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

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Apr 01, 2008

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).