Water loss, insensible

The above water deficit equation does not take into consideration continuous free water losses (i.e., insensible, renal, or gastrointestinal)... 

Normally, an individual excretes 1.0-1.5 L of water daily in the urine, about 100 ml each in feces and sweat, and up to 1 L in the expired air and from the skin (the latter quantity is also called insensible water loss). During the course of normal water metabolism, however, some 200 L water passes through the... 

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. 

Shahidullah, M., Raffle E., Frain-Bell, W., Insensible Water Loss in... 

Adults and older children tend to correct the disturbance at this point. Young children, however, quickly develop a fall in the blood pH (metabolic acidosis), due to salicylate stimulation of metabolism. The toxic effects of salicylate and the loss of buffer base interfere with metabolic processes, and ketosis develops. Because the respiratory alkalosis and metabolic acidosis occur simultaneously, a child may present with a mixed disturbance and a relatively normal pH, or with frank acidosis. The PC02 will be lower than expected. Children < 4 yr develop metabolic acidosis more rapidly, without concurrent respiratory alkalosis. Dehydrationj is a serious problem because of insensible water loss and increased renal water loss (from an increased urine solute load). Severe losses of sodium and potassium are not uncommon (15). 

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. 

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. 

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. 

The body is also continually losing water through the skin as perspiration, and from the lungs during respiration. This is called the insensible loss. This water loss is unregulated and amounts to between 5()0-85() ml/day. Water may also be lost in disease from fistulae. or in diarrhoea, or because of prolonged vomitine. 

Water depletion may arise from a dccrea.sed intake or excessive loss. A decreased water intake over a period of time when insensible los.ses have continued leads to decreased ECF and ICF volumes. The failure of intake to match the insensible water loss is the cause of the hypernatraemia. The toitil. sodium content of the ECF is unchanged. This is the most common reason for hypernatraemia. A frequently encountered example is the elderly person who becomes ill and is unable to get something to drink. 

Hypernatraemia is most commonly due to water loss (e.g. because of continuing insensible losses in the patient who is unable to drink). 

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. 

Transepidermal water loss (TEWL) includes a mixed phenomenon of passive diffusion (amount of water vapour passing the skin) and insensible perspiration (perspiration that evaporates before it is perceived as moisture on the skin). Mass or volume of water that evaporates in 1 h through 1 m of skin is measured. 

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. 

Increased insensible losses (from his fever caused by bacterial pneumonia) and lack of access to water (from his altered level of consciousness)... 

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... 

Water is lost Irom the body as urine and as obligatory insensible losses from the skin and lungs. 

Patients often become hypernatraemic because they are unable to complain of being thirsty. The comatose patient is a good example. He or she will be unable to communicate hi.s/licr needs, yet insensible losses of water will continue from lungs/skin and need to be replaced. 

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

Fig. 3. Alteration by kidney disease of water requirement and tolerance. The patient whose studies are represented here was loaded with water (left) and deprived (right) until the water concentration of serum (top) increased or fell below normal limits (shaded zone). Tlie corresponding urine water concentrations (second set of panels) define the limits of adjustment. Maximal and minimal urine flows are shown in the third set of panels from the top. The patient s maximum water tolerance and minimal requirements are given in the bottom panels which integrate the insensible loss and water gained from metabolic oxidations (Talbot t, 1959).

Fig. 9. The water requirement and tolerance of the infant as compared with those of the adult. Note that the infant s minimal water requirement is increased by relatively larger insensible and stool losses and an appreciable diversion of water to new protoplasm for growth. The water tolerance of the neonate is reduced by a slight reduction in the renal capacity to dilute accompanied by a substantially prolonged time for adaptation to a water load. Tolerance is also variably reduced by diet breast milk, for example, limits water excretion capacity because of its very small osmoti-cally active residue. (Talbot, 1959).

Insensible loss of water from the skin is greater in the infant than in the adult. The urine volume per kilogram of body weight is also greater in the infant, owing to the higher metabolic rate and the resultant increase in catabolites which must be excreted. Since ability to concentrate urine is limited in the infant, depletion of body water can occur more rapidly and be more serious than in the adult. 

Nearly one litre of water is lost per day from the lungs and in sweat (insensible loss). 

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