OBJECTIVE The aims of this study were to identify and analyze elements that affect duration of an interruption and likelihood of activity switch as experienced by nurses in an ICU. BACKGROUND Although interruptions in the ICU impact patient safety, little is known regarding the complex situations that drive them. METHODS RNs were observed in a 23-bed surgical ICU. We observed 206 interruptions, and analyzed for duration and activity switch. RESULTS RNs were interrupted on the average every 21.8 minutes. Attending physicians/residents caused fewer, but longer, interruptions to the RN. Longer interruptions were more likely to result in an activity switch. During complex situations such as when an RN is documenting, interruptions by a physician led to longer durations. Interruptions by a device led to higher switches. CONCLUSIONS A deeper understanding of individual factors and their complex interactions related to interruptions experienced by ICU RNs are vital to understanding the clinical significance of these interruptions and intervention design.