Arterial Blood gas analysis

Related Subjects: |Metabolic acidosis

Introduction

• We are all net producers of acid which is excreted both by the kidneys
• Also by removing carbon dioxide from the lungs.
• To assess Acid-base balance we can measure an arterial blood gas.

Look at pH: normal 7.35 - 7.45

• If the pH < 7.35 this is termed acidosis
• If the pH is 7.35 to 7.45 this is normal or compensated
• If the pH is > 7.45 then this is alkalosis

PaO₂: 11-13 kPa or 82.5 - 97.5 mmHg

• If the PaO₂ <: 8 Kpa or 60 mmHg then that is Respiratory failure
• Type 1 RF PaO₂ <8 kPa and PaCO₂ <6.0 kPa
• Type 2 RF PaO₂ <8 kPa with PaCO₂ >6.0 kPa

Look at PaCO₂: normal 4.7-6 Kpa 35-45 mmHg

• PaCO₂ > 44 mmHg or 6 KPa= respiratory acidosis or respiratory compensation for a metabolic alkalosis
• PaCO₂ < 40 mmHg or 4.5 KPa = respiratory alkalosis or respiratory compensation for metabolic acidosis

Look at HCO₃: normal 22-26 mmol/L

• HCO₃ < 22 mmol/l (mEq/L) = metabolic acidosis or renal compensation for respiratory alkalosis
• HCO₃ > 26 mmol/l (mEq/L) = metabolic alkalosis or renal compensation for respiratory acidosis

Base Excess from Normal -2 to 2

• BE > +2mmol/L suggests high HCO₃- due to primary metabolic alkalosis or a compensated respiratory acidosis.
• BE < -2mmol/L suggests low HCO₃- in the blood, suggesting either a primary metabolic acidosis or a compensated respiratory alkalosis.

Anion gap: normal value is 2-10 mmol/l.

• AG = Na + [Cl+ HCO₃]

Metabolic acidosis + Raised Anion gap:

• DKA, Alcoholic ketoacidosis, Methanol, Carbon monoxide, Cyanide, Metformin,
• Paraldeyde, Isoniazid, Iron, Ethylene glycol, salicylates

Metabolic acidosis + Normal Anion gap:

• Diarrhoea, small intestinal fistula, pancreatic alkali losses, Carbon anhydrase inhibitors
• Renal tubular acidosis, uretoenterostomy, rapid hydration with NaCl, Addison's disease

Metabolic alkalosis:

• Antacids, Dialysis, Milk-alkali syndrome, Pyloric stenosis/Gastric outlet obstruction with vomiting
• Bartter's syndrome, Adrenogenitial syndromes, Cushing's syndrome, COnn's syndrome.

Respiratory acidosis: CO₂ retention and acidosis

• Respiratory depression (e.g. opiates)
• Chest disease e.g. asthma, COPD airway obstruction, GBS
• Hypoventilation, respiratory nerve or muscle weakness

Respiratory alkalosis: High pH and low CO₂

• Hyperventilation, PE, Asthma
• CCF, increase in intracranial O₂/ V/Q scan, ventilation/perfusion mismatch.

Blood Gases (breathing air at sea level)

• Blood H+ 35-45 nmol/L from which is calculated pH 7.36-7.44
• PaO₂ 11.3-12.6 kPa
• PaCO₂ 4.7-6.0 kPa
• HCO₃ 22-26 mmol/l
• Base excess +/- 2 mmol/L
• Carboxyhaemoglobin: Non-smoker < 2%, Smoker 3-10%,

Interpretation

• pH < 7.36 = Acidosis
• pH > 7.44 = Alkalosis
• Acidosis + PaCO₂ > 6.0 Kpa suggests respiratory acidosis and there may be compensatory elevated HCO₃ to normalise pH
• Acidosis + low HCO₃ suggests Metabolic acidosis and there maybe a compensatory low PaCO₂ to normalise pH