Lab case 431 interpretation

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Answers:

Question 1:

PH = 7.58, that is severe alkalaemia.

pCO2 = 20 mmHg, so we have respiratory alkalosis.  Next we need to calculate the compensation. From the story, the condition is most likely acute.

For acute respiratory alkalosis, we expected HCO3 to drop by 2 mmol for every 10 pCO2 below 40. Accordingly expected HCO3 should be 20.

HCO3 is 18 mmol/L, that is lower than the expected so we have additional metabolic acidosis.

Since the main metabolic process is alkalosis, we can’t calculate the compensation for metabolic alkalosis. However, we are still able to calculate the anion gap to figure out what type of metabolic acidosis it is.

Anion gap is calculated as AG = Na – (CL + HCO3) = 15, So we have additional HAGMA here.

Other abnormal findings:

Cl = 108 mmol/L that is hyperchloraemia.

Glucose = 7.9 mmol/L, this is mildly elevated. Could be just a part of stress responce.

Lactate = 2.7 mmol/L, that is mild hyperlactateamia.

The phosphate level is very low.

 

This patient was hyperventilating, that caused the respiratory alkalosis. HAGMA was caused by high lactate and ketones. That patient was also dehydrated. Dehydration can explain the elevated chloride level.

 

 Question 2: ***

Low phosphate (Like any other electrolyte) is caused by:

  1. Increased excretion (GI or renal)
  2. Decreased intake (Dietary deficiency or malabsorption)
  3. Transcellular shift

Increased excretion is caused by:

  • Small bowel diarrhoea
  • Enteric fistula
  • Hyperparathyroidism
  • Vitamin D deficiency or resistance
  • Hypophosphataemia
  • Rickets
  • Oncogenic osteomalacia
  • Fanconi Syndrome
  • Osmotic diuresis
  • Acute volume expansion
  • Acetazolamide and thiazaid diuretics

Decreased intake is caused by:

  • Anorexia
  • Chronic alcoholism
  • Aluminium or Magnesium containing antiacids
  • Inflammatory bowel disease
  • Steatorrhoea
  • Chronic diarrhoea

Transcellular shift is caused by:

  • Recovery from DKA
  • Refeeding Syndrome
  • Acute respiratory alkalosis
  • Hypokalaemia
  • Hypomagnasaemia
  • Burns

From the list above, the most probable cause of hypophosphataemia in this patient is respiratory alkalosis.

 Question 3: 

Serum phosphate level doesn’t always reflects total body phosphate. Almost 100% of body phosphate is extravascular. 85% of body phosphate is in bones and 15% is intracellular (ATP and phospholipids of the cell wall).

Since the cause of hypophosphataemia in this patient is transcellular shift, there is not true phosphate deficiency.

The treatment should focus on correcting the cause and there are no needs to replace phosphate for this patient.

 

 

*** Manual of Medicine, Hyperphosphatemia and Hypophosphatemia: Clinical Features and Management. category:Acid Base and Electrolytes  published:April 7, 2021.