a) Describe the acid base abnormalities in the following results and suggest a possible cause.

(20% marks)

Parameter

Patient Value

Adult Normal Range

Fi02

0.5

pH

7.37

7.35 - 7.45

P02

90 mmHa (12 kPa)

PC02

25.0 mmHg (3.6

35 . 0- 45.0 

sp02

Bicarbonate

14.0 mmol/L*

22.0 - 26.0

Base Excess

-10.0 mmol/L*

Lactate

Sodium

1.2 mmol/L

145 mmol/L

135 - 145

Potassium 

4.2 mmol/L

Chloride

93 mmol/L*

95 - 105

Glucose

5.0 mmol/L

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College answer

a) 
Metabolic acidosis 
Concomitant respiratory alkalosis  
Elevated anion gap 
Delta ratio 2.6 – concomitant metabolic alkalosis 
 
Salicylate toxicity

Sepsis with vomiting/pain

Any other plausible. 
 

Discussion

Another disembodied gas, not even a stumpy end of a clinical setting. 

To approach this systematically:

  1. We weren't asked about the oxygenation ("describe the acid base abnormalities" they asked) and  so it would attract no marks to point out that there is a substantial A-a gradient (235, a P/F ratio of 180). 
  2. There is a normal pH
  3. There is a metabolic acidosis with a base deficit of -10. The expected CO2 is actually 30 (or 29 by the Boston rules); therefore there is also a respiratory alkalosis
  4. The anion gap is (145+4.2) - (93+14) = 42.2; or 38 sans potassium. Whichever rules you use, clearly there's an anion gap problem.

And so the SAQ unravels  with a tedious inevitability. What could give rise to a high anion gap without much of an acidosis?  For 20% of a 10-mark SAQ, you are only expected to give one differential ("suggest a possible cause", they asked).  The patient is also a bit hypoxic, so - with some stretch of  the imagination - one could generate a multiple myeloma scenario where there is renal failure with some fluid overload and pulmonary oedema, and  the additional anions are accounted for by a raised phosphate and paraprotein. Any other plausible. A reader has submitted euglycaemic DKA as a completely sensible alternative (Rawla et al, 2017)

References

References

Rawla, Prashanth, et al. "Euglycemic diabetic ketoacidosis: a diagnostic and therapeutic dilemma." Endocrinology, diabetes & metabolism case reports 2017.1 (2017).