A method for performing a diagnostic or therapeutic procedure on a partial occlusion within a blood vessel, includes positioning a guide having a sensor configured and adapted to measure at least one of pressure, volume or flow within a blood vessel, advancing the guide through an at least partial occlusion within the blood vessel such that the sensor is downstream of the occlusion, measuring at least one of pressure, volume or flow downstream of the occlusion using the sensor and analyzing data obtained from the sensor to assist in determining the viability of tissue of the blood vessel downstream of the occlusion.