Due to multiple reasons, emergency wards can become overloaded with patients, some of which can be in critical health conditions. To improve the emergency service and avoid deaths and serious adverse events that could be potentially prevented, it is mandatory to do a continuous monitoring of patients physiological parameters. This is a good fit for Internet of Things (IoT) technology, but the scenario imposes hard constraints on autonomy, connectivity, interoperability, and delay. In this paper, we propose a full Internet-based architecture using open protocols from the wearable sensors up to the monitoring system. Particularly, we use low-cost and low-power WiFi-enabled wearable physiological sensors that connect directly to the Internet infrastructure and run open communication protocols, namely, oneM2M. At the upper end, our architecture relies on openEHR for data semantics, storage, and monitoring. Overall, we show the feasibility of our open IoT architecture exhibiting 20–50 ms end-to-end latency and 30–50 h sensor autonomy at a fraction of the cost of current non-interoperable vertical solutions.