NONLINEARITY OF THE LEFT-VENTRICULAR END-SYSTOLIC WALL STRESS-VELOCITY OF FIBER SHORTENING RELATION IN YOUNG-PIGS - A POTENTIAL PITFALL IN ITS USE AS A SINGLE-BEAT INDEX OF CONTRACTILITY

Objectives. We sought to evaluate in the young heart the primary assum ptions on which the current use of the mean ''velocity of fiber shorte ning corrected far heart rate'' as a noninvasive index of contractilit y are based. Background. End-systolic wall stress-velocity of fiber sh ortening relation has been applied as a single beat, load independent index of contractility in children. This use is based on poorly valida ted assumptions of linearity, parallel shifts with changing contractil e state and inotropic sensitivity of the end systolic wall stress-velo city of fiber shortening relation. Methods. In eight anesthetized youn g piglets, 5F micromano metric catheters were placed in the ascending aorta and balloon occlusion catheters in the descending aorta. End sys tolic wall stress and velocity of fiber shortening were calculated fro m aortic pressure and M-mode echocardiography under six conditions: in three contractile states 1) baseline, 2) increased contractility duri ng dobutamine infusion (10 mu g/kg per min), and 3) decreased contract ility after propranolol injection (1 mg/kg), each at two afterload sta tes (normal and increased load by partial aortic occlusion). Results. Dobutamine increased and propranolol decreased afterload matched veloc ity of fiber shortening corrected for heart rate significantly to 140% and 77% of baseline, respectively. However, the slope of end-systolic wall stress-velocity of fiber shortening relation was much greater (2 51% of baseline) during dobutamine infusion, which also significantly decreased wall stress, and was much less (27% of baseline) after propr anolol injection, which increased wall stress. Conclusions. The veloci ty of fiber shortening corrected for heart rate did change predictably with changes in contractility and as such can be used noninvasively i n the temporal evaluation of individual patients undergoing therapeuti c interventions or to define the natural history of a disease process. However, the relation on which it is based is not defined by parallel straight lines across contractile states, so that abnormal single poi nt measurements may reflect only the nonlinearity of the relation rath er than abnormalities in contractility. Thus, we recommend that the en d systolic wall stress-velocity of fiber shortening relation should no t be used as a single-beat index of contractility.