Dynamic impedance measurements during radio-frequency heating of cornea

Hyperopia affects approximately 25% of the population. The aim of different heating modalities for the treatment of hyperopia is to steepen the central curvature of the cornea. Conductive keratoplasty (CK) involves the placement of radio-frequency (RF) lesions around a 7-mm-diameter ring concentric with the pupil of the eye. Dynamics of lesion formation during CK depend on corneal electrical impedance, which is expected to change during each 600-ms-long macropulse. The purpose of this study was to measure impedance dynamics during CK. RF lesions were made in in vitro porcine eyes at different power settings. Voltage and current measurements were acquired using a high-speed computer-based data acquisition system. Root-mean-square voltages (V_RMS) and currents (I_RMS) were calculated for each micropulse, and impedance was determined by calculating the quotient V_RMS/I_RMS. Initial corneal impedance in vitro was approximately 2000 Ω. During the macropulse, impedance decreased initially due to increased mobility of conductive ions. At higher power settings (e.g., >70%, or maximum peak-to-peak voltage of 233 V), impedance increased after the initial decrease, indicative of local water vaporization and/or tissue coagulation. Preliminary impedance data obtained for in vivo porcine eyes were similar in magnitude to the in vitro values.