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

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.