β-Adrenergic Influence on Increased Myocardial Oxygen Consumption during Hypoxemia in Awake Newborn Lambs

Abstract

ABSTRACT. The purpose of this investigation was to Abbreviations determine the role of increased P-adrenergic activity in regulating the increase in myocardial oxygen consumption LVEDP, left ventricular and diastolic pressure that occurs during hypoxemia in unanesthetized newborn dP/dt, first derivative of left ventricular pressure lambs. Through a left thoracotomy, fluid-filled catheters LV, left ventricle were placed in the ascending aorta, coronary sinus and left atrium, a pressure transducer was introduced into the left ventricle, and pacing wires were sutured onto the left atrium. The lambs were studied 3 d later by making Acute hypoxemia has important effects on the developing intermittent m a~~r e m e n t s of aortic and coronary sinus myocardium. We have demonstrated previously that myocardial blood oxygen saturations and Hb concentrations, left ven-oxygen consumption increased by 30-40% during moderate tricular myocardial blood flow, cardiac output, heart rate, hypoxemia (Fio2 = 0.08-0.10) in unanesthetized newborn lambs left atrial and aortic blood Pressures, the nmximal first (1). To understand the underlying mechanism, we reasoned that derivative of left ventricular pressure, aortic and coronary hypoxemia also has been associated with P-adrenergically mesinus blood oxygen contents, the arteriovenous difference diated increases in several of the hemodynamic determinants of of oxygen across the left ventricular myocardium, myocar-myocardial oxygen consumption. Thus, we postulated that indial oxygen consumption, and external cardiac work (aortic creased 0-adrenergic activity would be an important regulator of mean blood Pressure times cardiac output). In nine lambs the increase in myocardial oxygen consumption that occurs with an intact P-adrenergic nervous system (group I), the during hypoxemia (2-7). above measurements were made during a control period Previous studies have demonstrated that myocardial oxygen and during hypoxemia (Fioz = 0.08-0.10) with sponta-consumption increased during hypoxemia in @-adrenergically neous tachycardia. In another group of 12 lambs (group blocked anesthetized newborn lambs and mature dogs (8, 9). 2), the measurements were made during a control period, However, all of these experimental animals were studied under after P-blockade (1.5 mg/kg intravenous propranolol) with the influence of @-blockade. Thus, there is no information using pacing at the control heart rate, during hypoxemia with the same experimental preparation with and without @-blockade pacing at the control heart rate, and during hypoxemia that would permit a quantitative estimation of the role of the Pwith pacing-induced tachycardia. In the group 2 lambs, P-adrenergically mediated augmentation of cardiovascular funcblockade completely inhibited the increase in heart rate, tion on the increase in myocardial oxygen consumption during maximal first derivative of left ventricular pressure, cardiac hypoxemia. Furthermore, the previous studies were performed output and cardiac work that ~~c~r r e d during hypo...