A system for determining a heart failure status of a patient using an implantable medical device configured for subcutaneous implantation outside of a thorax of the patient, the system comprising: the implantable medical device comprising: at least one optical sensor; and a plurality of electrodes; and processing circuitry configured to: determine a current tissue oxygen saturation value of the patient based on a signal received from the at least one optical sensor; determine a current tissue impedance value of the patient based on a subcutaneous tissue impedance signal received from a first at least two of the plurality of electrodes; determine a current pulse transit time value of the patient based on a cardiac electrogram signal received from a second at least two of the plurality of electrodes and at least one of the signal received from the at least one optical sensor and the subcutaneous tissue impedance signal; compare the current tissue oxygen saturation value, the current tissue impedance value, and the current pulse transit time value to corresponding ones of a baseline tissue oxygenation saturation value, a baseline tissue impedance value, and a baseline pulse transit time value; and determine the heart failure status of the patient based on the comparison. the implantable medical device comprising: at least one optical sensor; and a plurality of electrodes; and at least one optical sensor; and a plurality of electrodes; and processing circuitry configured to: determine a current tissue oxygen saturation value of the patient based on a signal received from the at least one optical sensor; determine a current tissue impedance value of the patient based on a subcutaneous tissue impedance signal received from a first at least two of the plurality of electrodes; determine a current pulse transit time value of the patient based on a cardiac electrogram signal received from a second at least two of the plurality of electrodes and at least one of the signal received from the at least one optical sensor and the subcutaneous tissue impedance signal; compare the current tissue oxygen saturation value, the current tissue impedance value, and the current pulse transit time value to corresponding ones of a baseline tissue oxygenation saturation value, a baseline tissue impedance value, and a baseline pulse transit time value; and determine the heart failure status of the patient based on the comparison. determine a current tissue oxygen saturation value of the patient based on a signal received from the at least one optical sensor; determine a current tissue impedance value of the patient based on a subcutaneous tissue impedance signal received from a first at least two of the plurality of electrodes; determine a current pulse transit time value of the patient based on a cardiac electrogram signal received from a second at least two of the plurality of electrodes and at least one of the signal received from the at least one optical sensor and the subcutaneous tissue impedance signal; compare the current tissue oxygen saturation value, the current tissue impedance value, and the current pulse transit time value to corresponding ones of a baseline tissue oxygenation saturation value, a baseline tissue impedance value, and a baseline pulse transit time value; and determine the heart failure status of the patient based on the comparison.