Abstract 302: Transthoracic Impedance Is Poorly Predicted by Patient Characteristics but Significantly Reduced by Placement of Defibrillator Pads in the Anteroposterior Position

Abstract

Introduction The American Heart Association Resuscitation Guidelines advocate four pad positions to achieve defibrillation success following cardiac arrest. Transthoracic impedance (TTI) reflects the electrode-skin interface and patient impedance and affects current amplitude, energy delivered and defibrillation efficacy. Modern defibrillators utilise an impedance compensated waveform to account for changes in TTI to improve defibrillation success. However the effect of electrode pad position and patient characteristics on TTI remains unclear. Objective To evaluate the effect of patient characteristics and pad position on Transthoracic impedance (TTI). Methods Patients undergoing diagnostic cardiac catheterisation were recruited for the study following regional ethical approval. Standard self-adhesive defibrillator electrodes were applied to patients in both the anterolateral and anteroposterior positions and connected to Heartsine Samaritan defibrillators. TTI was measured by applying a 30kHz sinusoidal signal through the chest. The voltage was then converted to digital samples using an 8-bit microprocessor. The digital samples were then stored on the secure digital card to be retrieved using custom-made download data recovery software. TTI was measured at 10 second intervals in both positions for the duration of the clinical procedure. Results 32 patients (mean age 63), (66% male), were recruited. TTI measured at 10 seconds was significantly higher in the anterolateral position (88.3ohms vs 68.5ohms, p <0.001). Mean TTI reduction over 5 minutes was 2.4% in the anterolateral position and 1.9% in anteroposterior position. There was no significant correlation between TTI measured from either pad position and age, height, weight, body surface area and Haemoglobin (r<0.4 for all parameters). Conclusion TTI is significantly reduced by placement of defibrillator pads in the anteroposterior position. Furthermore patient characteristics are poorly predictive of TTI from either pad position. Patients suffering failed defibrillation attempts from pads positioned in the anterolateral position may benefit from prompt relocation to the anteroposterior position to improve defibrillation efficacy.