Time To Dp/Dt(Max) Reflects Both Inotropic And Chronotropic Properties Of Cardiac Contraction: A Conscious Dog Study

This study supports a mathematical model and previous findings indicating that t(d), the time from onset of contraction to dP/dt(max), 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 t(d)-HR (heart rate) relation. Both norepinephrine and isoproterenol decreased t(d) values, but norepinephrine, in contrast to isoproterenol, decreased the heart rate by a reflex response. Despite the remarkable decline in heart rate (25.8%) t(d) was decreased (16.5%). During wide spontaneous R-R variations longer preceding intervals gave shorter t(d) values. The latter two facts indicate the dependence of t(d) on the contractile state rather than it being merely interval dependent.