The advances in wireless low-power sensors and the increasing demand for portable healthcare monitoring systems have inspired the realization of Wireless Body Sensor Networks (WBSNs). IEEE Std. 802.15.6 defines the communication protocol for WBSN applications in terms of physical and medium access layers. The high generality and flexibility of standard requirements poses the question of how WBSN parameters, i.e. sampling frequency, node priority, superframe duration, have to be carefully set to guarantee a given accuracy level for the extraction and estimation of the biomedical information of interest. In this context, the present paper focuses on the most suitable design of network configuration by considering how the occurrence of missing packets affects the resulting quality of service. Throughout numerical simulations in Matlab, we reproduce a plausible WBSN scenario for monitoring the cardiac activity during physical exercise. The effect of missing packets on the recovered electrocardiographic (ECG) time-series at receiver side has been finally investigated, in terms of root-mean-square error and accuracy of the estimated heart rate, thus providing some criteria for choosing the most suitable network configuration.