A 58-year-old female presents with muscle weakness and fatigue. Her arterial blood gas is given below:

Parameter

Patient Value

Adult Normal Range

FiO2

0.21

pH

7.01*

7.35 – 7.45

pO2

88.0 mmHg (5.6 kPa)

pCO2

29.0 mmHg (3.9 kPa)*

35.0 – 45.0 (4.6 – 6.0)

Bicarbonate

7.0 mmol/L*

22.0 – 26.0

Base Excess

-21.0 mmol/L*

-2.0 to +2.0

Lactate

0.5 mmol/L

0.5 – 1.6

Sodium

120 mmol/L*

135 – 145

Potassium

2.3 mmol/L*

3.5 – 5.0

Chloride

97 mmol/L

95 – 105

Glucose

4.8 mmol/L

3.5 – 6.0

Urinary pH

6.1

a) Comment on the acid base status. What is the likely cause for her symptoms and give a rationale for your answer.                                                                                              (30% marks)

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College answer

  1. Metabolic and respiratory acidosis
    High anion gap acidosis (Anion gap 16) and normal anion gap acidosis, (Delta- Delta 0.24)
    Likely distal renal tubular acidosis due to non-anion gap acidosis, hypokalaemia and inadequate urinary acidification.    (3 marks)

Discussion

A systematic dissection:

  1. The A-a gradient is (0.21 x 713) - (29 x 1.25) - 88.0 = 25.48 mmHg, i.e essentially normal
  2. There is acidaemia
  3. The PaCO2 is compensating, as best it can.
  4. The SBE is -21, which suggests a severe metabolic acidosis
  5. The respiratory compensation is insufficient;  the expected PaCO2 is (7 ×  1.5) + 8 = 18.5, or 19mmHg by the Copenhagen rules.
  6. The anion gap is slightly raised; (120) - (97  + 7) = 16, or 18.3 when calculated with potassium.
  7. The delta ratio, without using potassium and assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (16 - 12) / (24 - 7) = 0.23. Where the college go the extra on-hundredth decimal point from, is anybody's guess. Also, the delta-delta is the delta gap rather than the delta ratio, which in this case would be (16 - 12) - (24 - 7) = - 15. 

Thus, this is a severe normal anion gap metabolic acidosis, as well as a respiratory acidosis because of inadequate respiratory compensation. The urinary pH, however, is 6.1, which is completely useless. What are you doing, kidneys? You're supposed to be acidifying the urine... Clearly, there is a renal tubular acidosis happening here. Judging by the potassium, you'd expect the problem to be in the distal tubule (i.e. a Type 1 RTA). 

References

References

Laing, Christopher M., et al. "Renal tubular acidosis: developments in our understanding of the molecular basis." The international journal of biochemistry & cell biology 37.6 (2005): 1151-1161.

Ring, Troels, Sebastian Frische, and Søren Nielsen. "Clinical review: Renal tubular acidosis–a physicochemical approach." Critical Care 9.6 (2005): 573.

McCurdy, Donna Kern, Myron Frederic, and J. Russell Elkinton. "Renal tubular acidosis due to amphotericin B." New England Journal of Medicine 278.3 (1968): 124-131.

Cohen, Eric P., et al. "Absence of H (+)-ATPase in cortical collecting tubules of a patient with Sjogren's syndrome and distal renal tubular acidosis." Journal of the American Society of Nephrology 3.2 (1992): 264-271.

Pitts, Robert F. "Renal production and excretion of ammonia." The American journal of medicine 36.5 (1964): 720-742.