When a fluid deficit state is encountered, assessment of the severity is usually categorized as percent dehydration, which is really the volume of fluid loss as a percentage of body weight. Mild dehydration is 5% or less, moderate is about 10%, and severe dehydration is about 15% or greater. This classification is relative and not well standardized. Ideally, one could use their baseline body weight to determine the percentage of fluid loss, but this is almost never useful because growing children almost never have a known baseline body weight just prior to becoming ill. Additionally, factors such as anorexia and the duration of illness may lead to loss of lean body mass as well which adversely affects the weight calculation. Clinical and laboratory criteria have been developed to estimate dehydration percentage categories, but these are similarly flawed. Unfortunately, there is no certain way to accurately determine the degree of dehydration, therefore all clinical information (including weight loss if known) should be used to ESTIMATE the dehydration severity. Criteria for 5% dehydration include: no tears when crying, oliguria, sticky (tacky) oral mucosa, less active than usual. Criteria for 10% dehydration include: sunken eyes, diminished skin turgor. Criteria for 15% dehydration include obvious shock (tachycardia, hypotension, cool extremities) and skin tenting. It should be noted that early signs of shock may appear as early as the 5% dehydration level. All of these clinical criteria have some flaws and they are not universally agreed upon. It is often not possible to estimate the urine output because of frequent diarrhea. The oral mucosa may appear to be moist if the patient has just vomited. Sunken eyes may be hard to determine if you dont know what the patient normally looks like. This is best assessed by asking the parent if the eyes look different. Parents will often use the word hollow to describe sunken eyes. A ketotic odor to the breath may signify ketosis due to poor oral intake which somewhat correlates with dehydration. The serum bicarbonate is a measure of metabolic acidosis, but this can be misleading as well since sodium bicarbonate can be lost directly from diarrhea. However, an increased anion gap (calculated as Na minus Cl minus bicarb, which should be less than 12) is almost always present in clinically significant dehydration since lactic acid is produced in a dehydrated state (due to cellular hypoperfusion and a relative increase in anaerobic metabolism). This requires some thinking. For example, in vomiting patients, their bicarbonate initially increases (because of gastric acid loss resulting in a metabolic alkalosis); however, as fluid loss continues, they become dehydrated and a metabolic acidosis would indicate the presence of dehydration. In a patient with diarrhea, the bicarbonate value may be low from diarrheal losses of bicarbonate. So if the serum bicarbonate is relatively low and an increased anion gap is not present, this may not signify dehydration. However, the presence of an increased anion gap would indicate the presence of lactic acid production and dehydration. Similarly in diabetic ketoacidosis, the production of ketoacids and lactic acid results in an increased anion gap. Other clinical situations could affect the bicarbonate value and the anion gap in unusual ways, but this discussion is beyond the scope of this chapter. The above examples pertain to gastroenteritis only. Replacing the fluid deficit (i.e., rehydration) can be done via oral rehydration or IV rehydration. Rehydration via a nasogastric tube is theoretically possible, but this option is not very popular since it possesses some of the negative characteristics of both oral and IV options. Oral hydration is generally preferable since this can be done at home, it is less invasive and it requires less costly resources. The AAP has published a practice guidelines on the management of acute gastroenteritis (3). Oral rehydration has been demonstrated to be successful in most (or perhaps nearly all) cases of gastroenteritis. The oral rehydration solution (ORS) developed by the World Health Organization takes advantage of the principle that glucose and sodium are co-transported in equimolar quantities across the GI mucosa. ORS contains this balance to optimize fluid absorption during gastroenteritis. Glucose in excess of sodium may remain in the bowel lumen as an unabsorbed osmotic particle which retains fluid in the bowel and inhibits fluid absorption. ORS has been demonstrated to be efficacious even in children who are vomiting. The standard strategy is to give a small amount of fluid at a time. Giving 5 cc every 1 to 2 minutes reduces the volume remaining in the stomach at any given time. Since the stomach is similar to a bag, it is difficult for the stomach to vomit if only a small fluid volume is present. Giving 5 cc every minute results in a maximum fluid administration rate of 300 cc per hour, but this is very labor intensive for parents who must do this continuously for it to work. More commonly, 30 cc (1 ounce) is given every 15 minutes which results in a maximum fluid administration rate of only 120 cc per hour. This is more within the realm of what most parents are willing to do at home. If the child is not vomiting, then ORS can be given ad lib. It should be noted that a major difference between the clinical utilization of oral rehydration in the U.S. and other countries, is that American parents are very different from parents in much poorer countries. While parents in other countries may be willing to administer 5 cc every 1 to minutes, while the child continues to have a few emesis episodes, American parents are not likely to be this persistent. Often, if their child is not tolerating 30 cc every 15 minutes, American parents will frequently utilize the option of going to an emergency department for IV rehydration. Children in poorer countries do not have this option and despite sustaining greater degrees of dehydration, they are satisfactorily rehydrated via the oral route. It can be said that oral rehydration usually works for parents who are willing to persevere. In poor countries where an IV is rare, rehydration with ORS is life-saving to a very large number of children. In the U.S. severe dehydration is less common (better hygiene and nutrition), yet IV rehydration is used frequently for mild dehydration. ORS is somewhat distasteful because it is rather salty and not very sweet. Even Pedialyte with much less sodium than ORS, is not very good tasting despite flavoring it. Significantly dehydrated children will usually drink ORS. Children who are not very dehydrated are not thirsty enough to be willing to drink ORS. However, some children who are significantly dehydrated do seem to refuse ORS or Pedialyte since they are either anorexic, too weak to drink, or are refusing because of behavioral reasons (i.e., spoiled). Children with mild dehydration can be placed on near normal diets (avoiding fat and excessive sugar), with good results in most instances. Many textbooks will indicate that children with severe dehydration should be given immediate IV fluid boluses. While this is the standard practice in the U.S., it should be noted that in poor countries, many children with severe dehydration are successfully rehydrated using ORS. However, because severe dehydration is likely associated with a greater mortality risk than mild dehydration, it is reasonable to aggressively treat severe dehydration using IV fluids to reduce this risk. Even moderate dehydration could be treated using IV fluids since this reduces the risk of progression toward severe dehydration with its associated higher mortality risk. In the U.S. where IV fluid infusion resources are plentiful, there should be no hesitation to utilize IV fluids for severe dehydration. For rapid IV rehydration, a fluid infusion utilizing normal saline (NS) or lactated Ringers (LR) of 20 cc/kg is a common starting point. For severe dehydration, this should be given as a rapid bolus (over less than 10 minutes), but for mild dehydration this can be given over one hour. The term isotonic fluid is often used, but this is actually a misnomer. NS and LR are isotonic, but so is D5-1/4NS. All of these solutions have measured osmolarities of approximately 290. NS and LR behave very similarly since both have sodium concentrations similar to that of the serum. The major difference between NS/LR and D5-1/4NS is that NS/LR stays within the vasculature, while D5-1/4NS does not. Since fluid follows osmotic particles, the fluid volume will go, where the osmotic particles go. When NS/LR are used, the osmotic particles are largely sodium and chloride in concentrations very close to that of the circulating plasma. These ions stay within the circulating plasma and thus, the fluid volume expands the intravascular space preferentially. D5-1/4NS has a glucose concentration of 5000 mg/dL (D5W = 5% glucose = 5 grams/100cc). The serum concentration of glucose is only about 100 mg/dL. Thus, when D5-1/4NS is infused, the excess glucose is taken up by cells and converted to glycogen and the fluid volume leaves the intravascular space to enter the intracellular space. This might promote cellular edema under some circumstances, but at the very least, the fluid does not effectively expand the intravascular space. Thus, rather than use the term isotonic IV fluids to describe NS and LR, it would be more accurate to use the term intravascular volume expanding IV fluids. It should be noted that 20 cc/kg is actually a small volume. Take for example a 4 year child who weighs about 20 kg. 20 cc/kg results in a 400 cc fluid infusion. For mild dehydration this can be given over 1 hour so the IV rate would be 400 cc/hr for one hour. While this sounds like a very fast IV rate for a small child, this is actually a small volume. 20 cc/kg only replaces 2% of the bodys weight, and thus it correctly for only 2% dehydration, which would be considered very mild and not generally in need of IV fluid rehydration. The 2% is determined by 400 cc divided by 20 kg (20,000 gms), or by 20 cc/kg (20 cc per 1000 cc = 2%). Another way to appreciate the truly small size of this fluid volume infusion is to equate this to soft drink cans, which are 12 ounce cans. Since 1 ounce equals 30 cc, a typical 12 ounce soft drink can contains 360 cc, which is similar to the 400 cc fluid infusion. One could say that we are giving a single can of IV fluid over an hour. Looking at it this way, most of us can see that this is not very much. Most 4 year olds can drink 3 or 4 soft drink cans on a hot day after a soccer game. Thus, 20 cc/kg fluid infusion volumes should almost always be repeated. For severe dehydration in the range of 15%, the patient would actually need 150 cc/kg to fully replace the fluid deficit. For a patient with 5% dehydration, the patient would actually need 50 cc/kg to fully replace the fluid deficit. In addition to the deficit replacement, maintenance fluid needs must be added in. Resuscitation of shock requires 20 cc/kg NS/LR as a rapid infusion and repeated until perfusion is restored. In most instances, fully rehydrating the patient very rapidly is not necessary and this may be harmful if excessive fluid shifts occur. Once satisfactory fluid resuscitation has stabilized the patient, continued rehydration and maintenance fluids can be administered more gradually. Some patients with mild dehydration will prefer IV rehydration instead of oral rehydration. Although IV rehydration requires more resources and is more invasive, it has some definite advantages. Once the IV is in, fluid infusion is comfortable, rapid, and is not dependent on GI cooperation for absorption. Studies comparing IV and oral rehydration need to compare an endpoint to determine if the endpoint is better in one group or the other. Mortality is the most objective endpoint to measure. For mild dehydration, mortality risk is very low regardless of whether rehydration occurs orally or IV. IV rehydration results in rehydration certainty with minimal work by parents. Oral rehydration requires more work on the part of parents and some uncertainty exists as to whether it will be successful. Most parents bringing their child to an emergency department for IV hydration, have already attempted oral rehydration and they are not fully satisfied with the results. Even though IV rehydration may not be required, it is reasonable to offer it. Put yourself in the body of the child who is experiencing the vomiting and diarrhea. Imagine that you/he/she has vomited 8 times and has had 7 episodes of diarrhea beginning 8 hours ago. You have tried oral rehydration with ORS, but the vomiting and diarrhea have continued. Would you prefer to continue drinking ORS or would you prefer an IV fluid infusion, during which you would have to lie down and get some rest? At some point, many of us would prefer the IV route even though it is not required to avoid mortality. A rule of thumb is that an IV fluid infusion can be considered if V+D (vomiting and diarrhea episodes) is greater than or equal to 10. At this level, sufficient discomfort has been sustained by the patient and mild dehydration is likely. Most mildly dehydrated patients who are given 20 cc/kg per hour for 2 hours (total 40 cc/kg), feel much better with less nausea and fatigue. For such mild patients, they can usually be discharged from the emergency department to catch up on some rest. After a nap or overnight rest, oral rehydration attempts can resume, which are likely to be successful. Compare this to a similar oral rehydration patient, who is not permitted a nap and a period of bowel rest, and who must continue oral rehydration. For inpatients who are hospitalized for IV rehydration, more time is available to gradually rehydrate the patient. Assuming that rapid IV fluid resuscitation has already taken place (or determined to be unnecessary), inpatient rehydration is a more complex calculation than emergency department rehydration. However, this knowledge is generally required for medical students and pediatric residents. Fluid administration over a 24 hour period consists of deficit replacement plus maintenance administration. This is best described with the example presented in the case at the beginning of the chapter. A 12 month old male with vomiting and diarrhea is assessed to be 5% dehydrated by clinical criteria. His weight is 10 kg at presentation, but his pre-illness weight is not known. The patients fluid deficit volume is 5% of 10 kg = 500 cc. The patients maintenance fluid volume is 1000 cc. Fluid administration is generally broken up into 8 hour blocks for the next 24 hours. The maintenance fluid volume is administered evenly over the three 8 hour blocks. Half of the deficit volume is given in the first 8 hours, with one-fourth of the deficit volume given in the next two 8 hour blocks. This is diagrammed below: First Second Third 8 hours 8 hours 8 hours Maintenance volume 1/3 1/3 1/3 Deficit volume 1/2 1/4 1/4
Posted on: Sat, 01 Nov 2014 09:38:14 +0000
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