Sepsis is the life-threatening organ dysfunction arising from a dysregulated host response to infection.Although the specific mechanisms leading to organ dysfunction are still debated, impaired tissue oxygenation appears to play a major role, and concomitant hemodynamic alterations are invariably present. The hemodynamic phenotype of affected individuals is highly variable for reasons that have been partially elucidated. Indeed, each patient's circulatory condition is shaped by the complex interplay between the medical history, the volemic status, the interval from disease onset, the pathogen, the site of infection and the attempted resuscitation. Moreover, the same hemodynamic pattern can be generated by different combinations of various pathophysiological processes, so the presence of a given hemodynamic pattern cannot be directly related to a unique cluster of alterations. Research based on endotoxin administration to healthy volunteers and animal models compensate, to an extent, for the scarcity of clinical studies on the evolution of sepsis hemodynamics. Their results, however, cannot be directly extrapolated to the clinical setting, due to fundamental differences between the septic patient, the healthy volunteer and the experimental model. Numerous microcirculatory derangements might exist in the septic host, even in the presence of a preserved macrocirculation. This dissociation between the macro-and the micro-circulation might account for the limited success of therapeutic interventions targeting typical hemodynamic parameters, such as arterial and cardiac filling pressures, and cardiac output. Finally, physiological studies point to an early contribution of cardiac dysfunction to the septic phenotype, however our defective diagnostic tools preclude its clinical recognition.) to anesthetized, vagotomized and mechanically ventilated Sprague-Dawley rats on the operating point of the baroreflex using a baroreflex diagram (SNA: sympathetic nervous activity; CSP: carotid sinus pressure; AP: arterial pressure). As time passes after lipopolysaccharide injection, the neural arc 9 progressively shifts rightwards, and the peripheral arc downwards, so the operating point moves from a (baseline) to b at 1 hour and to c at 2 hours, showing a progressive increase of SNA with little change of AP. From (523), with permission under the terms of the Creative Commons Attribution License. significantly (from 1.7±0.5 to 1.4±0.2 mmHg ml -1 min) (313). In the same preparation N w -nitro-L-methyl ester (L-NAME) increases SVR more than R VR both in the presence (+294 and +129%, respectively) and in the absence (+196 and +107%, respectively) of fluid-loading. In another study from the same group, L-NAME elicited similar effects (117). Overall, these experimental studies warn against the assumption that changes of SVR are always paralleled by similar changes of R VR .
