Chapter 036. Edema (Part 1)

Harrisons Internal Medicine Chapter 36. EdemaEdema: IntroductionEdema is defined as a clinically apparent increase in the interstitial fluid volume, which may expand by several liters before the abnormality is evident. Therefore, a weight gain of several kilograms usually precedes overt manifestations of edema, and a similar weight loss from diuresis can be induced in a slightly edematous patient before "dry weight" is achieved. Anasarca refers to gross, generalized edema. Ascites (Chap. 44) and hydrothorax refer to accumulation of excess fluid in the peritoneal and pleural cavities, respectively, and are considered to be special forms of edema. ...
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Chapter 036. Edema (Part 1) Chapter 036. Edema (Part 1) Harrisons Internal Medicine > Chapter 36. Edema Edema: Introduction Edema is defined as a clinically apparent increase in the interstitial fluidvolume, which may expand by several liters before the abnormality is evident.Therefore, a weight gain of several kilograms usually precedes overtmanifestations of edema, and a similar weight loss from diuresis can be induced ina slightly edematous patient before dry weight is achieved. Anasarca refers togross, generalized edema. Ascites (Chap. 44) and hydrothorax refer toaccumulation of excess fluid in the peritoneal and pleural cavities, respectively,and are considered to be special forms of edema. Depending on its cause and mechanism, edema may be localized or have ageneralized distribution; it is recognized in its generalized form by puffiness of theface, which is most readily apparent in the periorbital areas, and by the persistenceof an indentation of the skin following pressure; this is known as pitting edema.In its more subtle form, edema may be detected by noting that after thestethoscope is removed from the chest wall, the rim of the bell leaves anindentation on the skin of the chest for a few minutes. When the ring on a fingerfits more snugly than in the past or when a patient complains of difficulty inputting on shoes, particularly in the evening, edema may be present. Pathogenesis About one-third of total-body water is confined to the extracellular space.Approximately 75% of the latter, in turn, is interstitial fluid and the remainder isthe plasma. Starling Forces The forces that regulate the disposition of fluid between these twocomponents of the extracellular compartment are frequently referred to as theStarling forces. The hydrostatic pressure within the vascular system and thecolloid oncotic pressure in the interstitial fluid tend to promote movement of fluidfrom the vascular to the extravascular space. On the other hand, the colloid oncoticpressure contributed by plasma proteins and the hydrostatic pressure within theinterstitial fluid, referred to as the tissue tension, promote the movement of fluidinto the vascular compartment. As a consequence of these forces, there is a movement of water anddiffusible solutes from the vascular space at the arteriolar end of the capillaries.Fluid is returned from the interstitial space into the vascular system at the venousend of the capillaries and by way of the lymphatics. Unless these channels are obstructed, lymph flow rises with increases in netmovement of fluid from the vascular compartment to the interstitium. These flowsare usually balanced so that a steady state exists in the sizes of the intravascularand interstitial compartments, and, yet, a large exchange between them occurs. However, should either the hydrostatic or oncotic pressure gradient bealtered significantly, a further net movement of fluid between the two componentsof the extracellular space will take place. The development of edema, then, depends on one or more alterations in theStarling forces so that there is increased flow of fluid from the vascular systeminto the interstitium or into a body cavity. Edema due to an increase in capillary pressure may result from an elevationof venous pressure due to obstruction to venous and/or lymphatic drainage. Anincrease in capillary pressure may be generalized, as occurs in congestive heartfailure (see below). The Starling forces may also be imbalanced when the colloid oncoticpressure of the plasma is reduced, owing to any factor that may inducehypoalbuminemia, such as severe malnutrition, liver disease, loss of protein intothe urine or into the gastrointestinal tract, or a severe catabolic state. Edema maybe localized to one extremity when venous pressure is elevated due to unilateralthrombophlebitis (see below). Capillary Damage Edema may also result from damage to the capillary endothelium, whichincreases its permeability and permits the transfer of protein into the interstitialcompartment. Injury to the capillary wall can result from drugs, viral or bacterialagents, and thermal or mechanical trauma. Increased capillary permeability may also be a consequence of ahypersensitivity reaction and is characteristic of immune injury. Damage to thecapillary endothelium is presumably responsible for inflammatory edema, which isusually nonpitting, localized, and accompanied by other signs of inflammation—redness, heat, and tenderness.