Dehydration in Infancy and Childhood

Transcription

1 Article fluid & electrolytes Dehydration in Infancy and Childhood Laurence Finberg, MD* Objectives After completing this article, readers should be able to: 1. List the conditions under which can occur. 2. Describe the first step in therapy of. 3. Explain how to treat isonatremic and hypernatremic. 4. Delineate when oral hydration may be started and why. Definition The concept of did not enter clinical medicine until the 1830s and was not scientifically defined for some years after that. Although the word in general English usage means loss of water, in physiology and medicine, the unmodified word means a loss of water and salt or extracellular fluid (ECF), the most common of the clinically recognized types of. Depending on the type of pathophysiologic process, water and salts (primarily sodium chloride) may be lost in physiologic proportion or lost disparately, with each type producing a somewhat different clinical picture. We have found it useful to designate these types as isonatremic (classical), hypernatremic (hypertonic), or hyponatremic (hypotonic). The differential losses produce different clinical features because of the functional impermeability of the Na and Cl- ions of the ECF to the adjacent intracellular fluid (ICF). We use sodium rather than chloride in the nomenclature because chloride ions are simultaneously involved in a reciprocal relationship with bicarbonate ions. Such classification of the clinical entities has proved useful for understanding of the pathogenesis and especially for treatment. Etiology and Epidemiology Dehydration is a physiologic disturbance that occurs in a wide variety of circumstances affecting water and salt losses. The most common associated disease group among infants is the infectious diarrheas caused by viral and bacterial agents. The most frequent pathogens in the United States are the several rotavirus species, which are most prevalent in the winter months. Bacterial diarrhea, currently seen less commonly over the past 40 years, occurs more often in the summer months, as do some types of disease due to viral causes other than rotavirus. When malnutrition is present, diarrhea resulting in is a leading cause of death in parts of the world where poverty and poor hygiene are commonplace. These circumstances have become uncommon, although not unknown in the United States. Other causes of include diabetic ketoacidosis (DKA), diabetes insipidus, the stress of surgery, and simple water deprivation. Pathophysiology and Pathogenesis Understanding the disturbances of requires knowledge about body composition and the continuing requirements for water and electrolytes. Composition The lean body mass (LBM) of humans is 70% water, with 25% of the LBM in infants in the ECF and 45% in the ICF (Figs 1 and 2). Within the ECF is the plasma (about 6% of the LBM at all ages), which is contained within its compartment by the small osmotic (oncotic) concentration (1 to 2 mosm/kg) of protein molecules that limits their permeability *Clinical Professor of Pediatrics, University of California at San Francisco and Stanford University School of Medicine, San Francisco, CA. Pediatrics in Review Vol.23 No.8 August

2 Figure 1. Composition of lean body mass (fat-free) of the infant. Reprinted with permission from Finberg L, Kravath RE, Hellerstein S. Water and Electrolytes in Pediatrics. 2nd ed. Philadelphia, Pa: WB Saunders; markedly. The ECF and ICF compartments are maintained separately despite free permeability of water and Na and Cl- ions via the energy-dependent extrusion of sodium from cells and the obligate phosphate and negatively charged protein within cells. From 12 to 18 months to about 5 years of age, the proportions gradually evolve into 20% ECF and 50% ICF. Thus, the compositional maturity for body water components is complete at approximately 5 years of age. For the first6to12 months after birth and for all malnourished patients, the LBM and weight may be considered identical, with water content comprising 70% of weight. In older and optimally nourished children, about 10% of body weight is fat, making body water content 60% of total weight. Requirements To maintain a constant body temperature, humans produce heat and lose it at a controlled rate, in part by evaporation of water from the skin (30 ml/100 kcal expended) and from the lungs (15 ml/100 kcal expended), which is termed insensible water loss. Both of these amounts increase during hyperthermia, hyperventilation, and continuous muscle contractions. The losses may increase threefold if all three conditions occur simultaneously. Hyperthermia alone will double insensible losses, as will a very warm environment. When occurs because of diarrhea, homeostatic mechanisms usually adjust so that water and sodium chloride are lost in physiologic proportion, thus maintaining the sodium concentration in serum within the normal range. When vomiting also occurs, water intake is curtailed, making water loss proportionally greater than salt losses and resulting in hypernatremia. When massive stool loss of water and salt is ongoing and the only intake is water, salt losses predominate, resulting in a hyponatremic state. When occurs, several other disturbances come into play. The process of circulatory depletion from plasma loss and concomitant starvation lead to excess hydrogen ion accumulation and reduced urine output. Stool losses also contain intracellular ions, particularly potassium, and uncommonly, calcium homeostasis may be altered. These combined disturbances may be summed up in five categories: volume, osmolality (body space alterations), acid-base or hydrogen ion, potassium losses, and calcium disturbance. Clinical Aspects The first objective sign of is an increase in pulse rate as a response to reduced plasma volume; subjectively, there may be increased thirst. When a hypernatremic state is produced, tachycardia is less pronounced because ECF (hence plasma) is relatively protected, although fever may offset this difference. Thirst may be intense. Changes in acute diarrhea usually are not noted until approximately 5% of the body weight (7% of LBM) has been lost. If the process continues untreated, circulatory insufficiency predominates clinically. The 278 Pediatrics in Review Vol.23 No.8 August 2002

3 Figure 2. Ionic profiles of body fluids: approximate representation of cations and anions of the three principal body fluid compartments. All are electrically neutral and all have the same osmolality despite differences in total charges. The shaded areas represent large molecules or bound ions whose osmolal contribution (mosm/kg) is quantitatively much less than their electric charge (meq/kg), but which are of great importance to the distribution of ions because of their impermeability. Reprinted with permission from Finberg L, Kravath RE, Hellerstein S. Water and Electrolytes in Pediatrics. 2nd ed. Philadelphia, Pa: WB Saunders; most useful clinical sign is the capillary refill time (turgor): normal is less than 2 seconds, 2 to 2.9 seconds corresponds to 50 to 90 ml/kg loss, 3.0 to 3.5 seconds corresponds to 90 to 110 ml/kg, 3.5 to 3.9 seconds corresponds to 110 to 120 ml/kg, and more than 4 seconds corresponds to 150 ml/kg. Although not perfectly predictive, this is the only quantitatively useful sign. Even if the patient is febrile, the estimates are reliable if room temperature is not excessively hot. Other clinical signs of include mottled cool extremities, sunken fontanelle in infants, dry mucous membranes, receded eyeballs, hyperpnea, and loss of skin elasticity in infants (the degree of loss does not correlate with degree of volume loss). The sensorium usually remains intact until becomes moderate ( 6% of weight). A weak cry and stupor suggest a shock state (capillary refill 4 sec). Hypotension is a late manifestation of. Hypernatremic states are characterized by attenuated signs of circulatory deficiency because plasma volume is better preserved per degree of, but neurologic abnormalities appear. There may be a combination of lethargy, hyperresponsivness to stimuli, velvety feel to skin, a doughy feel to the subcutaneous tissue, increased deep tendon reflexes, and as the condition worsens, coma and convulsions. At the most extreme, plasma volume also may be compromised, producing a nearmoribund state that requires immediate resuscitation by restoration of the plasma volume. The hyponatremically dehydrated patient presents as an exaggeration of the isonatremically dehydrated state. Laboratory Analysis Very mildly dehydrated patients may be managed without laboratory determinations, but it is wise to confirm clinical impressions in moderate losses and always in severe illness. Sodium, chloride, bicarbonate, and urea nitrogen determinations are the most essential. An arterial blood gas may be helpful and may be obtained quickly in more severely ill patients. Potassium and calcium levels are occasionally helpful. The urea nitrogen level gives a rough estimate of renal compromise. It is the reduced renal function that produces the acidemia rather than the loss of base in the stool for which compensation readily occurs. Management Treatment is based on the severity of and the diagnosis of the differential losses of ECF and ICF (ie, isonatremic, hypernatremic, or hyponatremic). Severity and type of loss may be assessed best with a five-point diagnostic analysis: 1. Volume: That which has been and that which will continue to be lost. The first assessment is the deficit, which may be determined roughly from the rate of capillary refill corresponding to ECF loss, as noted previously. Estimating the volume begins by adding the obligatory daily replacement for each day of therapy. The Table shows the basal calorie expenditure at various ages that determines the obligatory maintenance requirement. The usual patient expends about l.5 times the basal Pediatrics in Review Vol.23 No.8 August

4 Table. Basal Caloric Expenditure for Infants and Children Age Weight (kg) Surface Area (M 2 ) Caloric Expenditure (kcal/kg) Newborn 2.5 to to wk to 6 mo 3 to to to 70 6 to 12 mo 8 to to to 60 1 to 2 y 10 to to to 50 2 to 5 y 15 to to to 10 y 20 to to to to 16 y 35 to to to 40 Adult to 20 Reprinted with permission from Finberg L, Kravath RE, Hellerstein S. Water and Electrolytes in Pediatrics. 2nd ed. Philadelphia, Pa: rate. High fever may increase this to twice the basal rate, and high fever plus hyperventilation and excessive muscle movement as in a convulsion may increase the requirement to three times the basal expenditure. 2. Osmolality: Circulatory deficits represent ECF, including plasma loss. Neurologic signs and a history of early abrupt cessation of water intake suggest hypernatremia within an estimated loss of 100 ml/kg. This can be confirmed or refuted by analysis of the sodium concentration. In the presence of DKA, this is determined by sodium plus glucose (mmol/l) or by osmolality (mosm/kg) of water. DKA is a hyperosmolal state and, thus, physiologically similar to hypernatremic states. 3. Hydrogen ion (acid-base equilibrium): Failing renal perfusion brings about acidosis or acidemia in diarrheal disease. Ketosis creates acidemia in DKA. Alkalotic occurs when there is high obstructive vomiting (eg, pyloric stenosis) or excessive bicarbonate ingestion or infusion plus. It is appropriate, therefore, to administer physiologic amounts of bicarbonate in the acidotic patient (ie, four chloride ions for each bicarbonate) and to use only chloride in the uncommon alkalotic patient. If the bicarbonate is omitted, the infant or child generally can compensate, but if there has been renal or pulmonary damage, a balanced solution may be important. Balanced (eg, sodium chloridebicarbonate) solution should be used routinely, except as noted previously. Lactate or acetate may be used as a substitute base for bicarbonate. 4. ICF ions: Potassium loss via the stool in diarrhea and via the urine in DKA must be replaced. This step is postponed until circulation is restored and urine production is determined. 5. Skeletal exchanges: Changes are uncommon. Among young infants, calcium ion concentration may be reduced because of phosphate retention. In hypernatremic states at any age, but especially in infants, ionized calcium concentration may decrease, although usually not to a clinically significant degree. Implementation of Analysis Serious Isonatremic or Hyponatremic Dehydration Treatment for losses of 100 ml/kg or greater consists of three phases for the patient in or near shock and requires an initial intravenous route of administration. EMERGENCY. This phase involves the quick restoration of plasma volume. Several options are available. 1. 5% albumin (20 ml/kg), quickly followed by 10% glucose in water (20 ml/kg) in 30 minutes. 2. Balanced isotonic sodium salt solution or 0.9% sodium chloride (40 ml/kg) over 30 to 60 minutes. 3. Balanced isotonic sodium salt solution (20 ml/kg) over 20 minutes, followed by 10% glucose solution (20 ml/kg) over 20 minutes. This regimen restores urine formation faster than does the previous one. REPLETION. This phase works to restore the ECF following the administration of 20 to 40 ml/kg of fluid. The aim is to restore the deficit in 24 hours while meeting the usual obligatory losses and any ongoing abnormal losses. For the infant whose estimated deficit is 100 ml/kg and maintenance requirement of 1.5 basal is another 100 ml/kg, 160 ml/kg remains to be delivered. Because 50% of this is for deficit and 50% for maintenance, the sodium concentration in delivered fluid should be 150/2 75 meq/l (75 mmol/l) (range, 60 to 80 meq/l [60 to 80 mmol/l]), chloride should be 50 to 55 meq/l (50 to 55 mmol/l), and bicarbonate or lactate should be 20 to 25 meq/l (20 to 25 mmol/l) in a 5% glucose solution to provide calories. The combined emergency and repletion phases should last about 8 hours, during which 50% of the day s calculated total should be delivered. When urine production has been assured, potassium chloride or potassium acetate may be added to the infusion at 20 meq/l (20 mmol/l). If acetate is used, the bicarbonate is reduced or eliminated. EARLY RECOVERY. ICF restoration is the focus of this phase. For the next 16 hours, the same solution may be continued at a slower rate to infuse the full 200 ml/kg (in this example). Any ongoing abnormal losses should be replaced by additional fluid of similar composition. 280 Pediatrics in Review Vol.23 No.8 August 2002

5 Hypernatremic Dehydration The principle in this condition is to extend the recovery period over 48 hours so as not to cause cerebral swelling with too rapid administration of the low-sodium concentration fluid. VOLUME. Estimated deficit plus 2 days of usual requirements are administered. For an infant, this would be 300 ml/kg. OSMOLALITY. The deficit of sodium may be replaced by a 100-mL/kg solution containing 80 to 100 meq/l (80 to 100 mmol/l) of sodium. Maintenance for 48 hours would be 200 ml/kg for an infant, which results in 25 to 33 meq/l (25 to 33 mmol/l) sodium in (preferably) 2% to 3% glucose. The lower concentration of glucose is preferred although not essential because hyperglycemia may appear in hypernatremic states. HYDROGEN ION. The anions should be a mixture of chloride and lactate. No bicarbonate is included if calcium is to be added, thereby avoiding precipitation. ICF IONS. A maximum safe concentration of potassium is infused to replace cellular losses and to keep the infused solution at an optimal concentration to avoid brain swelling. SKELETAL IONS. A 10-mL ampule of 10% calcium gluconate per 500 ml of infusion is recommended to combat occasional hypocalcemia. Oral intake of a glucose-electrolyte mixture may be initiated when tolerated and feeding resumed soon afterward. Moderate Dehydration For a 50- to 90-mL/kg deficit, there are two options: 1. Isotonic sodium solution administered intravenously in repeated 20-mL/kg increments until circulation is restored is followed by an oral glucose-electrolyte mixture for the remainder of the deficit and maintenance. The oral solution should contain 45 to 60 meq/l (45 to 60 mmol/l) of sodium and continued until a 24-hour volume (150 to 200 ml/kg) has been attained. Normal feeding then usually may be resumed. Use of this simple method should not preclude careful assessment of the nature and severity of the physiologic disturbance. 2. Complete oral rehydration with an initial solution of sodium concentration of 75 to 90 meq/l (75 to 90 mmol/l) (eg, the World Health Organization solution of 90 meq/l [90 mmol/l] of sodium or a commercially available solution of 75 meq/l [75 mmol/l] of sodium) that provides 40 to 50 ml/kg over the first 1 to 4 hours. Even in the presence of vomiting, slow administration with spoon or dropper (infants) allows success in 90% to 95% of patients. This phase of therapy should be under the direct supervision of a physician or nurse. When circulation has returned, as indicated by a normal pulse rate, the oral mixture is changed to a maintenance one of 40 to 50 meq/l (40 to 50 mmol/l) of sodium and 20 meq/l (20 mmol/l) of potassium. For either moderate or mild that is hyponatremic, the proportion of isotonic sodium solution is increased, keeping the volume the same as calculated for isonatremic states. Mild Dehydration The oral maintenance solution for a 50-mL/kg deficit or less can be administered for approximately 12 hours at the rate of 150 ml/kg per day. Usual feedings can be restarted when appropriate. Suggested Reading Finberg L, Kravath RE, Hellerstein S. Water and Electrolytes in Pediatrics. 2nd ed. Philadelphia, Pa: WB Saunders; 1993 Holliday M. The evolution of therapy for : should deficit therapy still be taught? Pediatrics. 1996;98: Pediatrics in Review Vol.23 No.8 August

6 PIR Quiz Quiz also available online at 6. A 6-week-old boy presents with vomiting for the past 2 weeks. Vomiting is described as forceful. It occurs 5 to 10 minutes after feeding and consists primarily of milk. Physical examination reveals: heart rate, 140 beats/min; respirations, 20 breaths/min; temperature, 98.2 F (36.8 C); and blood pressure, 90/50 mm Hg. The anterior fontanelle is depressed. The infant is alert, responds appropriately to stimuli, and sucks vigorously on a milk bottle. Skin turgor is diminished, with a capillary refill time of 2 to 3 seconds. The abdomen is soft and nontender. Laboratory examination shows: serum sodium, 125 meq/l (125 mmol/l); potassium, 3.8 meq/l (3.8 mmol/l); chloride, 85 meq/l (85 mmol/l); and bicarbonate, 30 meq/l (30 mmol/ L). Venous blood ph is 7.48 and PCO 2 is 55 torr. Blood urea nitrogen (BUN) is 48 mg/dl (17.1 mmol/l). Urine output is decreased. Which of the following is the most likely diagnosis? A. Congenital adrenal hyperplasia. B. Hypertrophic pyloric stenosis. C. Malrotation of intestine. D. Milk allergy. E. Obstructive uropathy. 7. A 3-month-old boy presents with vomiting and diarrhea for the past 3 days. He recently was switched from human milk to powdered milk formula. Physical examination shows a markedly irritable infant who has a heart rate of 170 beats/min, respirations of 50 breaths/min, temperature of 99.7 F (37.6 C), and blood pressure of 60/30 mm Hg. The anterior fontanelle is markedly depressed, and mucous membranes are dry. The skin has diminished turgor, with tenting and a doughy feel. The capillary refill time is 4 to 5 seconds. Bladder catheterization yields no urine output. Laboratory values are: serum sodium, 172 meq/l (172 mmol/l); potassium, 5.3 meq/l (5.3 mmol/l); chloride, 148 meq/l (148 mmol/l); and bicarbonate, 8 meq/l (8 mmol/l). BUN is 72 mg/dl (25.7 mmol/l). Which of the following is the best management strategy for this child? A. An intravenous bolus of 5% dextrose followed by rehydration over 48 hours. B. An intravenous bolus of 5% dextrose followed by rehydration over 24 hours. C. An intravenous bolus of 0.9% saline followed by rehydration over 48 hours. D. An intravenous bolus of 0.9% saline followed by rehydration over 24 hours. E. Rehydration over 48 hours without intravenous bolus. 8. Which of the following states is associated with greater fluid loss per kilogram of body weight for a given severity of clinical manifestations of in children? A. Fever. B. Hypernatremia. C. Male gender. D. Obesity. E. Older age. Please note that the deadline for submission of your answer sheets for the quizzes in the 2002 issues has been extended to January 31, 2003! 282 Pediatrics in Review Vol.23 No.8 August 2002