Programming of the basic parameters

Basic concepts

The main objectives of programming the basic settings of cardiac resynchronization therapy (CRT) are a) the maintenance of consistent BiV capture, and b) the optimization of hemodynamic function. CRT recipients suffer from a variety of underlying diseases. Therefore, the optimization of CRT must be tailored to each patient’s characteristics, as well as take into account the detection and treatment of ventricular tachyarrhythmia.

Minimal rate and pacing mode

Setting the basic pacing interval influences the percentage of atrial pacing. The left atrium is the only cardiac chamber that cannot be paced directly with a standard CRT system, although, in failing hearts, the left atrium plays a major role in determining the LV preload. The activation of the left atrium seems generally longer during atrial pacing (atrial latency + inter-atrial delay) than during sinus rhythm. As a consequence, the LV filling time may be shortened during atrial pacing. 

In absence of sinus node dysfunction (SND), the basic heart rate (HR) should be kept slow in order to limit the percentage of atrial pacing as much as possible and to preserve the physiological activation of the left atrium and spare the battery. While both the VDD and DDD pacing modes can be programmed, the DDI mode should be avoided in absence of SND. In presence of SND at rest, whether endogenous or drug-induced, atrial pacing is mandatory. The DDD mode with rate response is preferred (in case of chronotropic incompetence) and the VDD mode must be avoided.

While there is no consensus regarding a minimal HR applicable to all patients, a basic pacing rate of 60 bpm is probably associated with a higher cardiac output than a rate of 40 bpm, without increasing the myocardial oxygen consumption. A resting HR >70 bpm should be avoided, although in presence of frequent ventricular extrasystoles or slow VT, an increase in the minimal HR may alleviate the arrhythmic burden and increase the BiV stimulation rate. However, these effects are usually temporary or only partially effective. One can also program algorithms to lower the incidence of atrial fibrillation (AF) by increasing the HR and forcing atrial stimulation. The benefits conferred by these measures in recipients of CRT systems remain to be shown.

Algorithms have been developed by the device manufacturers to limit the percentage of ventricular stimulation, which is the inverse of the effect sought by resynchronization. It may be useful, however, when BiV stimulation needs to be interrupted, in case of lead dislodgement or in patients worsened by resynchronization. This kind of algorithm is programmable in CRT devices made by Sorin (Safe-R) or Saint Jude Medical (VIP), though not by Biotronik (Vp suppression will be available in future devices), Medtronic (MVP) or Boston Scientific (Rythmiq).

Upper tracking rate

The detailed characteristics of the upper tracking rate will be discussed in the paragraph dedicated to the programing during exercise. A large proportion of CRT system recipients are in sinus rhythm, with normal AV conduction and normal chronotropic response. Therefore, the upper tracking rate should be programmed just above the maximum sinus rate reached during exercise, thereby preserving BiV stimulation throughout exercise and preventing the development of Wenckebach periodicity. The programing of a slow upper tracking rate in CRT system recipients is a common error, which causes the loss of BiV stimulation at peak exercise, when it is probably most needed. The reliability of mode switches currently available in pacemakers is high enough to eliminate the risk of rapid pacing during atrial fibrillation. Therefore, it is safe to set the upper tracking rate as high as 150 bpm in many patients, particularly if a loss of BiV stimulation is observed during exercise. 

Rate responsive mode

The principles of the rate responsive algorithm are detailed in the chapter dedicated to the programming of CRT during exercise. Spontaneous as well as drug-induced chronotropic insufficiency is often observed in patients suffering from heart failure. The programming of a rate responsive function in these cases increases the patient’s exercise capacity and improves the quality of life. The DDDR pacing mode should be favored in patients whose heart rate at peak exercise is <70% of the maximum predicted. In contrast, in presence of a normal chronotropic function, a rate responsive algorithm should be avoided, as it may unnecessarily increase the atrial pacing rate, increase myocardial oxygen consumption, and disturb hemodynamic function.

Mode switch

Patients suffering from heart failure are at high risk of developing episodes of atrial fibrillation. Therefore, a mode switch function should be systematically programmed. If the patient is in atrial fibrillation, a slightly increased resting heart rate (70 bpm for example) along with rate responsiveness, increases the percentage of BiV stimulation.

Atrial and ventricular sensing

The main objective of device programming is to obtain a percentage of BiV stimulation approaching 100%. Therefore, any ventricular oversensing inhibiting BiV stimulation must be thoroughly eliminated. Ventricular sensing must be programmed to avoid P and T waves sensing, as well as ventricular double counting, which is common in these patients in whom conduction disorders are prominent, while preserving reliable sensing of the ventricular electrograms, a key to the flawless function of a defibrillator.

The defibrillators made by Medtronic allow a choice between standard and integrated bipolar sensing if the implanted lead is a standard bipolar lead. Atrial undersensing may also lower the percentage of BiV stimulation. Exercise and an increase in respiratory motion are often the cause of unreliable atrial sensing. Therefore, a wider safety margin must be programmed than for sensing under resting conditions.

LV sensing in a defibrillator

LV sensing is not available with the Medtronic, Saint Jude Medical and Sorin defibrillators. 

Specificities

Biotronik

An algorithm can be programmed to trigger LV stimulation by the sensing of a right ventricular lead signal.

The “LV-T wave Protection” is an additional programmable algorithm, that protects against ventricular stimulation on the T wave (arrhythmogenic risk) after a PVC originating from the left ventricle (sensed by the left before the right ventricular lead).

After the sensing of an event by the left ventricular lead, an interval begins that corresponds to the maximum tracking rate (maximum trigger interval) during which a signal sensed by the right-sided lead does not trigger a LV stimulus.

Example of a LV PVC sensed after a delay by the right-sided lead; the algorithm might trigger LV stimulation after right-sided sensing, causing the stimulus to fall in the vulnerable period of the PVC. The protection algorithm prevents this potentially arrhythmogenic stimulation.

Examples of problem due to the programming of this algorithm; after oversensing of the T wave by the left ventricular lead, an interval begins during which LV stimulation is precluded, explaining its absence (right-sided ventricular stimulation only). By perpetuating itself, this phenomenon causes a decrease in the percentage of LV stimulation.

Boston Scientific

LV sensing by a defibrillator operates as with a right-sided lead, with an automatic gain control. The nominal LV sensitivity is 1 mV. LV sensing can be programmed OFF. The aim of this sensing function is, like in the Biotronik devices, to prevent the delivery of a stimulus in the vulnerable period of a PVC originating from the left ventricle. A programmable LV refractory period prevents the inhibition of BiV stimulation after sensing of a T wave by the left-sided lead, for example. The programming of a long LV refractory period shortens the LV protection window, i.e. the interval during which stimulation is inhibited by sensing from the left-sided lead. The implantation of a quadripolar LV lead allows the programming of 8 separate left-sided sensing configurations.

Post ventricular atrial refractory period

Repetitive AR (atrial activity sensed in the post ventricular atrial refractory period) -VS sequences with loss of BiV stimulation may occur when a relatively long post ventricular atrial refractory period (PVARP) has been programmed: after a PVC or after oversensing of the T wave, the following P wave may fall in the PVARP. This P wave does not trigger an AV delay and is propagated to the ventricle (sensing of spontaneous QRS). If the PP interval is shorter than AR-VS + PVARP, the following P waves falls in the PVARP and the AR-VS cycles continue. The longer the PR interval (prolonged AR-VS), the greater the likelihood of a successive AR-VS cycles. Some algorithms, which temporarily lengthen the PVARP (after a PVC or when interrupting a pacemaker-mediated tachycardia), might facilitate this process. The automatic programming of the PVARP (its adaptation as a function of the on-going heart rate, i.e. the slower the heart rate, the longer the PVARP) may also facilitate its occurrence. Interrogation of the memories enables the identification of these repetitive (AR-VS) cycles within the episodes of ventricular sensing (which are available in the Medtronic devices). The programming of these algorithms must be reviewed on a case-by-case basis in recipients of CRT devices. Depending on the manufacturer, some algorithms can be programmed to interrupt this succession of AR-VS cycles and recover BiV stimulation

Biotronik

With Biotronik products, the PVC lock-in algorithm should terminate this kind of tachycardia, though it is no longer included in the new defibrillators.

Boston Scientific

The Tracking Preference algorithm designed to maintain atrial-tracked ventricular pacing (Nominal ON) can be programmed in these circumstances. If two consecutive cycles occur, in which a sensed ventricular event is preceded by an atrial event sensed in the PVARP, the pulse generator shortens the PVARP until normal atrial-tracked ventricular pacing is re-established. The PVARP is shortened to blanking A after RV sense, and the AV delay is lengthened in order to not exceed the programmed maximum rate. The shortened PVARP remains in effect until a ventricular paced event occurs at the programmed AV delay.

Medtronic

The algorithm "Atrial Tracking Recovery" restores atrial synchronization when lost because of several consecutive atrial events occurring inside the PVARP of sensed ventricular events. This function temporarily shortens the PVARP to allow the synchronization of sensed atrial events to the ventricle and the resumption of CRT. The function of this algorithm is explained by a tracing.

Abbott and Sorin

These manufacturers do not offer this type of algorithms.