Question 4.1

Last updated 30/11/2021 - 18:55
 Topic: Acid-Base Disorders
Pass rate
81.6%
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4.1
The following results are from a 35-year-old female with fever, shortness of breath and known renal calculi.

Parameter

Patient Value

Adult Normal Range

Fi02

0.21

pH

7.30*

7.35 - 7.45

pC02

25.0 mmHg (3.3 kPa)*

35.0 - 45.0 (4.6 - 6.0)

p02

117 mmHg (15.6 kPa) /

Sp02

98%

Bicarbonate

12.0 mmol/L*     /

22.0 - 26.0

Base Excess

-15.0 mmol/L*
'

-2.0 - +2.0

Lactate

1.7 mmol/L* .,

0.5 - 1.6

Sodium

135 mmol/L

135 - 145

Potassium

4.1 mmol

3.5 - 5.0

Chloride

105 mmol/L

95 - 105/

Glucose

5.8 mmol/L

3.5 - 6.0

Creatinine

324 µmol/L*

45 - 90

Urea

29.0 mmol/L*  '

3.0 - 8.0

Albumin

42 g/L

35 - 50


a) Describe the acid base abnormalities. (30% marks)


b) Suggest one likely aetiology. (20% marks)

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College answer

a)    Describe the acid base abnormalities.                               
 
Metabolic acidosis  
Anion gap elevated (18) 
Delta ratio 0.5 – so coexisting normal AG acidosis 
 
b)    Suggest one likely aetiology.                                    
 
Renal tubular acidosis (type 1) secondary to renal stones for NAGMA and urosepsis for HAGMA

 Guidance – any plausible answer that addresses all the acid-base abnormalities 
 

Discussion

A systematic approach:

  1. The patient is normoxic (PaO2 of 117 mmHg) and the A-a gradient is 1.5mmHg, which suggests that alveolar ventilation is satisfactory and there is probably no major gas exchange problems. 
  2. There is acidaemia.
  3. The CO2 is appropriately depressed. The expected CO2 is in fact 25 (by SBE method or by using the Boston rules).
  4. The SBE is -15, suggesting that there is a severe metabolic acidosis
  5. The anion gap is (135) - (105 + 12) = 18, or 22.1 when calculated with potassium
    The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (18 - 12) / (24 - 12) = 0.5
    Thus, . there is a mixed high and normal anion gap metabolic acidosis.

The urea and creatinine are significantly raised, suggesting acute kidney injury.

The lactate is minimally raised, suggesting that retained non-volatile acids of renal failure are mainly responsible for the raised anion gap.

Given the history of renal calculi and fever, one might surmise that this lady probably suffers from an untreated renal tubular acidosis (as RTA Type I and Type II are associated with nephrolithiasis) and now has obstructive uropathy because of the stones. The fever may mean the stones are infected, but we got no information about any of that, so it falls in the realm of wild speculation. "Any plausible answer", etc.

References

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