A medical system, comprising: a medical device having an elongate body with a distal portion, the distal portion including a plurality of electrodes for ablating tissue; and a control unit in communication with the plurality of electrodes, the control unit programmed to: deliver radiofrequency energy to each of the plurality of electrodes, wherein the delivered radiofrequency energy is sufficient to ablate tissue; measure a plurality of impedance characteristics between each of the plurality of electrodes during the delivery of the radiofrequency energy; calculate a rate of change in the impedance measurements for each of the plurality of electrodes over a pre-determined time period; compare impedance changes between the plurality of electrodes to determine whether there is any significant variation in the calculated rates of change for each electrode; attribute the calculated rate of change in the impedance measurements to a tissue contact condition; generate an alert signaling the tissue contact condition; determine whether at least one of the plurality of electrodes is positioned within at least one of the pulmonary vein and the antrum based on the calculated rate of change in the measured impedance; terminate the delivery of radiofrequency energy to the at least one of the plurality of electrodes determined to be positioned within the pulmonary vein. a medical device having an elongate body with a distal portion, the distal portion including a plurality of electrodes for ablating tissue; and a control unit in communication with the plurality of electrodes, the control unit programmed to: deliver radiofrequency energy to each of the plurality of electrodes, wherein the delivered radiofrequency energy is sufficient to ablate tissue; measure a plurality of impedance characteristics between each of the plurality of electrodes during the delivery of the radiofrequency energy; calculate a rate of change in the impedance measurements for each of the plurality of electrodes over a pre-determined time period; compare impedance changes between the plurality of electrodes to determine whether there is any significant variation in the calculated rates of change for each electrode; attribute the calculated rate of change in the impedance measurements to a tissue contact condition; generate an alert signaling the tissue contact condition; determine whether at least one of the plurality of electrodes is positioned within at least one of the pulmonary vein and the antrum based on the calculated rate of change in the measured impedance; terminate the delivery of radiofrequency energy to the at least one of the plurality of electrodes determined to be positioned within the pulmonary vein. deliver radiofrequency energy to each of the plurality of electrodes, wherein the delivered radiofrequency energy is sufficient to ablate tissue; measure a plurality of impedance characteristics between each of the plurality of electrodes during the delivery of the radiofrequency energy; calculate a rate of change in the impedance measurements for each of the plurality of electrodes over a pre-determined time period; compare impedance changes between the plurality of electrodes to determine whether there is any significant variation in the calculated rates of change for each electrode; attribute the calculated rate of change in the impedance measurements to a tissue contact condition; generate an alert signaling the tissue contact condition; determine whether at least one of the plurality of electrodes is positioned within at least one of the pulmonary vein and the antrum based on the calculated rate of change in the measured impedance; terminate the delivery of radiofrequency energy to the at least one of the plurality of electrodes determined to be positioned within the pulmonary vein.