ALTERED INOTROPIC AND LUSITROPIC RESPONSES TO HEART-RATE IN CONSCIOUSDOGS WITH TACHYCARDIA-INDUCED HEART-FAILURE

Objectives. The effects of increasing heart rate on left ventricular c ontraction and relaxation were examined in conscious dogs with tachyca rdia-induced heart failure under autonomically blocked conditions. Bac kground. Previous studies using isolated myocardium have shown attenua ted positive inotropic responses to stimulation frequency in heart fai lure. However, these responses have not been well examined in intact p reparations in the presence of heart failure with autonomic system blo ckade, where the intrinsic ventricular responses to increasing heart r ate could be revealed. Methods. Seven dogs mere instrumented with a mi cromanometer and a conductance volume catheter. After autonomic blocka de to eliminate neural reflexes, left ventricular contractile properti es mere quantified by the slope of the end-systolic pressure-volume re lation (ventricular elastance), and left ventricular relaxation was as sessed by the time constant of isovolumetric ventricular pressure deca y. Results. Increasing the heart rate by 60 beats/min enhanced ventric ular elastance by 71 +/- 18% (mean +/- SD) and decreased end-systolic volume by 6 +/- 5% in normal hearts. In failing hearts, ventricular el astance increased by only 21 +/- 20%, and end-systolic volume did not change appreciably. Although the reduction in left ventricular end-dia stolic and minimal pressures by tachycardia was smaller in the failing heart, ventricular relaxation rate remained unaltered both in the nor mal heart and in the failing heart. Conclusions. Under conscious but a utonomically blocked conditions, effects of increasing heart rate on t he failing left ventricle are characterized by a predominant attenuati on of the inotropic response rather than of the lusitropic response.