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EXPERIMENTAL STUDIES

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

Daniël De Wolf, MD, PhD, Luc Foubert, MD^*, Yves Van Belleghem, MD, Kathy Mareels, MD^*, Dirk Matthys, MD, PhD^a, Henri Verhaaren, MD, PhD, FACC^a and Guido Van Nooten, MD, PhD

^a Department of Pediatrics, Division of Pediatric Cardiology, University Hospital UZGhent, Ghent, Belgium
^* Department of Anesthesia, Division of Cardiac Anesthesia, University Hospital UZGhent, Ghent, Belgium
Department of Surgery, Division of Cardiac Surgery, University Hospital UZGhent, Ghent, Belgium

Manuscript received September 11, 1998; revised manuscript received May 5, 1999, accepted June 22, 1999.

Reprint requests and correspondence: Dr. Daniël De Wolf, Department of Pediatrics, Division of Pediatric Cardiology, University Hospital UZGhent, De Pintelaan 185, 9000 Ghent, Belgium
daniel.dewolf{at}rug.ac.be

OBJECTIVES

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 µg/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 µg/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.

Abbreviations and Acronyms   ANOVA = analysis of variance   BP = blood pressure   CO = cardiac output   ECG = electrocardiogram   EF = ejection fraction   ESWS = end-systolic meridional wall stress   ET = ejection time   ETc = rate-corrected LV ejection time   FS = fractional shortening   LV = left ventricular   LVID = left ventricular internal diameter   LVPW = left ventricular posterior wall thickness   SVR = systemic vascular resistance   VcFc = rate-corrected velocity of circumferential fiber shortening

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