Insensible Fluid Loss

Issues of Concern

Intravenous fluid therapy typically consists of using colloids, crystalloids, or a combination of both. Crystalloids such as normal saline contain usually contain salts with or without glucose. When crystalloids are given, they precipitously equilibrate with and disperse throughout the whole extracellular fluid space. In initial fluid resuscitation, crystalloids are classically the first line fluid used in most patients.[5] Colloids can be an option if the hemodynamic response is insufficient. When choosing a solution, the type of fluid loss is a consideration.[6] Losses primarily associated with water are replaced with a hypotonic solution. Losses involving both electrolytes and water are replaced with isotonic solutions. Glucose can be adding to either solution depending on the patient’s medical condition or needs. Overall in the operating room, isotonic solutions tend to be preferentially used because intraoperative fluid losses are isotonic. The two most frequently used solutions are normal saline and lactated ringers.[7]

Colloid solutions such as albumin typically contain proteins or large glucose polymers. When a colloid solution is given, the high protein content the solution keeps the solution intravascular for a more extended period. Secondarily, this type of solution works to support the intravascular plasma oncotic pressure. Colloids cost substantially more than crystalloids and are associated with more complications, both of which tend to limit their use.[6] Primary indications for colloids include fluid resuscitation in patients with significant intravascular fluid loss. Conditions associated with substantial protein loss, for example, are burn patients and patients with severe hypoalbuminemia.[8] Overall, the guidance of the choice of fluid administered intravenously is a function of the cardiovascular state, patient’s volume status, renal function, comorbidities, serum osmolarity, and any electrolyte imbalances.[8]

Clinical Significance

Evaluation of intravascular volume and clinical assessment are critical because fluid compartment volumes cannot easily be measured. Intravascular volume may undergo assessment by using laboratory/physical exams, and hemodynamic monitoring means.  Frequent evaluations are necessary despite the method used to confirm the initial impression to best manage fluid replacement. The physical examination is most reliable when assessing a patient’s baseline fluid status.[9]  Clues to hypovolemia include hydration of mucous membranes, skin turgor, resting heart rate, and intensity of peripheral pulses. Additionally, blood pressure particularly orthostatic changes and urinary output, are also important indicators of fluid status gleaned from a physical examination. Perioperatively, physiologic manipulation from surgical stress and anesthesia alter these signs (due to increased antidiuretic hormone secretion) rendering them unreliable.[9]

Insensible fluid loss is routinely a cause of concern when evaluating patients. Patients may exhibit nonspecific and specific signs on exams such as dry mucous membranes, poor skin turgor, poor capillary refill, tachycardia, and dyspnea. The cause of insensible fluid loss may be due to many diagnoses, including surgery, respiratory loss, dehydration, burns, metabolic, and vascular etiologies. While the physical exam findings may point to hypovolemia, the cause is difficult to know without proper laboratory and imaging studies.[10] [Level I/II]  Fortunately, the majority of patients will tolerate a fluid bolus challenge with quick re-examination after to assess for positive versus no change in their clinical status.  There has also recently been an explosion in the ability to noninvasively measure functions of fluid status (cardiac output, etc.) with medical devices that make estimates of these measurements. 

Nursing, Allied Health, and Interprofessional Team Interventions

Assessment of fluid status is taught in nursing school, medical school, and physician assistant school.  Knowledge of fluid status and insensible fluid losses is paramount in the care of critically ill patients.  There must be free-flowing information and communication regarding fluid status from bedside nurses to the other medical professionals that are part of the patient's care team.  Physicians across the United States are more commonly turning to point of care ultrasound (POCUS) to provide quick, accurate information on the fluid status of their patients by looking at the inferior vena cava and heart using transthoracic echocardiograms.  As this becomes more common, the healthcare team will be able to accurately assess and treat fluid status and monitor for insensible fluid losses.  Interestingly, there is level I/II evidence that radiologists can provide the healthcare team with pertinent imaging findings that may help to elude the cause of hypovolemia,[10] which point to a positive future for continued POCUS growth.