Devices and systems provide methods for detecting cardiac signals from head or facial biopotential sensors. In one embodiment, a facial biopotential signal is measured by a set of sensors. A processor derives a cardiac signal from the facial biopotential signal. Optionally, heart rate is detected from the cardiac signal. Heart rate variability may be determined and evaluated by the processor to generate warnings or messages from the evaluation. One or more head or facial biopotential electrodes for measuring the facial biopotential signal may be integrated in or at a contact surface of head support structures such as a headgear support or respiratory treatment mask. In some such embodiments a controller of a respiratory treatment apparatus may serve as a cardiac signal detector by processing the facial biopotential signal from the sensors and may make control adjustments or generate warnings based on an evaluation of detected signals.