LEFT ATRIAL ROLE IN LEFT VENTRICULAR FILLING: DOG AND COMPUTER STUDIES (DYNAMICS, CARDIAC, ATRIOVENTRICULAR DELAY, CARDIOVASCULAR CONTROL)

Abstract

A series of anesthetized and conscious dog experiments and computational model experiments was performed to investigate the role of the left atrium in filling the left ventricle. Computer hardware and software were designed to perform dual-chamber pacing experiments in which the effects of changing the paced atrioventricular delay on ventricular filling dynamics were investigated. The analysis of the in vivo data from both anesthetized and conscious dogs elucidated the specific mechanisms by which atrial pump function influences ventricular filling. After analyzing the immediate response to a transient change in atrial pump function (a perturbation of ventricular filling), a remote-controlled mitral valve was designed and constructed to investigate the transitional and steady state response to filling perturbations. By precisely controlling ventricular filling, a range of filling volume perturbations was produced to investigate intrinsic compensatory mechanisms due to atrioventricular interaction. An hydraulic, lumped parameter computational model of the pulmonary bed, left atrium, mitral orifice and valve, left ventricle, and aorta was designed. The model was validated by comparing the results of the in vivo atrioventricular pacing study with results of an analogous, simulated study by the computer model. After validation, the model was used to investigate intrinsic control mechanisms of ventricular filling, again from the viewpoint of the atrium. Atrial function was conceptually separated into reservoir, conduit, and pump functions. The concept of interfunction balance was introduced to describe the extent to which a perturbation of any one atrial function automatically affects the others, tending to restore ventricular filling volume toward its control value. Such an intrinsic regulation of ventricular filling may be important in beat-to-beat regulation of cardiac output. Three model studies of atrial function are included: (1) determinants of atrial conduit and reservoir function and their interrelationship; (2) determinants of atrial pump function in terms of atrial preload, afterload, and contractility; and (3) characteristics of early diastolic filling dynamics with emphasis on determinants of ventricular filling rate and on atrioventricular pressure gradient reversal.