1. A method of analyzing a physiological signal during application of chest compressions, the method comprising: acquiring a physiological signal during application of chest compressions; acquiring the output of a sensor from which information on the velocity of chest compressions can be determined; using the information on the velocity to reduce at least one signal artifact in the physiological signal resulting from the chest compressions; and using a ventricular fibrillation detection algorithm for processing the physiological signal with reduced artifact to estimate whether a ventricular fibrillation is present, wherein using the information on the velocity to reduce at least one signal artifact in the physiological signal comprises using an adaptive filter that is adjusted to remove chest compression artifacts, wherein a difference signal is produced, the difference signal being representative of the difference between the physiological signal fed into the adaptive filter and the physiological signal after artifact reduction by the adaptive filter, wherein the difference signal provides a measure of the amount of artifact in the physiological signal, and further comprising the step of using the difference signal to modify the subsequent processing of the physiological signal, wherein, if the difference signal indicates that the amount of artifact exceeds a first threshold, the ventricular fibrillation detection algorithm is modified to make it more resistant to being influenced by the artifact.