Nonlinear isochrones in murine left ventricular pressure-volume loops: how well does the time-varying elastance concept hold?
The linear time-varying elastance theory is frequently used to describe the change in ventricular stiffness during the cardiac cycle. The concept assumes that all isochrones (i.e., curves that connect pressure-volume data occurring at the same time) are linear and have a common volume intercept. Of specific interest is the steepest isochrone, the end-systolic pressure-volume relationship (ESPVR), of which the slope serves as an index for cardiac contractile function. Pressure-volume measurements, achieved with a combined pressure-conductance catheter in the left ventricle of 13 open-chest anesthetized mice, showed a marked curvilinearity of the isochrones. We therefore analyzed the shape of the isochrones by using six regression algorithms (two linear, two quadratic, and two logarithmic, each with a fixed or time-varying intercept) and discussed the consequences for the elastance concept. Our main observations were 1) the volume intercept varies considerably with time; 2) isochrones are equally well described by using quadratic or logarithmic regression; 3) linear regression with a fixed intercept shows poor correlation (R(2) < 0.75) during isovolumic relaxation and early filling; and 4) logarithmic regression is superior in estimating the fixed volume intercept of the ESPVR. In conclusion, the linear time-varying elastance fails to provide a sufficiently robust model to account for changes in pressure and volume during the cardiac cycle in the mouse ventricle. A new framework accounting for the nonlinear shape of the isochrones needs to be developed.
Claessens
T E
TE
Institute of Biomedical Technology, Ghent University, Sint-Pietersnieuwstr. 41, Gent B-9000, Belgium. tom.claessens@ugent.be
Georgakopoulos
D
D
Afanasyeva
M
M
Vermeersch
S J
SJ
Millar
H D
HD
Stergiopulos
N
N
Westerhof
N
N
Verdonck
P R
PR
Segers
P
P
D000465:Algorithms; D000818:Animals; D001794:Blood Pressure / Q000502:physiology*; D003198:Computer Simulation; D004548:Elasticity; D051379:Mice; D008810:Mice, Inbred C57BL; D008955:Models, Cardiovascular; D017711:Nonlinear Dynamics; D013314:Stress, Mechanical; D013318:Stroke Volume / Q000502:physiology*; D016276:Ventricular Function; D016277:Ventricular Function, Left / Q000502:physiology*