Introduction

Fluid management is a critical aspect of patient care, especially in the inpatient medical setting. What makes fluid management both challenging and interesting is that each patient demands careful consideration of their individual fluid needs. Unfortunately, it is impossible to apply a single, perfect formula universally to all patients. However, one general principle for all patient scenarios is to replace whatever fluid is being lost as accurately as possible. These fluid losses can differ depending on patients’ medical conditions and differ by both volume and composition. For example, a patient admitted to the hospital with severe burns will have much greater fluid losses than a relatively healthy patient who is allowed nothing by mouth and awaiting a procedure. A patient admitted for dehydration from severe diarrhea might require different fluid compositions than a patient admitted in hypovolemic shock from a brisk upper gastrointestinal (GI) bleed.

An important distinction in managing fluids is differentiating between maintenance fluids and fluid replacement. Maintenance fluids should address the basic physiologic needs of the patient including both sensible and insensible fluid losses. Sensible fluid losses refer to typical routes of excretion such as urination and defecation. Insensible losses refer to other routes of fluid loss such as in sweat and from the respiratory tract. Fluid replacement goes beyond the normal physiologic losses and includes such conditions as vomiting, diarrhea, or severe cutaneous burns. One must consider these 2 categories of fluid loss separately when devising a fluid management strategy for an individual patient.

Indications

Fluid management is an essential part for any patient admitted to the hospital. If possible, it is preferable that patients take fluids enterally since this is the natural route of fluid intake. However, many patients who are sick enough needing admission to the hospital might have a reason they cannot tolerate oral intake. Alternative routes of administration such as intravenous access can deliver fluids directly to the vascular system.

There are many ways to assess a patient’s volume status to determine their fluid needs. Often, one can determine the patient’s fluid status clinically based on a variety of physical exam findings and objective data from their vital signs. Laboratory markers are helpful as adjunctive data. The following is a list of findings which can be useful in determining whether a patient is fluid depleted or volume overloaded.[1]

Vital signs

• Weight: One of the most sensitive indicators of changes to the patient's volume status is their weight. Patient weight changes approximate a gold standard to determine fluid status. Unfortunately, due to differences in scales available to hospital staff, this can be a challenging target to measure. It is ideal to weigh a patient daily on the same scale to determine trends in his or her weight changes. One can see weight gain in states of fluid excess, and weight loss in states of fluid deficit. It is also helpful to look at patient records to see any recent outpatient visits before hospitalization which might provide an idea of a patient's normal baseline weight.
• Heart rate: Tachycardia can represent a compensatory physiologic response to maintain perfusion in the setting of hypovolemia. This can be an early finding in compensated hypovolemic shock. However, there are many other reasons for tachycardia such as pain, fever, and anxiety.
• Blood pressure: Falling blood pressure is an ominous finding in the setting of tachycardia indicating that the cardiovascular system can no longer compensate adequately for hypovolemia. Conversely, elevated blood pressures can be seen in hypervolemia.
• Orthostatic vital signs: A drop of at least 20 mm Hg systolic blood pressure or 10 mm Hg diastolic blood pressure within 2 to 5 minutes of quiet standing after 5 minutes of supine rest indicates orthostatic hypotension. Dehydrated or elderly patients who have lost sensitivity in their baroreceptors in their blood vessels might display these findings.
• Respiratory rate: Increased respiratory rate indicates a compensatory response to metabolic acidosis from lactic acidosis due to poor tissue perfusion. This is an early finding in hypovolemic shock.
• Urine output: Expect a minimum of 1.5 mL/kg per hour in children and greater than 1 mL/kg per hour in adults. Special situations such as administration of nephrotoxic medications such as acyclovir warrant higher thresholds for urine output to minimize renal toxicity.

Physical exam findings

• Capillary refill: Normally less than 2 seconds. Easy to test on fingertips and toes
• Fontanelle: Sunken fontanelle on the skull of an infant suggests hypovolemia
• Edema: Peripheral edema can be a sign of volume overload or third spacing of intravascular fluid      
• Tear production: Relevant in infants and children; Important to ask parents for their observations and evaluate the child while in the exam room
• Peripheral pulses: Check brachial and femoral pulses in infants; Check radial or dorsalis pedis pulses in older patients; Can see fast and thready pulses in dehydration states
• Skin turgor and eyeball appearance: Severe cases of dehydration might present with flaccid or tented skin; Eyeballs might also appear sunken back into orbital cavities
• Tactile temperature of skin: Classically find cool and clammy skin found in hypovolemic shock due to peripheral vasoconstriction causing hypoperfusion of skin especially at the extremities (i.e., hands or feet)
• Mucous membranes: Appreciate dry, sandpaper-like texture of the oral mucosa or tongue in states of dehydration
• Jugular vein appearance:  Appreciate a distended jugular vein in volume overload state; Can also be found in patients with congestive heart failure who are euvolemic but not pumping blood appropriately

Laboratory findings

• BUN/creatinine: Can be elevated secondary to prerenal acute kidney injury from decreased renal blood flow due to decreased intravascular volume
• Transaminases: Can see an elevation in AST or ALT due to hypoperfusion of hepatic tissue and subsequent tissue hypoxia causing hepatocyte injury, also known as “shock liver”
• Hemoconcentration: Can see elevated hematocrit due to a relative abundance of red blood cells relative to intravascular fluid volume

Equipment

Intravenous (IV) fluid solutions

• 5% Dextrose in 0.9% Sodium Chloride (Normal Saline) +/- potassium additive
• 5% Dextrose in 0.45% Sodium Chloride (Half Normal Saline) +/- potassium additive
• Lactated Ringer's

Enteral tubes

• Nasogastric tube
• Orogastric tube
• Gastric tube
• Nasoduodenal tube
• Gastrojejunal tube

Enteral fluid solutions

• Commerical rehydration solutions
• WHO rehydration solution
• Infant feeds (breastmilk or formula)
• Commercially available sports drinks

Preparation

The pediatric population demands careful consideration of a child’s size in determining their rate of fluid maintenance. A 3-month-old infant has much different fluid needs than those of a more fully grown 8-year-old child. In many cases, a simple calculation called the 4-2-1 rule can determine the hourly rate of fluid maintenance required for a child based on his or her weight.[2] The following example shows an application of this formula.

• First 10 kg = 4 mL/kg per hour
• Next 10-20 kg = 2 mL/kg per hour
• Any remaining weight over 20 kg = 1 mL/kg per hour

For example, a 22kg child would have the following maintenance fluid requirements.

• First 10 kg = 4 mL/kg per hour x 10kg = 40 mL per hour
• Next 10-20 kg = 2 mL/kg per hour x 10 kg= 20 mL per hour
• Remaining 2 kg = 1 mL/kg per hour x 2 kg = 2 mL per hour
• Total hourly rate = 40 + 20 + 2 = 66 mL per hour

Another commonly used formula predicts fluid needs over a 24-hour period. The following example shows an application of this formula. 

• First 10 kg = 100 ml/kg per day
• Next 10 to 20 kg = additional 50 mL/kg per day
• Any remaining weight over 20 kg = additional 20 mL/kg per day

For example, a 70-kg man would have the following maintenance fluid requirements.

• First 10 kg = 100 ml/kg/day x 10 kg = 1000 mL per day
• Next 10 to 20 kg = 50 ml/kg/day x 10 kg = 500 mL per day
• Remaining 50 kg = 20 ml/kg/day x 50 kg = 1000 mL per day
• Total fluids per day = 1000 + 500 + 1000 = 2500 mL per day
• Hourly fluid rate = 2500/24 = 104 mL per hour

One must exercise caution in applying these weight-based formulae to patients who are elderly or obese.[3]  Unfortunately, no standardized guidelines exist at this point to guide adult maintenance intravenous therapy. It is beyond the scope of this article to delve into the nuances of deciding between various tonicities and volumes of fluid administration. These choices demand clinical judgment based on the initial fluid status of the patient and predictions of ongoing fluid needs. The electrolyte derangements discussed below in the complications section show potential issues arising from certain fluid choices.

Technique

The strategy of managing a patient’s fluid differs depending on each patient’s clinical condition. If there can drink adequate fluid volumes by mouth, this should be the first choice. Some patient can tolerate other enteral options such as feeding tubes. IV plus oral orders are effective for those unable to meet their total daily fluid requirements enterally. Nursing staff can titrate the ratios accordingly depending on the patient’s ability to drink. Vital signs, physical exam, and adjunctive laboratory findings mentioned previously will show if each patient's fluid management strategy is appropriate. For example, a post-surgical patient with a new ileostomy might have additional fluid output from the stoma which clinicians must factor into the overall fluid management strategy.

Complications

Electrolyte Derangements

Hyponatremia

Monitor serum sodium regularly. This is more of a risk when using hypotonic solutions.  Many patients admitted to the hospital have risks of baseline elevated antidiuretic hormone (ADH) release leading to volume retention and worsening hyponatremia.[4] Isotonic fluids are preferred for maintenance fluids in patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Risks of hyponatremia include cerebral edema with potentially dangerous neurologic sequelae such as seizures. If significant hyponatremia develops, it is important not to correct the serum sodium too quickly to avoid the devastating neurologic complication of central pontine myelinolysis.[5]

Hyperkalemia

Patients with renal failure who receive a potassium-containing solution might not handle clearing the potassium load and develop life-threatening hyperkalemia causing cardiac arrhythmias.

Volume Overload

Monitor for peripheral edema, pulmonary edema, or hepatomegaly. It is important to consider underlying cardiac dysfunction or renal failure and adjust volumes of administration accordingly. These patients might require a lower maintenance fluid rate than expected for their body weight.

Metabolic Acidosis

Normal saline is a slightly acidic solution relative to normal body pH. This can precipitate metabolic acidosis.[6] Lactated ringers solution is a closer approximation to normal body pH; however, the use of lactated ringers vs. normal saline for fluid maintenance administration often depends on availability at each hospital institution and is an evolving paradigm undergoing discussion nationally.

Clinical Significance

Understating the importance of proper fluid management is difficult. Careful consideration of each patient’s current clinical status and relevant past medical history when determining a fluid management strategy is crucial to avoid iatrogenic problems such as dehydration, volume overload, electrolyte derangements, or pH imbalances. Close communication between all members of the healthcare team can mitigate these issues.

Enhancing Healthcare Team Outcomes

Interprofessional discussion within the healthcare team can optimize proper fluid management for patients admitted to the hospital.[7] Bedside nurses often spend more time than any other healthcare member at the bedside with their patients and can provide useful assessments of patients’ volume status through documentation of vital signs and frequent visual assessments. Nurses can also be very helpful in assessing patients’ ability to tolerate enteral fluids and encouraging patients to drink by mouth if there is no NPO order which would prevent them from doing so. Nutritionists and dietitians are very helpful in determining caloric needs for patients to ensure that they meet their metabolic demand which is especially important during acute illnesses so that the body can heal properly. This is especially important in the pediatric population for infants who drink breast or formula. Total parenteral nutrition (TPN) is sometimes necessary as a temporizing measure for patients unable to take enteral fluid intake, but it comes with a variety of challenges such as the need for central venous access and risk of central line-associated bloodstream infections (CLABSI).

Questions

To access free multiple choice questions on this topic, click here.

References

1.

Grainger JT, Maeda Y, Donnelly SC, Vaizey CJ. Assessment and management of patients with intestinal failure: a multidisciplinary approach. Clin Exp Gastroenterol. 2018;11:233-241. [PMC free article: PMC6003282] [PubMed: 29928141]

2.

Miller M. Syndromes of excess antidiuretic hormone release. Crit Care Clin. 2001 Jan;17(1):11-23, v. [PubMed: 11219224]

3.

Norenberg MD. Central pontine myelinolysis: historical and mechanistic considerations. Metab Brain Dis. 2010 Mar;25(1):97-106. [PubMed: 20182780]

4.

Kraut JA, Madias NE. Treatment of acute metabolic acidosis: a pathophysiologic approach. Nat Rev Nephrol. 2012 Oct;8(10):589-601. [PubMed: 22945490]

5.

Chesney CR. The maintenance need for water in parenteral fluid therapy, by Malcolm A. Holliday, MD, and William E. Segar, MD, Pediatrics, 1957;19:823-832. Pediatrics. 1998 Jul;102(1 Pt 2):229-30. [PubMed: 9651436]

6.

Shafiee MA, Bohn D, Hoorn EJ, Halperin ML. How to select optimal maintenance intravenous fluid therapy. QJM. 2003 Aug;96(8):601-10. [PubMed: 12897346]

7.

Friedman JN, Goldman RD, Srivastava R, Parkin PC. Development of a clinical dehydration scale for use in children between 1 and 36 months of age. J. Pediatr. 2004 Aug;145(2):201-7. [PubMed: 15289767]