MYOCARDIAL ENERGY-METABOLISM IN THE NEWBORN LAMB IN-VIVO DURING PACING-INDUCED CHANGES IN OXYGEN-CONSUMPTION

Myocardial energy metabolism was studied in newborn sheep to determine whether the metabolic responses to pacing-induced increases in heart rate were similar to those previously found during catecholamine stimu lation. Open-chest newborn sheep, 3 to 9 d old (n = 11), underwent atr ial pacing at a respiratory rate harmonic just above the intrinsic hea rt rate. Pacing rate was increased by 30 beats/min every 5 min until c onduction block or a drop in systemic arterial pressure occurred. Phos phorous metabolites were monitored simultaneously (n = 7) using a P-31 magnetic resonance surface coil over the heart within a magnet operat ing at 4.7 tesla. Myocardial oxygen consumption was monitored via an e xtracorporeal shunt from the coronary sinus. Rate pressure product inc reased with heart rate and was found to relate to myocardial oxygen co nsumption (r = 0.75), which increased maximally by 47 +/- 9% due to in creases in coronary blood flow. Phosphocreatine/ATP ratio decreased si gnificantly, and calculated ADP increased between baseline and peak pe rformance but returned to near baseline levels during recovery at the initial pacing rate. These findings indicate that intracellular high-e nergy phosphate concentrations do change with alterations in myocardia l oxygen consumption induced by cardiac pacing in the newborn. These c hanges are similar to those found during epinephrine infusion. Further more, the ATP hydrolysis products probably participate in myocardial r espiratory regulation in the newborn in vivo.