Emergency medical services (EMS) providers often face significant challenges in their work, including collecting, integrating, and making sense of a variety of information. Despite their criticality, EMS work is one of the very few medical domains with limited technical support. To design and implement effective decision support, it is essential to examine and gain a holistic understanding of the fine-grained process of sensemaking in the field. To that end, we reviewed 25 video recordings of EMS simulations to understand the nuances of EMS sensemaking work, including 1) the types of information and situation that are collected and made sense of in the field; 2) the work practices and temporal patterns of EMS sensemaking work; and 3) the challenges in EMS sensemaking and decision-making process. Based on the results, we discuss implications for technology opportunities to support rapid information acquisition and sensemaking in time-critical, high-risk medical settings such as EMS.