Author: Nicholas A. Kuehnel, MD, Medical College of Wisconsin
Editor: S. Margaret Paik, MD, FAAP FACEP , The University of Chicago
- Discuss the classifications of a patient’s degree of dehydration
- Identify clinical signs of dehydration
- Understand how laboratory data can supplement your clinical assessment of dehydration
- Determine the appropriate method and amount of rehydration
Dehydration is a symptom caused by a disease process leading to the reduction of one’s total body water content. Clinically, it is a physiologic disturbance leading to varying degrees of hypovolemic shock due to decreased intravascular fluid. Children, and particularly infants, are at increased risk of developing dehydration compared to adults due to: 1) larger total body water content, 2) higher metabolic turnover rate, 3) larger body surface area and in infants specifically 4) renal immaturity, and 5) inability to independently meet their own needs.
When determining specific causes of dehydration, all disease processes fit into one of three general categories: 1) decreased fluid intake, 2) increased fluid output (GI, renal, or insensible), or 3) fluid translocation (burns or ascites). For this reason, any disease process could cause varying degrees of dehydration, and it is important to assess for one of the aforementioned types of fluid loss with every patient. Worldwide, gastroenteritis is the most common cause of dehydration in infants and children, and is the leading cause of mortality in children less than 4 years of age.
In regards to classifying dehydration, it can be classified in two ways, by serum osmolarity and or severity, reflecting the degree of fluid losses. Determining whether a child has hyponatremic (sodium<130), isonatremic (sodium 130-150), or hypernatremic (sodium >150) dehydration can be useful for determining fluid management. However, from an emergency perspective, it is usually more useful to initially focus on determining severity of dehydration to help direct your interventions.
Initial Actions and Primary Survey
Initial evaluation of a child with dehydration should be directed towards determining the severity of dehydration. Degree of dehydration is usually classified into a mild, moderate, or severe category. Most children with significant dehydration will be noted to have one of the following four findings: 1) no tears, 2) dry mucous membranes (dry lips and mouth), 3) delayed capillary refill > 2 seconds, or 4) a generally ill appearance.
In children that are determined to have severe dehydration, correlating with a state of hypovolemic shock, it is important to restore their intravascular volume as quickly as possible, regardless of serum osmolarity or cause of dehydration. Intravenous (IV) access or intraosseous (IO) access should be obtained, and isotonic fluid, typically normal saline, should be infused as 20 mL per kg boluses over 15 – 20 minutes. The child’s degree of hydration and hypovolemia should be clinically reassessed after each fluid bolus.
After primary assessment of the patient, a thorough history is important to give clues to degree and cause of dehydration. One should focus their questioning on determining whether there is concern for decreased intake or increased output as a general cause. Once it is determined that there is historical concern for possible dehydration, you can probe further into signs of dehydration, and then examine for clinical signs.
Historically, it is important to ask about three factors:
- Tiredness or listlessness
- Absence of tears when crying in an infant
- Urine output (with decreased output suggesting worse degree of dehydration, and increased output suggesting possible cause of dehydration as seen in diabetes mellitus or insipidus)
From an examination standpoint, it is important to look for seven additional factors:
- Decreased skin elasticity
- Increased capillary refill > 2 seconds
- Abnormal respirations
- Dry mucous membranes
- Sunken eyes
- Abnormal pulses
All 10 clinical signs of dehydration need to be assessed in the context of the patient, as none of them individually are both sensitive and specific for degree of dehydration. Tachycardia is often the first sign, however it is neither sensitive nor specific (sen 0.46, sp 0.79) as it can be seen with fever, agitation, or pain. Alternatively, decreased skin elasticity is very specific (0.97) but not sensitive (0.35) as it is found in very few patients unless severely dehydrated. Conversely, parental history of decreased urine output is the most sensitive (0.85) but not specific (0.53).
Therefore, the general rule is:
|Findings||Degree of Dehydration|
|1 – 2 findings||Mild Dehydration (<5% total body water loss)|
|3 – 6 findings||Moderate Dehydration (5-10% total body water loss)|
|7 – 10 findings||Severe Dehydration (>10% total body water loss)|
To help supplement your historical and clinical assessment of dehydration in a patient, laboratory evaluation of urine and blood can be useful for patients deemed to have moderate or severe dehydration. The kidney’s natural response to progressive dehydration is to decrease urine production/output, increase urine osmolarity, and increase urine specific gravity. Normal specific gravity of the urine in a child is <1.015 and infants <1.010. A specific gravity >1.025 in any child is suggestive of dehydration and >1.015 in infants as the infantile kidney can only maximally concentrate urine to 800mOsm/L correlating to a specific gravity of about 1.020.
Concurrently, a Basic Metabolic Panel (consisting of Sodium, Potassium, Chloride, Bicarbonate, Blood Urea Nitrogen (BUN), Creatinine, and Glucose) can be very useful. To assist in determining severity of dehydration, it is important to focus on bicarbonate (HCO3), BUN, and creatinine. Early in the dehydration process, children may have an elevated HCO3 due to a contraction alkalosis (increased HCO3 reabsorption from the kidney due to solute loss). Over time, dehydration will lead to a primary metabolic acidosis with HCO3 < 16. BUN and creatinine additionally increase with degree of dehydration due to decreased afferent blood flow and kidney perfusion.
Additionally, depending on the mechanism of dehydration, there are often electrolyte abnormalities that may need to be corrected. Also, if there has been decreased intake or vomiting, children are at risk of developing hypoglycemia and a bedside accucheck using a glucometer should be used to rapidly evaluate a patient’s blood glucose level.
Depending on the degree of dehydration therapy consists of either oral or intravenous rehydration. For mild to moderate dehydration, oral rehydration therapy (ORT) is recommended. ORT consists of small amounts of solution given orally over a period of time. Solutions tend to have a 2:1 glucose: sodium ratio to both hydrate and replenish losses. Goal replacement is to replace half of a patient’s fluid deficit over the first 8 hours, and the remaining half over the next 16 hours. For those who are only mild to moderately dehydrated from vomiting and continue to be nauseated, ondansetron, a serotonin receptor antagonist, can be used at a dose of 0.1-0.15 mg per kg to abate nausea. Note that Zofran cannot be used under the age 6 months. As a quick rule of thumb, you can use 2mg for younger children under 5 years of age, and 4mg for older children and adolescents.
Calculating Amount of Fluid Replacement Needed:
- Weight (kg) / (1 – % dehydration) = Baseline weight
- Baseline Wt. – Current Wt. = Weight Loss (kg) = Equivalent to mL’s deficit
For those with severe dehydration, fluid loss should be replaced using IV or IO rehydration. As stated earlier, initial management consists of 20mL per kg boluses of isotonic normal saline solution until a patient’s signs of hypovolemic shock are stabilized. Once hemodynamically stabilized, fluids are adjusted to a maintenance rate, with glucose and potassium added to help replenish losses. Most commonly, patients are started on either D5.45 NS or D5.9 NS with 20 mEq per L of potassium chloride (exceptions are for burn patients who require ongoing NS fluid therapy, diabetic ketoacidosis patients who do not receive any glucose in their fluids initially, or for children with severe electrolyte abnormalities). Fluids can be adjusted as needed after initiated to replete sodium at an appropriate rate.
Calculating Maintenance Fluid Rate:
Method 1: (Weight based fluid replacement)
First 10 kg: 4 mL / kg / hr
Second 10 kg: 2 mL / kg / hr
Additional kg: 1 mL / kg / hr
A 40 kg child’s maintenance fluid rate would be 80 mL / hr
(40 mL for the first 10 kg, 20 mL for the second 10 kg, then 20 mL for the additional 20 kgs)
Method 2: (Body Surface Area based fluid replacement)
Replace 1200 mL per m2 over 24 hrs
Average 12 year old child: Weight – 40kg, Height – 150cm
BSA = √(cm x kg) / 3600: √(150cm x 40kg) / 3600 = 1.3 m2
Maintenance fluid rate: 1200 mL x 1.3 m2 / 24 hrs = 65 mL / hr
Pearls and Pitfalls
- Tachycardia in a calm, comfortable, and afebrile patient should raise concern for dehydration and compensated hypovolemic shock
- Patients with severe dehydration often have difficult IV access – don’t hesitate to utilize IO access for primary bolus rehydration (best placed in proximal tibia, distal femur, or proximal humerus)
- Utilize historical and clinical signs to determine severity of dehydration
- Mild: < 5% dehydration
- Moderate: 5 – 10% dehydration
- Severe: > 10% dehydration
- Urine specific gravity and serum electrolytes can help supplement your clinical decision to determine degree of dehydration and appropriate fluid rehydration
- In patients with nausea but requiring oral rehydration therapy, consider ondansetron 0.1 mg per kg early in the treatment course
- In patients requiring intravenous fluids, rehydration is initially with 20 mL per kg boluses of NS given over 15 – 20 minutes followed by maintenance fluids
- Normal minimum urine production for a child is expected to be 1 mL per kg per hour. Normal bladder size is calculated as follows:
- Under the age of two = 2 x age + 2 = capacity in ounces
- Over the age of two = age / 2 + 6 = capacity in ounces
Using these calculations, children at any age should fill their bladder at least every 12 hours. Thus, it is appropriate to give guidance to return to the Emergency Department for possible intravenous fluid therapy for dehydration if a child is not urinating at least twice in a 24 hour period
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