Fluid volume hypovolemia

Stimuli for Drinking. Thirst stimulation and the act of drinking are basic physiological responses. The three major circumstances known to stimulate thirst and drinking are (1) a deficit of body water (hypohydration and hypovolemia), (2) an increase in the osmolality of the extracellular fluid volume (hyperosmolality and hyperosmotemia), and (3) consumption of dry food (prandial thirst) (50). These three factors can function independently, but they are often interactive e.g., a hypovolemic subject is often hyperosmotic. In addition, the hormone angiotensin II acts as a stimulant for drinking (dipsogen) in animals, and possibly in man (51). 

CHAPTER 4 Fluid Volume Imbalances Hypovolemia and Hypervolemia... 

Chapter 4 focuses on fluid volume imbalances (i.e., hypervolemia and hypovolemia) and related symptoms and treatments. Chapters 5 through 9 present the major electrolytes and concepts related to excessive or insufficient blood levels of sodium, potassium, calcium, magnesium, and phosphate. Chapter 10 focuses on acid-base imbalances and discusses the procedures needed to determine the underlying source of the imbalance and the appropriate treatments and patient care needed to address the imbalance. Chapters 11 and 12 contain presentations of developmental conditions and disease conditions that involve imbalances in fluids, electrolytes, and acid-base, with the aim of enabling the reader to apply the concepts learned in earlier chapters of the book. 

Ruid volume regulation is necessary to maintain life. Decreased and inadequate fluid volume (i.e., hypovolemia) can result in decreased flow and perfusion to the tissues. Increased or excessive fluid volume (i.e., hypervolemia) can placed stress on the heart and cause dilutional electrolyte imbalance. It is clear that the renal system plays a vital role in fluid management. If the kidneys are not functioning fully, fluid excretion and retention will not occur appropriately in response to fluid adjustment needs. 2... 

Hematocrit levels also can indirectly indicate fluid volume in the blood. Since the test measures the number of blood cells per volume of blood, increased fluid in the blood, that is, hypervolemia, will dilute the blood cells and cause the hematocrit level to decrease. The normal range of values for men is 39 to 49 percent and for women is 35 to 45 percent. Consequently, too little fluid in the blood, that is, hypovolemia, will cause hemoconcentration and result in a high hematocrit level. It is therefore important to consider the patient s hydration level when interpreting laboratory values. For example, a hematocrit that falls within range or above range in a patient who is dehydrated actually may be low when the patient is fully hydrated. 2 Use other laboratory values, such as specific gravity, to see a full picture. 

Hypovolemia is a deficiency of body fluid that results when there is a total decrease in the fluid volume in the body or a relative decrease in body fluid owing to fluid loss from the blood vessels into the tissues. Hypovolemia can be classified as fluid volume deficit—the loss of water and sodium from the body—or as dehydration— the loss of water from the body in excess of sodium, resulting in an increased osmolality. While hypovolemia has significance relative to circulatory needs, loss of fluid accompanied by changes in osmolality and sodium concentration in the body has a more profound impact on the body and survival. The detrimental result is that... 

In dehydration, fluid volume is also decreased, but there is an increase in the osmolality of the blood because an equivalent amount of sodium was not lost. This form of hypovolemia occurs when fluid is lost but not replaced because the individual is unable to drink (e.g., an infant, an unconscious child or adult, or someone stranded without access to drinkable water) or did not experience the normal thirst impulse (e.g., elderly persons). The result would be a loss of water without replacement and without an equal loss of sodium, resulting in an elevation in sodium concentration in the blood and increased serum osmolality. Dehydration can occur through such mechanisms as profuse sweating, diuresis (e.g., in diabetes insipidus [deficient ADH] or diabetes mellitus and osmotic fluid loss), or excessive diuretic use. 2... 

Relative hypovolemia can occur when a large portion of the body s fluids escape into the tissues, most often owing to low oncotic pressures as a result of decreased protein. The circulating blood volume is decreased, which reduces venous return and results in symptoms similar to those of inadequate intake or fluid loss from the body. Symptoms of hypovolemia can occur when the total volume of fluid in the body is adequate. If an excess amount of fluid volume remains in the tissues, decreased volume in the blood vessels will result. 

Fluid volume imbalance has great implications for a patient. Excess or insufficient fluid volume could result in poor tissue perfusion. Some patients are at high risk for imbalance owing to young or extreme age or preexisting conditions, such as infection or gastrointestinal upset. The causes of hypovolemia or hypervolemia vary from altered intake— insuffient or excessive intake—to altered fluid reabsorption or excretion. Key points in this chapter include... 

Three days of vomiting with water for replacement likely will yield a hypovolemia and hemoconcentration, so the sodium level actually may be low once fluid volume is restored. 

Fluid volume is managed to avoid hypovolemia and the impact of hypervolemia. 

In patients with peritonitis, hypovolemia is often accompanied by acidosis, so large volumes of a solution such as lac-tated Ringers may be required initially to restore intravascular volume. Maintenance fluids should be instituted (after intravascular volume is restored) with 0.9% sodium chloride and potassium chloride (20 mEq/L) or 5% dextrose and 0.45% sodium chloride with potassium chloride (20 mEq/L). The administration rate should be based on estimated daily fluid loss through urine and nasogastric suction, including 0.5 to 1.0 L for insensible fluid loss. Potassium would not be included routinely if the patient is hyperkalemic or has renal insufficiency. Aggressive fluid therapy often must be continued in the postoperative period because fluid will continue to sequester in the peritoneal cavity, bowel wall, and lumen. 

Secondary ischemia is a frequent complication after SAH and is responsible for a substantial proportion of patients with poor outcome. The cause of secondary ischemia is unknown, but hypovolemia and fluid restriction are important risk factors. Hypovolemia should be avoided and intravenous fluids given, at least 3 liters per day, to reduce the likelihood of delayed ischemia. Indeed, volume expansion therapy is frequently used in patients with SAH to prevent or treat secondary ischemia. However, the risks and benefits of volume expansion therapy have been studied properly in only two trials of patients with aneurysmal SAH, with very small numbers (Rinkel et al. 2004). At present, there is no good evidence for the use of volume expansion therapy in patients with aneurysmal SAH. 

Conditions that result in the loss of large volumes of body fluids, such as high-volume diarrhea and gastric reflux, obviously require aggressive fluid therapy. However, many other horses may require fluid therapy because of prolonged mild-to-moderate fluid losses or prolonged reduced fluid intake. In neonatal foals, reduced fluid intake can rapidly result in hypovolemia and severe dehydration. This section addresses the identification of these horses and foals. 

The laboratory tests used most commonly to assess hypovolemia are packed cell volume (PCV) and plasma total solids. Unfortunately, these tests are neither sensitive nor specific (Hansen DeFrancesco 2002). The PCV may be increased substantially by splenic contraction, making small increases very hard to interpret. A PCV of >50% usually represents hypovolemia. The plasma total solids (protein measured by refractometer) or total protein concentration (measured by a chemistry analyzer) also increases with hypovolemia. How-ever significant protein loss can occur in disease (particularly colitis), resulting in a low or normal protein concentration despite hypovolemia. Further, hypergammaglobulinemia (e.g. in cyathostomiosis) can increase the plasma total protein concentration without the presence of hypovolemia. The PCV and plasma total solids are most useful when greatly increased or when used serially to monitor the response to fluid therapy. 

The resuscitation phase aims to restore circulating volume. There are two ways to think about the treatment of hypovolemia, both of which result in similar treatment patterns. Hypovolemic horses typically require 20-80 ml/kg of crystalloid fluids acutely. 

Hypernatremia in the setting of decreased ECF is caused by the renal or extrarenal loss of hypoosmotic fluid leading to dehydration. Thus once hypovolemia is established, measurement of urine Na" " and osmolality is used to determine the source of fluid loss. Patients who have large extrarenal losses have a concentrated urine (>800 mOsmol/L) with low urine Na (<20 mmol/L), reflecting the proper renal response to conserve Na and water as a means to restore ECF volume. Extrarenal causes include diarrhea, skin (burns or excessive sweating), or respiratory losses coupled with failure to replace the lost water. When gastrointestinal loss is excluded, and the patient has normal mental status and access to H2O, a hypothalamic disorder (tumor or granuloma) should be suspected, because the normal thirst response should always replace insensible water losses. 

The goals in treating patients with hypernatremia include correction of the serum sodium concentration at a rate that restores and maintains cell volume as close to normal as possible, as well as normalizing the ECF volume in states of ECF volume depletion and expansion. Adequate treatment should result in the resolution of symptoms associated with hypovolemia. Careful titration of fluids and medications should minimize the adverse effects from too rapid correction. Modulation of dietary sodium intake and sodium replacement may be necessary to prevent recurrence of hypernatremia. 

Hypoplasia - Incomplete development or underdevelopment of an organ or tissue. Hypotension - Abnormally low blood pressure. Hypovolemia - Abnormally decreased volume of circulating fluid in the body. 

Furosemide (Lasix) Inhibits chloride reabsorption in thick ascending loop of Henie. High loss of K+ in urine. Preferred diuretic in patients with low GFR and in hypertensive emergencies. Also, edema, pulmonary edema, and to mobilize large volumes of fluid. Sometimes used to reduce serum potassium levels. Hyponatremia, hypokalemia, dehydration, hypotension, hyperglycemia, hyperuricemia, hypocalcemia, ototoxicity, sulfonamide allergy, hypomagnesemia, hypochloremic alkalosis, hypovolemia. 

Blood loss through slow, prolonged bleeding or rapid bleeding also will cause hypovolemia. The degree and duration of the decreased fluid intake, prolonged vomiting or diarrhea, or blood loss will determine the severity of the hypovolemia and the amount of fluids needed to restore volume. 

Relatedly, if the fluid in the blood vessels pools owing to vasodilation, which decreases hydrostatic pressure in the arteries, blood volume circulating through the vessels and moving out to the tissues is decreased, and symptoms of hypovolemia result (Fig. 4-2). 

Thus any condition that reduces the protein level in the Wood will cause fluids to remain in the tissues. TWs causes massive edema and a reduction in circulating blood volume. In addition, any condition that causes massive vasodilation, for example, a systemic infection (sepsis), can cause a relative hypovolemia. In sepsis,... 

If the hypovolemia is relative and related to fluid moving into the tissues owing to a lack of protein and osmotic pressure in vessels, treatment will center on increasing protein (i.e., infusing albumin) in the blood vessels to bring volume into the blood vessels from the tissues. 

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