To help ensure successful outcomes of open-heart surgery, surgeon and perfusionist must coordinate their activities during management of cardioplegia. This research aims to understand the basis for this coordination. We employed the framework of distributed cognition and the methodology of cognitive ethnography to describe how cognitive resources are configured and utilized to accomplish successful cardioplegia management. Analysis identified six types of surgeon-perfusionist verbal exchange which collectively enable robust system performance through (a) making the current situation clear and mutually understood; (b) making goals and envisioned future situations clear and thereby anticipated; and (c) expanding upon the activity system's knowledge base through discovery and sharing of experience. We argue for the "activity system" as the appropriate unit of analysis, and distributed cognition as a powerful theoretical framework for studying the socio-technical work of health care.