10. Anaphylaxis Flashcards
Initial Mx
Anaphylaxis drill: Initial therapy
Stop administration of suspected agent.
Maintain airway: give 100% oxygen.
Lay the patient flat with feet elevated.
Give adrenaline: i.m. at a dose of 0.5–1 mg (repeated every 10 min if
required).
i.v. at a dose of 50–100 mcg for hypotension or cardiovascular collapse
titrated up to 0.5–1 mg as required.
Give i.v. fluids (crystalloid or colloid).
What secondary therapies would you administer or consider?
Antihistamines – give chlorpheniramine 10–20 mg by slow i.v. infusion.
Corticosteroids – give hydrocortisone 200 mg i.v.
Catecholamine infusion – if poor response to initial bolus.
Bronchodilators
Consider bicarbonate (0.5–1 mmol/kg).
How would you investigate this patient for suspected anaphylaxis?
Immediate Ix
Investigation can be divided into immediate and late.
The diagnosis of anaphylactic or anaphylactoid reaction hinges around the
plasma tryptase concentration.
Immediate
Identification of an anaphylactic/anaphylactoid reaction:
Take 10 ml venous blood into a plain tube to be spun down to separate the
serum and store at −20 ◦C. Send to reference laboratory for tryptase
estimation.
Take several samples:
Immediately if the clinical situation allows.
1 hour after start of reaction.
6–24 hours after the reaction.
The rise in tryptase is transient and so timing is important.
Tryptase concentration is thought to reach a peak at 1 hour after an anaphylactic
reaction, but there is evidence to suggest that the rise is earlier in reactions
with hypotension;
if the 1 hour sample only is taken, the rise may be missed. It
is essential to label the samples with the appropriate timings.
Later IX
Identification of suspected agent.
The patient should be referred to a regional allergy specialist.
Skin prick tests.
Anaesthetist is responsible for ensuring they are performed (either by
himself or, ideally, a specialist in interpretation of skin tests).
Tell me about plasma tryptase.
rotein contained in mast cells – 99% of total body tryptase is within mast
cells.
Released (along with histamine and other amines) in anaphylactic/
anaphylactoid reactions.
Sensitive and specific diagnostic test for anaphylactic/anaphylactoid
reactions.
Half-life 2.5 hours.
Basal level = 0.8–1.5 ng/ml.
Level >20 ng/ml seen in anaphylactic reactions
NB Urinary methylhistamine is the principal metabolite of histamine, but is
difficult to interpret outside the normal range, although it will be detectable
for longer than plasma tryptase.
How do you perform skin prick tests?
Testing should occur 4–6 weeks after the reaction.
Testing should include a wide range of anaesthetic drugs as well as the
suspected agents to establish those drugs that are safe for future use.
Performed with ‘neat’ drug and 1 in 10 dilution.
Positive control (histamine) and negative control (phenol saline) should be
included for each test.
A drop of drug is applied to the volar aspect of the forearm and a lancet
stabbed through it to break the skin but not draw blood.
Sites are inspected over 15 minutes for wheal and flare.
A wheal of >2mm larger than the negative control is regarded as positive.
A positive skin-prick test to 1 in 10 solution is a true positive. (True reactions
may have positive tests at much greater dilutions, e.g. 1 in 100 or 1 in 1000.)
A positive test to ‘neat’ drug may or may not be significant.
Reporting:
Give full explanation to the patient (Medic-alert bracelet).
Record in the case notes.
Inform GP.
Complete a ‘Yellow card’ to notify the MHRA.
