis or an alkalosis, we need to understand if it’s being driven via respiratory system, or if it’s metabolic
3. Check the Carbon Dioxide

Carbon dioxide can be thought of as ‘acid causing’. If a patient is retaining lots of carbon dioxide, the gas will bind with water in the blood to form carbonic acid, making things more acidic.

• There's lots of CO2 = acidosis/low pH
• Little CO2 = alkalosis/high pH
• If, however, the pH and the CO2 level don’t match – for example, if there is an acidosis but the carbon dioxide is low, it suggests the cause for the pH disruption is metabolic, not respiratory.

4. Check the Bicarbonate Ions
    

Bicarbonate ions are alkalotic.

• Lots of HCO3- = alkalosis/high pH
• Little HCO3- = acidosis/low pH
• Again, if the pH and the bicarbonate ion concentrations do not match, it suggests the cause

5. Check the Base Excess

• Base Excess is the amount of additional hydrogen ions you would need to return the pH to normal, given a normal pCO2. Therefore, base excess is a marker for metabolic acidosis/alkalosis.
• Base excess > +2 = You need additional hydrogen ions to bring the pH back to normal. That means there’s a metabolic alkalosis going on.
• Base excess < -2 = You need fewer hydrogen ions to bring the pH back to normal. That means there’s a metabolic acidosis.

6. Check for Compensation

In order to maintain a normal bodily pH, physiological processes try and compensate in one of two main ways.

• Respiratory Compensation: The rate of breathing can be changed to move the pH back towards a normal range. This type of compensation occurs quicker as it only involves changing of breathing rates.
   
  • Hyperventilation/increased breathing = ‘Blow off’ more CO2 = less acid
  • Hypoventilation/decreased breathing = Retain more CO2 = more acid
     
• Metabolic Compensation: This takes more time to change, as the kidneys are either increasing or decreasing the quantity of bicarbonate ions in the blood.
   
  • Increase levels of HCO3- = More alkaline
  • Decrease levels of HCO3- = More acidic

A partial compensation = A disturbance in either the respiratory/metabolic compensatory mechanism, but the pH remains abnormal.

Full compensation = A disturbance in either the respiratory/metabolic compensatory mechanism, and the pH is normal.

Causes

Anion Gap in Metabolic Acidosis

The anion gap is the difference between serum cations (positive ions) and serum anions (negative ions). Now, there are many different anions and cations, and we don’t routinely measure all of them e.g. we don’t always measure sulfate ions or albumin, which is negatively charged. However, we do routinely measure sodium, potassium, chloride and bicarbonate. This gives rise to the anion gap formula:

Anion Gap = (Na+ + K+) - (Cl- + HCO3-) = 8-12 mEq/L

At this point, if you’re wondering why the gap is normally 8-12, your confusion is warranted – after all, serum is not charged, so you’re probably wondering why this isn’t the formula:

(Na+ + K+) = (Cl- + HCO3-)

However, our normal equation doesn’t account for unmeasured ions. Normally, the unmeasured anions are greater than the unmeasured cations, and this is where the gap is i.e. it is the gap between the unmeasured stuff that we haven’t accounted for such as proteins. Since unmeasured anions are greater than unmeasured cations, the gap is positive i.e. the 8-12 mEq/L are all the anions that we have not measured.

• High Anion Gap: >12 mmol/L: Acidosis
  • A high anion gap means we have more anions floating around – but where are they coming from?
  • Acids are proton donors which makes them anions. Thus, if there are more unmeasured anions, it’s usually coming from greater acid ingestion, or greater acid production.
  • Common causes include diabetic ketoacidosis, lactic acidosis, rhabdomyolysis and salicylates (aspirin overdose)

• Normal Anion Gap Metabolic Acidosis
  • It is possible to get a metabolic acidosis with a normal anion gap.
     
  • If there is excessive loss of bicarbonate, the body compensates by retaining chloride ions in order to maintain an electroneutral serum (since you would’ve lost negative charge through bicarbonate ion loss). However, as the underlying cause of this is the loss of bicarbonate ions, there’s less buffer for the hydrogen ions, leading to an acidosis.
     
  • Bicarbonate loss can be caused by renal tubular acidosis, fluid loss from an ileostomy, carbonic anhydrase inhibitors such as acetazolamide and diarrhoea.

References

https://journals.lww.com/nursingcriticalcare/fulltext/2009/03000/assessing_the_anion_gap.5.aspx

https://emedicine.medscape.com/article/2087291-overview#a2