Automatic mode switching due to atrial arrhythmias in Boston Scientific pacemakers

Atrial Tachy Response (ATR)

The Atrial Tachy Response feature (ATR mode switch) is designed to limit tracking of atrial arrhythmias by automatically mode switching to a non-tracking mode when programmed ATR criteria are met.

Step 1: ATR trigger rate is exceeded

The ATR algorithm continually monitors for sensed atrial events which are equal to or above the programmed Trigger Rate.

• all refractory (PVARP) and non-refractory atrial events are included
• atrial events within the blanking period or noise windows are not counted

Step 2: entry count is met

The Entry Count is a count of atrial events that occur above the Trigger Rate and determines how quickly the atrial arrhythmia is initially detected.

• the entry counter increments (+1) for each sensed atrial event equal to or above the trigger Rate
• the entry counter decrements (-1) for each sensed or paced atrial event below the trigger rate and each sensed or paced ventricular event in which there has not been an atrial event within the last 2 seconds

Once the counter reaches the programmed entry count, duration begins. A lower programmed entry count requires fewer fast atrial events to fulfill this initial detection and begin duration.

 Step 3: duration criteria is met and the exit count begins

Duration determines the number of ventricular cycles during which the atrial arrhythmia will be monitored prior to mode switching.

• programming a short duration causes the device to mode switch more quickly whereas a long duration prevents mode switching due to shorter, non-sustained atrial arrhythmia episodes
• during duration, the exit count increments and decrements to ensure the count remains above zero for the programmed amount of ventricular cycles
• in the event of a short, non-sustained episode of atrial tachycardia, the exit count will reach zero prior to the end of duration and no mode switch will occur
• if the exit count is above zero at the end of duration, the fallback time and fallback mode will begin

Step 4: fallback mode is initiated

The fallback mode is the non-tracking pacing mode, which is initiated once duration is met and remains in effect until the exit count reaches zero.

• if both permanent and ATR fallback modes are rate responsive, the device will use the permanent sensor parameters
• if the permanent mode is not rate responsive, the ATR fallback mode may be programmed rate responsive using the accelerometer sensor

Step 5: ventricular paced rate decreases during fallback time

Fallback time determines how quickly the ventricular-paced rate will decrease to the fallback lower rate limit, sensor-indicated rate or VRR-indicated rate (whichever is faster).

• programming a shorter fallback time will result in a faster decrease of the ventricular-paced rate which physicians may consider for patients who are symptomatic at higher ventricular rates
• if exit count is met while device is falling back to fallback lower rate limit, it is possible for the rate to drop to normal Brady lower rate limit

Step 6: ventricular paced rate decreases to the fallback lower rate limit

The fallback lower rate limit is the separately programmable lower rate, which the ventricular paced rate will decrease to during fallback time.

• fallback lower rate limit may be greater than or less than the normal Brady lower rate limit

Step 7: exit count is met

The exit count is a count of atrial events that occur below the trigger rate and determines if the device will enter a mode switch following duration or if the device will exit an active mode switch once the atrial arrhythmia ends.

• the exit counter increments (+1) for each sensed atrial event equal to or above the trigger rate
• the exit counter decrements (-1) for each sensed or paced atrial event below the trigger rate and for each sensed or paced ventricular event in which there has not been an atrial event in the last two seconds

The lower the programmed exit count, the faster the device will return to a tracking mode once the atrial arrhythmia ends. All refractory and non-refractory atrial events are included while atrial events within the blanking period or noise windows are not counted.

• if the Exit Count reaches zero during Duration, no mode switch will occur
• if the count reaches zero during an active ATR episode, the mode switch will end and the device returns to normal programmed settings

Atrial Flutter Response (AFR)

Atrial Flutter Response provides immediate dissociation of the atrium and ventricle for atrial rates faster than the programmed AFR Trigger Rate. While AFR immediately prevents tracking of atrial arrhythmias and is sometimes referred to as a one beat mode switch, a true mode switch will not occur until ATR criteria are met.

• a single atrial sensed event occuring in PVARP triggers an AFR window equal to the programmed Trigger Rate (e.g., Trigger Rate of 230 bpm = AFR window of 260 ms)
• if an atrial-sensed event occurs within that AFR window (above the AFR trigger rate), it triggers another AFR window
• as atrial-sensed events continue to occur within AFR windows, they are classified as AS in refractory, are not tracked, and retrigger additional AFR windows

Dual-chamber operation resumes when both PVARP and the AFR window have expired without an atrial sense occurring within them.

• ventricular pacing is not affected by AFR and will always take place as scheduled
• atrial pacing is inhibited during AFR windows

APP/ProACt

The Atrial Pacing Preference (APP) and ProACt features are designed to promote atrial pacing by increasing the pacing rate. APP and ProACt are designed to decrease the number of atrial arrhythmic episodes. APP and ProACt use algorithms that function similarly, but the ProACt algorithm reacts to premature atrial contractions (PACs) while the APP algorithm reacts to non-PAC atrial senses.

Ventricular Rate Regulation (VRR)

VRR is designed to reduce the V–V cycle length variability during partially conducted atrial arrhythmias by increasing the ventricular pacing rate. The VRR algorithm calculates a VRR-indicated pacing interval based on a weighted sum of the current V–V cycle length and the previous VRR-indicated pacing intervals.