Emerging healthcare applications rely on personal mobile devices to monitor patient vital signs and to send it to the hospitals-backend servers for further analysis. However, these personal mobile devices have limited resources that must be used optimally in order to meet the requirements of healthcare applications end-users: healthcare professionals and their patients. This paper reports on a case study of a cardiac telemonitoring application delivered by the so-called MobiHealth system. This system relies on a commercial personal mobile device with multiple (wireless) network interfaces (NI). The study focuses on how the choice of a NI affects the end-to-end application's data delay (extremely important in case of patient's emergency) and the energy consumption of the device (relating to the service sustainability while a patient is mobile). Our results show the trade-off between battery savings and the delay achieved by various NI activation strategies in combination with application-data flow adaptation. For a given mobile device, our study shows a gain of 40-90% in battery savings, traded against the higher delays (therefore applicable mainly in non-emergency cases). The insights of our studies can be used for application-data flow adaptation aiming at battery saving and prolonging device's operation while patients being mobile.