Question 3.5

Last updated 25/07/2019 - 23:42
 Topic: Acid-Base Disorders
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For each set of the following biochemical and arterial blood gas parameters:

  • Describe the abnormalities.
  • Give one example of an associated clinical scenario.

Any reasonable scenario accepted that was both biochemically correct AND clinically likely.

Test

Value

Normal Range

Sodium

135 mmol/L

135 – 145

Potassium

4.0 mmol/L

3.2 – 4.5

Chloride

105 mmol/L

100 – 110

Bicarbonate*

22 mmol/L

24 – 32

pH*

7.60

7.35 – 7.45

pCO2*

23 mmHg (3.0 kPa)

35 – 45 (4.6 – 5.9)

 

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College Answer

Acute respiratory alkalosis. Clinical scenario – Psychogenic hyperventilation

Discussion

Let us dissect these results systematically.

  1. The A-a gradient cannot be calculated - the information is missing
  2. There is alkalaemia
  3. The PaCO2 is decreased, suggesting that it is contributing to the alkalosis
  4. The SBE is not supplied, but the bicarbonate is 22, suggestive of a metabolic acidosis or metabolic compensation
  5. The metabolic compensation may not have titrated the pH back to normality, but it appears to be adequate - the HCO3- is expected to fall by 2mmol/L for every 10mmHg of acute decrease in CO2 in an acute respiratory alkalosis, which would give us 20mmol/L. The fact that the HCO3- is 22mmol/L suggests that there is a mild metabolic alkalosis present.
  6. The anion gap is normal:
    (135) - (105 + 22) = 8, or 12 when calculated with potassium
    The delta ratio is of no use here.
  7. The urinary electrolytes and pH would not be helpful.

Thus, this is a respiratory alkalosis with virtually no other biochemical changes. A scenario which might explain these findings may be any cause of hyperventilation:

  • Anxiety
  • Fear
  • Pain
  • Dysregulation of central respiratory control (eg. severe brain injury)
  • Ridiculous ventilator settings

References

Barker, E. S., et al. "The renal response in man to acute experimental respiratory alkalosis and acidosis." Journal of Clinical Investigation 36.4 (1957): 515.

Foster, Guy T., Nostratola D. Vaziri, and C. S. Sassoon. "Respiratory alkalosis." Respiratory care 46.4 (2001): 384-391.

Giebisch, Gerhard, et al. "The extrarenal response to acute acid-base disturbances of respiratory origin." Journal of Clinical Investigation 34.2 (1955): 231.

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