A physiological monitoring system may receive a sensor signal from a physiological sensor. The system may determine a first and second change metric based on the sensor signal, and may determine a venous signal based on the change metrics. In some embodiments, the sensor signal may be a photoplethysmograph signal that includes both arterial and venous information. By subtracting a second change metric from a first change metric, arterial contributions may be substantially removed, resulting in a signal primarily comprising venous information. The venous signal may be indicative of changes in the venous blood, and may be used to determine a physiological parameter, for example, blood pressure. The venous signal may also be used to trigger an event, for example, calibration of a blood pressure measurement.