Regional differences in systolic active stress profiles in the normal beating heart. Assessment using an ultrasound based mathematical model.
Active stress (sigmaA) developed by cardiac muscle has been measured in isolated muscle preparations, under physiological loading conditions, by subtracting the passive stress (sigmaP) from the total stress (sigmaT). 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 sigmaA. 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. sigmaT 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 sigmaA = 0. These parameters were used to calculate sigmaP during systole and by subtracting this from sigmaT, sigmaA was calculated.


RESULTS
The timing and shape of sigmaA profiles match those obtained from isolated muscle experiments. SigmaA was higher and peaked sooner in the posterior wall than in the anterior wall.


CONCLUSIONS
Regional active stress estimation is possible in normal beating hearts.
McLaughlin
M
M
Cardiac Imaging Research, Department of Cardiology, Catholic University Leuven, Leuven, Belgium.
Langeland
S
S
Streb
W
W
Marciniak
M
M
D'Hooge
J
J
Bijnens
B
B
Claus
P
P