Regional Differences in Systolic Active Stress Profiles in the Normal Beating Heart. Assesment Using an Ultrasound Based Mathematical Model

Active stress (˙
A
) developed by cardiac muscle has been measured in isolated muscle preparations, under physiological loading conditions, by subtracting the passive stress (˙
P
) from the total stress (˙
T
). We previously developed a mechanical model based on M-mode ultrasound imaging to calculate these stresses in beating hearts. However, this model was based on one-dimensional imaging information and could not estimate regional differences in ˙
A
. In the current study this model was improved by including two-dimensional B-mode echocardiographic data. Methods: In a porcine model a micro-manometer tipped catheter was used to measure left-ventricular pressure (LVP) and B-mode ultrasound images were recorded in a short-axis view. On the ultrasound image points in the mid-wall were selected and tracked to completely define the deformation of the myocardium. A kinematic model of the LV was then constructed from the displacement vectors of these points. ˙
T
was calculated from the LVP. The material parameters for an exponential stress/strain relation were estimated during the diastolic E-wave when it was assumed that ˙
A
= 0. These parameters were used to calculate ˙
P
during systole and by subtracting this from ˙
T
, ˙
A
was calculated. Results: The timing and shape of ˙
A
profiles match those obtained from isolated muscle experiments. ˙
A
was higher and peaked sooner in the posterior wall than in the anterior wall. Conclusion: Regional active stress estimation is possible in normal beating hearts.