Monitoring biomedical signals using Wireless Body Sensor Networks (WSBN) is a growing need. It can be used to provide health care for patients who are in high risk. The portability, light weight and nonintrusive sensors of WSBN suits the acquisition and transmission of Electrocardiogram (ECG) signals for monitoring and tracking purposes of patients with cardiovascular problems. The ZigBee wireless technology can be used for the transmission of ECG signals because of its sufficient available bandwidth, the ad hoc organization, relative long range coverage and low power consumption. Three different WBSN topologies are possible to connect the end devices (the embedded body sensors), the routers and the coordinators; these are star, tree and mesh. In this work the performances of the network, under full packet length transmissions for the above topologies, are analyzed. This has been done in terms of throughput, end-to-end delay and packet loss. The results shows that for the same coverage area a star configuration has the best performance in terms of throughput, end-to-end delay and packet loss. In fact, in star configuration and packet transmission without acknowledgment we can obtain approximately a TDMA channel performance in terms of throughput. While the mesh network is highly loaded with high end-to-end delays observed in the simulation.