Abstract: A respiration monitor for measuring the variations of the thorax impedance of a patient due to respiration activity includes circuitry for suppressing the indication of unwanted signals caused by the activity of the heart. This circuitry is connected to receive separate signals corresponding to the heart beat rate and compare the periods of these signals to those of the signals measured at the thorax of the monitored person. If the periods of both signals are substantially equal, i.e. the signals measured at the thorax are actually due to heart activity rather than to respiration, the trigger threshold value of the monitor is automatically and stepwise raised so that the monitor does no longer respond to heart beat related signals but indicates that no respiration signals are measured.