Abstract: Medical apparatus for detecting the presence of artifact signals generated in the electrocardiogram of a patient by CPR and/or other patient motion. The presence of artifact signals is determined by analyzing variations in a measured electrical signal that represents the patient's transthoracic impedance. Such detection is important because the presence of CPR and/or motion artifacts can disrupt a patient's electrocardiogram (ECG) signal. The patient's impedance signal data is stored in the apparatus and analyzed to determine if the characteristics are indicative of the presence of CPR and/or motion artifacts. This analysis is performed independently of ECG data and may be used as an indicator of the underlying ECG rhythm classification. In essence, if the impedance exceeds some threshold amount, so as to indicate the presence of CPR or patient motion which can render the ECG data unreliable, the normal interpretation of the ECG data is interrupted.