4 month old female (corrected age 4 weeks) presents to ED after a 2 day history of non bilious vomiting  and diarrhoea.

Metabolic Acidosis – pH 7.14 and HCO3 12mmol/l

Expected PCO2 = 1.5 x HCO3 +8 =26

Anion Gap =Na – (Cl+HCO3) = 12

Description and Interpretation

There is a non anion gap metabolic acidosis with incomplete compensation or underlying respiratory acidosis, with an expected PCO2 of 26mmHg and actual pCO2 of 36mmHg. The patient has a moderately raised sodium and chloride and a mildly elevated lactate. The potassium is mildly elevated, which could be secondary to the acidosis or other underlying pathology (see below) The patient has a normal glucose and creatinine

In this clinical context the likely cause of the NAGMA would be the  diarrhoea. Other causes need to be considered

– addisons, but the patient has a high sodium, high potassium and a normal glucose

-RTA – typically produces a hyperchloraemic metabolic acidosis. There are  3 types. In the setting of hyperkalaemia – RTA type 1 or distal RTA

-excess chloride administration in the form of normal saline

-other rare causes are less likely in this case – pancreatic, small bowel and ureteric fistula, or carbonic anhydrase inhibitors

The higher than expected CO2 could be due to an underlying respiratory pathology – pneumonia, or poor respiratory effort with a low GCS. The mildly elevated lactate would also be contributing to the acidosis.

The moderately raised sodium and chloride would be related to increase water losses in the context of diarrhoea , however it could also be related to an underlying RTA, or inappropriate concetrated formula mixture.

Dehydration severity –

Mild (<3%)- dry mucous membranes, decreased urine output, thirst

Moderate (5%)- As above plus sunken eyes, diminished skin turgor, tachycardia, altered LOC (irritability/drowsiness)

Severe (10%)- as for moderate plus cool mottled skin with CPR>2sec, anuria and hypotension

This child is  tachycardic with sunken eyes, dry mucous membranes, irritable and a CPR>3 seconds. She is showing signs of moderate to severe dehydration. Her skin turgor is maintained most likely due to the hypernatraemia.

Fluid management

Resuscitation – Normal saline bolus 20mg/kg

Deficit volume – weight X %dehydration X 10 = 287ml

Maintenance – 100ml/kg/24hrs =410ml

As this child is hypernatraemic  the rehydration rate needs to be done over 48 hours instead of 24hours, therefore the hourly rate is 23ml/hr plus on going losses. The fluid used would be 0.9% NaCl and 5% dextrose