Acid base disturbances are indicators of serious underlying pathology. An arterial blood gas (ABG) analysis can provide vital information for a paediatric clinical assessment and aid in decision-making, especially in children suffering from respiratory distress. It is an essential tool in diagnosing and managing a child’s oxygenation status and acid base balance. A blood gas analysis also provides other vital information, such as blood sugar, haemoglobin, bilirubin, and electrolyte values, such as sodium, potassium, calcium and chloride levels. Blood gas sampling can be taken from an artery, a vein or a capillary. However, arterial sampling is considered to be more accurate.

There are two ways to obtain an ABG sample:

1. an aseptic direct arterial puncture with a heparinized syringe, using local anaesthesia. The syringe is heparinized to stop clotting.
   
2. an indwelling arterial cannula. The cannula should be situated in a radial artery by preference; however, the umbilical, brachial or femoral can be used.

• A 25 G needle or a 25 G butterfly needle.
  
• A 2 ml syringe with heparin.
  
• A cap for the syringe.
  
• A local anaesthetic (plus needle and syringe for administering it).
  
• Alcohol gel.
  
• Gauze.
  
• Gloves and apron.
  
• A sharps bin.
  
• A blood gas analyser.

Acid-base evaluation requires a focus on three of the reported components: pH, PaCO₂ and HCO₃.

• pH (potential hydrogen) = the pH determines the acidity or alkalinity of the blood. The normal pH of the blood is between 7.35 and 7.45.
  
  
  
  • Acidosis = pH less than 7.35;
    
  • Alkalosis = pH more than 7.45.
  
  
• PaCO₂ = this relates to the partial pressure of carbon dioxide (CO₂) dissolved in plasma. This is the respiratory component of the blood gas. The normal PaCO₂ values are 5–6 kPa or 38–42 mmHg. An increase in PaCO₂ will therefore indicate that, for some reason, carbon dioxide is not being eliminated.
  
• HCO₃ = bicarbonate is also known as a ‘base’ and is present in blood within a range of 22–28 mmol/l. This is the renal component of the blood gas.

If the pH and PaCO₂ are indeed moving in opposite directions, then the problem is primarily respiratory in nature. If the pH and HCO₃ are moving in the same direction, then the problem is primarily metabolic in nature.

The arterial PaO₂ normal range is 11–14 kPa (8–10 kPa in neonates). A high reading will likely be an indicator of respiratory alkalosis and a low reading will indicate respiratory acidosis.

A useful acronym to remember is ROME:

Respiratory

Opposite

Metabolic

Equal

Examples of acid base disturbances are:

• Respiratory acidosis: a pH less than 7.35 with a PaCO₂ greater than 45 mmHg. This occurs when there is inadequate ventilation and CO₂ production is greater than the CO₂ elimination. The common causes are airway obstruction, respiratory depression due to drugs, head injury and lung diseases.
  
• Respiratory alkalosis: a pH greater than 7.45 with a PaCO₂ less than 38 mmHg. This occurs with hyperventilation. Factors such as pain, anxiety, fear and medications such as respiratory stimulants can cause this.
  
• Metabolic acidosis: a bicarbonate level of less than 22 mEq/l with a pH of less than 7.35. Etiologies are loss of bicarbonate due to GIT losses or chronic renal disease, addition of inorganic acids such as diabetic ketoacidosis, lactic acidosis due to tissue hypoxia, salicylate, decreased acid excretion in renal failure and toxins.
  
• Metabolic alkalosis: a bicarbonate level greater than 28 mEq/l with a pH greater than 7.45. Usually associated with an excess of a base or loss of an acid. Examples are loss of gastric acid due to vomiting and diuretic therapy.