• AAI and VVI (included in this section) are single-chamber demand pacing modes; whereas DDI (included in “Dual-Chamber Pacing” section) is dual-chamber demand pacing mode.
• Demand pacing is pacing that occurs only when there is a “demand” for it, meaning when there is no intrinsic atrial or ventricular electrical activity sensed. When there is intrinsic atrial or ventricular electrical activity, pacing output is inhibited (”I” letter of the pacing code).
• VDD is a P-synchronous pacing mode because the ”D” function (which both inhibits “I” and triggers “T” pacing output) allows for atrioventricular (AV) synchrony (detailed mechanism is discussed below).
• In AAI mode, the right atrium is paced ”A” and sensed ”A”. When an intrinsic electrical activity is sensed, pacing output is inhibited “I”.
• ECG shows beats that are:
(1) Paced ”A” : have pacing spikes that appear before each paced P wave (atrial depolarization triggered by pacing output). Because impulses travel down the AV junction-His bundle-Purkinje system, the resulting QRS complexes are narrowed
(2) Non-paced (intrinsic): When an intrinsic beat is sensed, pacing output is inhibited “I” for that beat hence no pacing spike and intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
• This mode can only be used in patients with bradyarrhythmia without AV block but since most patients have a chance of developing AV block later down the line, a dual-chamber device is often placed instead.
• In VVI mode, the right ventricle is paced ”V” and sensed ”V”. When an intrinsic electrical activity is sensed, pacing output is inhibited “I”.
• ECG shows bbeats that are:
(1) Paced ”V”: have pacing spikes that appear before each widened QRS complex (impulses don’t travel down the AV junction-His bundle-Purkinje system). There is no P wave because atria are not paced
(2) Non-paced (intrinsic): When an intrinsic beat is sensed, pacing output is inhibited “I” for that beat hence no pacing spike and intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
• Because of AV dyssynchrony (uncoordinated contractions of atria and ventricles), this mode is reserved for patients with atrial fibrillation with slow ventricular response.
• In VDD mode, the right ventricle is paced ”V”, both right atrium and ventricle are sensed ”D”. When an intrinsic impulse 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 intrinsic impulse is successfully conducted down to the ventricle, pacing output in the ventricle is inhibited (the “I” component of ”D”).
• ECG shows both beats that are:
(1) Asynchronously paced ”V”: have pacing spikes before each widened QRS complex (impulses don’t travel down the AV junction-His bundle-Purkinje system). There is no P wave because atria are not paced
(2) P-synchronously paced “T”: have pacing spikes before each widened QRS complex, preceded by intrinsic P wave
(3) Non-paced (intrinsic) “I”: When an intrinsic beat is sensed, pacing output is inhibited for that beat hence no pacing spike and intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
• This mode allows for AV synchrony as compared to VVI therefore it is used in patients with AV block.
• In case of SVT, mode switching feature can convert VDD to VVI mode to prevent tracking of undesirably fast atrial beat.
