EGM recorded simultaneously

• VVI mode 90 beats/minute
• decrease in ventricular amplitude by 0.25V. every 2 beats
• loss of capture (0.75V/0.4 ms)
• threshold is 1V/0.4 ms.

Comments

• the pacing threshold is the lowest electrical pulse, delivered outside the natural refractory period, that causes consistent myocardial capture
• the pacing threshold is determined in terms of Volts (V) and pulse duration
• in clinical practice, the measurement of the pacing threshold is of major importance, since the programming of pulse amplitude and duration determines the pacing safety margin and the energy consumption of the pulse generator, hence the generator longevity
• energy consumption is directly proportional to the pulse width but increases with the square of voltage
• it is usual to program larger safety margins immediately after implant to compensate for post-implant threshold increase
• in patients with intact AV conduction, measurement of the atrial pacing threshold can be measured in the AAI mode (pacing mode above the intrinsic rate); atrial output is progressively lowered and a missing QRS complex indicates loss of atrial capture
• in patients with altered AV conduction, measurement of the atrial pacing threshold has to be measured in the DDD mode; atrial output is progressively lowered and reappearance of atrial sensed events on the marker channel indicates loss of atrial capture
• measurement of the ventricular pacing threshold can be measured in the VVI or DDD mode; DDD mode with a short AV delay is preferentially chosen in patients who feel poorly with loss of AV synchrony during VVI mode
• in pacemaker-dependent patients, loss of ventricular capture may result in asystole