Arterial Blood Gas Interpretation

Arterial blood gas (ABG to those in the know!) results are critical in establishing an overall picture of your patient’s condition.

Basic observations, history and examination are important in high-quality, patient care. ABG results can be used to complete the medical picture, confirm diagnostic suspicions and guide patient management.

ABG interpretation can seem tricky and overwhelming at first and you may feel your brain is about to implode from information overload!

Fear not, my good people, by following a structured approach and learning the relevance of each result, you will be performing like a pro in no time.

Normal Values

First off, it’s important to know normal range values, these can differ between analysers, but as a general rule, these parameters are listed below:

• pH: 7.35 – 7.45
• PaCO2: 4.7 – 6.0 kPa / 35.2 – 45 mmHg
• PaO2: 11 – 13 kPa / 82.5 – 97.5 mmHg
• HCO3–: 22 – 26 mEq/L
• Base excess (BE): -2 to +2 mmol/L

PaO2

This refers to the partial pressure of oxygen in arterial blood.

Is this patient hypoxic?

Back to basics:

• Airway
• Breathing
• Circulation

Hypoxia is immediately life-threatening and as such should be the first result to note.

Don’t forget to use context when interpreting these results, for example:

• Stanley, a 68-year-old admitted with breathlessness and is currently receiving high flow oxygen has returned a normal PaO2 result. This is abnormal in the context of Stanley who you would expect to return a raised PaO2.

• Beryl, who has a sore hip and has spent the last 10 minutes telling you about how her chihuahua won a beauty contest in 1969, has returned a low PaO2. This is abnormal for Beryl and could very likely indicate a venous sample has been taken and bloods may need to be repeated.

PaO2 of >10 kPa (75 mmHg) is a normal finding for a healthy patient breathing room air.

Patients on room air with a PaO2 of:

• <10 PaO2 are considered hypoxaemic

• <8 PaO2 are considered severely hypoxaemic

For patients receiving O2 therapy; PaO2 should be around 10 kPa less than the percentage of inspired concentration of oxygen. So if your patient is currently receiving 28% O2 via a nasal cannulae you should expect to see a PaO2 of around 18 kPa (135 mmHg).

Flow Rates and O2 Delivery Devices

Below is a table highlighting the delivery percentages of different devices commonly used, to help with context.

Questions:

• Is the patient hypoxic?

• Do the results match patient presentation, history and examination?

• Do the results match treatment therapies?

So now we know the partial pressure of oxygen in our patient, the next step is to look at blood pH.

• Normal pH values are tightly regulated but even a small shift can have major effects on the physiology of the human body.

• pH is the potential of hydrogen, which tells us the concentration of hydrogen in a liquid and informs how active hydrogen ions are.

• Carbon dioxide is excreted by the lungs and hydrogen ions are excreted by the kidneys.

• Hydrogen ions are primarily regulated by changes in the ventilation pattern.
  

• A disturbance causing a decrease in the excretion of hydrogen ions results in acid production, causing the pH to fall. This is known as metabolic acidosis.

• Similarly, a disturbance in the ability of the lungs to excrete CO2 results in respiratory acidosis.

There are 3 possible interpretations:

• Acidotic is <7.35
• Normal is between 7.35-7.45
• Alkalotic is >7.45

If the results display acidosis or alkalosis, you need to ascertain if this is through metabolic (HCO3-) or respiratory (CO2) mechanisms by working through the remaining results.

PaCO2

At this point you know if there are any abnormalities in the oxygen concentration and pH level, but do not necessarily know if the cause is a result of respiratory or metabolic derangements.

Interpreting CO2 levels will help you to either include or exclude the respiratory system as a cause of abnormal pH results.

Biochemistry Refresher

• Carbon dioxide (CO2) enters the bloodstream
  

• Co2 combines with water to form carbonic acid (H2CO3)

• This disassociates hydrogen ions (H+) and bicarbonate ions (HCO3-)

• This disassociation reduces pH
  

• A patient who is retaining CO2 will have increased carbonic acid, decreasing pH to become more acidic

• A patient who is “blowing off” or rapidly expelling CO2 will have decreased carbonic acid, raising the pH to become more alkalotic

• The body will also try to maintain equilibrium by adjusting other buffers to maintain pH, this is known as compensation

• If pH balance is deranged by the respiratory system, the body will sometimes, adjust the HCO3- to bring the pH closer to normal parameters

• Likewise, a metabolic cause may cause the respiratory system to compensate by increasing CO2 expulsion (think of Kussmaul’s breathing in DKA) or retaining CO2 to counterbalance and maintain normal pH

Respiratory Causes

Below is a table of ABG results you would expect to see in respiratory causes:

Questions?

• Is the result normal?

• If not, does the result correspond with the pH (E.g. raised CO2 = decreased pH and decreased CO2 = increased pH)? If yes, a respiratory cause is highly likely.

• If not normal and CO2 doesn’t correspond with pH, a metabolic cause is highly likely.

HCO3-

So at this point, we already have a lot of information and any derangements in pH have been discovered. An underlying cause has been established from the interpretation of the CO2 results.

So why bother with bicarbonate (HCO3-)?

• Well, we currently have our medical picture, but it’s black and white and we want a full-colour picture.

• Bicarbonate is produced in the kidneys and is a buffer responsible for maintaining the homeostasis of pH.

• Increased HCO3- indicates decreased free hydrogen ions and therefore increased pH leading to alkalosis.

• Decreased HCO3- indicates increased free hydrogen ions and therefore decreased pH leading to acidosis.

Questions

• Is the result normal?

• If not, does the result correspond with pH?
  

• If the result doesn’t correspond as described above, the cause is highly likely to be respiratory. We perhaps already knew this from CO2 results but this interpretation further confirms this fact.

Metabolic Causes

Below is a table of expected ABG results in metabolic causes:

Base Excess

So now we have our full-colour picture, let’s increase the resolution by checking the base excess.

• Base excess refers to the amount of acid that would need to be added or removed to maintain normal pH.

• Any value outside of the normal range indicates a metabolic cause.

• Base excess of >+2 mmol/L = increased HCO3- which indicates metabolic alkalosis or compensated respiratory acidosis

• So it should come as no surprise that a base excess of <-2 mmol/L = decreased HCO3- which indicates metabolic acidosis or compensated respiratory alkalosis.

Compensation

Of course in the medical field, nothing is easy, but the human body is amazing and will try to correct any derangements in pH, this response is called compensation.

• We have already discussed the compensation mechanism earlier in the article. Whereby, the body will compensate for derangements in pH by using different buffers to offset and maintain equilibrium.
  

• This, of course, affects ABG results and highlights the need to always look at the context of these results, not only with patient presentation but in context with each other.

• A patient with an underlying metabolic cause may compensate through respiratory mechanisms. This can occur very quickly through the recognition of abnormalities by chemoreceptors located in the medulla oblongata and carotid bodies. The body will respond with increases in the depth and rate of breathing
  

• An example of respiratory compensation is Kussmaul's breathing associated with diabetic ketoacidosis. These are deep, sighing respirations associated with the need to expel CO2 to correct metabolic acidosis.

• Metabolic compensation can take a few days to occur. The kidneys need time to adjust HCO3- production in response to pH derangement with an underlying respiratory cause.

• An example of metabolic compensation can be seen in ABG results of a patient with CO2 retention as a result of COPD. In response to increasing pH, the kidneys will begin to increase HCO3- production. This will show a normal pH but increased CO2 and bicarbonate results on their ABG. Due to the time needed for this compensation to occur, it is a fair assumption that this respiratory derangement has been present for days if not longer.

Mixed Underlying Causes

As previously mentioned, nothing in the medical field is simple!

• Overcompensation is not something that occurs, so should the results show this, you should highly suspect mixed aetiology.

• So you have an acidotic patient who appears to be overcompensating, it’s very likely they are experiencing both underlying metabolic and respiratory causes. Likewise with an alkalotic patient.

• With a mixed underlying acid-base balance the CO2 and HCO3- results will move in opposite directions. So a raised CO2 will result in a decreased HCO3- and vice versa.

Examples:

• Mixed acidosis in multi-organ failure
• Mixed alkalosis in excessive ventilation in COPD.

Summary

Phew, that was intense!! Remember to always interpret ABG results in the context of patient presentation, observations, examination, treatments and other ABG results.

1. PaO2

2. pH

3. PaCO2

4. HCO3-

5. Base Excess

6. Compensation?

Hopefully, you are now feeling more confident and knowledgeable in all things ABG and by using a methodical approach your interpretation skills will know no bounds.

Remember practice makes perfect and use context, context, context!