Presented are concepts for monitoring cardio-respiratory function of a patient. One such concept comprises detecting light or sound from the sublingual vasculature using a sublingual sensor unit adapted to be positioned at a sublingual vasculature of the patient's tongue and to generate a sensor output signal based on the detected light or sound. A processing unit adapted to receive at least one of the sensor unit output signal, wherein the sensor unit and the processing unit are arranged to analyze the venous component in the sensor output signal. An output signal from the sublingual sensor may then be used to provide information on cardio-respiratory parameters like respiration rate and respiration rate variability, for example.