In Wireless Sensor Networks (WSNs), nodes closer to an event are able to detect the event earlier and more accurately, thus contain more important information. Also, tiny nodes are usually scarce of energy and a major portion of their energy is used through communication. Therefore saving the energy by allowing only a limited number of communications is desirable in designing protocols for WSNs. We propose a data-driven Medium Access Control (MAC) protocol which allows only the more useful information to enter into the medium by using a modified contention mechanism and also suppressing other spatially-correlated data that are of less importance. Simulation results show that the proposed MAC outperforms the existing ones in terms of event reporting delay and packet delivery ratio for urgent data.