The arterial blood gas gives you information about two key areas of the patient s physiology: Gas exchange Acid-base balance

Size: px

Start display at page:

Download "The arterial blood gas gives you information about two key areas of the patient s physiology: Gas exchange Acid-base balance"

Noah Maxwell
7 years ago
Views:

1 Arterial blood gas (ABG) interpretation tutorial Introduction The arterial blood gas gives you information about two key areas of the patient s physiology: Gas exchange Acidbase balance Information about gas exchange is obtained by looking at the: PaO 2 PaCO 2 Information about acidbase balance is obtained by looking at the: ph PaCO 2 HCO 3 Base excess Some arterial blood gas machines will give you additional information, including: Lactate Glucose Sodium & Potassium Haemoglobin This tutorial will explain how to interpret this information. Normal values for the various ABG parameters are given below ph PaO kPa PaCO kPa HCO mmol/L Base excess 2 2 Lactate mmol/L

2 Step 1 Gas exchange Look at the PaO 2 and the PaCO 2. Determine whether respiratory failure is present. Is there type I respiratory failure? Type I respiratory failure occurs when: PaO2 < 8kPa PaCO2 < 6kPa (i.e. normal) Whilst breathing room air At sea level Is there type II respiratory failure? Type II respiratory failure occurs when: PaO2 < 8kPa PaCO2 > 6kPa Whilst breathing room air At sea level. Give two causes of type I respiratory failure. Give two causes of type II respiratory failure.

3 Step 2 Primary Acidbase balance problem Firstly, look at the ph. Determine whether acidosis or alkalosis is present. Remember: acidosis = ph < 7.38, alkalosis = ph > 7.42 Acidosis Look at the PaCO 2 : if this is high then there is a respiratory acidosis; if it is low then there is a metabolic acidosis. Alkalosis Look at the PaCO 2 : if this is low then there is a respiratory alkalosis; if it is high then there is a metabolic alkalosis. Why is the PaCO 2 low in metabolic acidosis? Step 3 Look at the metabolic component If you have already determined that the primary problem is metabolic then the HCO 3 and base excess will be abnormal: High HCO 3 and high base excess is in keeping with metabolic alkalosis Low HCO 3 and low base excess is in keeping with metabolic acidosis

4 If you have already determined that the primary problem is respiratory then looking at the metabolic component will tell you if there is any metabolic compensation present: High HCO 3 and high base excess can occur as metabolic compensation for respiratory acidosis Low HCO 3 and low base excess can occur as metabolic compensation for respiratory alkalosis (though it should be noted that this is very rare as respiratory alkalosis does not usually occur long enough for metabolic compensation to take place) Here are some results in a patient with stable COPD: ph 7.40 PaO kpa PaCO kpa HCO 3 38mmol/L Base excess 12 Why is the ph normal? There same patient is admitted with an acute exacerbation of COPD ph 7.26 PaO kpa PaCO kpa HCO 3 38mmol/L Base excess 12 What has happened now?

5 Step 4 When it doesn t make sense Most ABG s will be easy to interpret using these guidelines. Some patients have a mixed acidosis/alkalosis (usually acidosis) that doesn t quite fit with the rules above. Look at this example: ph 7.12 PaO kpa PaCO kpa HCO 3 18 mmol/l Base excess 12 Lactate 12 mmol/l If we follow the rules above we can see that the ph is low and the PaCO 2 is high, therefore the primary problem must be respiratory acidosis. However, the HCO 3 and base excess are low which is in keeping with metabolic acidosis which is normally accompanied by low PaCO 2. So what is happening? In this case the patient has a mixed metabolic and respiratory acidosis. There are a number of explanations as to why this might occur, but one example would be a patient with severe sepsis secondary to pneumonia who has been tachypnoeic because of pneumonia and because of respiratory compensation for metabolic acidosis. The combination of persistent tachypnoea, hypotension accompanying sepsis, acidosis impairing respiratory muscle function and hypoxia have led to the patient s respiratory muscles tiring, which has caused their PaCO 2 to increase. This patient is critically unwell. Deleted:

6 Acidbase disorders and their causes Respiratory acidosis Anything that causes hypoventilation CNS depression e.g. opiate overdose, head injury Nerve/neuromuscular disorders e.g. GuillianBarre syndrome, myaesthenia gravis, motor neurone disease Muscle problems e.g. fatigue, myopathy Airway obstruction e.g. COPD Respiratory alkalosis Anything that causes hyperventilation Anxiety is probably the most common cause Others include o Brain injuries o Salicylate (aspirin) overdose note this initially causes respiratory stimulation and respiratory alkalosis but later causes metabolic acidosis Deleted: CNS disease Metabolic acidosis Calculate the anion gap Anion gap = ([Na + ] + [K + ]) ([Cl ] + [HCO 3 ]) Normal anion gap = 8 16 If the anion gap is high the possible causes are: Urea Ketones Lactate Exogenous (ingested) acids e.g. methanol, ethylene glycol, isoniazid, salicylates (aspirin) Note that high anion gap metabolic acidosis is more common (and is more commonly due to one of the first three causes, rather than exogenous acids) If the anion gap is normal the possible causes are: Hyperchloraemia (e.g. diarrhoea) Acetazolamide Renal tubular acidosis Spironolactone Metabolic alkalosis (rare) Liquorice Conn s, Cushing s or Bartter s syndromes Vomiting Excess alkali ingestion (e.g. antacids) Diuretics Deleted: low Deleted:

Homeostasis. The body must maintain a delicate balance of acids and bases.

Homeostasis. The body must maintain a delicate balance of acids and bases. Homeostasis The body must maintain a delicate balance of acids and bases. Metabolic and respiratory processes must work together to keep hydrogen ion (H+) levels normal and stable. ph of Blood The ph of