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How to cite this article: Spiegel R. Stressed vs. unstressed volume and its relevance to critical care practitioners. What is already known In the emergency department there is an overwhelming focus on left ventricu-lar function when assessing circulatory failure. While little attention is paid to the factors which promote venous return. What is new in the current study This article highlights the factors that are responsible for venous return and discusses strategies to augment these factors. In the field of emergency medicine our view of the circulatory system, for better or worse, has primarily focused on the function of the left ventricle. And yet the vast majority of patients in shock that we encounter in the emergency department have normal intrinsic ventricular function , rather suffer from an insufficiency in venous return. In 1955 Dr. Arthur Guyton, the noted America physiologist, published a paper in which he described the factors that influence venous return physiology. 1 In this paper he highlighted three variables all of which independently affect venous return. These factors include: the right atrial pressure, the mean systemic pressure (Pms) and the vascular resistance. 1 The Guyton model of venous return is a complex and intricate system, and a detailed and expansive explanation is beyond the scope of this paper. Instead we will focus our attention on the Pms, its components, and its impact on venous return. The Pms is an elusive concept that likely does not get the attention it deserves. This may be due to the difficulties encountered when attempting to define its importance or measure its existence. The Pms is the driving pressure competing against right atrial pressure to create a gradient that promotes forward flow. It is essentially the pressure measured in the vascular system if all blood flow were to cease. 2 The Pms is determined by the total volume of blood present in the venous system, and the intrinsic compliance of the vascular bed. In reality it takes a certain volume of fluid to fill the vascular bed to the point where its presence exerts force on the vessel walls. This is what is known as the unstressed volume. Any volume above this level is the stressed volume, which will exert an increasing degree of pressure on the venous vascular bed (Fig. 1). The greater the stressed volume, the greater Pms , and in turn, the greater the venous return. 2 Imagine …