Myocardial function defined by strain rate and strain during alterations in inotropic states and heart rate

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Abstract
For porcine myocardium, ultrasonic regional deformation parameters, systolic strain (εsys) and peak systolic strain rate (SRsys), were compared with stroke volume (SV) and contractility [contractility index (CI)] measured as the ratio of end-systolic strain to end-systolic wall stress. Heart rate (HR) and contractility were varied by atrial pacing (AP = 120–180 beats/min, n = 7), incremental dobutamine infusion (DI = 2.5–20 μg · kg−1 · min−1, n = 7), or continuous esmolol infusion (0.5 mg · kg−1 · min−1) + subsequent pacing (120–180 beats/min) (EI group, n = 6). Baseline SRsys and εsys averaged 5.0 ± 0.4 s−1 and 60 ± 4%. SRsysand CI increased linearly with DI (20 μg · kg−1 · min−1; SRsys = 9.9 ± 0.7 s−1, P < 0.0001) and decreased with EI (SRsys = 3.4 ± 0.1 s−1, P < 0.01). During pacing, SRsys and CI remained unchanged in the AP and EI groups. During DI, εsys and SV initially increased (5 μg · kg−1 · min−1; εsys = 77 ± 6%, P < 0.01) and then progressively returned to baseline. During EI, SV and εsys decreased (εsys = 38 ± 2%, P < 0.001). Pacing also decreased SV and εsys in the AP (180 beats/min; εsys = 36 ± 2%, P < 0.001) and EI groups (180 beats/min; εsys = 25 ± 3%, P < 0.001). Thus, for normal myocardium, SRsys reflects regional contractile function (being relatively independent of HR), whereas εsys reflects changes in SV.

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