The insensible losses they refer to the loss of bodily fluids that are not easily evident; that is, they can not be measured and are part of the control in the balance of liquids administered and eliminated by the patient. The eliminated liquids are categorized mainly in sensible losses and insensitive losses.
Sensitive losses are those that can be measured without difficulty; for example, losses in diuresis or gastrointestinal losses. By contrast, insensitive losses are those that we can not measure and are virtually devoid of sodium, such as fluid lost through sweating or breathing.
Insensitive skin loss
Because they can not be measured directly, they are estimated according to the balance of the previous day taking into account factors that may influence its modification, such as the use of phototherapy, humidified ventilators, among others.
They suppose between 700 and 1000 ml day, approximately 35% of the total loss of water eliminated per day, and usually increase in the presence of some pathologies such as burns, fevers, sudden climatic changes, hyperventilation or in situations such as exhaustive exercise.
The water balance, with its sensitive and insensitive losses, can be calculated by pre-established formulas according to the weight, sex or pathology of the patient, but it must be borne in mind that the values are not exact but approximate, according to international consensus.
Types of insensitive losses
Cutaneous losses
These are produced by the thermoregulatory mechanism of convection, losing heat through the skin by diffusion. They must be differentiated from common sweat, since this contains solutes, while the insensible skin losses are imperceptible and can reach 400 ml in an adult.
Cutaneous insensible losses increase in the presence of elevated body temperature (fever) or an elevated ambient temperature.
Pulmonary losses
They are produced by the evaporation mechanism; When heating the air in the alveoli, it becomes saturated with water and is expelled through the expiration.
The temperature of the inspired air influences: the colder, the greater the loss in expiration.
The insensitive pulmonary losses increase in a hot and dry environment and during hyperventilation, as well as in patients with hyperthyroidism.
Causes of increase of insensible losses
The baseline insensitive losses are calculated using the formula 0.5 ml / kg / hour, and the exceptional situation must be added according to the pathology presented by the patient.
- Fever increases cutaneous losses by evaporation from 10 to 15% for every 1 ° C increase in temperature above 38 ° C.
- Very low birth weight premature infants can produce insensible losses of 100 to 200 ml / kg / 24 hrs. The lower the weight, the greater the volume of insensitive losses.
- In patients in ICU with intubation, 500 ml should be added every 24 hours of insensitive respiratory losses.
- In the case of hyperventilation or tachypnea, add 4 ml / hr for every 5 breaths above a normal respiratory rate (FR> 20).
- In the case of mild sweating, evaporation losses are calculated through evaporation at 10ml / hr, moderate sweating at 20cc / hr and, in the case of diaphoresis or profuse sweating, at 40cc / hr.
- In burned patients the loss of the cutaneous barrier generates an increase of insensible cutaneous losses. These losses are calculated by multiplying the total body surface area by 0.35 (constant), and the result is multiplied by the burned body surface area by 100. The result obtained is in ml.
- Other causes of increased insensitive losses can also be postoperative situations, suppurative wounds and cavity drainages, among other events.
How are the insensitive losses calculated?
The approximate daily insensible losses -in normal conditions, without stress or pathologies- are calculated by multiplying the patient's weight in kg by 0.7, and the result is multiplied by 24 hours a day.
The result obtained is in ml and represents an approximation of the insensitive losses expected in a day.
The calculation of insensitive losses is of vital importance in hospitalized patients, especially in the intensive care unit or in the burn care unit.
According to the data obtained, the necessary parenteral hydration of the patient should be adjusted, so that the sensible losses plus the insensible losses, do not exceed the ingested liquids and generate a dehydration.
Similarly, in cases in which the sum of the liquids eliminated (sensible losses + insensitive losses) is considerably lower than the sum of the ingested liquids -especially in cases of heart disease, nephrotic syndrome, cirrhosis, among others- the indication is the restriction of fluid intake in order to avoid edema.
References
- William N. Kelley. Diagnosis and treatment of kidney diseases and electrolyte disorders. Internal Medicine Volume 1. Second Edition. Editorial Panamericana Medical. Buenos Aires. 1992; 929-930.
- Nelson Treaty of Pediatrics. Volume I. 18 Edition. Chapter 52. Electrolytic and Acidobasic Disorders. 273
- Roberto Alcázar Arroyo. Algorithms in nephrology. Hydro-electrolytic and acid-base balance disorders. Module 01. Spanish Society of Nephrology. 2011. Retrieved from: elsevier.es
- American Academy of Pediatrics. Insensitive water loss in low birth weigth infants. Pediatric. August 1972 Volume 50 / Issue 2. Retrieved from: pediatrics.aappublications.org
- Spanish Society of Community Nutrition. Consensus Document. Hydration guidelines in comprehensive health care. Retrieved from: correofarmaceutico.com