Nominal Parameters

Device: CRT Field: Nominal parameters

1. Basic concepts

Basic parameters programming

BASIC PARAMETERS PROGRAMMING

The first goals of the basic CRT programming are to maintain a permanent biventricular capture and to optimize the patient hemodynamic. CRT recipients may present various form of underlying disease. Therefore CRT optimization should be tailored to the patient characteristics. CRT programming should also take into account the need for detecting and treating ventricular arrhythmias. 

Minimal rate and pacing mode 

Tuning the pacing basic interval will impact the percentage of atrial pacing. The left atrium is the only cavity that cannot be stimulated directly with a conventional CRT; however, it plays a major role in the preload of the LV failing heart. It seems that the activation time of the left atrium is longer in a majority of patients during atrial stimulation (atrial latency + inter-atrial delay) as compared with the timing during sinus rhythm. As a consequence, the LV filling time may be reduced during atrial pacing. In patients with no sinus node dysfunction, the basic heart rate should be kept low in order to reduce as much as possible the percentage of atrial pacing. This will allow for a more physiological activation of the left atrium but also for preserving the battery. Both the VDD and DDD pacing mode can be programmed. In patients with no sinus node dysfunction, the DDI pacing mode should be avoided.

In patients with a sinus node dysfunction at rest, either intrinsic or induced by a drug (beat-blockers…), atrial pacing is mandatory. The DDD pacing mode with a rate response (in case of chronotropic incompetence) should be preferred and the VDD mode should be avoided. There is no consensus on a minimal heart rate for all patients. At least, it is suggested that a basic pacing rate of 60bpm is associated with a better cardiac output without increasing the cardiac oxygen consumption when compared to a basic heart rate of 40bpm. Also, it can be said that higher resting heart rate (>70bpm) should be avoided. Still, in some patients with frequent ventricular extrasystole or slow VT, increasing the minimal heart rate may allow for reducing the burden of arrhythmias and increase the rate of biventricular pacing. However, these effects are usually temporarily observed or only partially useful. It is also possible to program some algorithm that may reduce the incidence of atrial fibrillation by increasing the heart rate and forcing the atrial stimulation. Potential benefits from these therapies in resynchronized patients remain to be demonstrated. 

Upper tracking rate

Nous reverrons dans le chapitre dédié à la programmation à l’effort, certaines spécificités du réglage de la fréquence cardiaque maximale synchrone chez le patient resynchronisé. Une proportion importante de patients resynchronisés présente un rythme sinusal, une conduction auriculo-ventriculaire normale et une fonction chronotrope préservée. La fréquence maximale synchrone doit donc être programmée supérieure à la fréquence sinusale maximale obtenue à l’effort de façon à maintenir une stimulation biventriculaire permanente à l’effort et éviter les phénomènes de pseudo-Wenckebach. En pratique, la fréquence cardiaque maximale est souvent programmée trop basse chez les patients resynchronisés. Cela s’accompagne d’une perte de la stimulation biventriculaire au maximum de l’effort, moment où une stimulation effective est probablement la plus requise. La qualité du repli dans les pacemakers est maintenant tout à fait satisfaisante ce qui limite le risque de stimulation rapide lors d’un passage en fibrillation auriculaire. Il n’y a donc pas de risque à monter les valeurs de fréquence cardiaque maximale et il ne faut pas hésiter à augmenter cette fréquence à 140, 150 battements/minute surtout si une épreuve d’effort documente une perte de capture à l’effort.

Rate responsive mode

Rate responsive algorithm principles are detailed in chapter 5, dedicated to the programing of CRT during exercise. Intrinsic and drug induced chronotropic insufficiency is frequently observed in HF patients. Programing a rate responsive function in this context clearly improves the patient’s exercise capacity and quality of life. The DDDR pacing mode should be favored in patients presenting a peak exercise HR below 70% of the predicted maximal HR. In contrast, in patients with a normal chronotropic function, rate response algorithm should be avoided because it may result in unnecessary increase of the atrial pacing, that may interfere with the physiological acceleration, increase the myocardial consumption, and even alter the patient hemodynamic. 

Switch mode

Heart failure patients are prone to present episodes of atrial fibrillation. Accordingly, the switch mode should be systematically programmed. If the patient is in atrial fibrillation, a resting HR slightly increased (70bpm for example) in conjunction with a rate response, allows for increasing the percentage of biventricular pacing.

Atrial and ventricular sensing

Atrial leads are bipolar. Ventricular leads are programmable either in "true" bipolar (dedicated) or in tip to coil configuration (integrated bipolar). There are two types of right ventricular leads: the true bipolar (tip electrode + ring electrode) and the integrated bipolar leads (no ring, sensing being performed between the tip electrode and the defibrillation coil). With a true bipolar lead, it is possible to choose between a true bipolar (tip – ring) or an integrated - pseudo bipolar (tip – coil) configuration. The sensing vector is therefore programmable via the polarity configuration. 
With an integrated bipolar lead, the sensing vector will occur only between the tip electrode and the defibrillation coil.
There is no left ventricular sensing on the Medtronic ICDs.
The primary goal of the CRT programing is to maintain the percentage of biventricular pacing around 100%. Therefore, ventricular oversensing and inappropriate inhibition should be minimized. The ventricular sensing should be programmed to avoid the detection of any P wave, T wave or ventricular double counting (frequently observed in patients with severe conduction delay), but also to detect ventricular EGM during ventricular arrhythmias (which is mandatory for the good functioning of the ICD).
Atrial undersensing may also cause a reduction in the rate of biventricular pacing. Exercise and the increase in the amplitude of the respiratory movements are often associated with a decrease in the atrial sensing. Therefore, we recommend programming a relatively high sensitivity as compared with the sensing at rest. 

Blanking periods

A blanking period follows a sensed or a paced event in the same cavity or a paced event in the other cavity. There is also a blanking period after a shock. Blanking periods following paced events are programmable with a value superior or equal to the blanking period following a sensed event.
After biventricular stimulation, the duration of the atrial blanking or the ventricular post stimulation blanking starts from the end of the second stimulus if a VV delay is programmed. The atrial detection is deactivated after an atrial-sensed event for the all duration of the atrial post atrial sensed blanking period. After a paced atrial event, the atrial detection is deactivated for the all duration of the atrial post atrial paced blanking period. The ventricular detection is deactivated after a sensed ventricular until the end of the ventricular post ventricular sensed blanking period. After a paced ventricular, the ventricular detection is deactivated for until the end of the ventricular post ventricular paced blanking period. The Atrial blanking period after a paced ventricular event (30 ms) and the Ventricular blanking period after an atrial paced event (30 ms) are not programmable.

Post ventricular atrial refractory period (PVARP) 

Recurrent sequences AR-VS with loss of biventricular stimulation may occur when relatively long PVARP are programmed classically after a premature ventricular complex or an oversensing of the T-wave. The following P wave falls in the PVARP and do not launch a new AV delay. This results in a conducted, spontaneous QRS. If the PP interval is shorter than the sum AR-VS + PVARP, the following P waves fall also in the PVARP and the AR-VS cycles continue. The probability that the phenomenon continue by itself increases with the duration of the spontaneous PR interval (prolonged AR-VS intervals) and the duration of the PVARP. Some algorithms allowing for a transient prolongation of the PVRAP (response to a VPB, interruption of PMT) can promote this phenomenon. Programing an automatic PVARP (adaptation of the PVARP duration to the heart rate: the slower is the HR, the longer is the PVRAP) can also facilitate this mechanism (PVARP auto).
Interrogating the device memory identifies repetitive cycles (AR-VS) in the ventricular sense episodes. With CRT patients, those algorithms should thus be carefully programed.
To interrupt this succession of AR-VS cycles, the atrial tracking recovery algorithm should be programmed. It allows for a quick recovery of the atrial synchronization in case of its loss due to several consecutive atrial events in the PVARP. The PVARP is temporarily shortened and the following P-wave (classified as AS instead of AR) launches an AV delay triggering biventricular stimulation. 

Ventriculo-atrial crosstalk prevention

VA crosstalk (oversensing of the ventricular stimulus or depolarization by the atrial channel) may alter the quality of the discrimination of the arrhythmias but also trigger inappropriate switch mode commutation and result in the loss of biventricular pacing
Typically, crosstalk problems present themself with an alternating pattern of two different morphologies and two different atrial cycles (short – long sequence). To reduce the risk of crosstalk, the atrial sensitivity may be reduced, with the risk of undersensing episodes of atrial fibrillation.     

2. Specificities by company