Beta-adrenoceptor antagonists increase sinus arrhythmia, a vagotonic effect

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation
• Pharmacology

Background:

• Sinus arrhythmia, a variation in heart rate, is influenced by both vagal and sympathetic nervous system activity.
• Understanding these influences is crucial for interpreting cardiovascular drug effects.

Purpose of the Study:

• To investigate the impact of vagal and sympathetic efferent activity on sinus arrhythmia in humans.
• To determine the role of intrinsic sympathomimetic activity (ISA) in beta-adrenoceptor blockers' effects on heart rate variability.

Main Methods:

• Six healthy subjects participated.
• Drugs administered included hyoscine butylbromide and various beta-adrenoceptor blocking agents (atenolol, metoprolol, oxprenolol, xamoterol).
• Heart rate variability, specifically sinus arrhythmia (quantified as standard deviation of R-R interval), was monitored using a microcomputer-linked electrocardiogram system.

Main Results:

• Hyoscine butylbromide nearly abolished sinus arrhythmia, regardless of concomitant beta-blocker use.
• Beta-blockers without ISA (atenolol, metoprolol) prolonged mean R-R interval and increased sinus arrhythmia.
• Beta-blockers with ISA (oxprenolol, xamoterol) had minimal or no effect on sinus arrhythmia, despite altering mean R-R interval.
• A non-linear relationship was observed between sinus arrhythmia and mean R-R interval.

Conclusions:

• Exaggerated sinus arrhythmia during beta-adrenoceptor blockade appears linked to the absence of ISA and resulting bradycardia.
• Findings support a hypothesis of a central vagotonic effect secondary to peripheral drug action.
• Vagal blockade partially altered drug-induced R-R interval changes, suggesting a role for vagal tone modulation in heart rate responses to these drugs.