Lab case 259 interpretation

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A 5 year old boy presents with ongoing vomiting and diarrhea. He was discharged the day before following a diagnosis of gastroenteritis and treatment with nasogastric rehydration. His father says that he seems very weak, to the point where he’s been having trouble standing up.

Vitals : HR 130, afebrile. RR 30, sats 98 RA, CRT 3 sec. GCS 15.

 

Answers:

1.     Important causes to consider in this scenario include:

  • dehydration resulting in fatigue and dizziness
  • electrolyte and metabolic abnormalities – e.g. hyper/hyponatremia, hyperkalemia/ hypokalemia, hypoglycemia, acidemia
  • nutritional deficiencies secondary to reduced intake.
  • neuromuscular complications associated with gastroenteritis-like illnesses — e.g. Hypokalemia, hyperthyroidism (myopathy); Guillain-Barre syndrome (peripheral neuropathy); Botulism (neuromuscular junction dysfunction)

 

2.     In this patient the hypokalemia is probably a result of diarrhea. Diarrheal stools usually have a high potassium content and volume losses may be up to about 10L per day (in adults). Decreased oral potassium intake may also contribute

Note that diarrhoeal stools also contain bicarbonate, which contributes to a normal anion gap metabolic acidosis (NAGMA). Theoretically, this should help compensate for hypokalemia, as acidemia promotes the transcellular shift of potassium (the exchange of extracellular hydrogen ions for intracellular potassium). Thus hypokalemia in this setting probably signifies an even more marked depletion of intracellular potassium.

The combination of hypokalemia and metabolic acidosis typical of a severe diarrhoeal illness is also seen in renal tubular acidosis.

3.

ECG :

  • flattened T wave
  • depressed ST segments
  • appearance of a U wave.
  • QT prolongation
  • Torsade and ventricular defibrillation.

serum Mg levels.

VBG for acid base status.

4.    Replacement :

Potassium replacement is indicated if:

  • serum potassium <3.0 mmol/L or
  • serum potassium <3.5 mmol/L with symptoms/signs/ECG changes

If serum potassium is 3.0 mmol/L – 3.4 mmol/L in a well child, it is reasonable to either:

  • monitor electrolytes,
  • increase maintenance potassium dose, or
  • replace potassium depending on the clinical situation

In children with stable hemodynamics and no ECG changes, aim for a gradual correction over 24-48 hours.

Correct serum magnesium as necessary

ORAL/ ENTERAL REPLACEMENT:

Acute replacement dose 1 – 2 mmol/kg/dose orally (maximum 20mmol per dose)

Dose may be repeated, after checking serum potassium level, to a maximum of 5mmol/kg/DAY (maximum daily dose 50mmol)
Maintenance dose

(if required)

 2 – 5 mmol/kg/DAY orally in divided doses (maximum 20mmol per dose)

INTRAVENOUS REPLACEMENT IN KIDS :

Acute replacement:  0.2mmol/kg/hour for 3-4 hours ( maximum 10 mmol per hour). For 20 KG child it will be 4mmol/per hour for next 4 hours followed by maintenance dose.

Maintenance: 1-4mmol/kg/day ( maximum 10 mmol/hour)

Repeat potassium 1 hour after replacement and then administer further potassium. ECG monitoring is required if K is less than 3mmol/L.

 

POTASSIUM DEFICIT TO BE REPLACED: Assuming that this child weighs 20kg and the usual total potassium body content is 50 mEq/kg, what amount (mmol) of potassium needs to be administered can be calculated as below:

A potassium deficit of 10% of the total body potassium stores is expected for every 1 mEq/L decrease in the serum potassium from 3.5 mmol/L.

Thus % potassium deficit is:

(3.5 – 2)mmol x 10% = 15%

The amount of potassium deficit is:

(15%/100) x 50 mmol/kg x 20 kg = 150 mmol

 

Given above, about one 5th of potassium will  be replaced with initial acute replacement and over all replacement takes about 24-48 hours.