title: The influence of low afterload on the nature of the stress-velocity relationship.
abstract: OBJECTIVE
  Correct assessment of contractility by conventional methods during manipulation of afterload is often disappointing. To this purpose, the stress-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 than two years, data in newborns or after administration of low-dose dobutamine 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 circumferential fiber shortening (VcFc) were measured in these piglets at baseline, after balloon occlusion of the descending aorta, and at nitroprusside infusion rates of 1, 2 and 5 microg/kg/min. To eliminate inotropic influences mediated by reflex tachycardia, we subsequently studied five piglets and six adult pigs after bilateral cervical vagotomy.
  
  
  RESULTS
  The ESWS changed from a baseline mean of 50 g/cm2 to 137 g/cm2 after balloon occlusion and to 19 g/cm2 at 5 microg/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 proved to be curvilinear instead of linear. The steeper slope at low afterload could suggest enhanced contractility compared to expected values had the relationship been linear.
  
  
  CONCLUSIONS
  Data from young piglets and adult pigs suggest a curvilinear relationship of the stress-velocity relationship. This could probably explain some of the "hypercontractile states" encountered in conditions with low afterload.
authors.affiliation: Department of Pediatrics, University Hospital UZGhent, Belgium. daniel.dewolf@rug.ac.be
authors.forename: D
authors.initials: D
authors.lastname: De Wolf
authors.forename: L
authors.initials: L
authors.lastname: Foubert
authors.forename: Y
authors.initials: Y
authors.lastname: Van Belleghem
authors.forename: K
authors.initials: K
authors.lastname: Mareels
authors.forename: D
authors.initials: D
authors.lastname: Matthys
authors.forename: H
authors.initials: H
authors.lastname: Verhaaren
authors.forename: G
authors.initials: G
authors.lastname: Van Nooten
mesh_terms: D000367:Age Factors; D000818:Animals; D001794:Blood Pressure / Q000502:physiology*; D006439:Hemodynamics / Q000502:physiology; D006801:Humans; D007231:Infant, Newborn; D008955:Models, Cardiovascular; D009200:Myocardial Contraction / Q000502:physiology*; D009599:Nitroprusside; D013318:Stroke Volume / Q000502:physiology*; D013552:Swine; D013599:Systole / Q000502:physiology
chemical_list: D009599:Nitroprusside