Lesson 19

Baroreceptor Reflex: Impairment by Drugs

• Impairment of baroreceptor reflex indicated by lack of compensatory heart rate response to blood pressure changes.

Drugs Known to Impair Baroreceptor Reflex:

• Volatile anesthetics (dose-dependent)
• Norepinephrine (dose-dependent)
• Propofol
• Beta-blockers (depends on level of ẞ1-blockade)
• Calcium channel blockers
• ACE inhibitors
• PDE inhibitors

Key Insights

• Preserved Reflex: If a drug does not alter baroreceptor function.
• Impaired Reflex: If a drug alters baroreceptor function; can cause hemodynamic instability in anesthesia.

Specific Drug Effects:

Volatile Anesthetics

• Impair reflex in dose-dependent manner; Isoflurane preserves reflex best due to mild ẞ1 agonism.

IV Induction Agents

• Propofol: Impairs function, can cause bradycardia/asystole.
• Ketamine: Activates SNS, increased HR, possible hypotension in critically ill patients.
• Etomidate: Usually no change in HR, slight decrease in SVR, hypotension in hypovolemic patients.
• Thiopental: Preserves function, decreases SVR with compensatory HR increase.

Vasodilators

• Hydralazine: Preserves reflex via compensatory HR increase.
• Nitroglycerine/nitroprusside: Preserve reflex.

Beta Adrenergic Blockers

• May impair reflex depending on the extent of ẞ1-blockade, risk of orthostatic hypotension with Labetalol.

Catecholamines

• NE effects are dose-dependent; lower doses favor ẞ1 effects (increase HR), higher doses favor a1 effects (decrease HR).
• Other catecholamines always increase HR.
• Phenylephrine: Non-catecholamine, can cause bradycardia reflecting preserved reflex.

Other Agents

• Calcium channel blockers and PDE inhibitors impair reflex.
• ACE inhibitors alone don’t impair reflex but may do so when combined with other vasodilators or in hypovolemia.