Question 5.1

College answer

a)

1. Severe HAGMA Anion gap 43 – sepsis, shock from any cause 2. Lactic acidosis – ischaemic bowel, sepsis, metformin toxicity 
3.    Delta ratio 1.6, pure high anion gap acidosis as per a. 
4.    Hyperglycaemia, stress response, not high enough to be primary cause of metabolic abnormalities 
5.    Renal impairment – sepsis, shock, impaired perfusion 
6.    Hyperkalaemia and hyperphosphatemia likely secondary to renal impairment 7. Anaemia – sepsis 
8.    Leucocytosis – sepsis, stress response 
9.    Elevated A-a Gradient @ 292mmHg – aspiration, pneumonia 

 (Any plausible answer acceptable.)

b)

1.    Serum Ketones 
2.    Measured osmolality 
3.    Lipase 
4.    Septic screen 
5.    CXR 
6.    ECG and troponin 
7.    Transthoracic echocardiogram 
8.    CT abdomen (or USS) 
9.    Renal USS 
10.    LFT’s 
 

Examiner Comments:

 Examiners noted a lack of detail in some answers with anion gap or Aa gradient not mentioned.

Discussion

a)

Let's dissect this systematically. First, the acid-base disturbance

• The patient is hypoxemic. The P/F ratio is 208.
• If the atmospheric pressure is 760 and the alveolar gas mixture has an 100% relative humidity, the alveolar O₂ partial pressure is 405 mmHg:
  0.6 × (760-47) - (18 / 0.8) = 405
  Thus, the A-a gradient is (405-125) = 280 mmHg. Where they got 192 from, one can only guess.
• There is acidaemia.
• There is a metabolic acidosis (the SBE is -23).
• The CO₂ is making an appropriate attempt at compensation; the expected CO₂ is 17 mmHg according to the Copenhagen method, or (5 × 1.5) +8 = 15.5 mmHg according to the Boston rules  
• The anion gap is raised, no matter how you calculate it (48.4 if you include potassium, and 43 if you do not).
• The delta ratio is (43-12) / (24-5) = 1.63, which suggests this is a pure high anion gap acidosis. If you used potassium in your anion gap equation, you get 1.91 which still suggests a pure high anion gap acidosis.
• The lactate is elevated, but the 11 mmol of lactate does not account for all of the anion gap or base deficit (in terms of mole-per-mole stoichiometry).

So, there is a high anion gap metabolic acidosis with appropriate respiratory compensation, which is only partly explained by the raised lactate. Scenarios which could explain this are:

• Shock of any cause with acute renal failure
• DKA/HHS
• Intoxication with toxic alcohol (including ethylene glycol, in which case the lactate is spurious)

The other abnormalities (and explanations) are:

• High potassium (renal failure, acidosis)
• High phosphate (renal failure)
• High glucose (stress response, or diabetes)
• High urea and creatinine (acute or chronic renal failure)
• Anaemia (chronic renal failure, acute blood loss, iron deficiency)
• Elevated WCC (sepsis, steroids, stress response, vomiting, etc)

b)

Biochemistry

• LFTs
• Lipase
• Inflammatory markers
• Laboratory lactate
• Serum ketones
• Osmolality
• Glycolic acid level

Imaging

• CXR (respiratory source of sepsis)
• ECG (myocardial infarction, tricyclic toxicity)
• TTE (cardiogenic cause of shock)
• CT of the abdomen
• Renal tract ultrasound