Fluid & Electrolyte Therapy

Acid-Base BalancePart 1

• Physiology of Body H₂O:• Efforts to normalize amount, composition,

distribution, & pH of body fluids, by internal homeostasic processes or by externally applied therapeutic measures, are aimed @ restoring & maintaining an environment in which body functions can proceed normally.

• H₂O ~ 60% body mass

• Body H₂O is divided into 2 main compartments: 1) intracellular fluid (ICF) 2) extracellular fluid (ECF):

a. interstitial (H₂O between cells)b. vascular (H₂O inside walls of blood vessels)

*Body H₂O is constantly circulating between the 3 compartments • The exchange of molecules across water permeable cell

membranes under stable conditions is regulated by differences in hydrostatic & oncotic (protein) pressure on the arteriole & the venule side of the capillaries

• @ the heart: CO & arterial tone determine the IV or hydrostatic pressure• @ the arteriole side of capillary: this hydrostatic pressure ~ 17mmHg & the free H₂O is pushed out into the interstitial or 3rd space. Proteins, mainly albumin, & (-) hydrostatic pressure in 3rd space @ the same time pull H₂O from the vascular space into the 3rd space.• The combined effect of these 2 processes account for

the movement of H₂O out of the vascular space.

• On the venule side of the capillary bed: IV oncotic pressure pulls most of the H₂O back into the vascular space.• The volume of the 3 compartments remain standard only as long as the hydrostatic & oncotic forces remain constant relative to each other

• Dehydration: Fluid volume is low in all 3 compartments. • Hypovolemia: Low IV volume but interstitial & intracellular volumes are not described or quantified• TBW Overload: TBW > 60% without describing the volume of any specific compartment• Edema: Fluid accumulation in interstitial space if blood (hydrostatic) pressure is normal but oncotic pressure is low

• Low albumin conc.→gradual expansion of 3rd space @ expense of vascular space

• Severe hypoalbuminemia→hypovolemia + edema & TBW>60% →death

• More fluid must be given to support the circulating vascular volume but edema & TBW will↑.

• Only raising the low albumin or ↓’ing BP → normalize the volumes of the 3 compartments

• Physiology of Body Solute Balance:• The conc. of dissolved ions (electrolytes) in each

compartment creates the Osmotic Pressure responsible for containing the H₂O in each space.

• Normal serum Osmolality= 280-300 mmol/kg (mOsm/L)• Na & Cl →main ions in ECF• K & PO₄ →main ions in ICF• Other important osmotically active: Glucose, urea, phopholipids, cholestrol, & neutral fats (nonelectrolytes contribute little to the osmolality unless their values are abnormally high)• Osmolality (mmol/kg)= 2 X Na (mmol/L) + Glucose (mmol/L)/18 + BUN(mmol/L)/2.8

• Change in ions conc. in any compartment→ H₂O moves across cell membranes→ reestablish osmotic equilibrium

• If serum Na↑ →osmolality of vascular is momentarily > that of the interstitial & intracellular spaces → H₂O moves from these 2 areas to dilute the vascular space until correct relative osmolalities of all 3 compartments are reestablished → ↓size of interstitial & intracellular spaces + ↑ vascular space

• If serum Na ↓ → opposite occurs

• MAINTENANCE FLUID & ELECTROLYTE NEEDS:• The kidneys ↑ or↓ their fluid & electrolyte output by the

action of ADH & aldosterone which compensates for daily variations in fluid & electrolyte intake.

• ADH(Vasopressin): regulates the amount of H₂O reabsorbed in the distal tubule of the kidney by assessing the volume & ECF osmolality .

• If ECF osmolality > normal or the blood volume low, ADH is released & H₂O is reabsorbed in the renal tubules & the converse is true.

• Aldosterone:↑’s Na reabsorption, and its release is stimulated by low total body Na.

• These 2 hormones, with stimulation of thirst centers in the brain, enable the body to maintain TBW and sodium within normal over wide variations in daily intake.

• Fluid Requirement (ml)=1,500 + 20 ( wt – 20)

• Maintenance Needs per 24 Hrs• Water• 0-10 kg 100 mL/kg• 10–20 kg 50 mL/kg• 20 kg 20 mL/kg• Electrolytes• Na 3 mmol (3 mEq)/100 mL H2O need• K 2 mmol (2 mEq)/100 mL H2O need• Cl 2 mmol (2 mEq)/100 mL H2O need• Ca 0.05–0.1 mmol (0.1–0.2 mEq)/kg• Mg 0.05 mmol (0.1 mEq)/kg• PO4 0.1 mmol (2.8 mg)/kg• Use ideal body weight in obese patients.

• DISORDERS OF BODY WATER & SOLUTE:• SODIUM: 135-145mmol/L (135-145 mEq /L). • Hypernatremia:( Hypertonic) 1) Hypovolemic hypernatremia: from free H₂O deficit with sodium loss 2) Isovolemic hypernatremia: from free H₂O deficit without sodium loss3) Hypervolemic hypernatremia: ↑’d total body Na from excessive H₂O & salt

** Clinically, complications of hypernatremia generally appear when serum [N] >160 mmol/L & result from CNS dehydration.

• Treatment of Hypernatremia:• If due to H₂O loss → Calculate the amount of free H₂O needed

to correct the conc. of all electrolyte values & return the volumes of all 3 compartments to normal :

• H₂O deficit (L)=[(Measured serum Na(mmol/L)-140) - 1] x Body wt (kg) x 0.6

1) Hypovolemic hypernatremia: Na loss accompanies the H₂O loss →* Administer 0.9% saline(NS) to correct the ECF.• When the volume status is correct, electrolyte-free or

hypotonic solutions, i.e. D5W or 0.45% saline, may be given.

2) Isovolemic hypernatremia:* If Na loss does not accompany free H₂O loss→ hypotonic solutions are the Tx of choice.

3) Hypervolemic hypernatremia: Tx of actual ↑ in Na in the body is directed @ removal of Na from the body which may involve the use of diuretics & D5W to ↑ renal elimination while maintaining a normal TBW.

• Aggressive Tx is not indicated because it may CNS problems. Equilibration across the blood–brain barrier is slower than across other semipermeable membranes in the body, & rapid changes may cause seizures.

• If hypernatremia has developed rapidly over a few hrs, Tx to ↓[Na] should NOT be > 1 mEq/L/hr. • If hypernatremia has developed slowly, correction of [Na] should NOT be > 0.5 mEq/L/hr.

• Hyponatremia: [Na]<135 mmol/L• symptoms appear when [Na] <125 mmol/L & result from CNS H₂O intoxication. • Hyponatremia: hypotonic, isotonic, or

hypertonic. • Hyponatremia can result from dilution or depletion or may be translocational. • The cause must always be determined

before Tx begins.

1)Hypertonic (translocational) Hyponatremia: • Manifests when a gain of impermeable solutes other

than Na occurs in the vascular compartment. • The most common solute responsible is glucose• Mannitol and glycine have also been identified. • Solutes shift H₂O from the ICF → ECF compartment• Since these solutes contribute to the serum

osmolality → hypertonic hyponatremia. • Treatment:• involves correcting the elevated blood glucose

concentrations or eliminating the mannitol or glycine.

• Hypotonic Hypervolemic (dilutional) Hyponatremia:• When the total body Na is actually high, but TBW is ↑’d to a

greater degree. (i.e. in cirrhosis & CHF when effective CO reaching the kidneys is diminished). This, along with the low [Na], stimulate ADH & aldosterone secretion → further salt & H₂O retention →edema

• Initial treatments: * Salt & H₂O restriction, *bed rest (to ↑ venous return to heart),

*correction of the 1⁰ order *Measures must not be so aggressive

• Diuretics may be necessary but should be used carefully to prevent exacerbation of the hyponatremia by causing Na excretion that exceeds H₂O elimination.

• Hypotonic Euvolemic Hyponatremia: • When both the ECF & ICF compartments expand because of

H₂O retention, but the body Na stores are nl or slightly ↓’d.• ↑’s in the ECF are usually not sufficient to cause edema. • Commonly associated with syndrome of inappropriate

antidiuretic hormone (SIADH), but other disorders may cause it i.e. adrenal insufficiency, hypothyroidism, & 1⁰ psychogenic polydipsia.

• The underlying cause must be identified and treated.• Many medications cause SIADH .• Treatment: Fluid restriction is often 1st-line Tx for SIADH.• Demeclocycline 900 - 1,200 mg/ day may also be initiated in

pts. with chronic complications (it reverses excessive fluid intake, restores Na balance, & inhibits ADH effect by causing nephrogenic diabetes insipidus) , but it is NOT indicated for acute situations because of its long t1/2.

• Hypotonic Hypovolemic Hyponatremia:• ↓in TBW, with a greater ↓in Na • Commonly results from GI losses caused by

vomiting, excessive, diuretic therapy, or replacement of losses with electrolyte fluid.

• Signs of dehydration (dry mucous membranes, skin tenting, lethargy) and, in extreme cases,

hypovolemia. • Treatment: Volume replacement with NS is usually 1st -line Tx.

• Treatment of Severe Hyponatremia:• Symptomatic hyponatremia or [Na] < 120 mmol/ L (~50%

mortality) require IV Na. • Na deficit (mmol) = [140 - Measured serum Na (mmol/L)] X

Body wt (kg) X 0.6• Appropriate solutions to replace Na deficit are:• 0.9% saline or 3% hypertonic saline • ~50% of the deficit is adm. over a 12-hour period @ rate that ↑’s

serum [Na] by no > 2 mEq/L/hr, or max. of 12 mEq/L/d. • Rapid infusions of 3% saline at 1 to 2 mL/kg/hr over 2 to 3hrs is

used only in seizures or coma pts.• Severe chronic hyponatremia should be corrected more slowly @

rate not > 0.5 mEq/L/hr. • The deficit may be replaced over several days to avoid IV volume

overload, seizures, pulm. edema, or CHF

• ALTERATIONS IN FLUID COMPARTMENT INTEGRITY: • Causes:• Trauma, tissue ischemia, endotoxemia, hypoalbuminemia, &

↓’d CO cause hypovolemia by damaging capillary membranes or disrupting the hydrostatic & oncotic forces

• Symptoms of hypovolemia: • ↑HR, low CVP, low PCWP, and ↓’d UO. • BP: NOT a good measure of here because ↑’s in SNS tone can maintain the BP @ near-nl in hypovolemia• In trauma, ischemia, and endotoxemia, capillary pore size ↑’s

& H₂O, solute, & plasma proteins, flow into the interstitial space. Leak may be localized or generalized throughout the body.

• The amount of fluid lost from IV space can →CV collapse & death with severe hypovolemia is not maintained.

• Treatment for Hypovolemia: • Correcting underlying disorder to normalize capillary

permeability while replacing the lost IV fluid with a solution of the same composition.

• 2 categories of replacement solutions:1) Crystalloid:* General term for aqu. sol’ns that contain electrolytes. *NS & lactated Ringer (LR) sol’ns: most commonly used for treating hypovolemia because isotonic with ECF and are most effective in maintaining the circulating volume.

2) Colloid:*General term for a solution that contains plasma proteins or other colloidal molecules.• 3 colloid solutions: a) Plasma protein fraction (PPF) (~ 85% albumin & 15% globulin) as a 5% sol’ n b)Albumin as a 5% or 25% sol’nc)Hetastarch as a 6% sol’n (mix of ethoxylated amylopectin molecules) exerts same hemodynamic effect as albumin & < expensive than PPF or albumin, but was assoc. with severe bleeding

• All three products also contain 130-160 mmol Na & Cl/L.

• The use of colloid infusions is controversial. • Colloids act by temporarily ↓’ing the rate of fluid migrating

into the 3rd space, but hypovolemia may be↑’d 24 to 36 hrs later as colloids move into the interstitial space and begin to draw fluid there.

• Colloids are recommended ONLY if: 1)The actual serum [albumin]< conc. needed to maintain the capillary venule oncotic pressure gradient 2)Aggressive crystalloid therapy is not restoring IV volume.

• During treatment of hypovolemia pts, the rate of fluid adm. into the vascular space must ≥ rate of loss if to maintain tissue perfusion.

• In extreme cases, >1 L/ hr. • Keep UO @ a min. of 0.5 mL/kg/hr (for

tissue perfusion)• Monitoring Parameters indicating nl. intravascular volume: • Nl HR & CVP & UO• During resuscitation, rate of fluid adm. is

adjusted hourly to maintain adequate IV vol.

• Hypoalbuminemia: without capillary leak can → hypovolemia with edema → low capillary venule oncotic pressure, and fluid that moved into the interstitial space on the arterial side of the capillary bed is not returned to vascular system. * Treatment:• IV albumin in sufficient amounts

*Hypovolemia by ↓’d CO:* (e.g., CHF, cardiomyopathy, MI)• Not corrected by fluid adm. (can cause edema)• Treatment: *Agents that improve CO, i.e. inotropes.

• Hypovolemia from blood loss:• Treatment:• Whole blood or packed RBCs with NS or LR to

reestablish the hematocrit at above 0.25 g per L (25%) with a normal circulating volume.

• LOSSES OF BODY H₂O & SOLUTE: • With prolonged vomiting, diarrhea, or losses through perforations in

organs or the GI tract leading to the skin (fistulas)• The compositions of these fluids vary according to the area affected.• When the exact origin of the fluid is not known, lab. analysis helps in

selecting appropriate replacement sol’n. esp. in enterocutaneous fistula (communication between the GI tract and the skin).

• Diarrhea is most often of distal small-bowel or colonic origin, but in cases of secretory diarrhea,

i.e., in giardiasis or AIDS, the fluid may be from the duodenum or jejunum.• In all cases of abnormal loss, the fluid composition should be determined. • Appropriate replacement soln’s, & maintenance quantities, are given @

rate designed to restore & maintain nl body water & solute.