The following arterial blood gas results are from a 72-year-old male admitted for investigation of nausea, vomiting and severe abdominal pain. He has a history of type 2 diabetes and atrial fibrillation. 


a)    Comment on the abnormalities on this arterial blood gas.               (15% marks) 
 
b)    List five likely causes for the acid-base disturbance.                  (15% marks) 

Parameter

Patient Value

Adult Normal Range

FiO2

0.6

pH

6.98*

7.35 – 7.45

pO2  

92 mmHg  (12.3 kPa)

pCO2

31.0 mmHg (4.1 kPa)*

35.0 – 45.0 (4.6 – 6.0)

SpO2

99%

Bicarbonate 

7.0 mmol/L*

22.0 – 26.0 

Base Excess 

-22.0 mmol/L*

-2.0 – +2.0 

Lactate 

14.5 mmol/L*

0.5 – 1.6

Sodium 

146 mmol/L*

135 – 145 

Potassium 

5.3 mmol/L*

3.5 – 5.0

Chloride 

103 mmol/L

95 – 105

Glucose 

7.7 mmol/L*

3.5 – 6.0

Creatinine

711 μmol/L*

60 – 110

Haemoglobin

108 g/L*

135 – 180

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College answer

a) 
Elevated Aa gradient 
Profound lactic acidosis 
High Anion Gap Metabolic Acidosis (36)  
Associated respiratory acidosis or incomplete compensation  
Delta ratio 1.41 – suggests pure elevated anion gap acidosis 
Renal impairment  
 
b) 

Metformin induced 
Ischaemic gut 
Pancreatitis 
Sepsis 
Cardiogenic shock 

Discussion

Let us dissect these results systematically:

  • The A-a gradient is raised (the expected alveolar O2 concentration is = (0.6 × 713) - (3 ×0.8) = 389, so the A-a gradient must be around 297) 
  • There is acidaemia.
  • There is a metabolic acidosis (SBE is -22). Lactate is responsible for virtually all of  it (lactate is 14 mmol/L)
  • The CO2 is appropriately decreased, but not enough. The expected CO2 for "full compensation" would be 28 mmHg by the Copenhagen rules or (7 × 1.5) +8 = 18.5 mmHg by Winter's rule. Thus, there is also a respiratory acidosis
  • The anion gap is raised; it is 36 if you omit potassium and 41.3 if you include it (the consequences of using different equations for this are discussed elsewhere).
  • Assuming an albumin value of 40g/L, the delta ratio is either 1.41 or 1.72, but either way there is a pure HAGMA.
  • Urea is not available, but the creatinine of over 700 strongly suggests that something is wrong with the kidneys.

The college answer gives up only five causes, so one would be expected to at least include these in their list. Here they are again with justifications:

  • Ischaemic gut (AF, nausea, vomiting, abdominal pain)
  • Sepsis (safe differential for everything)
  • Pancreatitis (abdominal pain)
  • Cardiogenic shock (diabetic, AF suggests structural cardiac disease)
  • Metformin (diabetic, therefore presumably on metformin)

Other unofficial possibilities include:

  • Portal vein thrombosis with ascites and hepatorenal syndrome (explains lactate, abdominal pain, and to some extent the creatinine)
  • Malignancy (eg. perforated colorectal cancer with bowel obstruction, which explains everything)

References

References