Acid-Base Physiology

Case 30 : A child ingesting Windscreen washer fluid

Updated 05Sep2015

Clinical Details

History: A 5 yr old 24.5kg child ingested an unknown amount of windscreen washer fluid (containing 40% methanol). He presented within an hour at the Emergency Department.

Examination: Temp 37C, BP 101/59, Pr 101/min, Resp rate 22/min. There was no evidence of intoxication.

Investigations: Na+ 134, K+ 3.7, Cl 110, HCO3 23, urea 4.3, blood glucose 11.7 mmol/l. Creatinine was normal. Serum osmolality 320 mOsm/kg H2O and calculated serum osmolarity was 284 mOsm/Kg H2O. Toxicology investigations: Plasma aspirin, paracetamol & ethanol levels were zero. Serum methanol was 35 g/dl.

The child was transferred to a Paediatric ICU at a tertiary centre. By this time, the child was confused, tachypnoeic and complaining of abdominal pain. The admission arterial blood results were:

Arterial Blood Gases

pH 7.43

pCO2 36 mmHg

pO2 137 mmHg

HCO3 20 mmol/l

Assessment

First: Initial clinical assessment

From the history, the diagnosis is methanol ingestion. Methanol itself is not toxic but both the major metabolites (formaldehyde & formic Acid) interfere with oxidative phosphorylation and it is these metabolites that cause the toxic effects. If untreated, patients develop a metabolic acidosis due to the formic acid. As methanol has a low affinity for alcohol dehydrogenase, metabolism is slow and there is a lag period of usually 12 to 18 hours before development of symptoms. This patient has presented early and was initially asymptomatic. With significant ingestions, there may be respiratory depresion and development of a respiratory acidosis. This patient has presented early and had minimal symptoms at the time of the ABG collection. ABG results would be expected to be normal.

The minimum lethal dose of methanol in an adult is often stated as 12g (or 30mls of a 40% solution). This child is at great risk. However, the early presentation allowing optimal medical therapy suggests an optimistic prognosis in this case

Second: The acid-base diagnosis

Proceeding systematically:

  1. pH: The pH is within the reference range. This suggests either no acid-base disorder or compensating disorders
  2. Pattern: The pCO2 is normal and the HCO3 is only slightly decreased. It could still be possible to have a mixed disorder with a metabolic acidosis balancing a metabolic alkalosis.
  3. Clues: The anion gap is normal. The osmolar gap is 36 mOsm/kg - this is significantly elevated. The Cl- is slightly elevated indicating the presence of a large amount of low molecular weight solute.
  4. Compensation: There is no compensation to consider as no acid-base disorder has been detected.
  5. Formulation: No acid base disorder is present
  6. Confirmation: Nil

Finally: the Clinical Diagnosis

Because the patient has presented early, there has not been sufficient time to develop a metabolic acidosis. The diagnosis is methanol ingestion. There is no evidence of any acid-base disorder.

Comment

This patient was treated with fomepizole. This is a competitive inhibitor of alcohol dehydrogenase and delays the conversion of methanol to formaldeyde. In this case the methanol level was 28 mg/dl at 14 hours post-ingestion. The fomepizole had effectively prevented generation of toxic metabolites.

Methanol levels >20mg/dl are expected to generate sufficient metabolites to cause ocular injury (formaldehyde) and metabolic acidosis (formic acid). In this case, it was decided to start haemodialysis for 4 hours. After this, methanol level was 0 mg/dl at 18 hours post-ingestion. The child was discharged the following day without any sequalae.

There are certain practical difficulties in dealing with patients with methanol ingestion. For example:

The osmolarity in this case has been determined as below:

Osmolarity = 2 x [Na+] + urea + glucose (where units are all in mmols/l)
= (2 x 134) + 4.3 + 11.7 = 284 mmol/l.

An alternative formula is used for the calculated osmolarity in many biochemistry labs in Australia:

Haemodialysis is generally required for severe ingestions.

To be completed .....