805 FAILURE OF RYTHMIQ, AN ALGORITHM FAVOURING INTRINSIC CONDUCTION: WHEN IS TOO MUCH OF A GOOD THING?


 An 80-year-old woman received a dual chamber pacemaker (Boston Scientific Accolade MRI DR) for pre-syncopal episodes associated with transient type 1 and 2:1 II degree atrioventricular block, recorded in 24-hour Holter monitoring. Due to residual AV conduction, pacemaker was set with RYTHMIQ algorithm, in order to reduce inappropriate ventricular pacing. A month later, the patient began to complain of severe asthenia and bradycardia (46-48 bpm). Telemetry-supported pacemaker control revealed III degree AV block with junctional escape rhythm, unmasking failing to switch of RYTHMIQ algorithm.
 Why did it happen?
 The RythmIQ algorithm has been designed to promote intrinsic conduction in Boston Scientific devices. With RythmIQ the device provides AAI(R)[i] at the lower rate limit and/or sensor indicated rate. It provides backup VVI pacing at a rate 15 bpm slower than the programmed lower rate limit, with backup VVI pacing rate limited to no slower than 30 bpm and no faster than 60 bpm. If 3 slow ventricular beats are detected in a window of 11 beats, the device automatically switches to DDD(R) mode. A slow beat is defined as a ventricular paced or ventricular sensed event that is at least 150 ms slower than the AAI(R) pacing rate. Slow ventricular beats are any of the following: - V paced event- V sensed event > AAI lower rate limit + 150 ms- V sensed event > AAI(R) sensor indicated rate + 150 ms
 In our case LRL was set at 50 bpm (1200 ms). During III degree AV block, patient's ventricular rate was always about 46-48 bpm, due to stable junctional escape rhythm. Thus no ventricular sensed event occurred with a V-V cycle greater than AAI lower rate limit + 150 ms, namely 1350 ms (around 44 bpm). Moreover the device didn't provide backup VVI pacing at a rate 15 bpm slower, namely 35 bpm. This two conditions impaired Rhythm IQ switching from AAI with VVI backup to DDD.
 The device was set manually in DDD with restoring of AV synchrony and adequate ventricular rate followed by complete resolution of symptoms.
 Rhythm-IQ algorithm doesn't check for AV synchrony, providing switch from AAI with VVI backup to DDD just in case of sudden and marked bradycardia. Loss of AV synchrony doesn't represent a switching criteria and moreover it is not recognized by the device. This condition could be dangerous failing to correct AV conduction disturbances in pacemaker recipient, especially if LRL was set ≤ 50 bpm. For this reason, adequate knowledge of the algorithm would indicate greater caution in using RythmIQ in certain categories of patients, such as the one in the case just described.