Method and pulse oximeter system for determining blood characteristics of a subject are disclosed. A pulse oximeter sensor for collecting plethysmographic data is also disclosed. In order to reduce the power consumption, time instants of systolic rises are estimated in at least one plethysmographic waveform of a subject and light emitting elements of a sensor are controlled according to the estimated time instants, thereby to collect signal samples from a plurality of plethysmographic waveforms of the subject during the systolic rises. A desired blood parameter, typically oxygen saturation, is then defined based on the signal samples collected during the systolic rises.