• AOO, VOO, and DOO are asynchronous pacing modes because paced beats are released at their own pace and separate from the intrinsic beats (due to the lack of sensing).
• On ECG, two distinctive rhythms (paced and intrinsic) co-exist and compete with each other. Because there is an increased risk of ventricular arrhythmias when pacing occurs during intrinsic T wave (“R on T” phenomenon), these modes are now only used in cases where sensing function is not desired (during surgery when electrocautery is used).
• In AOO mode, the right atrium is paced ”A” and there is no sensing “O”.
• ECG shows beats that are:
(1) Paced ”A”: have pacing spikes at regular intervals 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) Intrinsic: Due to the lack of sensing, an intrinsic beat is conducted normally, and in some cases, outcompete the paced beat. As a result, pacing output is released at scheduled intervals but can’t trigger a beat therefore there will be pacing spike without a paced beat but rather an intrinsic beat with intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
• In VOO mode, the right ventricle is paced ”V” and there is no sensing “O”.
• ECG shows beats that are:
(1) Paced ”V”: have pacing spikes at regular intervals 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) Intrinsic: Due to the lack of sensing, an intrinsic beat is conducted normally, and in some cases, outcompete the paced beat. As a result, pacing output is released at scheduled intervals but can’t trigger a beat therefore there will be pacing spike without a paced beat but rather an intrinsic beat with intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
• In DOO mode, both right atrium and right ventricle are paced “D” and there is no sensing “O”.
• ECG shows beats that are:
(1) Paced “D”: have both atrial and ventricular pacing spikes at regular intervals that appear before each paced P wave and widened QRS complex, respectively
(2) Intrinsic: Due to the lack of sensing, an intrinsic beat is conducted normally, and in some cases, outcompete the paced beat. As a result, pacing output are released at scheduled intervals but can’t trigger a beat therefore there will be pacing spikes without a paced beat but rather an intrinsic beat with intrinsic P-QRS (which can be normal sinus beat or PVC as illustrated below)
