The influence of low afterload on the nature of the stress-velocity relationship

OBJECTIVES Correct assessment of contractility by conventional methods duri ng manipulation of afterload is often disappointing. To this purpose, the s tress-velocity relationship offers assessment of contractility at different levels of afterload. We decided to study the influence of afterload on the nature of the stress-velocity relation. BACKGROUND Although linear at baseline conditions in a population older tha n two years, data in newborns or after administration of low-dose dobutamin e suggest a different nature of this relationship at low afterload. METHODS Ten healthy piglets (five to six weeks; 11 to 13 kg) were studied. End-systolic meridional wall stress (ESWS) and rate-corrected velocity of c ircumferential fiber shortening (VcFc) were measured in these piglets at ba seline, after balloon occlusion of the descending aorta, and at nitroprussi de infusion rates of 1, 2 and 5 mu g/kg/min. To eliminate inotropic influen ces mediated by reflex tachycardia, we, subsequently studied five piglets a nd six adult pigs after bilateral cervical vagotomy. RESULTS The ESWS changed from a baseline mean of 50 g/cm(2) to 137 g/cm(2) after balloon occlusion and to 19 g/cm(2) at 5 mu g/kg/min of nitroprusside . The VcFc changed from 1.19 c/s (circumference/second) to values of 0.9 c/ s and 1.73 c/s, respectively. The ensuing stress velocity regression line p roved to be curvilinear instead of linear. The steeper slope at low afterlo ad could suggest enhanced contractility compared to expected values had the relationship been linear. CONCLUSIONS Data from young piglets aid adult pigs suggest a curvilinear re lationship of the stress-velocity relationship. This could probably explain some of the "hypercontractile states" encountered in conditions with low a fterload. (J Am Coll Cardiol 1999;34:1219-25) (C) 1999 by the American Coll ege of Cardiology.