Visual assessment of a coronary artery narrowing (called stenosis) seen on angiography is conventionally used to infer how likely the stenosis will limit blood flow (called ischemia) under conditions of increased demand (e.g exercise). This is based on animal work and data from humans with simple single vessel disease with no co-existing conditions. These data have been extrapolated to more complex patients/ complex disease but clearly over-simplifies the situation in the majority of patients cardiologists treat.