Water is essential for the maintenance of life. As the major element of fluid compartments in the body, water makes up approximately 60% of body weight. Gender, age, physical activity, and adiposity are pertinent factors that may alter this percentage. In the body, water subdivides between extracellular (33%) and intracellular (67%) spaces. Thirst and hormonal mechanisms are both responsible for maintaining total body water within a tight range. A decrease in plasma volume, increase in plasma osmolality, or decrease in blood pressure all stimulate a patient’s thirst drive. The hormone vasopressin is responsible for controlling daily water balance, and hyperosmolality stimulates its release.
The body uses water for a variety of mechanisms from transporting nutrients to excreting wastes and tissue structure viability. Water balance occurs by matching the daily water input/output to and from the body. The primary means of water intake is by consumption of food and fluids. Daily fluid maintenance requirements for adults are approximately 1.5 to 2.5L of water. The majority of fluid loss occurs in urine, stool, and sweat but is not limited to those avenues. Insensible fluid loss is the amount of body fluid lost daily that is not easily measured, from the respiratory system, skin, and water in the excreted stool. The exact amount is unmeasurable but is estimated to be between 40 to 800mL/day in the average adult without comorbidities. A total loss of approximately 600 to 800mL/day characterizes 30 to 50% of all water loss, contingent on the level of water consumed. Thus insensible water loss is a significant component of water balance and needs to be routinely monitored.
During the perioperative period, the goals of fluid management are to provide a suitable volume of parenteral fluid to support cardiac preload, intravascular volume, oxygen carrying capacity, and electrolyte balance. Additionally, parenteral fluid replenishment focuses on equipping the body with enough fluid to meet both insensible and sensible physiologic losses. Maintenance fluid rate replacement is calculated by using the “4-2-1” rule, which came from the 1950s work published in Pediatrics. For the first 10kg of the patient, fluid replacement is at a rate is 4mL/kg/h. For the next 10kg, the rate is 2mL/kg/h, and for each kg, after 20kg the rate is 1mL/kg/h. Preoperative fasting causes a fluid deficit leading to a slight decrease in the extracellular fluid while maintaining intravascular volume. Without any fluid intake overnight, a patient’s fluid deficit is proportionate to the duration of the fast. This deficit is estimated by multiplying the normal maintenance rate by the length of the fast. Fluid replacement during surgery centers on the type/extent of surgery as well as a patient's hourly needs.
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. Colloids can be an option if the hemodynamic response is insufficient. When choosing a solution, the type of fluid loss is a consideration. 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.
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. 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. 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.
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. 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.
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. [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.
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, which point to a positive future for continued POCUS growth.
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