Question 20.2

Last updated 28/04/2016 - 22:12
 Topic: Acid-Base Disorders
Pass rate
71%
Other SAQs in this topic:
Other SAQs in this paper:

A 28-year-old previously fit male presents with a two-day history of fever, headache and a widespread rash.

Results of investigations are as follows:

Parameter

Patient Value

Normal Adult Range

FiO2

0.3

pH

6.99*

7.35 – 7.45

PCO2

26* mmHg (3.4 kPa)*

35

– 45 (4.6 – 6.0)

PO2

78 mmHg (10.3 kPa)

SpO2

96%

95

– 100

Base Excess

-27.5 mmol/L*

-3.0 – +3.0

Bicarbonate

6 mmol/L*

22

– 27

Sodium

126 mmol/L*

135 – 145

Potassium

5.1 mmol/L*

3.5 – 4.5

Creatinine

186 micromol/L*

60

– 110

Glucose

2.4 mmol/L*

3.6 – 7.7

Lactate

16.0 mmol/L*

0.2 – 2.0


Blood cultures show Gram-negative cocci.

a)    List the abnormalities shown by the ABG.    (10% marks)

b)    Give the most likely diagnosis.    (5% marks)

c)    What complication of this condition may have occurred?    (5% marks)

[Click here to toggle visibility of the answers]

College Answer

a)

Severe lactic acidosis with inadequate respiratory compensation and acute renal impairment and hypoglycaemia.

b)
Meningococcal septicaemia

c)
Waterhouse Friderichsen syndrome.

Multi-organ failure with liver and renal dysfunction is a reasonable answer and was given some credit.

Discussion

a) Let us dissect the results systematically:

  1. The A-a gradient is high:
    The alveolar oxygen tension is (0.3 × 713) - (26 × 1.25) =
    Thus, the A-a gradient is 103.4.
  2. There is acidaemia.
  3. The PaCO2 is low, which is a move in the appropriate direction given the metabolic acidosis
  4. The SBE is -27.5, suggesting a severe metabolic acidosis.
  5. The respiratory compensation is insufficient.
    According to the Boston rules the expected PaCO2 is (6 × 1.5) + 8 = 17mmHg, and so there is also a respiratory acidosis.
    According to the Copenhagen rules, the the expected PaCO2 = (40 - SBE) = 12.5mmHg, which is an unrealistic number and which demonstrates the breakdown of these rule sets in the setting of truly extreme situations.
  6. The anion gap cannot be calculated, because no chloride is offered. One can only expect that it is  high, given the extremely low bicarbonate value and the extremely high lactate.

Thus, in summary:

  • Severe acidaemia
  • Severe lactic acidosis
  • Inadequate respiratory compensation (thus, respiratory acidosis)
  • Widened A-a gradient
  • Hyponatremia and mild hypokalemia
  • Renal failure (impossible to say whether acute or chronic)
  • Hypoglycaemia
  • gram negative cocci in the blood (and that could only be a few different things: namely, Moraxella or Neisseria among the aerobes, and Veilonella among the anaerobes)

b)

The trainees were expected to identify the meningococcaemia on the basis of "fever, headache and a widespread rash". This is probably somewhat unfair. The college, in recognition of this fact, acknowledged as correct any answer which explained the lactic acidosis by blaming it on sepsis-induced liver failure.

c)

The patient has features of hypoadrenalism, consistent with Waterhouse-Friedrichsen syndrome. If the trainee managed to connect the dots (gram negative cocci, low sodium, high potassium) they would have only earned a paltry 5% of the total marks for Question 20, which is a poor marks to effort ratio.

References

Rosenstein, Nancy E., et al. "Meningococcal disease." New England Journal of Medicine 344.18 (2001): 1378-1388.

Mautner, L. S., and W. Prokopec. "Waterhouse-Friderichsen Syndrome."Canadian Medical Association journal 69.2 (1953): 156.

Ferguson, J. Howard, and Orren D. Chapman. "Fulminating meningococcic infections and the so-called Waterhouse-Friderichsen syndrome." The American journal of pathology 24.4 (1948): 763.

[Submit a comment or correction]