FORCE AND OXYGEN-CONSUMPTION IN THE IMMATURE RABBIT HEART

We speculated that there are important age-related differences in the economy of left ventricular force development in the isolated heart. To assess this, we: evaluated oxygen consumption and force development in newborn (less than 1 wk old) (n = 26), juvenile (4-6 wk] old) (n = 26), and adult (5-7 mo old) (n = 26) isolated,; isovolumic rabbit hearts. Measurements were obtained 1 with three different interventions, including I) changes in heart rate, 2) inotropic stimulation with isoproterenol, and] 3) changes in end-diastolic pressure. We found no significant baseline differences in the economy of force development. However, when heart rate was increased by 20%, the force/oxygen consumption ratio (economy) increased in newborn hearts by approximately 37%, whereas there was a decrease in juvenile and adult hearts of |27%. In addition, with increases in end-diastolic pressure above 10 mm Hg, newborn hearts increased their force/myocardial oxygen consumption ratio to 300% of the baseline value, 1 whereas adults increased to only 160% of baseline. Isoproterenol produced no significant age-related differences in the force/myocardial oxygen consumption ratio. We con-] elude that there are important age-related differences in the economy of left ventricular force development in this] model, but these differences are apparent only at higher heart rates and end-diastolic pressures. © 1990 International Pediatric Research Foundation, Inc.