Diphenhydramine in Cardiac Surgery Patients: Contraindications and Risks
Diphenhydramine (Benadryl) should be avoided in cardiac surgery patients due to its potential for cardiac side effects, including QT prolongation and arrhythmias, and the availability of safer alternatives.
Cardiac Risks of Diphenhydramine
Diphenhydramine poses several specific concerns for cardiac surgery patients:
QT Prolongation: Diphenhydramine can inhibit repolarizing potassium channels, leading to prolongation of the action potential and QT interval 1. This effect is particularly concerning in cardiac surgery patients who may already have electrolyte abnormalities or be taking other medications affecting cardiac conduction.
Cardiac Toxicity: In higher doses or with intravenous administration, diphenhydramine has been associated with serious cardiac effects, including cardiac arrest 2. While the FDA label doesn't explicitly contraindicate diphenhydramine in cardiac surgery patients, it does warn about potential sedative effects that could complicate recovery 3.
Hemodynamic Effects: First-generation antihistamines like diphenhydramine can cause anticholinergic effects that may lead to tachycardia, which could be problematic in the post-cardiac surgery setting where heart rate control is often a priority 4.
Sedation Considerations in Cardiac Surgery Patients
The 2018 Critical Care Medicine guidelines specifically address sedation in cardiac surgery patients:
Preferred Agents: The guidelines suggest using propofol over benzodiazepines for sedation in mechanically ventilated adults after cardiac surgery (conditional recommendation, low quality of evidence) 4.
Alternative Agents: Dexmedetomidine is also recommended as an alternative to benzodiazepines, with evidence suggesting it may reduce the risk of delirium, ventricular tachycardia, and hyperglycemia in post-cardiac surgery patients 5.
Practical Approach
When managing cardiac surgery patients requiring antihistamine effects or mild sedation:
Consider second-generation antihistamines if an antihistamine effect is needed, as they have fewer cardiac and anticholinergic side effects 6.
For sedation purposes, follow the Critical Care Medicine guidelines recommending propofol or dexmedetomidine 4.
Monitor cardiac parameters closely if diphenhydramine must be used, including continuous ECG monitoring for QT prolongation or arrhythmias.
Special Considerations
Elderly patients are at higher risk for adverse effects from diphenhydramine, including confusion and anticholinergic effects.
Patients with pre-existing conduction abnormalities should absolutely avoid diphenhydramine due to the increased risk of QT prolongation.
Patients on multiple medications may experience drug interactions with diphenhydramine, particularly with other sedatives, tranquilizers, or medications that affect cardiac conduction.
While not absolutely contraindicated in all cardiac surgery patients, the potential risks of diphenhydramine and the availability of safer alternatives make it a medication that should generally be avoided in this population, particularly in the immediate post-operative period when cardiac stability is crucial.