Sedation, anticholinergic and cardiotoxity are possible depending on dose and whether older (sedating) or newer (non-sedating) generation of antihistmines.
Competitive inhibition at H1 receptors. Sedating AH are lipophilic and cross BBB, they also block M1, & 5HT receptors. Non-sedating AH are less lipophilic and H1 selective, although OD may selectivity and still cause CNS sedation. Cardiac Na & K channel blockade occurs in large/massive OD.
Well abs. Lipophilic with large Vd (Sedating AH), or small Vd (Non-sedating). Variable liver met. Variable T½.
CNS depression/sedation: minor with non-sedating AH
Anticholinergic toxidrome: minor with non-sedating AH
Seizures: rare – only really a risk with sedating AH
Hypotension 2º to NaBlockade rare after massive OD of sedating AH.
QTc 2º to KBlockade rare after large OD of non-sedating AH.
Screening: serial ECG, paracetamol, BSL
Large OD may CVS toxicity otherwise sedation/anticholinergic effects more likely with sedating AH. Children may gate anticholinergic delirium.
Rarely required but should be cardiac monitored for 6h (12h non-sedating).
Treat seizures with BDZs.
For QRS/NaBlockade: Sodium bicarbonate 1–2mmol/kg boluses over 1–2min.
For QTc: MgSO4, isoprenaline or overdrive pacing.
Antidote: Physostigmine if severe anticholinergic delirium unresponsive to BDZ.
If asymptomatic and normal ECG at 6hr d/c else admit. If sig. delirium/sedation or dysrhythmia HDU/ICU.