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Quick Test posted on 2.25.10:
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Fluid and Electrolyte Therapy
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The development of a rational plan of fluid and electrolyte therapy requires an understanding of the principles developed earlier in this chapter. First, maintenance fluid requirements must be determined. Second, existing deficits of volume or composition should be calculated. This involves the analysis of four aspects of the patient's fluid and electrolyte status based on weight changes, serum electrolyte concentrations, and blood pH and Pco2: (1) the magnitude of the volume deficit present, (2) the pathogenesis and treatment of abnormal sodium concentration, (3) assessment of any potassium requirement, and (4) management of any coexistent acid-base disturbance. Finally, therapy must also recognize the presence of ongoing obligatory fluid losses and include these losses in the daily plan of treatment.
Normal maintenance requirements can be determined using standard guidelines. Fever or elevated ambient temperature will increase insensible losses and thereby increase these requirements. The normal response to the stress of surgery is to conserve water and electrolytes, so maintenance requirements are decreased in the immediate postoperative period. In addition, increased catabolism will deliver more potassium to the circulation, so that this ion can be omitted from maintenance solutions for several days postoperatively.
Correction of preexisting deficits must be based on the four factors listed above. Volume deficit is best estimated on the basis of acute changes in weight or from clinical estimates; the clinician should remember that deficits less than 5% of body water will not be detectable and that loss of 15% of body water will be associated with severe circulatory compromise. The relationship of net sodium to net fluid deficit is calculable. If the serum sodium concentration is normal, fluid losses have been isotonic; if hyponatremia is present, more sodium than water has been lost. In either case, initial replacement should be with isotonic saline solutions. Any potassium excess or deficit must be assessed in the light of the blood pH. If hypokalemia exists at normal pH, the magnitude of the total body potassium deficit can also be estimated. For a normal 70-kg man, total potassium capacity is 45 × 70, or 3150 meq; the deficit is 20% of this, or 630 meq, and this amount must be considered in therapy calculations.
Two rules of thumb should be applied in prescribing parenteral therapy for fluid and electrolyte deficits. The first is that for most problems, half of the calculated deficits should be replaced in a 24-hour period, with subsequent reassessment of the clinical situation. The second is that a fluid or electrolyte abnormality should take as long to correct as it took to develop. By adherence to these guidelines, overly vigorous replacement will be avoided and, along with it, the production of a different (iatrogenic) electrolyte abnormality.
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| Fig. 9-1. Electrolyte composition of human body fluids. Note that the values are in meq/L of water, not of body fluid. (From Leaf A, Newburgh LH: Significance of the Body Fluids in Clinical Medicine, 2nd ed. Thomas, 1955. Reproduced by permission from Blackwell Publishing.) |
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Quick Test Questions QUESTION 1: A hypovolemic patient with a serum sodium
of 158 should initially be treated with
QUESTION 2: The initial treatment in a patient with
a serum potassium of 6.4 with electrocardiogram changes is
QUESTION 3: An alcoholic patient with a serum albumin
of 3.9, K of 3.1, Mg of 2.4, Ca of 7.8, and PO4 of 3.2 receives
three boluses of IV potassium and has a serum potassium of 3.3.
You should
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