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Shock (hemodynamic failure) is ubiquitous in the modern intensive care unit (ICU). Venodilation, transudation of fluid from the vascular space into the interstitium and increased insensible losses will all result in hypovolemia in the course of patients with sepsis. Absolute hypovolemia is defined as a reduction of total circulating blood volume, while relative hypovolemia is an inadequate distribution of blood volume between the central and peripheral compartments.
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Early goal-directed therapy emphasizes aggressive fluid resuscitation of septic patients during the initial 6 h of presentation. Persistent hypotension after initial fluid resuscitation is common and poses the dilemma of whether the patient should receive additional fluid boluses, positive inotropic agent or a vasopressor. Persistent signs of organ hypoperfusion such as oliguria make timely decision making crucial. While a number of technologies including pulse counter analysis, transpulmonary thermodilution, and bioreatance have all shown promise in the evaluation of volume status of septic patients, bedside ultrasonography has already established itself as a useful tool for evaluating cardiac function. Applying the same echocardiographic techniques to dynamically assess the physiological response to spontaneous or mechanical ventilation, bedside maneuvers and the response to therapeutic interventions will likely become a cornerstone of hemodynamic monitoring in the modern ICU. This chapter reviews the utility of echocardiography for identification of the volume-responsive patient with hemodynamic failure.
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Benefits and Pitfalls of Fluid Resuscitation
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When hypovolemia (either absolute or relative) is present, fluid resuscitation will provide benefit to the patient by increasing venous return, left ventricular diastolic volume, cardiac output, arterial blood pressure, and ultimately tissue perfusion. The rapidity with which euvolemia is reestablished may be a decisive factor in the eventual outcome. That being said, there is an increasing body of evidence suggesting that fluid resuscitation is not without serious and possibly lethal complications. Those complications may be related to preexisting conditions such as systolic or diastolic heart failure, acute cor pulmonale (ACP), or the development of sepsis-related cardiac dysfunction. In patients with ACP, volume resuscitation may be particularly harmful as it may cause further right ventricular (RV) enlargement and left ventricle (LV) compression, thus worsening the shock state (see Chapter 11 and Videos 10-1 and 10-2). ACP can be readily recognized by the clinician with basic-level echocardiography skills. Extravasation of prescribed fluids may result in worsening of acute respiratory distress syndrome (ARDS) and prolonged mechanical ventilation. Anemia and clotting disorders occur with hemodilution. Excessive fluid resuscitation can be positively correlated with increased mortality in the ICU. Given the risk to benefit ratio of volume expansion, the key question is whether the patient would benefit from additional fluid boluses. It is essential to make this determination as clinical studies have repeatedly demonstrated that only about 50% of hemodynamically unstable ICU patients are volume responsive (see definitions below).
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