Independent effects of preload, afterload, and contractility on left ventricular torsion

Shortening of oblique left ventricular (LV) fibers results in torsion. A un ique relationship between volume and torsion is therefore expected, and the effects of load and contractility on torsion should be predictable. Howeve r, volume-independent behavior of torsion has been observed, and the effect s of load on this deformation remain controversial. We used magnetic resona nce imaging (MRI) with tagging to study the relationships between load and contractility, and torsion. In ten isolated, blood-perfused canine hearts, ejection was controlled by a servopump: end-diastolic volume (EDV) was cont rolled by manipulating preload parameters and end-systolic volume (ESV) by manipulating afterload using a three-element wind-kessel model. MRI was obt ained at baseline, two levels of preload alteration, two levels of afterloa d alteration, and dobutamine infusion. An increase in EDV resulted in an in crease in torsion at constant ESV (preload effect), whereas an increase in ESV resulted in a decrease in torsion at constant EDV (afterload effect). D obutamine infusion increased torsion in association with an increase in LV peak-systolic pressure (PSP), even at identical EDV and ESV. Multiple regre ssion showed correlation of torsion with preload (EDV), afterload (ESV), an d contractility (PSP; r = 0.67). Furthermore, there was a close linear rela tionship between torsion and stroke volume (SV) and ejection fraction (EF) during load alteration, but torsion during dobutamine infusion was greater than expected for the extent of ejection. Preload and afterload influence t orsion through their effects on SV and EF, and there is an additional direc t inotropic effect on torsion that is independent of changes in volume but rather is force dependent. There is therefore potential for the torsion-vol ume relation to provide a load-independent measure of contractility that co uld be measured noninvasively.