Phase 1 - Conventional CS lead placement (Conventional subgroup). The first 100 patients will be implanted with a CRT device with or without defibrillator (P/D) as per standard clinical practice, without CS lead pacing specific optimization. In these patients RLD is measured blind to implanting physician who will place CS lead according to his/her clinical practice. Since these patients follow strictly standard clinical practice, their implantation data can be collected prospectively or even retrospectively when available.