Ultra wideband (UWB) antennas are used extensively in the area of biomedical communication devices. The main reasons behind that are their robust performance for indoor environments, high data rates, inherently encrypted data…etc. UWB antennas propagate EM-signals of up to few GHz in bandwidth. However, the dispersive nature of human tissues means a significant change in tissues EM properties over that large spectrum. In this article, we present a time-domain computational scheme for specific absorption rate (SAR) evaluation and temperature elevations when tissues are exposed to UWB signals. Moreover, since the EM power is mainly absorbed in the first few skin depths we present a methodology for efficiently computing the SAR at very high frequencies using a time-dependent impedance boundary conditions (IBCs) model. Two examples will be presented in the final submission to show the validity of the proposed approaches and CPU/memory gain as compared to the upto- date approaches.