Abstract: Apparatus and method for filtering the electrocardiograph (ECG) signal of a patient of unwanted signals, such as contamination signals produced by the use of nuclear magnetic resonance imaging system or muscle artifact signals. In a preferred embodiment, variable filter (2) is used to filter an input ECG signal (1). The filtering characteristics applied to the ECG signal by variable filter (2) vary in response to the logical state of a synchronization signal indicating a patient's biophysical state which is input to variable filter (2) via line (3). In alternative embodiments, a QRS detector (22), a delay circuit (20), and a pulse stretcher (21) may be used to provide a synchronization signal. In another alternative embodiment, a microprocessor (24) with an input ECG signal and with input data is used to control the filtering of the ECG signal in synchronization with a patient's ECG signal.