A key goal for patient safety is to improve the early recognition and management of patients whose conditions deteriorate whilst in hospital. Paper-based observation charts are the main means of recording and monitoring patients' physiological stability, yet observations (e.g., blood pressure, heart rate, and respiratory rate) are not always correctly recorded or appropriately acted upon. No prior published study has applied usability heuristics to systematically compare the usability of multiple observation chart designs. In this study, five evaluators with human factors, applied psychology, or medical expertise inspected 25 observation charts for usability problems. Every chart was found to have substantial usability problems, potentially affecting the ability of hospital staff to accurately record observations or recognize patient deterioration. We proposed a new observation chart design, which avoids many of the previously observed usability problems.