The objective of this study is to collect clinical data on safety and performance of ACUITY X4® leads when used in a standard clinical setting.

It is a prospective, non-randomized, observational multicenter study evaluating standard of care.

For Post Market Clinical Follow up (PMCF) purposes the 3 month implant success rate, adverse events and basic parameters of the lead will be assessed. The cohort of subjects included in this evaluation will be the first 200 subjects which are indicated for PMCF in Rally X4 to receive an ACUITY X4® lead implant.

Study endpoints:

Phrenic Nerve Stimulation (PNS) related CFR through 6 months post-implant (Defined as: rate of freedom from loss of function or operative system revision due to unacceptable PNS threshold) Lead-related Complication-Free Rate (CFR) from Implant through 3 months post-implant.

Clinic visits will occur at:

• Enrollment and Consenting Clinic Visit (≤ 30 days prior to implant procedure)
• Implant Procedure (Day 0; all future follow ups based on this date)
• Pre-Discharge Clinic Visit (≤ 7 days post implant procedure) (Required)
• One to 6 Month Clinic Visit (20 to 180 days post implant procedure) (Required)
• Interim Visit(s) (Any time between the 1 to 6 Month Clinic Visit and Close-out Clinic Visit) (Following study center specific standard of care) (Device follow up optional) AE - reporting required
• Close-out Clinic Visit (30 months ± 90 days, OR 180 days ± 90 days after the study is closed to enrollment, whichever comes first) (Required)
• During the trial all AEs, deaths, and changes in the device system must be reported
• Devices of subjects who have received a Latitude device will be followed by the Boston Scientific (BSC) Latitude team. Device Data as defined in the Clinical Investigation Plan (CIP), device alerts, and diagnostic data from the standard Latitude database may be collected and entered into the study database at any time.

Inclusion Criteria:

1. Subject is willing and capable of providing informed consent
2. Subject is planned to be implanted with an ACUITY X4® lead for left-ventricular pacing and sensing via the coronary venous system in conjunction with a compatible BSC pulse generator
3. Subject is willing and capable of participating in all visits associated with this study at an approved clinical study center and at the intervals defined by this CIP
4. Subject is age 18 or above, or of legal age to give informed consent specific to state and national law

Exclusion Criteria:

1. Subjects with a hypersensitivity to a maximum single dose of 0.51 mg dexamethasone acetate

2. Subject is enrolled in any other concurrent study without prior written approval from BSC, with the exception of local mandatory governmental registries and observational studies/registries that are not in conflict and do not affect the following:

   • Schedule of procedures for the RALLY X4 Study (i.e. should not cause additional or missed visits);
   • RALLY X4 Study outcome (i.e. involve medications that could affect the heart rate of the subject);
   • Conduct of the RALLY X4 Study per GCP/ ISO 14155:2011/ local regulations as applicable

3. Per the implanting physician's discretion, the subject is not a suitable candidate to receive the study device as determined during the implant procedure

4. Women of childbearing potential who are or might be pregnant at the time of study enrollment or ACUITY X4® lead implant.

5. Subject is unwilling or unable to participate in all scheduled study follow up visits at an approved study center

6. Subject does not anticipate being a resident of the area for the scheduled duration of the trial

7. Subject's physician does not allow participation

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The BSC Cardiac Resynchronization Therapy Defibrillators (CRT-Ds) are indicated for subjects with art failure who receive stable optimal pharmacologic therapy (OPT) for heart failure and who meet any one of the following classifications:

• Moderate to severe heart failure (New York Heart Association (NYHA) Class III-IV) with ejection fraction (EF) ≤ 35% and QRS duration ≥ 120 ms
• Left bundle branch block (LBBB) with QRS duration ≥ 130 ms, EF ≤ 30%, and mild (NYHA Class II) ischemic or non-ischemic heart failure or asymptomatic (NYHA Class I) ischemic heart failure BSCCRT-Ds are also intended to provide ventricular antitachycardia pacing and ventricular defibrillation for automated treatment of life-threatening ventricular arrhythmias.