The Influence of Preload Measured as Diastolic Mural Force on Myocardial Contractility Indices

Abstract

There have been reports in the recent literature citing the influence of ventricular preload on indices of myocardial contractility. Most of these reports used left ventricular end-diastolic pressure (LVEDP) as an indicator of preload (1-6), but few have used an indicator which incorporated a geometry factor. Since ventricular radius contributes to the force in the ventricular wall and, therefore to the load on the myocardial fiber, directly measured mural force (MF = PπR) should provide a more accurate indication of preload changes than pressure alone. Therefore, the purpose of this investigation was to examine the influence of preload changes as measured by either MF or LVEDP on five simultaneously recorded indices of contractility. These indices were peak rate of pressure rise of left ventricular pressure (dP/dt) (1, 2), maximum physiological velocity of the contractile elements (Vpm) (3-6), time from onset of contraction to VPM (t-Vpm) (3, 4), the ratio of dP/dt to instantaneous pressure at 5 mmHg developed pressure [(dP/dt)/K·5] (7), and the ratio of dP/dt to common peak isovolumic developed pressure [(dP/dt)/CPIF] (1,4). Method. Eighteen mongrel dogs weighing from 18 to 31 kg (mean = 21.4 kg) were anesthetized with 20 mg/kg pentobarbital sodium. After institution of positive pressure respiration with room air, each animal received a supplemental dose of barbital sodium (150 mg/kg) intraperitoneally to maintain a steady state of anesthesia. Polyethylene catheters were inserted in a femoral vein and artery. The tip of the arterial catheter was positioned in the ascending aorta for the measurement of aortic blood pressure with a Statham P23Dd pressure transducer. Through a ventral midline incision in the neck, both vagii were isolated and severed. A left lateral thoracotomy was performed and the pericardium widely incised.