Study objective: Previous work has shown that a passive membrane model using a parallel resistor-capacitor circuit is capable of predicting optimal waveforms for transvenous defibrillation. This study tested the ability of that model to predict optimal waveforms for transthoracic defibrillation. Methods: This study was divided into 3 parts, each of which determined transthoracic defibrillation thresholds (DFTs) in 6 dogs for several different waveform shapes and durations. For each part, strength-duration relationships were determined from both experimental and model data and then compared with test model predictions. Part 1 DFTs were determined at various durations for 3 different monophasic waveforms—the ascending ramp, descending ramp, and square waveform. Part 2 DFTs were determined for 3 biphasic waveforms. Phase 1 was a 30-ms ascending ramp, and phase 2 was an ascending ramp, a descending ramp, or a square waveform. Part 3 DFTs were determined for 3 biphasic waveforms with very short second-phase durations. Phase 1 was a 30-ms ascending ramp, and phase 2 was a descending ramp. Results: For part 1, the model was able to predict the relative defibrillation efficacy of the 3 monophasic waveforms ( P <.05). For parts 2 and 3, the model was able to predict the biphasic waveforms with the lowest DFTs. These predictions were based on the criterion that the model response at the end of the second phase should return to or slightly pass the model response value at the beginning of the first phase. Conclusion: The resistor-capacitor model successfully predicted the relative defibrillation efficacy of several different waveforms delivered transthoracically. [White JB, Walcott GP, Wayland JL Jr, Smith WM, Ideker RE: Predicting the relative efficacy of shock waveforms for transthoracic defibrillation in dogs. Ann Emerg Med September 1999;34:309-320.]