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ABG Interpretation Dr Azlan Tsia 13th May 2025 •Acid-base status •pH •PCO2 •HCO3- •Anion gap •Base excess/deficit Index •Oxygenation •PO2 (FiO2) •SO2 •CasespH • Acidaemia is a pH <7.35 • Acidosis is a process that leads to an acidaemia Acid base • Alkalaemia is a pH >7.45 • Alkalosis is a process that leads to an alkalaemia status Normal range: pH 7.35-7.45PCO2 • alkalosis7 kPa – respiratory • acidosis0 kPa - respiratory • Increased alveolar ventilation : • Reduced alveolar ventilation: PCO2 Normal range: 4.7-6.0 kPa • PCO2 < 4.7 kPa – respiratory • PCO2 >6.0 kPa - respiratory alkalosis acidosis • Increased alveolar ventilation : • Reduced alveolar ventilation: • Anxiety/panic attack • Hypoxemia • Salicylate excess (early PCO2 stages) • Compensation for metabolic Normal range: acidosis e.g. DKA 4.7-6.0 kPa • PCO2 < 4.7 kPa – respiratory • PCO2 >6.0 kPa - respiratory alkalosis acidosis • Increased alveolar ventilation : • Reduced alveolar ventilation: • Anxiety/panic attack • Chronic lung diseases e.g. • Hypoxaemia COPD • Salicylate excess (early • Acute lung diseases e.g. tiring PCO2 stages) due to increased WOB • Opioid excess • Compensation for metabolic • Weakened respiratory muscles Normal range: acidosis e.g. DKA • Compensation for metabolic 4.7-6.0 kPa alkalosisHCO3- • HCO3- > 26 mEq/L – • HCO3- < 20 mEq/L – metabolic metabolic alkalosis acidosis • Loss of H+ / gain of HCO3- •slide) the anion gap (see next • Vomitingretics HCO3- • Antacid use • Hyperaldosteronism Normal range: 22-26 mmol/LAnion Gap • Blood is electrochemically neutral – Ucations:ble Uanions:able positively charged ions (cations) must Mg++ and PO43-, be balanced by negatively charged ions CK++ Anion proteins e.g. gap albumin, (anions) organic acids • Main cation: Na+ and K+ Anion gap HCO3- • Unmeasured cations: Mg++ and Ca++ • Main anions: Cl- and HCO3- Normal range: • Unmeasured anions: PO43-, SO42-, Na+ proteins e.g. albumin, organic acids Cl- 8-12 mEq/L • There are more unmeasurable anions compared to unmeasurable cations Anion gap = (Na+ + K+) - (Cl- + HCO3-) • Raised anion gap (>12 mEq/L) • Normal anion gap (8-12 mEq/L) metabolic acidosis metabolic acidosis • Gain of unmeasured acid • Loss of HCO3- and gain of Cl- CAT MUDPILE HARDASS • Carbon monoxide • Hyperalimentation (TPN) • Aspirin • Addison’s disease • Theophylline • Renal tubular acidosis Anion gap • Methanol (formic acid) • Diarrhoea • Ureamia • Acetazolamide • Diabetic ketoacidosis • Spironolactone • Propylene glycol • Saline Normal range: • Iron tablets • Lactic acidosis 8-12 mEq/L • Ethylene glycol (Oxalic acid)Base Excess • The dose of acid that needs to be added to the return to a normal pH (7.40) under standard conditions Base excess • Only looks at the metabolic component Normal range: • Positive base excess – indicates metabolic alkalosis • Acid needs to be added to return to normal pH -2 to +2 mmol/L • Negative base excess – indicates metabolic acidosis • Acid needs to be removed to return to normal pHPO2 • PO2 <8 kPa – hypoxia • Type 1 respiratory failure PO2 • Need to know the fraction of • Type 1 respiratory failure: Normal range: inspired oxygen (FiO2) • Low O2 • PO2 should be 10 kPa less than • Normal or low CO2 11-13kPa the FiO2 • Type 2 respiratory failure: • 1 L/min – 24% • Low O2 • 3 L/min – 32% • High CO2 • 4 L/min – 36% SO2 Normal range: • SO2 should be the same as 94-98% displayed on your SpO2 trace • If not: • ?mixed or venous sample • ?cool peripheries • ?nail varnish • Lactate – 0.5-1.8 mmol/L • Glucose – 6.0-8.0 mmol/L • Na+ Other • K+ parameters • Ca++ • COHb • Look at all parameters • pH – 7.49 Reference ranges: pH 7.35 – 7.45 • PCO2 – 2.6 kPa pCO 4.7-6.0 • PO2 – 8.1 kPa pO2 10 – 13 HCO3 22-26 • HCO3- - 24 mmol/L BE -2 – 2 Case 1 Lac 0.5-1.8 • Anion gap – 9 mEq/L Glu 6.0-8.0 • Base excess - -1 mmol/L • Lactate - 4.3 mmol/L • Glucose – 9.8 mmol/L • pH – 7.49 Alkalosis • PCO2 – 2.6 kPa Hyperventilating Reference ranges: • PO2 – 8.1 kPa Normal pH 7.35 – 7.45 pCO 4.7-6.0 • HCO3- - 24 mmol/L Normal pO2 10 – 13 • Anion gap – 9 mEq/L Normal HCO3 22-26 Case 1 Anion gap 8-12 • Base excess - -1 mmol/L Normal BE -2 – 2 Lac 0.5-1.8 • Lactate - 4.3 mmol/L Raised Glu 6.0-8.0 • Glucose – 8.0 mmol/L Normal Respiratory alkalosis Uncompensated • Hypoxia leads to hypocarbia and a respiratory alkalosis due to Type 1 increased respiratory effort respiratory • Salbutamol nebulisers often lead to raised lactate and tachycardia failure • Normal or high CO2 is a very bad sign in asthma – it shows that the patient is tiring ▯ senior help will be required secondary to • In acute respiratory compromise there is no time for metabolic acute asthma compensation • pH – 6.9 Reference ranges: pH 7.35 – 7.45 • PCO2 – 3.1 kPa pCO 4.7-6.0 pO2 10 – 13 • PO2 – 12.0 kPa HCO3 22-26 • HCO3- - 15 mmol/L Anion gap 8-12 Case 2 BE -2 – 2 • Anion gap – 18 mEq/L Lac 0.5-1.8 • Base excess - -16 mmol/L Glu 6.0-8.0 • Lactate - 3.2 mmol/L • Glucose – 22 mmol/L • pH – 6.9 Acidosis Reference ranges: • PCO2 – 3.1 kPa Compensating pH 7.35 – 7.45 • PO2 – 12.0 kPa Normal pCO 4.7-6.0 • HCO3- - 15 mmol/L Low pO2 10 – 13 HCO3 22-26 • Anion gap – 18 mEq/L Raised anion gap Anion gap 8-12 Case 2 • Base excess - -16 mmol/L Negative BE -2 – 2 Lac 0.5-1.8 • Lactate - 3.2 mmol/L Raised Glu 6.0-8.0 • Glucose – 22 mmol/L Raised ↑↑ Partially compensated metabolic acidosis with raised anion gap • Always check ketones in anyone with a raised glucose (even if they are not a type 1 diabetic) • DKAprotocol: • Fluid resuscitation • Fluid resuscitation with fixed rate insulin Diabetic • K+ replacement as required • Glucose replacement as required ketoacidosis • DKAcure – normalisation of pH and ketones • Often go onto a sliding scale initially after cure • Hyperchloraemic metabolic acidosis is common • Very high K+ requirements due to depletion of total body K+ • pH – 7.1 Reference ranges: pH 7.35 – 7.45 • PCO2 – 4.2 kPa pCO 4.7-6.0 • PO2 – 11.0 kPa pO2 10 – 13 HCO3 22-26 • HCO3- - 19 mmol/L BE -2 – 2 Case 3 Lac 0.5-1.8 • Anion gap – 17 mEq/L Glu 6.0-8.0 • Bae excess - - 10 mmol/L • Lactate – 8.2 mmol/L • Glucose – 7 mmol/L • pH – 7.1 Acidosis Reference ranges: • PCO2 – 4.2 kPa Slightly low, compensating pH 7.35 – 7.45 • PO2 – 11.0 kPa Normal pCO 4.7-6.0 • HCO3- - 19 mmol/L Low pO2 10 – 13 HCO3 22-26 • Anion gap – 17 mEq/L Raised Anion gap 8-12 Case 3 • Bae excess - - 10 mmol/L Negative BE -2 – 2 Lac 0.5-1.8 • Lactate – 8.2 mmol/L Raised Glu 6.0-8.0 • Glucose – 7 mmol/L Normal Metabolic acidosis with partial respiratory compensation • Lactate is a product of anaerobic glycolysis • About 50% of lactate is cleared by the liver – so rises in liver failure • Causes of raised lactate: • Type A: inadequate oxygen delivery • Tissue hypoperfusion: shock, cardiac arrest, bowel ischaemia • Excessive muscle activity: convulsions • Reduced O2 delivery: anaemia, hypoxia Lactic acidosis • Type B: no evidence of inadequate tissue oxygen delivery; occurs due to altered metabolism • Underlying disease: liver, kidney, lymphoma • Drugs and toxins: paracetamol, salbutamol • Inborn errors of metabolism • pH – 7.32 Reference ranges: pH 7.35 – 7.45 • PCO2 – 6.8 kPa pCO 4.7-6.0 pO2 10 – 13 • PO2 – 8.6 kPa HCO3 22-26 • HCO3- - 30 mmol/L Anion gap 8-12 Case 4 • Anion gap – 12 mEq/L BE -2 – 2 Lac 0.5-1.8 • Base excess - +7 mmol/L Glu 6.0-8.0 • Lactate – 1.0 mmol/L • Glucose – 10 mmol/L • pH – 7.32 Acidosis Reference ranges: • PCO2 – 6.8 kPa High, retaining CO2 pH 7.35 – 7.45 • PO2 – 8.6 kPa Low pCO 4.7-6.0 • HCO3- - 30 mmol/L Compensating pO2 10 – 13 HCO3 22-26 • Anion gap – 12 mEq/L Raised Anion gap 8-12 Case 4 • Base excess - +7 mmol/L Positive BE -2 – 2 Lac 0.5-1.8 • Lactate – 1.0 mmol/L Normal Glu 6.0-8.0 • Glucose – 10 mmol/L High Respiratory acidosis with metabolic compensation •chronic type 2 respiratoryve failure with metabolic compensation • If you see metabolic compensation you know that the Compensated minimum of 12+ hours – but a likely days to weeks type 2 respiratory • These patients should be given O2 cautiously – at risk of CO2 failure retention •that in the acute setting hypoxia is the greatest risk • Should be aiming for SpO2 88- 92% • Chronic hypoxia causes right shift in the O2-Hb dissociation curve – more O2 availability to the tissues Reference ranges: • pH – 7.52 pH 7.35 – 7.45 pCO 4.7-6.0 • PCO2 – 6.3 kPa pO2 10 – 13 • PO2 – 12. kPa HCO3 22-26 Case 5 • HCO3- - 34 mmol/L • Na+ - 130 Anion gap 8-12 • K+ - 3.3 BE -2 – 2 • Base excess - +12 mmol/L Lac 0.5-1.8 • Lactate – 2.0 mmol/L Glu 6.0-8.0 Na 135 – 145 • Glucose – 6 mmol/L K 3.5 – 5.0 • pH – 7.52 Alkalosis • PCO2 – 6.3 kPa High, compensating Reference ranges: • PO2 – 12. kPa Normal pH 7.35 – 7.45 pCO 4.7-6.0 • HCO3- - 34 mmol/L Raised ↑↑ pO2 10 – 13 • Base excess - +12 mmol/L Very positive HCO3 22-26 Case 5 • Lactate – 2.0 mmol/L Raised Anion gap 8-12 BE -2 – 2 • Glucose – 6 mmol/L Normal Lac 0.5-1.8 • Na+ - 130 Hyponatraemic Glu 6.0-8.0 Na 135 – 145 • K+ - 3.3 Hypokalaemic K 3.5 – 5.0 Metabolic alkalosis with respiratory compensation • Loop diuretics lead to increased excretion of Na+, Cl- and H+ • HCO3- acts to maintain electrochemical neutrality Diuretic- • Hyponatraemia and hypokalaemia are common complications of loop induced diuretic use • Lactate rise can be due to hypovolaemia metabolic alkalosis • Type 1 respiratory failure secondary to acute asthma • Diabetic ketoacidosis Cases • Lactic acidosis due to bowel ischaemia • Compensated type 2 respiratory failure secondary to COPD • Metabolic alkalosis due diuretic use • Clinical context is VERY IMPORTANT when interpreting ABG • Rapid POC test • VBG>ABG Summary • Practice interpreting results on the ward/ED • Credits to Dr Winnie Ko for sharing slidesQuestions Feedback https://app.medall.org/feedback/feedback-flow? keyword=13f5e24c42b0c09def071432&organisation=bits