Fluid loss insensible

The early phase of SBS is associated with large day-to-day variations in fluid and electrolyte losses. Strict output records should be assessed, as well as all intake including intravenous medications. Initially, it is recommended to start a standard PN solution that meets the patient s maintenance metabolic, fluid, and electrolyte needs, and a separate intravenous replacement solution is typically necessary to keep the patient euvolemic based on actual fluid losses. Insensible losses should be estimated between 300 and 800 mL/day above measured output, and daily urine output should be kept at least 1 L. As fluid and electrolyte losses stabilize over time it becomes possible to incorporate these replacement requirements into the PN solution. The PN solution typically is composed of standard crystalline amino acids, glucose, and intravenous lipids. A generic caloric breakdown for SBS patients based on a need of 30 to 40 kcaV kg per day may be 1.5 g/kg of protein per day, approximately 20% to 30% of calories from intravenous lipids, and the remainder of calories from carbohydrates. An example of a PN formula for the patient with SBS is given in Table 139-2. 

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. 

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. 

In order to prescribe appropriate fluid therapy for this woman, one needs to estimate her sodium, potassium and water deficits from.her fluid balance charts. Particular note must be taken of losses that are relatively rich in sodium, such as drainage fluid, losses from fistulae, stomas or by nasogastric aspiration. Insensible water loss and urinary losses must also be taken into account. 

Instruct the client to drink water to replace insensible fluid loss. 

TBW depletion (often referred to as dehydration ) is typically a more gradual, chronic problem compared to ECF depletion. Because TBW depletion represents a loss of hypotonic fluid (proportionally more water is lost than sodium) from all body compartments, a primary disturbance of osmolality is usually seen. The signs and symptoms of TBW depletion include CNS disturbances (mental status changes, seizures, and coma), excessive thirst, dry mucous membranes, decreased skin turgor, elevated serum sodium, increased plasma osmolality, concentrated urine, and acute weight loss. Common causes of TBW depletion include insufficient oral intake, excessive insensible losses, diabetes insipidus, excessive osmotic diuresis, and impaired renal concentrating mechanisms. Long-term care residents are frequently admitted to the acute care hospital with TBW depletion secondary to lack of adequate oral intake, often with concurrent excessive insensible losses. 

In summary, the settings in which fluid replacement is used are hypovolemic patients (e.g., sepsis or pneumonia) hypervolemic patients [e.g., congestive heart failure (CHF), cirrhosis, or renal failure] euvolemic patients who are unable to take oral fluids in proportion to insensible losses (e.g., the perioperative period) and patients with electrolyte abnormalities (see below). 

In the next 24 hours, the medical team wants to give 50% of the fluid deficit plus an extra 240 mL to account for insensible losses in addition to the patient s maintenance needs. Using the equation (1500 mL + 20 mL for each kg greater than 20 kg), calculate the rate of fluid administration for the total fluids needed in this 24-hour period and over the next 48 hours. 

Fluid requirements increase with increased insensible or GI losses, fever, sweating, and increased metabolism. Fluid requirements decrease with kidney or cardiac failure and hypoalbuminemia with starvation. 

I.c.2.1. Fluid intake. This includes restriction of fluid intake to less than 1 liter per day if, as in oliguric renal failure, daily urine volumes are 500 ml or less and daily insensible losses are estimated to be 500-700 ml. In non-oliguric renal failure daily urine losses plus insensible losses may be in excess of 2 1/day and daily intake obviously has to be adjusted accordingly. Careful balance of intake and output of fluid and electrolytes is extremely important in ARF patients, both oliguric and non-oliguric. 

The body gains water via food and fluid intake plus the metabolic production of water. Routes of water loss include urine, sweat, faeces and insensible losses via the skin and lung. 

Except for respiratory and dermal insensible water-vapor losses, all remaining water lost by the body contains electrolytes, mainly sodium and chloride. The normal cation and anion constituent composition of the fluid spaces is given in Table IV. In the extracellular fluid space, sodium is the major cation and chloride the major anion. Those two ions constitute 95 of the extracellular fluid osmolality. Changes in plasma sodium concentration reflect changes in extracellular fluid volume. Potassium is the major cellular cation and phosphates and proteins comprise the major anions. The total cellular osmolality (175 + 135 = 310 mosraol/kg H2O) is equal to the total extracellular osmolality (155 + 155 = 310 mosmol/kg HaO) therefore, equal total osmotic concentrations are maintained between two fluid compartments of widely different ionic contents (Table IV). 

The minimum daily requirement for water can be estimated from renal (-1200 mL in urine) and insensible losses (-200 mL due to evaporation from the skin and respiratory tract). Activity, environmental conditions, and disease all have dramatic effects on daily water (and electrolyte) requirements. However, on average, an adult must take in 1.0 to 1.5 L of water daily to maintain fluid balance. Because primary regulatory mechanisms are designed to first maintain intracellular hydration status, uncorrected imbalances in TBW are initially reflected in the ECF compartment. Table 46-1 lists common causes and clinical... 

Infants and young children may be mildly dehydrated owing to increased insensible loss, vomiting, and decreased intake. Unless dehydration has occurred, increased fluid therapy is not indicated in acute... 

Patients with a contracted ECF volume and a low urine output include those who have sustained insensible water losses that exceed intake, as well as those with extrarenal losses of hypotonic fluids. On physical exam, one should search for postural hypotension, diminished skin turgor, and delayed capillary refill. The daily urine output is typically less than 1 L. 

The inlet supply represents fluids taken orally or by intravenous infusion, while the outlet is normally the urinary tract. Insensible loss can be thought of as. surface evaporation. 

The client should drink adequate amounts of fluids to replace insensible loss of fluids and to help prevent dehydration. 

Comment In these premature infants with idiopathic respiratory distress syndrome (RDS) many problems are present. The decrease in insensible water loss resulting from humidification compounds the problem of inappropriate vasopressin secretion associated with positive pressure respirator therapy (K jekshus t, 1972). Fluid and caloric supply is limited by these factors. 

In critical illness suitable adjustment to administered fluid load must be made to allow for changes in insensible loss and mode of therapy. 

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