Time to reflects both inotropic and chronotropic properties of cardiac contraction: a conscious dog study

This study supports a mathematical model and previous findings indicating that td, the time from onset of contraction to dP/dtmax, reflects the time-dependent aspects of contraction and hence decreases with increasing contractility. Combined data from 20 conscious instrumented dogs create a highly significant inverse and linear td-HR (heart rate) relation. Both norepinephrine and isoproterenol decreased td values, but norepinephrine, in contrast to isoproterenol, decreased the heart rate by a reflex response. Despite the remarkable decline in heart rate (25.8%) td was decreased (16.5%). During wide spontaneous R-R variations longer preceding intervals gave shorter td values. The latter two facts indicate the dependence of td on the contractile state rather than it being merely interval dependent.