The interactions between the respiratory and cardiovascular systems are complex. Clinical terms as “cor pulmonale” or “cardiopulmonary unit” attest to the intimate anatomic and physiologic relationship between heart and lung. In the intact circulation changes in intrathoracic pressure and/or lung volume will simultaneously induce alterations in venous return, abdominal pressure, cardiac volumes, lung vascular restistance, lung vascular capacity, cardiac output and contractility, aortic pressure, and surface pressures surrounding the various intrathoracic vascular and cardiac structures [1]. While the analysis of steady state effects of pertubations in intrathoracic pressure such as positive or negative pressure breathing on global cardiovascular parameters in health and disease states is very important clinically, it can contribute little to our understanding of the specific mechanisms involved in cardiopulmonary interactions from a physiologic point of view. In the past ten years therefore, emphasis was laid on investigation of the effects of shorter pertubations in pleural pressure, e.g. a single spontaneous inspiratory effort with and without changes in lung volume or a single positive pressure ventilation [2–11]. This approach led to the emergence of basic concepts of cardiorespiratory interactions [see 1 for review].