A medical device including a probe configured to be orally inserted into a lumen extending into the thorax of the subject, a plurality of electrodes, and a control circuit. The probe includes a first electrode. The plurality of electrodes includes at least one second electrode and at least one third electrode configured to be disposed externally on the thorax of the subject on a first side of a sternum of the subject and a second side of the sternum of the subject, respectively, the second side opposite the first side. The control circuit is electrically coupled to the first electrode and the at least one second and third electrodes and configured to measure an impedance between the first electrode and each of the at least one second and third electrodes and determine a ratio of arterial blood volume relative to venous blood volume based upon the measured impedance.