A transesophageal endoscopic system for determining a mixed venous oxygen saturation of a pulmonary artery, comprising: a light source configured to generate light having a time-varying intensity and at least two different wavelengths; a transesophageal probe comprising a light guide having a proximal and a distal end and being configured to collect the light from the light source at the proximal end and to forward the light to the pulmonary artery and an aorta through a wall of an esophagus at the distal end; and an acoustic transducer configured to detect at least one acoustic signal generated from each of the pulmonary artery and the aorta in response to the forwarded light; wherein the distal end of the light guide and the acoustic transducer are assembled into a capsule configured to be introduced into the esophagus through a mouth or a nose and to be navigated by advancing, rotating or flexing the probe at a height between an arch of the aorta and the pulmonary artery such that the acoustic transducer faces the pulmonary artery and the aorta in a sagittal plane; an acoustic pulser/receiver configured to amplify and digitize the acoustic signal detected by the acoustic transducer; wherein the system further comprises a processor configured to (i) obtain calibration data from the acoustic signal generated from the aorta, based on a known arterial blood oxygen saturation; and (ii) determine the mixed venous oxygen saturation of the pulmonary artery using the calibration data and the acoustic signal generated from the pulmonary artery; and a graphic user interface configured to display the determined mixed venous oxygen saturation. a light source configured to generate light having a time-varying intensity and at least two different wavelengths; a transesophageal probe comprising a light guide having a proximal and a distal end and being configured to collect the light from the light source at the proximal end and to forward the light to the pulmonary artery and an aorta through a wall of an esophagus at the distal end; and an acoustic transducer configured to detect at least one acoustic signal generated from each of the pulmonary artery and the aorta in response to the forwarded light; wherein the distal end of the light guide and the acoustic transducer are assembled into a capsule configured to be introduced into the esophagus through a mouth or a nose and to be navigated by advancing, rotating or flexing the probe at a height between an arch of the aorta and the pulmonary artery such that the acoustic transducer faces the pulmonary artery and the aorta in a sagittal plane; a light guide having a proximal and a distal end and being configured to collect the light from the light source at the proximal end and to forward the light to the pulmonary artery and an aorta through a wall of an esophagus at the distal end; and an acoustic transducer configured to detect at least one acoustic signal generated from each of the pulmonary artery and the aorta in response to the forwarded light; wherein the distal end of the light guide and the acoustic transducer are assembled into a capsule configured to be introduced into the esophagus through a mouth or a nose and to be navigated by advancing, rotating or flexing the probe at a height between an arch of the aorta and the pulmonary artery such that the acoustic transducer faces the pulmonary artery and the aorta in a sagittal plane; an acoustic pulser/receiver configured to amplify and digitize the acoustic signal detected by the acoustic transducer; wherein the system further comprises a processor configured to (i) obtain calibration data from the acoustic signal generated from the aorta, based on a known arterial blood oxygen saturation; and (ii) determine the mixed venous oxygen saturation of the pulmonary artery using the calibration data and the acoustic signal generated from the pulmonary artery; and a graphic user interface configured to display the determined mixed venous oxygen saturation.