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In our article “We need to know more about autonomic dysfunction in our critically ill patients” [] in the 1999 issue of this Yearbook, we postulated that autonomic dysfunction may have a relevance beyond just being an epiphenomen in these patients, and listed the questions to which answers may prove or disprove this hypothesis: Now, eight years later, we have gained many new insights into the intriguing interplay of cardiovascular reflexes that mediate the autonomic dysfunction seen in MODS and sepsis, so that we can now answer at least some of the aforementioned questions.

Hemodynamics is the physiology concerned with movements of blood and the forces involved in the circulation []. Hemodynamic monitoring involves the study of this physiology, with various forms of technology to understand these forces and the movement of blood, and put them into a clinical context that can be assessed and used to direct therapy. The main function of these hemodynamic forces is to transport substrates to, and clear metabolites from, the cells in order to allow adequate cellular function. The assessment of hemodynamics must, therefore, also take into account the metabolic status of the cell in particular in relation to its supply of oxygen. A relative lack of oxygen at the cellular level is known as tissue hypoxia. The identification and correction of tissue hypoxia remains one of the central facets of any protocol that aims to resuscitate patients from shock conditions. This is because tissue hypoxia has both pathological relevance [] and an association with poor outcome []. When monitoring the circulation, therefore, an estimate must be made of the adequacy of the circulation with respect to the likelihood of there being underlying tissue hypoxia. With most currently available monitors for routine practice it is impossible to assess tissue hypoxia at either a local or a cellular level. An extrapolation is, therefore, made from a number of globally measured parameters that can provide an estimate of the likelihood of underlying disturbance. Clinicians can then use this information to direct therapeutic decisions in order to benefit their patients.

Intensive care medicine is one of the areas of medicine most closely linked to applied physiology. Furthermore, it has a long tradition of being the forefront of advanced physiologic measurement technologies. The associated volume of quantitative data about a patient’s physiologic status, therapy, together with the output of off-line analyses, creates an information overload that profoundly reduces efficient and effective information processing. To a certain extent, this disconnection is a reason for the slow progress in utilizing such information across patients and hospital systems to improve patient care, perhaps most prominently evidenced by the failure of the physiologically valuable information provided by pulmonary artery catheterization to improve outcome in the critical care setting [, ]. In fact, for newer and more advanced monitoring equipment, evaluations of utility and ability to fit into proven treatment protocols is often lacking. Although the difficulty in translating the increased amount of available patient-specific information into patient benefit may in part be due to the lack of adequate therapeutic options, where clear benefit is known, actual translation of this information into practice is a primary barrier to improving patient care.

The development of noninvasive devices to manage hemodynamics in patients with shock is directly prompted by the results of recent studies in the intensive care unit (ICU), and during the perioperative period, which demonstrated the inability of an invasive approach, based on right heart catheterization, to improve prognosis []. Some authors have suggested that these results were largely due to inaccurate use of right heart catheterization, without clear goals or protocol [], However, previous studies have demonstrated that optimization of cardiac output and mixed venous oxygen saturation (SvO) with clear endpoints also fails to improve prognosis []. So, the lack of efficacy is inherent in the device. In 1985, Eugene Robin suggested that using right heart catheterization in patients with shock led physicians to give fluids plus diuretics whatever the wedge pressure []. In 2003, Francois Jardin claimed that we were going to move from the “age of oil lamps” to the “age of elec-tricity” []. In fact, we are going to change our practices in the management of shock, from an invasive and quantitative approach of hemodynamics to a non-invasive one, more functional and especially qualitative, mainly thanks to the use of echocardiography. This leads us to think about the meaning of hemodynamic monitoring.

Since the introduction of the pulmonary artery catheter (PAC) into clinical practice in the 1970s, this device has been considered to be the gold standard for cardiac out-put measurement and advanced hemodynamic monitoring. Nevertheless, in the last 10 years, its risk-to-benefit ratio has become a subject of controversy. One recent meta-analysis on the impact of the PAC in critically ill patients [] has presented conclusive results showing that the PAC does not bring any clinical benefit, although its use does not prolong hospital length of stay or increase the mortality rate, as was previously claimed by Connors et al. []. Another recent prospective multicenter study on 1041 critical patients came to the same conclusions as the meta-analysis []. Finally, in a randomized trial comparing hemodynamic management guided by a PAC with hemodynamic management guided by a central venous catheter (CVC), using an explicit management protocol in 1000 patients with established acute lung injury (ALI), PAC-guided therapy did not improve survival or organ function, but was associated with more complications than the CVC-guided therapy. The authors concluded that these results, when considered with those of previous studies, suggested that the PAC should not be routinely used for the management of patients with ALI []. The negative results of the PAC studies have led to a gradual decrease in the use of this monitoring modality. In fact, a survey in Germany in 2006 showed that, in a population of 3877 critically ill patients, less than 15% of patients with the criteria of severe sepsis or septic shock were monitored with a PAC [].

The basic mechanism underlying functional preload indices, such as stroke volume variation (SVV), pulse pressure variation (PPV), or systolic pressure variation (SPV), is that mechanical ventilation induces cyclic alterations in ventricular filling and, in consequence, in stroke volume and cardiac output. This phenomenon is most easily recognized in clinical practice as periodical variations in the arterial pressure signal. Based on the understanding of the Frank-Starling-relationship, i.e., the relation of cardiac preload and stroke volume, the ventilation-synchronous variations of cardiac output, or the indices named above, which serve as surrogates, allow assessment of left ventricular (LV) filling, and, more importantly the evaluation of the steepness of the patient-individual LV function curve []. The usefulness of these functional preload indices in assessing cardiac preload and in predicting whether a patient will respond to fluid administration with an increase in cardiac output (fluid responsiveness) has been demonstrated in many studies.

Volemia is the total blood volume of the body (plasma and cells) and is normally situated in the range of 65 to 75 ml/kg. Hypovolemia is a very frequent clinical situation in intensive care. Two types of hypovolemia are distinguished: Absolute and relative hypovolemia. Absolute hypovolemia is defined as a reduction in total circulating blood volume [, ], which may be related to blood loss (hemorrhage) or plasma loss (gastrointestinal, renal, cutaneous, extravasation into interstitial tissues). Relative hypovolemia is defined as an inadequate distribution of blood volume between the central and peripheral compartments (venodilatation or during positive pressure ventilation).

Assessment of volume responsiveness is an important issue in patients with spontaneous breathing activity. The difficulty in predicting the response to fluid infusion in this population of patients is variable and depends on the clinical situation. Three different scenarios must be distinguished:

Passive leg raising involves the elevation of the lower limbs from the horizontal plane. It was used as an empiric rescue therapy for acute hypotension long before intensive care units (ICUs) were created. The hemodynamic effects of passive leg raising have been progressively elucidated. In view of its simplicity, there is renewed interest in passive leg raising as a means of predicting fluid responsiveness in the critically ill.

Extravascular lung water (EVLW) is the term used to describe water within the lungs but outside the pulmonary vasculature. Excessive EVLW volume is a common and serious feature of critical illness. However, clinical assessment of the extent of pulmonary capillary leakage is difficult and inconsistent [, ]. Traditional methods of reducing EVLW volume include the use of loop diuretics and vasodilator drugs. The choice of these interventions is very much at the discretion of the clinician; pharmacological therapy is titrated to achieve a subjective clinical improvement rather than a quantitative EVLW volume target. The critically ill patient may also require fluid resuscitation to correct hypovolemia and to maintain oxygen delivery to the major organs. However, in the presence of increased pulmonary capillary permeability or impaired myocardial function, the administration of large volumes of intravenous fluid is associated with a significant risk of pulmonary edema. Effective fluid resuscitation, therefore, involves a fine balance between the harmful effects of inadequate tissue oxygen delivery on the one hand and excessive EVLW volume on the other.