Provided are methods and a system for the transbronchial reflectance oximetric measurement of mixed venous oxygen saturation in the pulmonary artery of a patient, and a device therefor. Thus, provided is a quick, minimally invasive, method and system for determining an important endpoint of resuscitation, particularly for a patient in shock, because the probe used to measure the venous oxygen saturation is configured to permit insertion through an endotracheal tube until the probe is placed in contact with the bronchial luminus mucosa of the patient's right bronchus, left bronchus or distal trachea, directly adjacent to the patient's pulmonary artery.