A catheter system and related method for measuring the oxidation state of biological molecules in a region of a measurement site of a subject. The system may include a catheter device having one or more emitters in mechanical communication with the catheter and configured to make contact with a tissue wall of the subject. The catheter device also includes one or more detectors in mechanical communication with the catheter and configured to make contact with the tissue wall of the subject. The emitters and the detector are in electromagnetic radiation communication with one another, whereby the electromagnetic radiation communication allows visible radiation or near infrared radiation emitted by emitters to be detected by the detector to determine tissue oxidation state of the region of the measurement site of the subject.