Guidelines for daily fluid therapy planning: The basics (Proceedings)

Once life-threatening hypovolemia and electrolyte problems have been corrected or if they were not judged to be present in the first place, the remaining fluid and electrolyte abnormalities can be dealt with. There are three categories to be considered when developing a fluid therapy plan: 1) the deficit - how much fluid volume and of what type will it take to restore the patient to normal; 2) the normal ongoing losses (commonly referred to as maintenance) - how much (and what type) of fluid will it take to accommodate the normal ongoing losses; 3) the abnormal ongoing losses - how much (and what type) of fluid will it take to accommodate the abnormal ongoing losses.

Fluid therapy planning guide

• Deficit

-Calculate a quantitative estimate of the deficit volume.

-Start with an ECF replacement crystalloid solution.

-Supplement potassium, or don't.

-Adjust the sodium concentration, or don't

-Add bicarbonate, or don't

• Normal ongoing losses

-Look up on the chart or calculate the maintenance volume

-Start with an ECF replacement crystalloid or a real maintenance fluid (low sodium conc)

-Supplement with potassium (usually 20 mEq/L)

• Estimate the abnormal ongoing losses (or don't; they can be added later, once it has been established that the abnormal ongoing losses are going to continue).

-Start with an ECF replacement solution

-Supplement with potassium (usually 10 mEq/L)

• Decide on a route of fluid administration (intravenous CRI; subcutaneous intermittent; per GI)

-Calculate hourly infusion rate or per dose volume

• Mathematically average the above fluid "prescription" and administer as one fluid or administer each in series.

• Monitor the patient during the day to make sure that all is going according to plan.

The deficit fluid volume

The deficit volume is usually determined by the magnitude of the decrease in skin elasticity. Normally the skin over the thorax, after being lifted into a fold, will snap immediately back to its resting position when released. When the skin fold returns a little slowly, the animal is said to be 5% of its body weight dehydrated. When the skin fold stands in the fold after it is released, the animal is said to be 12% of its body weight dehydrated. Intermediate "skin fold return rates"are extrapolated between 5 and 12 %. Unfortunately, this sign, as a quantitative estimate of the magnitude of dehydration is not very accurate; we use it because it is one of the few quantitative estimates available at presentation. Obesity will obscure the sign (obese animals may have no decrease in skin elasticity even though dehydrated); emaciation will amplify the sign (emaciated animals will have a decrease in skin elasticity even though they are not dehydrated). In addition, there is considerable patient-to-patient variation in this sign.

An acute change in body weight provides a quantitative guide to the volume of the deficit. At initial examination, however, the pre-illness body weight is seldom known. Body weight is a good way to track the adequacy of the ongoing fluid therapy plan. Lean body mass is normally neither lost nor gained rapidly enough to affect major day-to-day changes in body weight. Large volumes of fluid can, however, accumulate in cavities such as the intestinal lumen, the peritoneal or pleural cavities, or in tissues around fracture or trauma sites and effectively decrease extracellular fluid volume without a change in body weight.

Determining the magnitude of dehydration in a patient is, at best, an inaccurate science, and at worst, a wild guess. In matters of hydration status, it is always important to assess assess as many parameters as possible, and to correlate them with each other and other patient abnormalities. The end-product of this process will be to determine whether the animal is under- or over-hydrated, and, if dehydrated, to establish a quantitative estimate of the magnitude of the dehydration. If the patient is deemed to be dehydrated, the clinician may pick a number between "5 and 12% and multiply this by the animal's body weight to determine the volume of fluids predicted to remedy the dehydration. Alternately, the clinician may estimated a categorical magnitude of dehydration (mild, moderate, or severe) and then equate this with or a specific percentage (6, 9, and 12%, respectively).

Once the volume of the deficit has been estimated, the time period over which it is to be corrected must be determined. This is entirely at the discretion of the clinician (there is no formula to guide this decision). Mild degrees of dehydration are usually repleted over the entire day while severe degrees of dehydration are usually front-loaded (a large portion of the estimated deficit volume is administered over the initial 2 to 6 hours) (if there is an associated hypovolemia).

The deficit repair fluid

An iso-osmolar, polyionic ECF replacement crystalloid such as lactated Ringer's or plasmalyte 148, or an equivalent solution with about normal extracellular concentrations of sodium, potassium, chloride, and a "bicarbonate-like" anion (bicarbonate, lactate, gluconate, or acetate), should be used to restore the deficit. These replacement fluids should be administered without alteration if front-end loading of the fluid volume is planned and when it is known or suspected that there are not major electrolyte abnormalities.

Potassium abnormalities are common in critically ill patients (vomiting, diarrhrea, no food intake) and abnormalities are often severe enough to warrant the addition of potassium to the deficit repair solution. Potassium should not be added if the animal is normokalemic or hyperkalemia. Unfortunately most of the body's potassium inside the cells and is unmeasurable. Plasma potassium is used as a window to the total body potassium balance; the presumption being that intracellular and extracellular potassium stores are depleted somewhat proportionately.

Potassium deficits are common enough that the magnitude of the total body potassium depletion should be estimated (based upon the magnitude and duration of the abnormal losses and the duration of the malnutrition) if plasma potassium measurements are not available.

Guidelines for potassium supplementation of deficit replacement solutions

Potassium can be administered no faster than it can redistribute from the vascular fluid compartment into the intracellular fluid comparment (otherwise an iatrogenic hyperkalemia will occur). The potassium infusion rate should be calculated if higher potassium concentrations or higher infusion rates are used. The maximum initial infusion rate for potassium is 0.5 mEq/kg/hr. The guidelines are estimates for where to start with the potassium supplementation. If the animal is not responding, the potassium concentration of the fluids and/or the infusion rate can be increased. The administration of higher potassium concentrations and higher infusion rates should be monitored by regular or continuous electrocardiographic monitoring. The electrical changes associated with iatrogenic hyperkalemia are tall, spiked T waves; small P waves; prolonged P-R intervals; bradycardia; and widened QRS waves and QT intervals.

Patients with diabetic ketoacidosis and chronic malnutrition may have reasonably normal plasma potassium concentrations in the face of severe total body potassium depletion. Insulin therapy or refeeding may unmask a serious potassium depletion. Nonorganic acidemia (bicarbonate loss, hydrogen ion retention), but not organic acidemia (lactic acidosis, ketoacidosis), will increase the plasma potassium, again hiding an underlying total body potassium deficit. Alkalinization therapy may unmask a serious potassium deficit.

Volume restoration with an iso-osmotic, polyionic ECF crystalloid fluid with a "bicarbonate-like" anion enables most patients with reasonable renal function to self-correct a mild to moderate metabolic acidosis (these fluids, per se, have minimal impact on acid-base balance). Patients with moderate to severe metabolic acidosis may benefit from bicarbonate therapy. The metabolic contribution to the acid-base balance is easily identified by the base deficit ("SBE" on blood gas analyzer printouts) or by the quantitative change in the measured bicarbonate or total carbon dioxide concentrations (24 and 25 mMq/L, respectively). The amount of bicarbonate to add to the deficit repair solution can be calculated by the equation: (goal base deficit or bicarbonate concentration - current base deficit or bicarbonate concentration) x 0.3 x kg of body weight. Bicarbonate therapy must be conservative; give less as opposed to more. The goal is often to restore the base deficit or bicarbonate concentration to somewhat below normal, allowing correction of the underlying disease process to adjust the remaining acid-base imbalance. If base deficit, bicarbonate, or total carbon dioxide measurements are not available, one can estimate the magnitude of acid-base disturbance, based upon the magnitude of the underlying disturbance: in mild, moderate, or severe metabolic acidosis might estimate a bicarbonate dose of 1, 3 or 5 mEq of sodium bicarbonate/kg of body weight.

If the measured sodium concentration is between about 130 and 165 mEq/L and the animal has reasonable renal function, volume restoration with an iso-osmotic, polyionic ECF crystalloid fluid will allow the animal to restore its own sodium balance in most circumstances. If the sodium concentration is below 130 or above 165, special precautions are required so that the plasma sodium concentration is not normalized too rapidly. A rapid decrease in sodium causes water intoxication; a rapid increase causes central myelinolysis.

The normal ongoing loss (maintenance) volume

The volume of fluids required to replace the normal ongoing losses can be determined from predictive charts for the dog (Table 9-4) and cat (9-5). If a chart is not available, the maintenance volume can be calculated as 132 x BWkg0.75 for dogs and 80 X BWkg0.75 for cats. In lew of calculations, the volume can be estimated as 50, 75, or 100 ml/kg per day (2, 3, or 4 ml/kg/hr) for large (> 30 kg), medium(8-12 kg), and small (< 4 kg)dogs and 50 to 75 ml/kg/day for cats (< 2 and > 4 kg), respectively.

The normal ongoing loss replacement fluid

The net sodium concentration of normal urine and insensible losses are 50 to 60 mEq/L while the potassium concentration is 15 to 20 mEq/L. Administering an ECF replacement solution predisposes to hypernatremia (it is more sodium than the patient is losing) and hypokalemia (almost always a problem if the animal is not eating because the kidney is not very good at conserving potassium). A low sodium, high potassium solution such as plasmalyte 56 would be an ideal fluid for replacing the normal ongoing losses. An isotonic, polyionic ECF replacement solution, supplemented with potassium (20 mEq/L), however, is most frequently used for replacing normal ongoing losses, and works well most of the time. Hypernatremia is usually not a problem in a well hydrated patient with reasonable renal function because the kidneys can readily eliminate the excess sodium. Hypokalemia, however, is almost always a problem if the animal is not eating because the kidney is not very good at conserving potassium.

Estimated crystalloid fluid volume to replace normal ongoing losses in dogs (132 x Kg0.75)

Estimated crystalloid fluid volume to replace normal ongoing losses in dogs (132 x Kg0.75)

A real normal ongoing loss replacement solution would be relatively more important in a patient with poor renal function. While ECF replacement solutions (with potassium) may be used for maintenance fluid therapy, the opposite is not true; low sodium maintenance solutions must not be used to replace extracellular volume deficits because they will cause hyponatremia and hyperkalemia when administered in large volumes.

Estimated crystalloid fluid volume to replace normal ongoing losses in cats (80 x Kg0.75)

The abnormal ongoing loss

When a fluid therapy plan is first being constructed, it is usually not known how much fluids the animal will lose over the day. One could either leave this category blank for the time being and then as losses occur during the day, add equivalent volumes of an appropriate fluid to the fluid therapy plan. Alternatively, if the patient has a disease which is known to be associated with unrelenting fluid losses, an estimated volume could be factored in at the time of the initial construction of the fluid plan and then adjusted upward or downward as the day progresses.

Ongoing losses which occur via transudation into one of the major body cavities, into the tissues, or via burn wounds, are electrolytically in equilibrium with the extracellular fluid compartment and should be replaced with an unadulterated isotonic, polyionic ECF replacement solution. Vomition, diarrhea, or diuresis usually have a measured sodium concentration of 60 to 120 mEq/L and a potassium concentration of 10 to 20 mEq/L and should be replaced with an isotonic, polyionic ECF replacement solution which has been supplemented with potassium to 10 mEq/L.

An alternative approach to building a fluid therapy plan

An alternate method of building a fluid plan

It is common to utilize an abbreviated version of the above described fluid therapy plan. The replacement volume for the normal ongoing losses is determined as described above. Deficit and abnormal ongoing losses are estimated as a multiple of this maintenance volume.

Implementation of the fluid therapy plan

What has been completed so far is the best guess as to the requirements of the patient. Each categorical decision has been made without reference to the other categories and without reference to how these fluids are actually going to be administered. When we try to assimilate these individual decisions into a functional therapy plan, we occasionally find that it is inconvenient, difficult, or impossible to administer the fluids exactly as prescribed. While it is important to try to enact a fluid therapy program that is close to the prescription, given the inherent inaccuracies in the assumptions utilized in the construction of the plan, it is not imperative that what is actually administered exactly duplicates what has been prescribed.

There are many acceptable ways to administer the prescribed fluids: intravenous or intraosseous, subcutaneously, orally, or even intraperitoneal. The intravenous administration of fluids is often the preferred route since its effects are immediate and reliable. It may be inconvenient in some practice settings to do this, and there are some reasonable alternatives. The fluids could also be administered via the intramedullary route, if venous access is not possible due to the small size of the patient. A regular hypodermic needle works well in the very young animals, while a bone marrow biopsy needle works will in the older animal. The bony prominence at the proximal humerus and proximal tibia, and the trochanteric fossa of the femur are common sites for needle introduction. The fluid prescription can be administered subcutaneously in several divided daily dosages. This route is usually well tolerated by patients and therapy is often efficacious. The subcutaneous route is, however, slower in onset than the intravenous route, less efficacious than the intravenous route because some patients (particularly those that are severely dehydrated and vasoconstricted) will not absorb the fluids well or at all. The fluids could also be administered orally or via stomach tube, in several divided daily doses, as long as the gastrointestinal tract is functional. The fluids could also be administered intraperitoneal, a route that is characterized by the same advantages and disadvantages as the subcutaneous route, except, in addition, the danger of injury or perforation of an abdominal organ.

If the fluids are to be administered by the intravenously, one could literally or arithmetically mix all of the fluids and additives from each category into one container and administer them at an even rate throughout the day or fluids could be administered simultaneously but separately by conjoining one administration line with another. The fluids could also be administered in series (e.g. first the deficit fluid, then the maintenance fluid, and finally the abnormal ongoing loss fluid).

The intravenous infusion rate is determined by dividing the total fluids to be administered by the number of hours available to administer them. A longer, slower infusion is preferrable to a faster infusion (notwithstanding hypovolemia issues) to allow the patient to have as much time as possible to fully utilize the administered fluids and electrolytes. Faster administration of larger volumes are likely to induce a diuresis.

Fluids administered by peripheral veins cannot be too hyperosmolar; below 600 mOsm/l is safe; above 700 mOsm/l will cause phlebitis and thrombosis.

The administration of fluids by gravity is plagued by difficulties in controlling the rate of infusion. This problem is virtually eliminated by the use of a fluid pump.