Abstract: Systems and methods for estimating a plethysmograph waveform are provided. In some aspects, a system includes a detector module configured to receive, from a single channel, an oximeter output signal indicative of light absorption in a patient. The oximeter output signal includes infrared light components and red light components. The system also includes a processing module configured to determine an indicator of a ratio of (i) an indicator of at least one of the infrared light components to (ii) an indicator of at least one of the red light components. The processing module is configured to determine, based on the indicator of the ratio, an indicator of a plethysmograph waveform of the patient.