ABSTRACT
OBJECTIVE: Critical myocardial oxygen imbalance as indicated by elevated interstitial lactate levels may occur in cases of rapidly elevated end-diastolic myocardial wall tension during elevated myocardial contractility in the intact myocardium. Simultaneous administration of beta-adrenergic receptor agonist and antagonist reliably allows for investigating the myocardial response. DESIGN: Experimental using an in vivo animal model. SETTING: Research institution. PARTICIPANTS: Animal model. INTERVENTIONS: Not applicable. MEASUREMENTS AND RESULTS: Fifteen minipigs were investigated in an open-chest model. After a midline sternotomy, a thin dialysis tube was implanted into the LV midmyocardium. Extracellular lactate in perfusate was analyzed every 5 minutes. End-systolic time-varying elastance and end-diastolic wall tension were measured. After a stable period, dobutamine (10 microg/kg/min) was administered to 8 animals. After 20 minutes, esmolol (0.5-mg/kg bolus, repetitively) was added until heart rate decreased to <100 beats/min. For 20 minutes, esmolol was infused at a rate of 3 mg/kg/h, and then dobutamine alone was continued for 10 minutes. With dobutamine, the lactate level did not change, but wall tension decreased and contractility increased. Simultaneous esmolol initially (in the first 10 minutes) increased lactate, whereas LV end-diastolic wall tension and contractility both increased; but after 10 minutes, lactate and contractility decreased significantly. Lactate again increased within 10 minutes after stopping esmolol. A group of 7 animals received esmolol for 20 minutes and showed no changes in lactate; myocardial wall tension increased and contractility decreased. CONCLUSION: Results suggest that oxygen demand/supply is balanced until both end-diastolic wall tension and myocardial contractility are elevated to critical levels.
