Question 4.2

Last updated Fri, 07/26/2019 - 01:43
 Topic: Acid-Base Disorders
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A 69-year-old male has been intubated and ventilated  in the Emergency Department for worsening respiratory distress and abdominal pain. He was diagnosed with oesophageal cancer 3 months ago and has received chemotherapy followed by an oesophageal stent. He has non-insulin dependent diabetes.

The following blood results were obtained:

Parameter               Patient Value         Adult Normal Range
Fi02               0.5                      
pH               7.13*               7.35 - 7.45
p02               253 mmHg (33 kPa)              
pC02               25.0 mmHg (3.3 kPa)*     35.0 - 45.0 (4.6 - 6.0)
Sp02               99%                      
Bicarbonate             8.0 mmol/L*           22.0 - 26.0
Base Excess             -19.0 mmol/L*         -2.0 - +2.0
Lactate               10.0 mmol/L* ./         0.5 - 1.6  
                                         
Sodium               136 mmol/L           135 - 145
Potassium               4.6 mmol/L           3.5 - 5.0  
Chloride               103 mmol/L           95 - 105  
Glucose               15.5 mmol/L*           3.5 - 6.0  
Urea               54.0 mmol/L*           3.0 - 8.0  
Creatinine               644 µmol/L*           45 - 90  
Albumin               20 g/L*             35 - 50  
Ionised calcium             1.15 mmol/L           1.10 - 1.35

Interpret the data provided and give likely causes for the abnormalities in this patient.    (50% marks)

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College answer

Interpret the data provided and give likely causes for the abnormalities in this patient      
 
Increased Aa gradient – aspiration, ARDS, fluid overload – any plausible cause 
High anion gap metabolic acidosis, elevated lactate – sepsis in immunosuppressed individual, consider oesophageal perforation, cardiac failure, metformin toxicity 
Respiratory acidosis – primary lung pathology, inadequate ventilator settings for degree of acidosis 
Delta ratio 1.1 (taking into account albumin)  
Renal impairment- sepsis, dehydration,  
Hyperglycaemia, low albumin – diabetes, stress response, malnutrition. 
 
Guidance to examiners: answers which are more specific to the known patient problems score more 
– e.g. oesophageal perforation, metformin toxicity 
 

Discussion

The history offered here suggests strongly that the gas exchange and metabolic problem is probably related to the oesophageal stent somehow.

A systematic dissection of these results:

  1. The patient is hyperoxic (PaO2 of 253 mmHg) and the A-a gradient is raised (72.3mmHg)
  2. There is acidaemia.
  3. The CO2 is appropriately depressed.  The expected CO2 is in fact 21 (by SBE method) or 20 (by using the Boston rules), and so there is also be a mild respiratory acidosis.
  4. The SBE is -19, suggesting that there is a severe metabolic acidosis
  5. The anion gap is  (136) - (103 + 8) = 25, or 29.6 when calculated with potassium.
    The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (25 - 12) / (24 - 8) = 0.81. Even though we have been recently instructed not to correct the anion gap for albumin "unless otherwise specified in the question stem",  the college examiners here clearly did expect the candidates to do this even though the stem does not mention it.  Seeing that the albumin is 20 we would expect an anion gap of 7.0, which means that the delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (25 - 12) / (24 - 8) = 1.12.
    Thus, unless one corrected the anion gap, one would not realise that this is a pure high anion gap metabolic acidosis.

The lactate is 10, which does not completely account for the change in the anion gap. One might interpret the raised glucose and sky-high urea and creatinine as signs that the patient is severely dehydrated and has some contribution from diabetic ketoacidosis (even though NIDDM is stated in the college question). The alternative explanation is uraemia, i.e. the retention of non-volatile acids in renal failure.  

References

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