Apparatus for monitoring cardio-respiratory function of a patient, the apparatus comprising: a sensor unit adapted to generate sensor output signals; a respiratory support unit adapted to provide a controllable positive airway pressure to an airway of the patient; a control unit adapted to control the respiratory support unit; and a processing unit adapted to determine venous information of the patient based on the sensor output signals, and characterised in that: the sensor unit comprises a sublingual optical sensor unit adapted to be positioned at sublingual vasculature of the patient's tongue, to detect light from the sublingual vasculature and to generate the sensor output signals based on the detected light; the control unit is adapted to control the respiratory support unit to a provide a first positive airway pressure to the airway of the patient and to receive a first sensor output signal from the optical sensor unit, the first sensor output signal being based on light detected from the tissue in response to provision of the first positive airway pressure, and further adapted to control the respiratory support unit to provide a second, different positive airway pressure to the airway of the patient and to receive a second output signal from the optical sensor unit, the second sensor output signal being based on light detected from the tissue in response to provision of the second positive airway pressure; and a processing unit adapted to determine venous information of the patient based on how the first and second sensor output signals vary from provision of the first positive airway pressure to provision of the second positive airway pressure. a sensor unit adapted to generate sensor output signals; a respiratory support unit adapted to provide a controllable positive airway pressure to an airway of the patient; a control unit adapted to control the respiratory support unit; and a processing unit adapted to determine venous information of the patient based on the sensor output signals, and characterised in that: the sensor unit comprises a sublingual optical sensor unit adapted to be positioned at sublingual vasculature of the patient's tongue, to detect light from the sublingual vasculature and to generate the sensor output signals based on the detected light; the control unit is adapted to control the respiratory support unit to a provide a first positive airway pressure to the airway of the patient and to receive a first sensor output signal from the optical sensor unit, the first sensor output signal being based on light detected from the tissue in response to provision of the first positive airway pressure, and further adapted to control the respiratory support unit to provide a second, different positive airway pressure to the airway of the patient and to receive a second output signal from the optical sensor unit, the second sensor output signal being based on light detected from the tissue in response to provision of the second positive airway pressure; and a processing unit adapted to determine venous information of the patient based on how the first and second sensor output signals vary from provision of the first positive airway pressure to provision of the second positive airway pressure. the sensor unit comprises a sublingual optical sensor unit adapted to be positioned at sublingual vasculature of the patient's tongue, to detect light from the sublingual vasculature and to generate the sensor output signals based on the detected light; the control unit is adapted to control the respiratory support unit to a provide a first positive airway pressure to the airway of the patient and to receive a first sensor output signal from the optical sensor unit, the first sensor output signal being based on light detected from the tissue in response to provision of the first positive airway pressure, and further adapted to control the respiratory support unit to provide a second, different positive airway pressure to the airway of the patient and to receive a second output signal from the optical sensor unit, the second sensor output signal being based on light detected from the tissue in response to provision of the second positive airway pressure; and a processing unit adapted to determine venous information of the patient based on how the first and second sensor output signals vary from provision of the first positive airway pressure to provision of the second positive airway pressure.