Acid base teaching – Case 3

A young woman was admitted as an emergency, having been found by her flatmate. Although she had been well when last seen a few hours earlier, she was semiconscious, hyperventilating, and responding only to painful stimuli. An empty, unlabeled pill container was found next to her.

Her results were:
[H+] = 46 nmol/L   pH = 7.34
PaCO2 = 3.0 kPa
[HCO3] = 12 mmol/L
[Na+] = 137 mmol/L
[K+] = 4.7 mmol/L
[CI] = 107 mmol/L

Question 1

What is the patient’s acid base picture at presentation?

FALSE – Polydipsia alone should not present with any acid base disturbance.

FALSE – No suggestion of anorexia which usually presents with normal acid base picture or metabolic alkalaemia due to vomiting or use of diuretics.

TRUE – In view of her age, symptoms and laboratory results it is the most likely diagnosis.

FALSE – Severe diarrhoea usually presents with metabolic alkalaemia due to volume contraction.

FALSE – Severe anaemia should not change the acid base picture.

Over a relatively short time period, as here, respiratory compensation will occur, lowering the PCO2, but this will not restore the [H+] to such a nearly normal result, which would require renal tubular bicarbonate regeneration, which is a slower process. Another way of thinking about this combination of results is that in metabolic acidaemia it is the raised [H+] which drives the hyperventilation, causing the respiratory compensation.

The [H+] here is no longer particularly high so another reason needs to be considered for the low PCO2. She also has a respiratory alkalaemia. Some of the symptoms might suggest diabetes, but this is unlikely since [glucose] is normal. Renal failure might produce a similar picture (though [K+] might then be increased)

 


Question 2

What substance is the most likely cause of this patient’s condition?

TRUE / FALSE – Insert answer.

TRUE / FALSE – Insert answer.

TRUE / FALSE – Insert answer.

TRUE / FALSE – Insert answer.

Overdoses can cause metabolic acidosis, respiratory alkalosis (by stimulation of the respiratory centre causing hyperventilation), or both. She had taken an aspirin overdose.


Question 3

What is her anion gap in this case?

The anion gap can be calculated from this equation:

AG = ([Na+] + [K+]) – ([Cl-] + [HCO3-])

Causes of metabolic acidaemia with increased anion gap:
Ketoacidosis (Diabetes Mellitus type 1, starvation)
Araemia (as a result of renal failure, with retention of strong acids) Salicylate (derived from Aspirin)
Methanol
Alcohol (including ethylene glycol)
Lactic acid (exercise, shock, hypoxia, liver failure, trauma, biguanides)

Causes of metabolic acidaemia and normal anion gap:
1) [HCO3-] loss
GI loss (diarrhoea)
Proximal Tubular Acidosis
Acetozolamide (a.k.a. diamox,
a carbonic anhydrase inhibitor)

2) Failure to excrete acid
Distal Tubular Acidosis
Hyperkalaemia

 

TRUE / FALSE – Insert answer.

TRUE / FALSE – Insert answer.

TRUE / FALSE – Insert answer.

Overdoses can cause metabolic acidosis, respiratory alkalosis (by stimulation of the respiratory centre causing hyperventilation), or both. She had taken an aspirin overdose.


Question 3

What is her anion gap in this case?

The anion gap can be calculated from this equation:

AG = ([Na+] + [K+]) – ([Cl-] + [HCO3-])

Causes of metabolic acidaemia with increased anion gap:
Ketoacidosis (Diabetes Mellitus type 1, starvation)
Araemia (as a result of renal failure, with retention of strong acids) Salicylate (derived from Aspirin)
Methanol
Alcohol (including ethylene glycol)
Lactic acid (exercise, shock, hypoxia, liver failure, trauma, biguanides)

Causes of metabolic acidaemia and normal anion gap:
1) [HCO3-] loss
GI loss (diarrhoea)
Proximal Tubular Acidosis
Acetozolamide (a.k.a. diamox,
a carbonic anhydrase inhibitor)

2) Failure to excrete acid
Distal Tubular Acidosis
Hyperkalaemia