Question 11.3

Last updated 27/11/2021 - 17:00
 Topic: Acid-Base Disorders
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A 72-year-old female is admitted to ICU after decortication of her left lung, due to empyema. Her background history includes insulin dependent diabetes mellitus, rheumatoid arthritis and hypertension. She was previously hospitalised 2 weeks ago for a methicillin sensitive Staphylococcus aureus bacteremia, and is on high dose intravenous flucloxacillin. She is persistently febrile.

Parameter Patient Value Adult Normal Range
FiO2 0.6  
pH 6.94* 7.35 – 7.45
pO2 85.0 mmHg (11.3 kPa)       
pCO2 43.0 mmHg (5.7 kPa) 35.0 – 45.0 (4.6 – 6.0)
SpO2 98%  
Bicarbonate       9.0 mmol/L* 22.0 – 26.0
Base Excess -15 mmol/L* -2.0 – +2.0
Lactate 4.0 mmol/L* 0.5 – 1.6
Sodium 141 mmol/L 135 – 145
Potassium 5.0 mmol/L 3.5 – 5.0
Chloride 92 mmol/L* 95 – 105
Glucose 3.8 mmol/L 3.5 – 6.0
Urea 18.0 mmol/L* 3.0 – 8.0
Creatinine 145 μmol/L* 45 – 90

a)    List the abnormalities on the blood gas analysis.    (15% marks)

b)    Outline how you would investigate this further.    (15% marks)
 

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College answer

Not available.

Discussion

Let us dissect these results systematically:

  1. The A-a gradient is markedly raised:
    PAO2 = (0.6 × 713) - (43× 1.25) = 374
    Thus, A-a = (374 - 85) = 289 mmHg.
  2. There is severe acidaemia
  3. The PaCO2 is relatively normal, which is unexpected considering the pH.
  4. The SBE is -15, suggesting a metabolic acidosis. The bicarbonate is only 9.
  5. There is no attempt to compensate; the expected CO2 is (40 - 15) = 25, or (1.5 × 9) + 8 = 21.5 mmHg. 
    Thus, there is a respiratory acidosis as well.
  6. The anion gap is raised:
    141-(92+9) = 40. 
  7. The delta ratio is:
    (40-12)/(24-9) = 1.89, i.e. a pure high anion gap metabolic acidosis

So, on this gas, there is:

  • Profound hypoxemia
  • Severe high anion gap metabolic acidosis
  • Hyperlactataemia, which does not completely explain the acidosis
  • Raised urea and creatinine

So: wherever the stem volunteers a history of flucloxacillin or MSSA, the expectation is that the trainees will mention pyroglutamic acidosis in their list of differentials. However, this is a diabetic and she has renal failure. So: relevant investigations should include:

  • Blood ketone levels
  • Urine dipstick for ketones
  • 5-oxoproline level

References

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