What effect would 25 mg of atenolol (beta-blocker) have on a heart rate of 94 beats per minute (tachycardia)?

Effect of 25 mg Atenolol on Heart Rate of 94 bpm

Atenolol 25 mg would reduce a heart rate of 94 bpm by approximately 15-20 beats per minute within 2-4 hours of administration, with effects persisting for at least 24 hours.

Mechanism of Action and Pharmacodynamics

• Atenolol is a beta1-selective (cardioselective) beta-adrenergic receptor blocking agent that reduces heart rate by blocking catecholamine effects on cardiac adrenergic receptors 1
• The drug demonstrates a significant beta-blocking effect within one hour of oral administration, with maximal effect occurring at 2-4 hours post-dose 1
• Beta-blockers reduce heart rate by: (1) reducing resting and exercise heart rate, (2) reducing cardiac output, (3) inhibiting isoproterenol-induced tachycardia, and (4) reducing reflex orthostatic tachycardia 1

Expected Heart Rate Reduction

• For a heart rate of 94 bpm (mild tachycardia), 25 mg of atenolol would be expected to reduce the heart rate by approximately 15-20 bpm, bringing it to approximately 74-79 bpm 2, 3
• Studies have shown that even low doses of atenolol (25 mg) produce significant reductions in exercise tachycardia, with greater reductions seen at higher doses 3
• The heart rate-lowering effect of atenolol is dose-dependent, with 25 mg being the typical starting dose for rate control 4

Timing and Duration of Effect

• The onset of action begins within 1 hour of oral administration 1
• Peak effect occurs between 2-4 hours after ingestion 1
• The beta-blocking activity of even a single 25 mg dose persists for at least 24 hours, making it suitable for once-daily dosing 1, 5
• The elimination half-life of oral atenolol is approximately 6-7 hours 1

Clinical Considerations for Rate Control

• Atenolol is indicated for control of ventricular rate in patients with atrial fibrillation or other supraventricular tachycardias 4
• For rate control in tachyarrhythmias, the typical starting dose is 25-50 mg daily, which can be titrated up to 100 mg daily if needed 4
• Atenolol has been shown to effectively prevent or control supraventricular tachycardias in approximately 68% of cases 6

Potential Side Effects and Precautions

• Common side effects include hypotension, bradycardia, heart block, and worsening of heart failure 4
• Atenolol should be used with caution in patients with:
  • AV block greater than first degree or SA node dysfunction (in absence of pacemaker) 4
  • Decompensated systolic heart failure 4
  • Reactive airway disease (though atenolol is more cardioselective than non-selective beta blockers) 1
  • Severe renal dysfunction (dose adjustment required) 1

Comparison with Other Beta Blockers

• Atenolol has a longer duration of action compared to metoprolol, making it more suitable for once-daily dosing 3, 5
• In comparative studies, atenolol 100 mg provided more effective blood pressure and heart rate reduction at 25 hours post-dose than metoprolol 100 mg 5
• Atenolol is more cardioselective at lower doses compared to non-selective beta blockers like propranolol 1

In summary, 25 mg of atenolol would effectively reduce a heart rate of 94 bpm to a normal range within a few hours of administration, with the effect persisting for at least 24 hours. This makes it an appropriate choice for rate control in patients with mild tachycardia.