A method of detecting a suction condition during operation of a rotary blood pump with an inlet connected to a ventricle of the heart of a patient, an outlet connected to an artery of the patient, a rotor, and a control circuit configured maintain the rotor at a set rotational speed. The method includes measuring the rotational speed of the rotor at a plurality of times during each of a plurality of speed measurement intervals. A speed range is determined between a minimum measured speed and a maximum measured speed during each of the plurality of speed measurement intervals. At least one additional parameter relating to the operation of the blood pump is derived. A suction detection signal is generated if both at least one determined speed range is above a speed range limit and the at least one additional parameter is indicative of a suction condition.