Unexpected deterioration of sick patient Hypoxaemia on sats monitoring Reduced conscious level Exacerbation of COPD Monitoring of ventilated patient Sepsis Metabolic or electrolyte problem e.g. DKA Drug Overdose
Normal PaO 2 11.5-13.5 kPa ON AIR › Correct hypoxaemia immediately (target SpO 2 ) Respiratory failure defined as PaO 2 <8kPa (SpO 2 <93%) › Type I – normal PaCO 2 › Type 2 – elevated PaCO 2 (ventilatory failure) Significant respiratory failure may be present despite ‘normal’ or high PaO 2 › Predicted PaO 2 normally ~10kPa below FiO 2 › e.g. 40% venturi, PaO 2 should be ~30kPa › Document oxygen use on ABG result!
Look at the pH (normal range 7.35-7.45) pH <7.35 = acidaemia/acidosis pH >7.45 = alkaemia/alkalosis Acidosis or alkalosis may still be present with a ‘normal’ pH if the body has already buffered = compensation CO 2 is acidic and HCO 3 - is alkaline Normal compensation for acidosis is to decrease CO 2 (rapid) and increase HCO 3 - (takes longer) Normal compensation for alkalosis is to decrease HCO 3 - (and increase CO 2 )
Normal range for HCO 3 - is 22-26mmol/L › Normal range for base excess (BE) is -2 to +2 Acidosis (pH <7.35) › HCO 3 - <22mmol/L (BE < -2) = metabolic acidosis › HCO 3 - >22mmol/L (BE > -2) = respiratory acidosis Alkalosis (pH >7.45) › HCO 3 - >26mmol/L (BE > +2) = metabolic alkalosis › HCO 3 - <26mmol/L (BE < +2) = respiratory alkalosis
What is the primary disturbance? Is there any compensation? Is there a mixed picture? e.g. in acidosis (pH <7.35) › PaCO 2 >6.0kPa with HCO 3 - >26mmol/L = › Respiratory acidosis with partial metabolic compensation › PaCO 2 >6.0kPa with HCO3- <22mmol/L = › Mixed respiratory and metabolic acidosis
Lactate (normal range 0-2mmol/L) › Elevated levels often associated with acidosis › tissue hypoperfusion/anaerobic metabolism, liver/renal failure or drugs (e.g. metformin) › Degree of elevation correlates directly with mortality in sepsis › Response to fluids also important Haemoglobin (Hb) Potassium (K + ) and Sodium (Na + ) Glucose (not on AMU analyzer)
78-year-old male admitted with IECOPD becomes more drowsy and confused whilst on AMU. Oxygen is being delivered via a non-rebreathe bag and mask.
FiO2 0.4 (40%) 0.21 (air) pH 7.21 7.35-7.45 PaO 2 15.7kPa 11.5-13.5kPa PaCO 2 8.9kPa 4.5-6.0kPa HCO 3 - 31mmol/L 22-26mmol/L BE +8 -2 to +2 Lactate 1.8mmol/L <2.0mmol/L
Relative hypoxaemia (PaO2 should be ~30kPa) Respiratory failure = type 2 (elevated PaCO2) Acidosis (pH <7.35) High PaCO 2 = respiratory acidosis High HCO 3 - = partial metabolic compensation (likely chronic) Acute-on-chronic type 2 respiratory failure with respiratory acidosis and partial metabolic compensation
29-year-old female with type 1 diabetes. Admitted with 48h history of diarrhoea and vomiting. Rapid respiratory rate. CBG 27mmol/L and blood ketones 4.9mmol/L.
FiO2 0.21 (21%) 0.21 (air) pH 7.36 7.35-7.45 PaO 2 13.7kPa 11.5-13.5kPa PaCO 2 3.2kPa 4.5-6.0kPa HCO 3 - 14mmol/L 22-26mmol/L BE -12 -2 to +2 Lactate 2.8mmol/L <2.0mmol/L
Normal PaO 2 on air i.e. no respiratory failure Normal pH however; Low HCO 3 - and BE with high lactate and ketones = metabolic acidosis Low PaCO 2 = respiratory compensation (Kussmaul respiration) Fully compensated metabolic acidosis due to DKA
36-year-old male with alcohol dependence and ALD. Admitted to AMU following a staggered co-codamol overdose. GCS 8/15 with small pupils and respiratory rate of 10/min.
FiO2 0.35 (35%)0.21 (air) pH7.197.35-7.45 PaO 2 11.7kPa11.5-13.5kPa PaCO 2 7.5kPa4.5-6.0kPa HCO 3 - 17mmol/L22-26mmol/L BE-8-2 to +2 Lactate4.2mmol/L<2.0mmol/L
Relative hypoxaemia (PaO 2 should be ~25kPa) Respiratory failure = type 2 (elevated PaCO 2 ) Acidosis (pH <7.35) High PaCO 2 = respiratory acidosis Low HCO 3 - and BE = metabolic acidosis Mixed respiratory and metabolic (lactic) acidosis Hypoventilation due to reduced GCS (hepatic encephalopathy) and opiate overdose Lactic acidosis due to liver failure/drug overdose
56-year-old female admitted with acute breathlessness. Recent marital stress. Heart rate 120/min, respiratory rate 28/min, SpO 2 96% on air. Chest clear. Looks anxious ++
FiO2 0.21 (40%) 0.21 (air) pH 7.49 7.35-7.45 PaO 2 10.1kPa 11.5-13.5kPa PaCO 2 3.8kPa 4.5-6.0kPa HCO 3 - 25mmol/L 22-26mmol/L BE +2 -2 to +2 Lactate 0.7mmol/L <2.0mmol/L
Hyperventilation with respiratory alkalosis No metabolic compensation May be due to anxiety/panic attack but relative hypoxaemia in this case suggests alternative cause e.g. PE, pneumonia, acute asthma
Cannot be used to assess pO 2 or pCO 2 › Normal pCO 2 on VBG excludes hypercapnia Good correlation with ABG for other parameters › pH, lactate, BE, HCO 3 -, electrolytes, Hb › except if patient shocked/peri-arrest VBG usually adequate in all other situations › Obtain VBG in all acutely unwell patients › Especially sepsis, DKA, UGIB, AKI, overdose
Sample must not be shaken and should be analysed immediately (<10min) to prevent haemolysis › lowers pH and increases K + K + up to 0.5mmol lower than lab value Na + up to 6mmol lower than lab value Hb 5g/L higher than lab value on average