Topics in Progressive Care Third-spacing: When body fluid shifts By Susan Simmons Holcomb, ARNP-BC, PhD In a healthy adult, nearly all fluid is contained in Water, water the intracellular, intravascular, or interstitial everywhere spaces, with the intracellular space holding about two-thirds of total body water. Normally, fluid moves freely between these three spaces to main- Intracellular fluid tain fluid balance (see Water, water everywhere). Third-spacing occurs when too much fluid moves Ifrom the intravascular space (blood vessels) into the interstitial or “third” space—the nonfunctional Intravascular fluid area between cells. This can cause potentially seri- ous problems such as edema, reduced cardiac output, and hypotension. Interstitial fluid In this article, I’ll describe why third-spacing occurs and how to intervene to restore balance. Let’s start with a brief physiology review. Body fluids are distributed between the intracellular What’s behind third-spacing? and extracellular fluid compartments. The intracellular Fluid volume, pressure, and levels of sodium compartment consists of fluid contained within all the and albumin are the keys to maintaining fluid body cells. The extracellular compartment contains all balance between the intracellular and extracellu- the fluids outside the cells, including fluid in the inter- lar (intravascular and interstitial) spaces. Capillary stitial (tissue) spaces, and that in the intravascular permeability and the lymphatic system also play space (blood vessels). a role. A problem with any of these components can cause fluid to shift from the intravascular space losses during diarrhea or fluid losses caused by to the interstitial space. Let’s look more closely at medications such as diuretics. Hyponatremia can each component. also arise from volume overload. Also called dilu- • Increased fluid volume can be caused by overzealous tional or hypervolemic hyponatremia, this can fluid replacement or renal dysfunction. Volume occur with overzealous fluid replacement, heart overload can lead to peripheral edema, pulmonary failure, hepatic cirrhosis, renal disease, hypothy- edema, hepatic dysfunction, cerebral edema and roidism, or administration of vasopressin. mental changes, and decreased cardiac output. Other • Albumin losses disrupt colloidal osmotic pressure. signs of fluid overload include jugular vein distension, Plasma proteins are crucial to maintaining colloidal hypertension, and a pathologic S3. osmotic pressure. Albumin, the major protein con- • Increased capillary hydrostatic pressure often stituent of the intravascular space, accounts for up accompanies heart failure. Right-sided heart to 60% of total protein. Any condition that destroys failure is characterized by an increase in venous tissue or reduces protein intake can lead to protein pressure that causes edema in the liver and the losses and third-spacing. Some examples are periphery. Left-sided heart failure causes pul- hypocalcemia, decreased iron intake, severe liver monary edema. diseases, alcoholism, hypothyroidism, malabsorption, • Decreased sodium level, or hyponatremia, may result malnutrition, renal disease, diarrhea, immobility, from sodium loss; for example, gastrointestinal burns, and cancer. www.nursingcenter.com March l Nursing2009Critical Care l 9 Topics in Progressive Care • Increased capillary permeability results from burns and other forms of tissue The role of the lymph system trauma. Edema due to an increase in Normally the forces moving fluid out of the capillaries into the interstitial capillary permeability can be local, as space are greater than those returning fluid to the capillaries. The lym- with a localized trauma, or systemic as phatic system usually returns excess fluids and osmotically active plas- with anaphylaxis or disseminated ma proteins to the circulation. But if the lymphatic system is obstructed, intravascular coagulation. fluid and plasma proteins accumulate in the interstitial space. • Lymphatic system obstruction is com- monly caused by lymph node removal to treat cancer. An obstruc- Venous end tion typically leads to localized Arterial end Capillary edema; fluid and plasma proteins accumulate and can’t be drained into Excess fluid and proteins accumulate in interstitial space the general circulation because of the lymphatic obstruction (see The role of the lymph system). Postmastectomy Obstructed lymphedema is an example of this lymphatic vessel type of third-spacing. Phases of third-spacing hypovolemia; decreased values may indicate hyper- Third-spacing has two distinct phases—loss and volemia. The metabolic panel will give clues to renal reabsorption. and hepatic function as well as electrolyte balance In the loss phase, increased capillary permeability (especially sodium), and levels of protein, including leads to a loss of proteins and fluids from the albumin. intravascular space to the interstitial space. This The albumin-to-globulin ratio (normally slightly phase lasts 24 to 72 hours after the initial insult greater than 1:1) will elicit more information about that led to the increased capillary permeability colloidal osmotic pressure than total protein and (for example, surgery, trauma, burns, or sepsis). albumin levels alone. Albumin molecules are large Fluid loss from diarrhea, vomiting, or bleeding can and don’t diffuse freely through the vascular be measured, but fluid loss from third-spacing isn’t so endothelium, making this protein a major source easy to quantify. Signs and symptoms include weight of plasma colloid osmotic pressures. gain, decreased urinary output, and signs of hypo- Noninvasive assessment tools include an echocar- volemia, such as tachycardia and hypotension. diogram, which may yield information on cardiac During the reabsorption phase, tissues begin to heal function and volume status, and weighing the and fluid is transported back into the intravascular patient daily. Invasive hemodynamic monitoring of space. Signs of hypovolemia resolve, urine output central venous pressure, right atrial pressure, and increases, the patient’s weight stabilizes, and signs pulmonary artery occlusive pressure also help track of shock (if any) begin to reverse. If the patient volume status and the patient’s response to treat- was given fluid resuscitation during the loss phase, ment for hypervolemia or hypovolemia. However, monitor for fluid overload as interstitial fluid shifts some patients aren’t candidates for hemodynamic back to the intravascular space. monitoring, and some facilities aren’t equipped for this type of monitoring. Determining the cause Treatment of third-spacing depends on the cause, In some cases, the cause of third-spacing may be the phase, and the factors involved. Stabilizing your subtle and require a diagnostic workup, including a patient’s hemodynamic status is the first priority. complete blood cell count (CBC), complete metabol- During the loss phase, your focus is on preventing ic profile, and serum osmolality. The CBC may give hypovolemia and hypotension, which can lead to clues to volume status and factors contributing to shock and renal failure. During the reabsorption third-spacing, such as infection or necrosis. Elevated phase, focus on preventing circulatory overload and hemoglobin and hematocrit values may indicate hypertension, which can lead to pulmonary edema. 10 l Nursing2009Critical Care l Volume 4, Number 2 www.nursingcenter.com Which fluid is best? To stabilize the patient’s volume status, you’ll administer crystalloids, colloids, or a combination of these. Crystalloids replace electrolytes and restore normal serum osmolality; colloids replace the proteins responsible for maintaining plasma colloid osmotic pressure. Crystalloids are most commonly used, and can also treat hyponatremia. Remember, you’re trying to replenish intravascu- lar volume, not deplete the third space. Crystalloid fluids can be hypotonic, isotonic, or hypertonic. Hypotonic solutions, such as 0.45% sodium chloride solution, aren’t appropriate for volume resuscitation because very little of the THE GENTLE GIANT fluid would remain in the intravascular space. Isotonic solutions such as lactated Ringer’s solu- tion and 0.9% sodium chloride solution, which are There’s always been a genuine comfort level between similar to plasma in tonicity and osmolality, are used health care professionals and the powerful pink for resuscitation, with 0.9% sodium chloride solution preferred if the patient is hyponatremic. presence of Hy•Tape. And with good reason. No Hypertonic solutions, such as 3% sodium chloride other surgical tape sizes up to the formidable array of solution, contain large amounts of sodium and have easy working properties neatly rolled into every spool. been rarely used for resuscitation because of their potential for cellular dehydration and overexpansion This industry giant is all about sensitivity. of the intravascular space. However, a recent study found that hypertonic crystalloids were better than Hy•Tape is durable, yet flexible, so it’s perfect isotonic crystalloids for reducing abdominal third- for extended wearing while it gently accommodates spacing and abdominal compartment syndrome that often occur with massive fluid resuscitation in underlying tissue shifts. Its waterproof surface can be patients with extensive burns.1 Another study of washed with soap and water without slipping or critically ill patients found that even though smaller detaching from wet, oily or hairy surfaces. Remarkably, volumes of hypertonic solutions are needed