Key result
Norepinephrine reduces contraction-to-dP/dtmax time by ~17%, reflecting altered contractile state over cycle length alone.
Why the study?
The time from onset of contraction to dP/dtmax (td) reflects time-dependent aspects of contraction and its relation to contractility and heart rate needed further investigation.
The time from onset of contraction to dP/dtmax (td) reflects the contractile state of the heart independent of heart rate in a conscious dog model.
Answers are grounded in this paper and cite their sources.
td may reflect contractility beyond interval dependence; leaves open validation as a clinical biomarker.
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.
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Adler et al. (1996) studied this question. Norepinephrine and isoproterenol was evaluated on td (time from onset of contraction to dP/dtmax).
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