title: Mechanical restitution curves: a possible load independent assessment of contractile function.
abstract: OBJECTIVE
  The time constant of mechanical restitution (T((MRC))), proposed to reflect changes in calcium release and uptake, has been shown to increase in left ventricular (LV) failure, and might have a potential as an index of contractile function. However, in vivo studies of the effect on T((MRC)) of changing loading conditions in the normal and failing heart have not been reported. Consequently, in this study, we tested the hypothesis that the increase in T((MRC)) in vivo is independent of preload and afterload.
  
  
  METHODS
  Left ventricular pressure-volume loops were assessed at baseline in eight open chest pigs using the combined pressure-volume conductance catheter technique during right atrial pacing at 120b/min. Mechanical restitution curves (MRC) were constructed during four different loading conditions in all eight animals: uninfluenced load, reduced preload (balloon catheter in v. cava inferior), increased afterload (balloon catheter in descending aorta), and increased preload combined with reduced afterload (aortocaval shunting). Acute LV failure was then induced by microembolization through the left main coronary artery, and the experimental protocol was repeated. Contractile response was defined as the maximal first derivative of pressure (dP/dt(max)), and T((MRC)) was calculated using a least square approximation algorithm.
  
  
  RESULTS
  Hemodynamic data 30min after microembolization showed decreased mean arterial pressure (98+/-14-67+/-10mmHg, (mean+/-SD) P<0.0001) and dP/dt(max) (1482+/-193-1001+/-125mmHg/s, P=0.001). Stroke volume decreased from 30+/-5 to 20+/-5ml (P<0.0001) compared to baseline, and preload recruitable stroke work decreased from 52+/-7 to 31+/-10mmHg (P=0.002). T((MRC)) increased in all eight animals after induction of LV failure at all loading conditions. There was no difference between the different loading conditions at baseline, nor at LV heart failure, but T((MRC)) increased significantly after the induction of heart failure (ANOVA, two ways).
  
  
  CONCLUSIONS
  We have shown that the left ventricular T((MRC)) increases after developed heart failure. The increase in T((MRC)) was independent on loading conditions and thus have a potential for a contractility index.
authors.affiliation: Department of Cardiothoracic and Vascular Surgery, University Hospital North Norway, N-9038 Tromsø, Norway. knutek@fagmed.uit.no
authors.forename: Knut E
authors.initials: KE
authors.lastname: Kjørstad
authors.forename: Dag Ole
authors.initials: DO
authors.lastname: Nordhaug
authors.forename: Christian
authors.initials: C
authors.lastname: Korvald
authors.forename: Stig
authors.initials: S
authors.lastname: Müller
authors.forename: Tor
authors.initials: T
authors.lastname: Steensrud
authors.forename: Truls
authors.initials: T
authors.lastname: Myrmel
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000502:physiology; D002303:Cardiac Output, Low / Q000503:physiopathology; D002304:Cardiac Pacing, Artificial / Q000379:methods; D003326:Coronary Circulation / Q000502:physiology; D005260:Female; D006334:Heart Function Tests / Q000379:methods; D008297:Male; D008954:Models, Biological; D009200:Myocardial Contraction / Q000502:physiology*; D017202:Myocardial Ischemia / Q000503:physiopathology; D013314:Stress, Mechanical; D013318:Stroke Volume / Q000502:physiology; D013552:Swine; D018487:Ventricular Dysfunction, Left / Q000503:physiopathology