Question 9.2

Last updated 09/08/2022 - 15:36
 Topic: Acid-Base Disorders
Pass rate
??
Other SAQs in this topic:
Other SAQs in this paper:

A 21-year-old patient presents with fainting spells and lethargy.

a) Explain the following results and provide a rationale for the abnormalities.    (40% marks)
 

Parameter

Patient Value

Adult Normal Range

FiO2

0.21

pH

7.51*

7.35 – 7.45

pO2

72 mmHg (9.5 kPa)

pCO2

69.0 mmHg (9.1 kPa)*

35.0 – 45.0 (4.7 – 6.0)

SpO2

97%

Bicarbonate

55.0 mmol/L*

22.0 – 26.0

Lactate

1.0 mmol/L

0.5 – 1.3

Sodium

128 mmol/L*

135 – 145

Potassium

2.0 mmol/L*

3.5 – 5.0

Chloride

71 mmol/L*

95 – 105

Glucose

4.4 mmol/L

3.5 – 6.0

Urea

15.2 mmol/L*

3.0 – 8.0

Creatinine

82 μmol/L

45 – 90

[Click here to toggle visibility of the answers]

College answer

Not available.

Discussion

The abnormalities are:

  • The A-a gradient is actually negative: (713 × 0.21) - (69  / 0.8) - 72 = -8.52. This is usually an error of measurement (eg. excess air bubbles in the sample).
  • There is alkalaemia
  • There is a metabolic alkalosis; the SBE is not offered, but the bicarbonate is 55.
  • The CO2 is excessive. We cannot apply the SBE equation here, but empirical rules for respiratory compensation in metabolic alkalosis suggest the CO2 should be (0.7×55)+20 = 58.5 mmHg, i.e. there is also a respiratory acidosis here.
  • There are also electrolyte abnormalities:
    • Hyponatremia
    • Hypokalemia
    • Hypoglycaemia
  • Lastly, the urea and creatinine suggest dehydration.

The possible differentials include:

  • Bartter syndrome
  • Laxative abuse
  • Diuretic abuse

References

[Submit a comment or correction]