Common Pacer Modes
Choosing the right pacemaker mode depends on numerous factors. These include the patient’s intrinsic cardiac rhythm, age, exercise capacity, comorbidities, and chronotropic response. The modes can be divided into three categories:
Asynchronous Modes
Single Chamber Modes
Dual Chamber Modes
Asynchronous modes will set specific pacing rate regardless of the underlying rhythm. This can either be applied to the atrium,(AOO), the ventricle (VOO), or both (DOO). It is typically a short-term pacing mode, usually set at the backup mode for pacemaker dependent patients undergoing surgery that have risk of pacemaker interference from things like electrocautery. Interference usually occurs if the electrocautery is in close proximity to the pacemaker (ie. Above the umbilicus). It has potential to be “sensed” by the pacer and inhibit pacemaker output causing a profound bradycardia or asystole. This can manifest as hypotension during cautery. There is also the risk for an “R on T phenomenon”. This is when a ventricular pacing stimulus occurs during a native T-wave, leading to a lethal ventricular tachyarrhythmia.
Single chamber modes allow for the pacing of either the atrium or the ventricle if the patient’s underlying rhythms slows down below the threshold for the pacer to take over. The more common mode of this categories is VVI. The ventricle is both paced and sensed here. Let’s say the baseline pacing is set for 70 bpm. If the patient’s ventricular rate is greater than 70bpm, the pacer maker will be off. However, if the ventricular rate drops below 70, then the pacemaker will pace the ventricle at 70 bpm. This mode of pacing is vulnerable to AV dysynchrony since the atrium is not being paced. AAI modes can be used if the AV nodal function is still good, minimizing ventricular pacing. DDD modes can be programmed to switch in and out of AAI mode depending on the sensing of the AV nodal conduction.
Dual chamber modes allow for a lot of flexibility in terms of what the pacemaker can do. The most common pacer mode is DDD. This allows for both the atrium and ventricle to be based, both be sensed, and for the pacemaker to both inhibit and pace the chambers. There are 4 different rhythms that can be observed in DDD.
AsVs: Atrial sensing and Ventricular sensing - this is normal sinus rhythm
ApVs: Atrial pacing and Ventricular sensing - this is when the sinus node is nonfunctional but the AV node still works
AsVp: Atrial sensing and Ventricular pacing - this is when there is a natural P-wave (working sinus node) with a paced ventricular beat
ApVp: Atrial pacing and Ventricular pacing - this is when both chambers are being paced when both the sinus and AV nodes are not functioning
