The working heart

Abstract

Since the advent of genetically altered mice, a vast amount of fascinating cardiovascular phenotypes have been described with even more remaining to be explored (Doevendans et al. 1998; Gödecke and Schrader 2000). In addition to the many ways of assessing cardiac function in the intact animal through acute or chronic instrumentation or even non-invasively by echocardiography or MRI, the isolated, perfused heart remains a prominent investigative tool. This is because it offers a whole array of unique advantages: The isolated heart provides a highly reproducible preparation that can be studied in a time and cost efficient manner. It allows a broad spectrum of biochemical, physiological, morphological and pharmacological indices to be measured, permitting detailed analysis of intrinsic ventricular mechanics, metabolism and coronary vascular responses. These measurements can be obtained without the confinements of systemic interference and side effects encountered in whole animal studies such as sympathetic and vagal stimulation, circulating neurohormonal factors and changes in substrate supply as well as alterations in systemic and pulmonary vascular resistance and left and right ventricular loading. Furthermore, experimental conditions such as ventricular pre- and after-load, perfusate oxygenation, substrate supply, coronary flow, heart rate and temperature, to name but a few, can be altered with ease and great precision to address the experimental question of interest. In addition, the measurement of physiological parameters is facilitated by the convenient exposure of the isolated heart, as is the application of pharmacological agents directly into the coronary circulation. The latter aspect also makes the working heart ideally suited for metabolic studies. Labelled precursors can be readily applied via the coronary system and their fate within the heart can be studied in a time-dependent manner. The preparation also readily allows the induction of whole heart or regional ischemia at various degrees of flow and a t various degrees of oxygen deprivation. Thus, the isolated heart preparation is amenable to reperfusion or reoxygenation at various rates and with various reperfusate compositions, providing a powerful tool for assessing many aspects of ischemia- and reperfusion-induced injury. The isolated, working heart preparation established under strict standards represents a functionally and metabolically stable system well suited for several hours of studies (Decking et al., 1997). However, as an ex vivo preparation, the isolated heart demands the application of appropriate precautions to maintain the organ's stability (see below). In addition, rigorous testing of the preparation must be performed and organs failing to live up to previously determined standards should be excluded from experimental analysis. These procedures should be repeated intermittently throughout the experimental course. In principle two different isolated heart models exist: the isolated heart according to Langendorff (1895), in which hearts re supplied with coronary flow through retrograde perfusion (described in the previous chapter) and the working, fluid-ejecting heart, in which hearts are perfused via the left atrium and eject fluid through the left ventricle into the aorta thus perfusing their own coronaries. The latter method was first described by Neely (Neely et al. 1967) and has since been adapted to a multitude of species including mice (Bardenheuer and Schrader 1986; Ng et al. 1991; Grupp et al. 1993; De Windt et al. 1999). The working, fluid-ejecting heart performs pressure-volume work, an important distinction from its Langendorff counterpart, which performs energetically less demanding isovolumetric contractions. Because the left ventricle is filled with perfusate, left ventricular pressure and its derivative parameters can be obtained directly through the application of a pressure transducer. This eliminates the need for a balloon to be inserted into the left ventricle and thus avoids the confinements inherent to balloons such a s: differences between ventricle and balloon geometry, compliance issues of balloon material, signal dampening due to minute air inclusions in the balloon system and the like. With the working heart ejecting fluid, cardiac output, being the sum of aortic and coronary flow, can be measured continuously. Preload and afterload may be adjusted over a wide range of feasible loading conditions enabling detailed studies of ventricular function. Thus the isolated working heart adds functionally important parameters to the already broad arsenal provided by Langendorff preparations, while retaining ease of access to the organ and versatility in experimental design. © Springer-Verlag Berlin Heidelberg 2005.