What is the mechanism of action of VVIR (Ventricular Pacing with Inhibited Response) stimulation?

Mechanism of Action of VVIR Stimulation

VVIR pacing is a ventricular pacing mode that provides rate-responsive ventricular pacing with inhibition by sensed ventricular activity, allowing the pacemaker to adjust heart rate based on physiological demand without requiring atrial sensing or pacing.

Basic Components of VVIR Pacing

VVIR pacing consists of two fundamental components:

1. VVI Component (Ventricular Pacing with Inhibited Response):

   • Paces the ventricle when no intrinsic ventricular activity is detected
   • Senses ventricular activity
   • Inhibits pacing output when intrinsic ventricular activity is detected 1
   • Functions only in the ventricle without atrial involvement

2. Rate-Responsive Component (R):

   • Utilizes sensors to detect physical activity or physiological changes
   • Automatically adjusts pacing rate based on detected physiological demand
   • Compensates for chronotropic incompetence (inability of the heart to increase rate appropriately during exercise) 1

Detailed Mechanism

Sensing Mechanism

• The pacemaker continuously monitors ventricular electrical activity
• When intrinsic ventricular depolarization occurs, the pacemaker detects this and inhibits scheduled pacing output
• If no intrinsic ventricular activity is detected within the programmed interval, the pacemaker delivers a pacing stimulus

Rate Response Mechanism

• Activity sensors (accelerometers) detect body movement
• Minute ventilation sensors measure changes in respiration
• Some devices use dual sensors for more physiologic response 2
• The pacemaker algorithm translates sensor input into appropriate heart rate changes
• Pacing rate increases during physical activity and decreases during rest

Clinical Applications

VVIR pacing is indicated for:

• Symptomatic bradyarrhythmias with chronotropic incompetence and anticipated moderate to high level of physical activity 1
• Patients with persistent or paroxysmal atrial fibrillation/flutter where atrial contribution is not significant 1
• Situations where pacing simplicity is a prime concern (terminal illness, senility, remote location from follow-up center) 1
• Patients without retrograde ventriculoatrial conduction (to avoid pacemaker syndrome) 1, 3

Hemodynamic Effects and Limitations

• VVIR pacing lacks atrioventricular synchrony, which can lead to:

  • Reduced cardiac output compared to modes with AV synchrony 4
  • Pacemaker syndrome in susceptible patients 3
  • Increased risk of atrial fibrillation development 5

• Pacemaker syndrome may develop due to:

  • Loss of atrial contribution to ventricular filling
  • Cannon A waves (atria contracting against closed AV valves)
  • VA conduction causing atrial contraction against closed valves 3

Important Contraindications

VVIR pacing is particularly contraindicated in:

• Patients with retrograde ventriculoatrial conduction (increases risk of pacemaker syndrome) 1
• Patients with angina pectoris or congestive heart failure that may be aggravated by fast rates 1
• Patients who would significantly benefit from atrial contribution to cardiac output 1

Modern Considerations

• When comparing VVIR to dual-chamber pacing modes in patients with normal ventricular function and QRS duration, VVIR shows similar rates of death, stroke, and heart failure hospitalization, but significantly increases the risk of atrial fibrillation development 5
• For patients with chronotropic incompetence requiring rate response, dual-chamber rate-responsive pacing (DDDR) may provide better hemodynamics while maintaining AV synchrony 1

VVIR pacing represents a balance between the simplicity of single-chamber pacing and the physiological benefit of rate responsiveness, making it suitable for specific patient populations where atrial sensing or pacing is not feasible or necessary.