A model based approach to estimate contractile force development using myocardial velocity imaging: a validation study during alterations in contractility and heart rate

Forces within the myocardial wall may be described by a stress tensor that is the sum of passive/elastic (σ_P) and active (σ_A) stresses. We developed a mechanical model based approach to calculate these stress components separately using myocardial velocity imaging (MVI). The timing and shape of the extracted σ_A profiles match those in isolated muscle experiments. We wanted to validate further this model based approach to estimating active stress and see whether it could detect changes in active force development. Therefore, we compared peak active stress (σ_Amax) with peak positive rule of pressure rise (dPdt_max), a common clinical contractility index, during changes in heart rate (HR) and contractility. In closed-chest pigs, MVI and pressure data were recorded. HR was varied by atrial pacing (AP=120-180 bpm, n=9). Contractility was increased by incremental dobutamine infusion (DI=5-20 μg/kg/min, n=9). Finally, contractility was decreased by esmolol infusion (0.5±0.15 mg/kg/min) plus subsequent pacing (120-180 bpm) (EI group, n=6). σ_Amax increased significantly with each stage of DI. During AP, σ_Amax remained relatively constant while it decreased significantly with EI. σ_Amax correlates with dPdt_max (r=0.79 P<0.01). σ_Amax reflects changes in contractility while being relatively independent of heart rate.