Hemodynamic failure is a common problem in the intensive care unit (ICU). Echocardiography helps the intensivist establish a diagnosis, develop a therapeutic plan, and monitor the results of therapeutic intervention for patients with shock. A common question is whether the patient should receive volume resuscitation. This chapter will review the utility of echocardiography for identification of the volume-responsive patient with hemodynamic failure.
In many clinical situations, such as hypotension, shock, functional renal failure, oligoanuria, and clinical or laboratory signs of dehydration, hypovolemia may be suspected. Two types of hypovolemia can be distinguished: absolute and relative. Absolute hypovolemia is defined as a reduction of 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. This is commonly seen during septic shock. Hypovolemia results in a reduction in venous return, which may diminish preload and stroke volume (SV). This response depends on the Frank-Starling relationship, which relates left ventricular (LV) preload to stroke volume. The LV Frank-Starling curve has two parts (Figure 10.1): a steep first part (Segment A), where preload and SV are linearly related and any preload change is accompanied by an SV change (preload-dependent part); and a flat second part (Segment B), in which modifications of ventricular preload do not change SV (preload-independent part).
(A) The Frank-Starling curve indicating a patient with preload responsiveness; the increase of preload is followed by a significant increase of stroke volume, indicating that the patient is in the ascending part of the Frank-Starling curve. (B) A patient without preload responsiveness; the increase of preload is not followed by a significant increase of stroke volume, indicating that the patient is in the horizontal part of the Frank-Starling curve.
Significant central hypovolemia reduces preload, SV, mean arterial pressure, and cardiac output. From a hemodynamic perspective, central hypovolemia is clinically relevant when it is of sufficient severity to reduce venous return and ventricular preload to the extent that there is a threat to tissue perfusion. The determination of volume or preload sensitivity is a critical management issue for the patient in shock. If indicated, volume resuscitation may be beneficial; however, inappropriate volume resuscitation in the setting of adequate intravascular volume may be harmful to the patient.
Benefits of Fluid Resuscitation
When central hypovolemia is suspected, the clinical challenge is to assess whether volume resuscitation is needed. The expected benefits of volume expansion are to increase venous return, preload, SV, cardiac output, arterial blood pressure (systolic, mean, and pulse pressure), and tissue oxygen delivery. The rapidity with which these objectives are achieved during the management of hypovolemia constitutes a decisive prognostic element in terms of morbidity and mortality.