Total body water in a normal-weight dog is estimated to be 534 to 660 mL\/kg.1<\/sup> Two-thirds (about 40%) of the total body water is stored in cells within the intercellular fluid (ICF), and one third of total body water is stored in the extracellular fluid (ECF).3<\/sup> ECF is composed of both interstitial fluid volume (ISF, ~15%), transcellular fluid (~1%), and intravascular fluid volume (IVF, ~5%).3<\/sup> Approximately three fourths of the ECF consists of ISF, with the remaining fourth being IVF.3<\/sup> The interstitial area is the space between cells where fluid is held in a gel-like form in a proteoglycan\/collagen matrix. The electrolyte composition of ICF and ECF are very different and play a key role in fluid balance. Fluids between all of these compartments are under a continual dynamic equilibrium. Fluids and electrolytes continually shift, primarily because of osmosis from a low- to a high-sodium concentration. Osmolar, hydrostatic, and oncotic pressures all regulate the balance of fluid movement. Osmotic pressure is the primary determinant of this exchange.1<\/sup> Osmotic pressure is the potential pressure of a solution directly related to its solute concentration.<\/p>\n Normal fluid losses that occur daily are considered to be approximately 50 mL\/kg per day.2<\/sup> Urination makes up approximately 20 mL\/kg per day, fecal losses make up approximately 10 mL\/kg per day, and respiratory and skin losses make up the additional 20 mL\/kg per day.2<\/sup> In healthy animals, some of these losses are replaced with food intake. Fluid losses not compensated for by food intake can stimulate thirst mechanisms or other compensatory means, such as the sympathetic nervous system, angiotensin II, and renal sodium excretion.<\/p>\n Hypertonicity, hypovolemia, and hypotension can be detected by various means in the body in attempt to compensate. Hypertonicity occurs when enough free water has been lost to increase the concentration of solutes in body fluids to a level greater than normal plasma osmolality (285 to 295 mOsm\/kg).1<\/sup> The hypothalamus detects the hypertonicity (increase in plasma concentration) and stimulates thirst. In patients in which hyper- or hypo-osmolality is suspected, osmolality can be calculated by using the following formula:1<\/sup><\/p>\n Posm<\/sub>= [2(Na+ + K+)] + [glucose (mg\/dL)\/18] + [blood urea nitrogen (mg\/dL)\/2.8]<\/strong><\/p>\n In a healthy patient, hypertonicity, any detectable decrease in extracellular fluid volume, or circulating volume can be detected by the following:<\/p>\n Therefore, when a patient is presenting to the veterinary hospital in shock with dehydration deficits or with other fluid imbalances, these mechanisms can\u2019t keep that patient\u2019s fluid balance in check (FIGURE 1<\/strong>). It\u2019s up to the veterinary professional to intervene quickly and fluid support.<\/p>\n FIGURE 1. Critical patients can be especially challenging. Heatstroke causes hypertonicity but may also cause fluid overload with lung injuries.<\/p><\/div>\n Fluid imbalances have many causes. Water losses and dehydration can also result in a variety of electrolyte and acid\/base imbalances. Pure water deficits may also result from excessive panting, water deprivation, or diseases such as diabetes insipidus. It\u2019s important to assess the type of fluid and\/or electrolyte losses that occur in order to determine appropriate replacement fluids. If fluid losses are hypotonic, the ECF will become hypertonic.4<\/sup> Isotonic losses will allow the ECF to remain in an isotonic state. Hypertonic losses result in the ECF becoming hypotonic. The resulting osmolality of the body is an important factor to consider in fluid replacement in order to monitor any potential shifts in osmolality.<\/p>\n Electrolytes play a large role in the balance of fluids within the body. The table provides approximate compositions of electrolytes in the various fluid compartments and protein (primarily albumin in g\/dL) levels, which are involved in maintaining oncotic pressure (TABLE 1<\/strong>).<\/p>\n Evaluation of urine specific gravity (USG) can help determine a patient\u2019s fluid balance (FIGURE 2<\/strong>). USG is a measure of solute concentration in the urine, which is affected by the ability of the renal tubules to dilute glomerular filtrate. The number of molecules in urine (as well as their weight and size) can also affect USG. Isosthenuria (USG, 1.008 to 1.015) occurs when the USG and osmolality of the urine are equal to those of the plasma. Hyposthenuria (USG < 1.008) occurs when USG and osmolality decrease. Hypersthenuria (USG > 1.015) occurs when the USG and osmolality increase or are greater than what is present in the plasma. A USG of >1.030 in dogs and >1.035 in cats indicates adequate concentrating ability of the kidneys.<\/p>\n FIGURE 2. Closed collection sets are the best way to accurately monitor urine production.<\/p><\/div>\n Dehydration cannot be detected clinically until a patient is at least 5% dehydrated. Patients can appear clinically normal, so the patient\u2019s history must be factored in to consider whether losses incurred may have resulted in subclinical (<5%) dehydration.6<\/sup> A thorough history, including information on frequency and approximate amounts of abnormal losses (eg, vomiting, diarrhea, and drainage from wounds), and presence of polyuria and \/or anorexia, should be obtained. Fever or excessive panting may be additional causes of excessive fluid loss. Previously recorded body weight from medical records can be helpful to assess total body water losses.<\/p>\n Dehydration above 5% may be observed clinically, so a physical exam must be performed. The exam should include evaluation of mucus membrane color, capillary refill time, heart rate, respiratory rate, pulse quality, warmth of extremities, skin turgor, and assessment for blood loss. In assessing skin turgor, it\u2019s important to note the patient\u2019s age and body condition score (BCS). Older cachectic patients have reduced skin turgor without dehydration. Overweight or obese patients, on the other hand, may not reveal decreased skin turgor despite significant dehydration.<\/p>\n Packed cell volume, total protein, blood urea nitrogen, and creatinine in relation to USG may all be evaluated as part of this assessment. Elevations in any of these laboratory tests may indicate dehydration. Some signs of severe fluid depletion include:<\/p>\n By combining all the information from the patient\u2019s history and complete physical exam, the degree of dehydration of the patient may be assessed. However, this assessment is only an approximation and may indicate substantially less than the actual loss (FIGURE 3<\/strong>). See TABLE 2<\/strong> for a guide on dehydration estimates.<\/p>\n FIGURE 3. Dehydration deficits should be calculated in order to ensure deficit replacement.<\/p><\/div>\n Once the degree of dehydration is approximated, the amount of fluid volume required to resuscitate the patient may be calculated. Fluid deficits can be calculated by using the following formulas5<\/sup> (1 lb of water = 454 mL; 1 kg of water = 1000 mL):<\/p>\n Percentage dehydration \u00d7 body weight (lb)<\/strong> The fluid deficit is multiplied by 0.80 because we want to give 75% to 80% of a patient\u2019s total fluid deficit within the first 24 hours of resuscitation. Daily maintenance volumes must be added to the deficit volume to provide the total fluid to administer in the first 24 hours. Maintenance volumes of fluids are considered to replace normal ongoing losses.<\/p>\n Maintenance volumes have been estimated as 65 to 75 mL\/kg per day.4<\/sup> Some practitioners calculate water requirements in the same fashion as used for calculating basal energy requirements4<\/sup> (1 kcal energy = 1 mL water) by using a formula such as (30 \u00d7 body weight in kg) + 70. The alternate calculation (70 \u00d7 body weight in kg0.75<\/sup>) is recommended for very small patients (under 2 kg) or very large patients (over 50 kg). Ongoing additional losses must also be considered and factored into the fluid therapy plan. Any calculation is an approximation and is no substitution for monitoring throughout fluid resuscitation\u2014these calculations may underdose one patient or overdose another.<\/p>\n For the first 24 hours of fluid therapy, use the following formula:4<\/sup><\/p>\n Dehydration deficit \u00d7 0.80 + maintenance If shock is present, shock doses must be provided rapidly, in addition to the other replacement volumes. Shock volumes may be up to 60 to 90 mL\/kg for dogs and 40 to 60 mL\/kg for cats.4<\/sup> This volume should be given in increments of 20 mL\/kg (dogs) or 10 mL\/kg (cats) rapidly over 10 to 20 minutes and the response monitored. If there is no response, then boluses can be repeated up to the maximum shock volumes if necessary. If the response after any bolus is sufficient, then the fluid rate should be adjusted. If the patient\u2019s response to crystalloid is not sufficient, then colloids may be added, starting with IV boluses of 5 mL\/kg, up to 20 mL\/kg.4<\/sup> Continuous rate infusions of hetastarch may be continued when needed at rates of 20 to 40 mL\/kg per day.<\/p>\n Crystalloids are the most commonly used replacement fluids (TABLE 3<\/strong>). Crystalloids that are isotonic and noncolloidal are considered to be balanced, with a composition close to that of ECF, with sodium as the primary electrolyte. Crystalloids contain solutes composed of both electrolytes and nonelectrolyte solutes. Isotonic crystalloids can be given rapidly to expand the intravascular space and improve circulation, improving oxygenation to tissues in cases of shock, as they pass readily through cell membranes with up to 75% of crystalloids moving out of the intravascular space within 1 hour of administration. This feature makes them especially indicated for patients with dehydration. Normosol-R (hospira.com<\/strong><\/a>), 0.9% NaCl, lactated Ringer\u2019s solution, and Plasma-Lyte (baxterhealthcare.com<\/strong><\/a>) are all considered to be isotonic crystalloid replacement fluids and are commonly used in veterinary medicine. However, 0.9% NaCl is considered an unbalanced isotonic solution because its composition is not similar to that of ECF (FIGURE 4<\/strong>).<\/p>\n FIGURE 4. Sodium Chloride 0.9% is an isotonic replacement fluid.<\/p><\/div>\n The choice of fluid to administer to a patient depends on the disease process and the type of fluids that may have been lost. Additional electrolyte supplementation may be required. Fluids such as lactated Ringer\u2019s solution, Normosol, and Plasma-Lyte are considered alkalinizing fluids because they contain buffers that the body can convert to bicarbonate after administration: Lactated Ringer\u2019s solution contains lactate, and Normosol (FIGURE 5<\/strong>) and Plasma-Lyte contain acetate. Lactated Ringer\u2019s solution should not be used in patients receiving blood products because of the calcium content in this fluid.<\/p>\n\n
![\"TVN1803F02Fig01\"](\"https:\/\/todaysveterinarynurse.com\/wp-content\/uploads\/sites\/3\/2018\/03\/TVN1803F02Fig01.jpg\")
<\/a>Assessing Fluid Imbalances<\/h2>\n
<\/a><\/p>\n<\/a>\n
<\/a><\/a><\/p>\nEstimating Fluid Replacement Therapy<\/h2>\n
\n \u00d7 454 \u00d7 0.80 g<\/strong>
\nor
\nPercentage dehydration \u00d7 body weight (kg)<\/strong>
\n \u00d7 1000 \u00d7 0.80 g<\/strong><\/p>\n
\nvolume + ongoing losses<\/p>\nFluid Types<\/h2>\n
Crystalloids<\/h3>\n
<\/a><\/p>\n<\/a>