Arterial Blood Gas Analysis Part 2 – Interpreting the Results

 

In Part 1 of ‘Arterial Blood Gas Analysis’ we learnt about the components of the arterial blood gas, and the principles underlying compensation and respiratory failure. If you have not already read this article, it is important that you go back and do so before reading this one.

A simple stepwise approach should be taken when interpreting the results of an arterial blood gas (ABG):

1. Assess the patient
2. Assess the oxygenation
3. Assess the pH status
4. Assess the respiratory component of the pH
5. Assess the metabolic component of the pH

 

Step 1: Assess the patient

An ABG should always be used as an adjunct to a thorough clinical assessment of the patient. The patient’s clinical condition will add context to the result of the ABG and make interpretation much easier. The history and examination will also provide clues as to the underlying cause for any abnormalities present.

 

Step 2: Assess the oxygenation

After assessing the patient, the first aspect of the ABG that should be assessed is the oxygenation status of the patient. While breathing room air the partial pressure of oxygen in arterial blood (PaO₂) should be 10-14 kPa. A PaO₂ of less than 10 kPa constitutes hypoxaemia, and a PaO₂ of less than 8 kPa constitutes severe hypoxaemia.

If the patient is being administered supplemental oxygen, this should be taken into account when interpreting the results of the ABG. The PaO₂ should be approximately 10 kPa less than the inspired concentration (%) of oxygen. For example a patient breathing 40% oxygen would be expected to have a PaO₂ of approximately 30 kPa. If there is a difference of greater than 10 between the two values, it is suggestive that a defect in oxygenation is present.

 

Step 3: Assess the pH status

The next aspect of the ABG that should be inspected is the pH status. The pH should be within the range of 7.35 to 7.45 under normal circumstances. A pH of less than 7.35 constitutes acidosis, and a pH of more than 7.45 constitutes alkalosis. The pH can also be within normal range in the presence of a chronic condition when compensation has occurred.

 

Step 4: Assess the respiratory component of the pH

Once the pH status has been assessed, the respiratory component of the pH should be inspected. This can be done by looking at the partial pressure of carbon dioxide in arterial blood (PaCO₂).

If acidosis is present (pH <7.35) and the PaCO₂ is elevated (>6.0 kPa), then this is indicative that the acidosis is at least partially respiratory in nature (respiratory acidosis).

If alkalosis is present (pH >7.45) and the PaCO₂ is lowered (<4.5 kPa), then this is indicative that the alkalosis is respiratory in nature (respiratory alkalosis).

 

Step 5: Assess the metabolic component of the pH

Finally, once the respiratory component of the pH has been assessed, the metabolic component of the pH should be inspected. This can be done by looking at the bicarbonate (HCO₃-) and base excess (BE).

If acidosis is present (pH <7.35) and the bicarbonate is reduced (HCO₃– <22 mmol/l), or there is a base deficit (BE <-2mmol/l), then this is indicative that the acidosis is at least partially metabolic in nature (metabolic acidosis).

If alkalosis is present (pH >7.45) and the bicarbonate is elevated (HCO₃– >26 mmol/l), or there is a base excess (BE >+2mmol/l), then this is indicative that the alkalosis is at least partially metabolic in nature (metabolic alkalosis).

 

Causes of acid-base abnormalities

Some common causes of acid-base abnormalities are summarised in the table below:

Acid-base abnormality	Causes
Respiratory acidosis	COPD and asthma Respiratory depression (e.g. opiates) Neuromuscular disease (e.g. Guillain-Barré syndrome, muscular dystrophy) Incorrect ventilator settings (hypoventilation)
Respiratory alkalosis	Anxiety, pain, fever Pneumothorax Pulmonary embolism Salicylate poisoning (early) High altitude Incorrect ventilator settings (hyperventilation)
Metabolic acidosis	Diabetic ketoacidosis Lactic acidosis Salicylate poisoning (late) Other drug overdose (e.g. methanol, ethanol, ethylene glycol) Chronic renal failure Rhabdomylosis
Metabolic alkalosis	Vomiting Cardiac arrest Multi-organ failure Cystic fibrosis

 

Example 1

A 48-year-old businessman returns from a trip to Los Angeles with shortness of breath and pleuritic chest pain. The results of his arterial blood gas (ABG) on air are shown below:

• pH: 7.48
• pO₂: 7.4 kPa
• PCO₂: 3.1 kPa
• HCO₃-: 24.5 mmol/l

 

Interpretation:

• Step 1: In this case the patient’s history should concern you about a possible diagnosis of pulmonary embolus.
• Step 2: Significant hypoxia is present (type 1 respiratory failure)
• Step 3: Raised pH is present (alkalaemia)
• Step 4: Low PCO₂ is present
• Step 5: Normal bicarbonate is present

 

It can therefore be seen that he has a primary respiratory alkalosis.

 

Example 2

A 30-year-old woman presents having taken an overdose of an unknown substance. She is drowsy and slurring her speech. Her observations are as follows: HR 116, BP 91/57, SaO₂ 96% on air. GCS 11/15. The results of her arterial blood gas (ABG) on air are shown below:

• pH: 7.24
• pO₂: 9.4 kPa
• PCO₂: 4.6 kPa
• HCO₃-: 16 mmol/l

 

Interpretation:

• Step 1: In this case the patient’s history alerts us to the fact that she has taken an overdose.
• Step 2: Mild hypoxia is present
• Step 3: Lowered pH is present (acidaemia)
• Step 4: PCO₂ is normal
• Step 5: Low bicarbonate is present

 

It can therefore be seen that she has a primary metabolic acidosis.

 

Example 3

An 18-year-old woman presents with polyuria, polydipsia and abdominal pain. The results of her arterial blood gas (ABG) on air are shown below:

• pH: 7.21
• pO₂: 11.4 kPa
• PCO₂: 3.1 kPa
• HCO₃-: 15 mmol/l

 

Interpretation:

• Step 1: In this case the patient’s history should alert us to the possibility of a diagnosis of diabetic ketoacidosis (DKA).
• Step 2: The pO₂ is normal
• Step 3: Lowered pH is present (acidaemia)
• Step 4: Low PCO₂ is present
• Step 5: Low bicarbonate is present

 

It can therefore be seen that she has a primary metabolic acidosis with respiratory compensation.

 

Example 4

A 60-year-old patient with COPD presents with worsening breathlessness. The results of her arterial blood gas (ABG) on air are shown below:

• pH: 7.25
• pO₂: 8.8 kPa
• PCO₂: 9.1 kPa
• HCO₃-: 34 mmol/l

 

Interpretation:

• Step 1: In this the patient has a clinical presentation that is suggestive of an exacerbation of COPD.
• Step 2: Mild hypoxia is present (type I2 respiratory failure)
• Step 3: Lowered pH is present (acidaemia)
• Step 4: Raised PCO₂ is present
• Step 5: Raised bicarbonate is present

 

The combination of acidaemia, a high PCO₂ and high bicarbonate indicates that this patient has an acute exacerbation of COPD superimposed on a chronic, compensated respiratory acidosis.

 

Header image used on licence from Shutterstock