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Support Line December 2005 Volume 27 No. 6 Fluid …

13 AbstractAn understanding of basic Fluid andelectrolyte physiology can aid cliniciansin administering parenteral nutrition(PN). disturbances in electrolytes, Fluid ,and acid-base balance require changesin therapy. Therefore, close monitoringof these parameters is essential duringthe administration of nutrition (PN) is a com-plex therapy containing more than 40components, including dextrose, aminoacids, fat emulsions, water, electrolytes,trace elements, and vitamins. To orderPN appropriately, clinicians must havea good understanding of body compo-sition, Fluid balance, electrolyte assess-ment, and acid-base balance. Body Fluid CompartmentsTotal body water (TBW) comprisesapproximately 60% of body weight inmen and 50% in women.

– 13 – Abstract An understanding of basic fluid and electrolyte physiology can aid clinicians in administering parenteral nutrition (PN). Disturbances in electrolytes, fluid,

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Transcription of Support Line December 2005 Volume 27 No. 6 Fluid …

1 13 AbstractAn understanding of basic Fluid andelectrolyte physiology can aid cliniciansin administering parenteral nutrition(PN). disturbances in electrolytes, Fluid ,and acid-base balance require changesin therapy. Therefore, close monitoringof these parameters is essential duringthe administration of nutrition (PN) is a com-plex therapy containing more than 40components, including dextrose, aminoacids, fat emulsions, water, electrolytes,trace elements, and vitamins. To orderPN appropriately, clinicians must havea good understanding of body compo-sition, Fluid balance, electrolyte assess-ment, and acid-base balance. Body Fluid CompartmentsTotal body water (TBW) comprisesapproximately 60% of body weight inmen and 50% in women.

2 There aretwo primary compartments of water inthe body. Intracellular Fluid Volume (ICFV) contains two thirds of TBW,and extracellular Fluid Volume (ECFV)contains the remaining one third (1).The ECFV contains interstitial fluidand plasma. Interstitial Fluid surroundscells and comprises three quarters ofthe ECFV Volume . Plasma makes upthe remaining one quarter of proteins maintain plasmavolume primarily through oncoticpressure (1). In response to changes in solute concentrations, water passesthroughout these compartments tomaintain osmotic equilibrium (1).Water BalanceMaintaining an appropriate waterbalance is crucial for optimal metabolicfunction. Thirst, antidiuretic hormone(ADH), and aldosterone regulate waterbalance.

3 During periods of acute illness,ADH and aldosterone production areoften increased. Hypotension, stress,decreased intravascular Volume , pain,surgery, and increased plasma osmolalityall increase ADH output, favoringwater retention. Aldosterone productionincreases when kidney perfusiondecreases, causing sodium and waterretention (1). An aging individual,generally defined as older than 65years, has a diminished ability to adjust to hemodynamic changescompared with a younger , in a hospitalized, acutely ill elderly patient, extra care must betaken with total Fluid provision, fluidbalance, and monitoring of serumchemistry values (2). Assessing Fluid Requirements An average adult requires approxi-mately 2,000 to 2,500 mL/d of water (3).

4 Oral fluids provide 1,100 to 1,400 mL;solid foods provide 800 to 1,000 mL;and oxidation of protein, carbohy-drate, and fats yields approximately300 mL water. Fluid output consists oflosses from urine (1,200 to 1,500 mL)and stool (~100 to 200 mL) (4). Otheroutput is termed insensible because itcannot be measured, but it has beenestimated that the average adult loses450 mL of Fluid per day (5).During illness, there are multiple othersources of input and output, includingintravenous (IV) fluids, medications, anddrips. IV medications and drips shouldbe subtracted from estimated fluidrequirements before calculating thePN formulation. Variables for outputinclude nasogastric suctioning losses/vomiting, diarrhea, fistula drainage (4),and sequestration of fluids resultingfrom injury, infection, peritonitis, andburns (6).

5 Such losses must be replacedto prevent Fluid deficits. When accountingfor significant losses from the gastro-intestinal tract, it is important to havea general understanding of the amountsof electrolytes lost that require replace-ment(Table 1) (4). The PN solutioncan be adjusted to provide for theseongoing electrolyte losses. Insensible losses occur primarilywith fever; Fluid needs increase each degree increase in bodytemperature greater than 37 C (4) orapproximately 60 to 80 mL daily foreach degree over normal ( F) (2). Fluid and electrolyte Management in Parenteral NutritionJodi Kingley, MS, RD, CNSDS upport LineDecember 2005 Volume 27 No. 6(Continued on page 16)Table 1. electrolyte Composition of Body Fluids (6)VolumeSodiumPotassiumChlorideBicarbon ateType of (mL/24 h)(mEq/L)(mEq/L)(mEq/L)(mEq/L)Secretion( range)(range)(range)(range)(range)Tonici tySaliva1,00030 to 80207030 HypotonicSweat200 to 1,00020 to 705 to 1040 to 600 HypotonicStomach1,000 to 2,00060 to 80151000 HypotonicSmall bowel2,000 to 5,0001402010025 to 50 IsotonicColon200 to 1,5006030400 HypotonicBile1,0001405 to 1010040 IsotonicPancreas1,0001405 to 1060 to 9040 to 110 IsotonicWriter sMentoring Program 16 Increased insensible losses occur fromhyperventilation, low humidity, andincreased ambient room temperature(4).

6 Fluid requirements vary with age,sex, body weight, disease state, anddegree of insensible loss. Table 2 lists several methods todetermine Fluid requirements. Methods1 and 3 should be used carefully becausetheycan result in insufficient fluidprovision for those with low bodyweights and excess Fluid provision inobese patients. Conversely, method 4(the Adequate Intake) is based on healthyindividuals (5) and, therefore, may notbe applicable during acute illness. Theliterature suggests a minimum of 1,500 mL/d of fluids for most elderlypersons (7). Fluid restrictions may berequired for those with cardiac, liver,and renal FluidsA basic understanding of IV solutionsand what they provide can assist whenevaluating sodium, electrolyte , andfluid provision in nutrition supportregimens.

7 IV fluids provide maintenanceor repletion of Fluid and electrolytes,especially sodium chloride and potas-sium (Table 3). Some contain dextroseas a protein-sparing source. IV fluidsare also used to provide IV antibiotics,which are mixed in dextrose or normalsaline (9). Electrolytes Plasma electrolyte abnormalities arecommon in acutely ill patients. Thenutrition Support clinician shouldunderstand the factors that causeirregular electrolyte levels because PNis often the primary Fluid source forthose receiving the therapy. PN fre-quently is used to help correct suchdeficits or excesses, unless the patientis unstable. Separate IV administrationof electrolytes is more practical forfrequent changes in Volume andelectrolyte requirements (10).

8 SodiumEvaluating abnormal sodium concen-trations is not always sodium concentration is theprimary measurement used to deter-mine the volumes of the ICF and ECF,not the actual amount of total bodysodium (11). Sodium is the main cationin the ECFV. If the total amount ofsodium in the ECFV is elevated, thesize of the ECFV also increases, whichmay lead to a state of Volume sodium in the ECFV compartment results in volumedepletion (1). HyponatremiaHyponatremia is defined as a serumsodium concentration of less than 135 mEq/L. The mortality of patientswith hyponatremia is approximatelydouble that of patients with normalplasma sodium concentrations (11). It isalso the most common electrolyte dis-order seen in hospitalized patients (4).

9 Hyponatremia is associated with low,normal, or high tonicity. Tonicity is thecombined effort of solutes such assodium and glucose that causes watermovement from one body compart-ment to another (1). When evaluatinghyponatremia, the first step is tocalculate the serum tonicity or requesta plasma osmolality. Calculated tonicityand plasma osmolality are viewed asinterchangeable because the only dif-ference between the two is blood ureanitrogen, which is the smallest contrib-utor to plasma osmolality (Table 4). Ifplasma osmolality is normal (275 to295 mOsm/kg), the hyponatremia isusually due to hyperlipidemia orhyperproteinemia; this is rarely seenwith modern laboratory assays. If plasmaosmolality is high (>295 mOsm), thehyponatremia is likely due to hyper-glycemia or intravenous infusions ofglucose or mannitol (4).

10 The pseudo-hyponatremia value should be corrected:for every 100-mg/dL increase inglucose, plasma sodium decreases to 2 mEq/L (13). If hypertonic and isotonic hypona-tremia are ruled out, hypotonichyponatremia is present. This is themost common cause of hyponatremiaand requires assessment of the hyponatremia results froman excess of water in relation to exist-ing sodium stores and usually is due to decreased renal water of hypotonic hypona-tremia are more severe when thedecrease in serum sodium concentrationis large or rapid (occurring over hours).Most patients with serum sodium con-centrations greater than 125 mEq/Lare asymptomatic. If clinical symptomsoccur, they include headache, nausea,vomiting, muscle cramps, lethargy,disorientation, depressed reflexes,seizure, or coma (14).


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