This document discusses, among other things, an apparatus comprising, a plurality of electrodes configured to deliver defibrillation countershock energy to a subject, an impedance measurement circuit communicatively coupled to the electrodes and configured to measure the impedance between any two of the electrodes using a non-stimulating excitation signal, and a controller communicatively coupled to the impedance measurement circuit and configured to calculate the impedance of a shock vector, wherein the shock vector includes a first electrode and a second electrode electrically connected together, and a third electrode, and wherein the controller calculates the impedance using measured impedances between the three electrodes when none of them are electrically connected.