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Questions Answered: 300

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229
71

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Pain & Sedation

Question 293 of 300

A 21 year old student presents to the Emergency Department complaining of shortness of breath, lethargy and "numb feet". She has no significant past medical history but reports using recreational nitrous oxide daily for the last year. Which of the following findings would you most expect to see on laboratory testing?

Answer:

Exposure to nitrous oxide for prolonged periods, either by continuous or by intermittent administration, may result in interference of the action of vitamin B12. This can result in clinical features such as megaloblastic anaemia and neurological sequelae e.g. subacute combined degeneration of the spinal cord. Laboratory results may show a low serum vitamin B12 concentration, anaemia, macrocytosis and raised serum homocysteine concentration. Note that clinical features of vitamin B12 deficiency can also occur without anaemia and without low serum levels of vitamin B12 (functional deficiency).

Nitrous Oxide (Entonox) Pharmacology

Pain & Sedation

Last Updated: 28th August 2021

Nitrous oxide is used for maintenance of anaesthesia and, in sub-anaesthetic concentrations, for analgesia.

Uses

For anaesthesia, nitrous oxide is commonly used in a concentration of around 50 - 66% in oxygen in association with other inhalational or intravenous agents. Nitrous oxide cannot be used as the sole anaesthetic agent due to lack of potency, but is useful as part of a combination of drugs since it allows reduction in dosage of other agents.

For analgesic purposes without the induction of anaesthesia, a mixture of nitrous oxide and oxygen containing 50% of each gas (Entonox®, Equanox®) is used. Self-administration using a demand valve is popular in obstetric practice, for changing painful dressings, as an aid to postoperative physiotherapy, and in emergency ambulances. Entonox bottles are blue or blue/white in colour and should have a blue/white quartered collar.

Contraindications

Nitrous oxide must not be used in patients with an air-containing closed space as nitrous oxide diffuses into these spaces with a resulting increase in pressure. This includes conditions such as pneumothorax, the presence of intracranial air after head injury, entrapped air following recent underwater diving, recent intraocular gas injection or intestinal obstruction. Nitrous oxide also increases cerebral blood flow and should be avoided in patients with, or at risk of, raised intracranial pressure.

Adverse effects

Hypoxia can occur immediately following the administration of nitrous oxide; additional oxygen should always be given for several minutes after stopping the flow of nitrous oxide.

Exposure to nitrous oxide for prolonged periods, either by continuous or by intermittent administration, may result in megaloblastic anaemia as a result of interference with the action of vitamin B12; neurological toxic effects can occur without preceding overt haematological changes. Depression of white cell formation may also occur.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l
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