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 (ε sys ) and peak systolic strain rate (SR sys ), 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 SR sys and ε sys averaged 5.0 ± 0.4 s −1 and 60 ± 4%. SR sys and CI increased linearly with DI (20 μg · kg −1 · min −1 ; SR sys = 9.9 ± 0.7 s −1 , P < 0.0001) and decreased with EI (SR sys = 3.4 ± 0.1 s −1 , P < 0.01). During pacing, SR sys and CI remained unchanged in the AP and EI groups. During DI, ε sys and SV initially increased (5 μg · kg −1 · min −1 ; ε sys = 77 ± 6%, P < 0.01) and then progressively returned to baseline. During EI, SV and ε sys decreased (ε sys = 38 ± 2%, P < 0.001). Pacing also decreased SV and ε sys in the AP (180 beats/min; ε sys = 36 ± 2%, P < 0.001) and EI groups (180 beats/min; ε sys = 25 ± 3%, P< 0.001). Thus, for normal myocardium, SR sys reflects regional contractile function (being relatively independent of HR), whereas ε sys reflects changes in SV.