Clinical factors and echocardiographic techniques related to the presence, size, and location of acoustic windows for leadless cardiac pacing.

Aims: Temporary leadless cardiac pacing using ultrasound energy is feasible in patients. An implantable left ventricular stimulation system being developed for cardiac resynchronization therapy transfers energy from a subcutaneous transmitter to an endocardial receiver through tissue free of interfering lung or rib ('acoustic window'). The aim was to use transthoracic echocardiography to evaluate acoustic window (AW) locations and sizes to determine the implant site for a transmitter, and to investigate clinical predictors of AW location and size.
Methods and Results: Inclusion criteria were ejection fraction ≤35%, and New York Heart Association functional class III or IV. Acoustic windows were evaluated in intercostal spaces (ICSs) measured in the supine, right lateral, sitting, and standing position during normal respiration and held inspiration. Among 42 patients, at least one adequate AW (≥2 cm(2)) was identified in 41, 19 patients had adequate AWs in 2 ICSs and 20 patients had adequate AWs in 3. Acoustic window areas were generally smallest in the lateral position with held inspiration and largest in the standing position with normal respiration. Patients with ischaemic cardiomyopathy compared with non-ischaemic cardiomyopathy had smaller heart size [left ventricular end-systolic volume index (LVESVI) 78 ± 38 mL/m(2) vs. 104 ± 46 mL/m(2), P = 0.03] but larger AWs in the right lateral position (11.4 ± 6.5 cm(2) vs. 7.3 ± 3.4 cm(2), P = 0.01) and standing position (14.0 ± 7.2 cm(2) vs. 9.4 ± 3.3 cm(2), P = 0.02).
Conclusions: Adequate AWs were present in nearly all patients. Despite smaller hearts, ischaemic cardiomyopathy patients had adequate AWs. A simple procedure performed as an adjunct to pre-implant echocardiography can screen patients and identify transmitter implant locations for an ultrasound-mediated leadless pacing system.