dual-chamber pacing

• In DDI mode, both chambers paced ”D” and sensed ”D”. When an intrinsic electrical activity is sensed, pacing output is inhibited “I”.
• ECG shows beats that are:
   (1) AV sequentially paced ”D” (kicks in when heart rate drops too slow): have both atrial and ventricular pacing spikes at regular intervals that appear before each paced P wave and widened QRS complex, respectively
   (2) Non-paced (intrinsic): When an intrinsic signals are sensed in both right atrium & ventricle, pacing output from both chambers are inhibited “I” hence no pacing spike and intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
   (3) Atrially paced only: Pacing output is released from the right atrium and this impulse is sucessfully conducted down the AV junction-His bundle-Purkinje system resulting in a paced P wave preceded by a pacing spike and followed by a narrowed QRS complex. This also leads to inhibition of ventricular pacing output “I”
   4) Ventricularlly paced only: An intrinsic impulse is released in the atrium (intrinsic P wave) leading to inhibition “I” of atrial pacing output but cannot travel down the AV junction-His bundle-Purkinje system. And because it’s over the maximal AV delay (prolonged PR), ventricular pacing output is released resulting in widened QRS complex preceded by a pacing spike
• Because of AV dyssynchrony (uncoordinated contractions of atria and ventricles), this mode is largely replaced by DDD (which now has the capability of mode switching to DDI and VVI during SVT).

• In DDD mode, both chambers paced ”D” and sensed ”D”. When an impulse (paced or intrinsic) is released in the atrium but cannot be conducted down to the ventricle, pacing out is triggered in the ventricle (the “T” component of ”D”). But if the impulse is successfully conducted down to the ventricle, pacing output in the ventricle is inhibited (the “I” component of ”D”).
• ECG shows beats that are:
   (1) AV sequentially paced ”D” (kicks in when heart rate drops too slow): have both atrial and ventricular pacing spikes at regular intervals that appear before each paced P wave and widened QRS complex, respectively
   (2) Non-paced (intrinsic): When an intrinsic signals are sensed in both right atrium & ventricle, pacing output from both chambers are inhibited “I” hence no pacing spike and intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
   (3) Atrially paced only: Pacing output is released from the right atrium and this impulse is sucessfully conducted down the AV junction-His bundle-Purkinje system resulting in a paced P wave preceded by a pacing spike and followed by a narrowed QRS complex. This impulse also leads to inhibition of ventricular pacing output “I”
   (4) Atrially sensed-ventricularlly paced: An intrinsic impulse is released in the atrium (intrinsic P wave) leading to inhibition “I” of atrial pacing output but cannot travel down the AV junction-His bundle-Purkinje system and synchronous triggering “T” of ventricular pacing output resulting in widened QRS complex preceded by a pacing spike
• This mode allows for AV synchrony as compared to DDI and therefore it is often used in patients with AV block.
• In case of SVT, mode switching feature can convert DDD to DDI or VVI mode to prevent tracking of undesirably fast atrial beat.