An impedance compensated passive implantable atrial defibrillator is reported. The two-part system consists of a handheld lithium-ion powered base unit (external power transmitter) and a passive (battery free) implantable coil (power receiver), with integrated rectifilter and power control unit, electrocardiogram (ECG) and bioimpedance measurement circuits, data communications circuitry and atrial connection leads. The system is designed to operate in two distinct modes: cardiac sense mode (wake-up, measure the impedance of the cardiac substrate and communicate data to the external base unit) and shock mode (delivery of an ECG synchronised impedance compensated monophasic very low tilt rectilinear shock waveform). A prototype was implemented and tested. In the sense mode, up to 5 W of sustained DC power was delivered across a 2.5 cm air-skin barrier with approximately 40% DC-to-DC power transfer efficiency at a transmission frequency of 185 kHz achieved, thereby providing 15.9 VDC (320 mA) to the implant side for measurement and communication at 433 MHz with the base unit. In the shock delivery mode, >186.9 W (rectilinear monophasic shock pulse: 100 V, 1.9 A, 12 ms duration) was repeatedly and reliably delivered transcutaneously to a 50 Ω cardiac load. Further testing in ten porcine models verified the in vivo operation, with intercatheter impedance variations of ±20.1% measured between successive defibrillation attempts.Introduction: Atrial fibrillation (AF) accounts for 30-40% of all cardiac arrhythmia-related hospital admissions and is associated with increased morbidity and mortality [1,2]. Consequently, the need for continued improvements in AF therapies remains self-evident. Recent publications indicate that the advancement of treatment for chronic AF sufferers (patients where pharmacological interventions have failed) may result from two specific lines of enquiry: (i) optimisation of the electrical defibrillation shock waveform for the lowest possible energy to achieve successful internal cardioversion (<1 J would potentially avoid the need for patient sedation) and (ii) the development of battery-free semi-passive implantable atrial defibrillators (facilitating AF arrhythmia detection and synchronised self-cardioversion, under controlled circumstances, in a minimal critical care setting) [3][4][5][6][7][8]. Controlled transcutaneous delivery of moderate amounts of electrical energy to the cardiac substrate is therefore of key importance in developing strategies for lowenergy treatment of AF [4,5,8]. Here we report a passive (battery-free) implantable atrial defibrillator architecture that facilitates measurement of the impedance spectrum of the cardiac substrate as a means of precisely controlling the energy delivered to the heart during ECG synchronised internal defibrillation.