Left ventricular pacing
Different access routes have been proposed for the pacing of the left ventricle: the transvenous route, by far the most commonly used, with positioning of the lead in a branch of the coronary sinus; the transseptal route which allows endocardial pacing; the epicardial route after surgical approach, which constitutes the most common recourse in the event of failure of the traditional approach, and finally, a still experimental access route with direct puncture of the pericardial space by transcutaneous approach. Venous access is the first-line approach used in a majority of centers given its high feasibility and limited risk of complications. The stability of the lead, however, remains a limitation with a higher risk of micro or macro-dislodgement than for an endocardial lead.
A percentage of biventricular pacing nearing 100% is a necessary prerequisite but not sufficient for a good response to resynchronization. Indeed, biventricular pacing does not equate to effective biventricular capture. In this patient, memory interrogation revealed permanent biventricular pacing (100% pacing), whereas the electrocardiogram revealed a characteristic pattern of apical right ventricular pacing: positive QRS in lead I, negative in leads II, III and aVF, negative in V1, wide QRS. The left ventricular lead had moved and receded to a position near a left atrial vein explaining the particular pattern of the left ventricular pacing threshold test. This loss of left ventricular capture accounts for this patient’s non-response. Future CRT defibrillator platforms could ultimately be equipped with algorithms enabling to confirm effective capture as opposed to pacing.
Bipolar LV threshold test
In a patient implanted with a MedtronicTM triple-chamber defibrillator connected to a bipolar left ventricular lead, 4 left ventricular pacing configurations can be programmed: LV tip – RV coil, LV tip – LV ring (anode), LV ring –RV coil, LV ring – LV tip. Various elements can more or less influence the choice of programming to privilege: presence of diaphragmatic pacing, hemodynamic impact, pacing threshold and impedance. The selected left ventricular pacing configuration must enable effective and permanent capture by maintaining sufficient safety without stimulation of the phrenic nerve, while limiting the energy used in order to increase the longevity of the device.
Quadripolar LV lead threshold test
Given the multitude of possible configurations, the choice of programming can be complex even if there are a certain number of known fundamentals. In a first step, it appears logical to avoid all configurations associated with phrenic stimulation which is often poorly supported by the patient. Similarly, given the substantial variation in pacing thresholds according to the electrode used, e.g. from a very satisfactory value (<1 volt for 0.4 ms) to values nearing the maximum capacities of the device, it would thus seem advantageous not to select a configuration leading to premature wear of the batteries. A certain number of configurations correspond to these first two criteria (absence of phrenic stimulation, necessity of small pacing amplitude). The hemodynamic criterion could then be taken into account for deciding which configuration to choose. The current literature is rich in terms of publications assessing the impact of the left ventricular pacing site on the degree of response to resynchronization. Unfortunately, the results are controversial and suggest the existence of considerable inter-individual variations without the possibility of defining a same universal optimal site. It seems, however, that an apical position should be avoided, which allows eliminating a certain number of configurations. The 4 electrodes on this type of lead are relatively interspaced: in this patient, the distal electrode (LV1) was positioned in a lateral region whereas the proximal electrode (LV4) was positioned opposite a more apical region (lead positioned in a lower vein continuing into a more basal lateral vein). The electrocardiographic variations are relatively invisible on the lead used for the threshold assessment. In contrast, the 12-lead electrocardiogram found significant changes in the paced QRS pattern according to the chosen configuration. Since the electrical activation is different, it would seem plausible to also envision a different degree of response according to the selected configuration. However, given the absence of a validated method to compare the degree of response to be expected according to the pacing site chosen, it remains very difficult to choose a programming based on a possible superior clinical benefit. The possibility of choosing a pacing site without phrenic stimulation with an acceptable pacing threshold while reducing the risk of dislodgement (possibility of pushing the leads more distally in the vein) probably constitutes the main advantage provided by the quadripolar leads. This likely increases the percentage of patients with effective left ventricular pacing. The new devices allow dual-LV pacing with 2 simultaneously-active ventricular lead electrodes. The clinical value of this type of programming remains to be demonstrated.