What does a pacing rate of 75 with a pulse rate of 70 indicate?

Understanding Pacing Rate vs. Pulse Rate in Cardiac Pacing

A pacing rate of 75 with a pulse rate of 70 indicates that there is electrical capture of the heart but incomplete mechanical capture, resulting in a loss of 5 beats per minute between electrical stimulation and mechanical cardiac output.

Electrical vs. Mechanical Capture

When a pacemaker delivers an electrical stimulus to the heart, two distinct processes must occur for effective cardiac function:

1. Electrical Capture: This refers to the successful depolarization of cardiac tissue by the pacemaker stimulus. The pacing rate (75 bpm in this case) represents how frequently the pacemaker is delivering electrical impulses to the heart.

2. Mechanical Capture: This refers to the heart's mechanical contraction in response to electrical stimulation. The pulse rate (70 bpm in this case) represents the actual mechanical contractions of the heart that produce a palpable pulse.

Causes of Discrepancy Between Pacing and Pulse Rates

The 5 bpm difference between pacing rate and pulse rate can be attributed to several factors:

• Intermittent Failure to Capture: Some pacemaker impulses may fail to trigger ventricular contraction 1
• Pacemaker Syndrome: Particularly in patients with ventricular pacing, where AV synchrony is lost 1
• Hemodynamic Compromise: Reduced stroke volume during some beats may result in pulses too weak to be detected peripherally 2
• Fusion or Pseudofusion Beats: Where intrinsic cardiac activity and pacemaker stimulation occur simultaneously

Clinical Significance

This discrepancy has important implications for patient management:

• Reduced Cardiac Output: The loss of 5 effective beats per minute may impact hemodynamic status, especially in patients with underlying cardiac dysfunction
• Need for Pacemaker Adjustment: The settings may need optimization to improve mechanical capture 1
• Possible Underlying Pathology: May indicate myocardial issues such as ischemia, fibrosis, or electrolyte abnormalities affecting cardiac contractility

Assessment and Management

To address this discrepancy:

1. Verify Electrical Capture: Review ECG to confirm that all pacemaker spikes are followed by appropriate QRS complexes

2. Evaluate Pacemaker Settings:

   • Check pacing threshold and increase output if necessary
   • Consider adjusting the AV delay in dual-chamber systems
   • Evaluate rate-responsive settings if applicable 1

3. Assess for Underlying Causes:

   • Electrolyte abnormalities (particularly potassium, calcium)
   • Medication effects (beta-blockers, calcium channel blockers)
   • Myocardial ischemia or infarction
   • Progression of underlying heart disease

4. Consider Pacemaker Syndrome: If the patient has symptoms such as dizziness, fatigue, or hypotension, pacemaker syndrome may be present, particularly with ventricular pacing 1, 3

Optimization Strategies

For patients with this discrepancy:

• Adjust Pacing Output: Increase voltage or pulse width to ensure consistent capture
• Consider Mode Changes: In dual-chamber systems, optimizing AV delay or switching to atrial-based pacing if appropriate
• Rate-Responsive Features: May need adjustment to better match physiologic needs 4
• Evaluate for Biventricular Pacing: In patients with heart failure, cardiac resynchronization therapy may improve mechanical efficiency 5

This discrepancy between pacing rate and pulse rate highlights the important distinction between electrical and mechanical cardiac function, and requires careful evaluation to ensure optimal pacemaker function and patient outcomes.