This work investigates various methods to expand the deployment of Wireless Sensor-Actuator Networks (WSANs) to mission critical applications. We address the neglected open issues of hard deadline and deterministic delivery for event-triggered traffic in safety and closed-loop regulatory systems. DeMAC algorithm is proposed by utilizing TDMA-based subslots in emergency access periods, group acknowledgement, alternative packet structure, and relay node. The proposed algorithm displayed promising results in comparison with contention-based methods, in improving delivery rate of critical data within the deadline, and lowering the worst-case delay