MRI with Pacemakers: VI – Pacemaker Modes Reprogrammed to during MRI

MRI with Pacemakers: VI – Pacemaker Modes Reprogrammed to during MRI 

Pacemaker modes are broadly divided into asynchronous and synchronous modes.  Asynchronous mode: pacemaker delivers pacing signal at a fixed rate independent of the natural or spontaneous contractions of the heart. Rarely used except in some procedures such as MRI.

Synchronous mode: pacemaker sets up timing windows (or timer) for the atrium and for the ventricle. For the atrium, it will only deliver a pacing signal if within the timing window for atrium, a natural or spontaneous contraction does not occur; likewise for the ventricle. This is also called in-demand mode. An example is when the heart rate is not up to the rate set by the pacemaker. In this case, the pacemaker will deliver pacing signals to make the heart beats faster.

In asynchronous mode, since the pacemaker’s pacing and the spontaneous pacing are independent to each other, there could be a conflict if one pacing starts before the other pacing ends. For example, the ventricle is in the process to polarize (charge up), but at the same time, there is a pacing signal asking it to depolarize.  Another conflict is when both the intrinsic and the extrinsic pacings occur at almost the same time.

(A) Why the pacemaker mode needs to be changed during MRI?

The following descriptions from literature give the reasons.

“Use of “MRI  Mode” with disabling of arrhythmia sensing/treatment and ICD shocking”

“All ICDs had a programmable MRI mode to provide pacing without the risk of inappropriate inhibition by electromagnetic fields and to eliminate functional interference during the MRI procedure by temporary deactivation of anti-tachycardia “

 “Before the MRI scans, the ICD devices were programmed to MRI mode to disable detection and therapy of ventricular tachyarrhythmia. During the scans, the patients were continuously monitored by electrocardio-gram (ECG), pulse oximetry, blood pressure unit, or a combination of these methods. At least one physician had to be present during the procedure (radiologist, cardiologist, or electrophysiologist), and a second person (that could also be an authorized technical MRI assistant or cardiology technician) in order that one staff member could focus on the MRI aspects (e.g. radiologist, cardiologist experienced with MRI, or authorized technical MRI assistant) and that the other staff member could focus on the monitoring and the patient.”

One modern brand of pacemakers has a timer for the MRI mode. When the timer expires, the PM reverts back to the original mode used before MRI.

The manufacturers do not publish what the MRI modes are, but I was able to find one pacemaker uses AOO, VOO or DOO as MRI modes.

Below are the modes used in MRI with pacemakers studies that I have come across in the literature.

1) OOO: the pacemaker is turned off.

2) VOO:  This is a popular setting for MRI. In this mode, the ventricle is paced at a fixed (but often elevated) rate.  Sensings and responses to sensing are disabled.  It ignores the intrinsic pacings in the atria or ventricle. So conflict occurs when the ventricle is in the spontaneous process to polarize while the extrinsic pacing commands it to depolarize.

“Ventricular asynchronous (VOO) pacing is the simplest of all pacing modes because there is neither sensing nor mode of response. The timing cycle is shown in Figure 6.1. Irrespective of any other events, the ventricular pacing artifacts occur at the programmed rate. The timing cycle cannot be reset by any intrinsic event. In the absence of sensing, there is no defined refractory period.” (Ref 1)

3) AOO: same as VOO, except pacing takes place in the atrium. 

4) DOO: In this mode, sensing and response to sensing are disabled. The generator starts 2 pacing sequences at the same fixed rate. The first sequence is for the atrium and the second sequence, offset from the first by an amount equal to the AV delay, is for the ventricle.  Since both sensings are off, there could be conflict between spontaneous and extrinsic pacings.

 “Dual-chamber, or AV sequential asynchronous (DOO), pacing has an equally simple timing cycle. The interval from atrial artifact to ventricular artifact (atrioventricular interval, AVI) and the interval from the ventricular artifact to the subsequent atrial pacing artifact (ventriculoatrial interval, VAI, or atrial escape interval, AEI) are both fixed. The intervals never change, because the pacing mode is insensitive to any atrial or ventricular activity, and the timers are never reset (Fig. 6.2).” (Ref 1)

“Asynchronous modes, VOO or DOO: These are asynchronous pacing modes in which the pulse generator delivers a pacing stimulus at a fixed rate, without any sensing capabilities. Therefore, the pacemaker is not “in sync” with the patient’s native rhythm and continues to deliver a pacing stimulus regardless of what the native conduction is doing.

These modes are rarely used for extended periods of time. They are typically used when a pacemaker dependent patient is undergoing a surgical procedure that uses electrocautery that could be sensed by the pacemaker as native electrical conduction, which would inhibit pacemaker output and subsequently the patient could have profound bradycardia or even asystole. There is a small possibility that pacing in an asynchronous mode could induce a pacing stimulus in the vulnerable period (on the T wave), which could potentially induce a lethal ventricular tachyarrhythmia.” (ref 2)

“VOO/DOO are asynchronous modes, which are used to prevent oversensing during episodes of electromagnetic interference (EMI), such as during magnetic resonance imaging (MRI) or electrocautery. In dual-chamber systems, an atrial tachyarrhythmia sensed in the DDD mode can lead to ventricular pacing at rates up to the upper rate limit because atrial events are tracked in the ventricle. Mode-switching algorithms switch to a nontracking mode (VVI, DVI, or DDI). Mode-switch events indicate the potential need for anticoagulation.” (ref 3)

5) VVI: Ventriculal pacing inhibited when there is spontaneous ventricular contraction

“Ventricular Inhibited Pacing

By definition, ventricular demand inhibited (VVI) pacing incorporates sensing on the ventricular channel, and pacemaker output is inhibited by a sensed ventricular event (Fig. 6.3).VVI pacemakers are refractory after a paced or sensed ventricular event, a period known as the ventricular refractory period (VRP). Any ventricular event occurring within the VRP is not sensed and does not reset the ventricular timer (Fig. 6.4).” (Ref 1)

“VVI(R) is ventricular demand pacing. The ventricle is paced, sensed, and the pulse generator inhibits pacing output in response to a sensed ventricular event. This mode of pacing prevents ventricular bradycardia and is primarily indicated in patients with atrial fibrillation with a slow ventricular response.” (Ref 2)

6) DDI: Non–P–Tracking-Synchronous Pacing

“DDD = dual-chamber antibradycardia pacing; if atria fails to fire, it is paced. If the ventricle fails to fire after an atrial event (sensed or paced) the ventricle will be paced. DDI = Like above, but the atrial activity is tracked into the ventricle only when the atria is paced.”  (ref 4)

“DDI pacing mode uses two pacing leads, into right atrium and right ventricle. It monitors the two cavities and maintains a minimum rate in each of these cavities. It is mostly used in the presence of sinus bradycardia with possible AV conduction disorders or in the presence of frequent atrial arrhythmia.” (ref 5)

 “The DDI mode of response is inhibition only; that is, no tracking of P waves can occur. Therefore, the paced ventricular rate cannot be greater than the programmed LRL(lower rate limit). The timing cycle consists of the LRL, AVI(AV  delay), postventricular atrial refractory period (PVARP), and VRP (ventricular refractory period). The PVARP is the period after a sensed or paced ventricular event during which the atrial sensing circuit is refractory. The atrial sensing circuit does not sense any atrial event occurring during the PVARP. If a P wave occurs after the PVARP and is sensed, no atrial pacing artifact is delivered at the end of the VAI(AV Interval). The subsequent ventricular pacing artifact cannot occur until the VV interval has been completed; that is, the LRL cannot be violated (Fig. 6.11).

It bears repeating that, because P-wave tracking does not occur with the DDI mode, the paced rate is never greater than the programmed LRL.” (Ref 1)

“To avoid inappropriate activation of pacing due to tracking of radiofrequency pulses, we suggest device programming in patients without pacemaker dependence to a non-tracking ventricular (VVI) or dual chamber inhibited pacing mode (DDI). If the patient’s device was programmed to AAI-DDD, we would recommend programming to VVI or DDI during scanning.” (Ref 6)


Ref 1 “Pacemaker Timing Cycles”, David L. Hayes and Paul A. Levine

Ref 2: “Modes of cardiac pacing: Nomenclature, Selection and Indications for permanent cardiac pacing”, Roger A. Freedman;Daniel Noonan”

Ref 3: “Cardiac Pacemakers: Part 1”, Thomas C. Crawford

Ref 4: “Pacemaker Designation”,when%20the%20atria%20is%20paced.

Ref 5:

Ref 6: “Magnetic resonance imaging safety in patients with cardiac implantable electronic devices”, Eunice Yang and others


Programming PM for MRI

by Rch - 2023-03-21 23:26:49

Thanks for the summary on the various PM modes for different indications including MR. It was very educational 

FYI: The following is the actual guided approach for BS- MR conditional pms to device techs preparing a patient for MRI. Might be of interest to some!

Very helpful info

by brady - 2023-03-22 15:57:08

Thank you the link, the information is very useful for people that want to MRI!

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