Basic Fluids and Electrolytes

Douglas P. Slakey

Why Listen to This?• Essential for surgeons• Based upon physiology

– Disturbances understood as pathophysiology

Most abnormalities are relatively simple, and many iatrogenic

Purpose of this Talk

To Encourage Thought

Not

Mechanical Reaction

You Have to Read!

It’s All About Balance

• Gains and Losses– Losses

• Sensible and Insensible

• Typical adult, typical day– Skin 600 ml

– Lungs 400 ml

– Kidneys 1500 ml

– Feces 100 ml

• Balance can be dramatically impacted by illness and medical care

Fluid Compartments

• Total Body Water– Relatively constant– Depends upon fat content and varies with age

• Men 60% (neonate 80%, 70 year old 45%)

• Women 50%

TOTAL BODY WATERTOTAL BODY WATER60% BODY WEIGHT60% BODY WEIGHT

ICF

2/3

Predominant solute

K+

ECF

1/3

Predominant solute

Na+

HH22OO

(mEq/L) Plasma IntracellularNa 140 12K 4 150Ca 5 0.0000001Mg 2 7Cl 103 3

HCO3 24 10Protein 16 40

Electrolytes

Fluid Movement

• Is a continuous process• Diffusion

– Solutes move from high to low concentration

• Osmosis– Fluid moves from low to high solute concentration.

• Active Transport– Solutes kept in high concentration compartment– Requires ATP

Movement of Water• Osmotic activity

– Most important factor– Determined by concentration of solutes

Plasma (mOsm/L)

2 X Na + Glc + BUN

18 2.8

Third Space

• Abnormal shifts of fluid into tissues

• Not readily exchangeable

• Etiologies– Tissue trauma– Burns– Sepsis

Fluid Status• Blood pressure

• Check for orthostatic changes

• Physical exam

• Invasive monitoring– Arterial line– CVP– PA catheter– Foley

Remember JVD?

Fluid Imbalances

• Must assess organ function– Renal failure– Heart failure– Respiratory failure• Excessive GI fluid losses• Burns

Volume Deficit

• Most common surgical disorder• Signs and symptoms

– CNS: sleepiness, apathy, reflexes, coma– GI: anorexia, N/V, ileus– CV: orthostatic hypotension, tachycardia with

peripheral pulses– Skin: turgor– Metabolic: temperature

Dehydration

Chronic Volume Depletion

Affects all fluid components

Solutes become concentrated

Increased osmolarity

Hct can increase 6-8 pts for 1 L deficit

Patients at risk:

Cannot respond to thirst stimuli

Diabetes insipidus

Treatment: typically low Na fluids

HypovolemiaAcute Volume Depletion

Isotonic fluid loss, from extracellular compartmentDetermine etiology

Hemorrhage, NG, fistulas, aggressive diuretic therapyThird space shifting, burns, crush injuries, ascites

Replace with blood/isotonic fluid» Appropriate monitoring

» Physical Exam» Foley (u/o > 0.5 ml/kg/min)» Hemodynamic monitoring

Fluid Replacement

Fluid Replacement

• Isotonic/physiologic– NS, LR

• Less concentrated– 0.45NS, 0.2NS– Maintenance

• Hypertonic Na

Fluid Replacement

• Plasma Expanders– For special situations– Will increase oncotic pressure– If abnormal microvasculature, will extravasate

into “third space”Then may take a long time to return to circulation

Fluid Replacement

• Maintenance– 4,2,1 “rule”

• Other losses (fistulas, NG, etc)– Can measure volume and composition!!!– Should be thoughtfully assessed and

prescribed separately if pathologic

Maintenance Fluid

• Daily Na requirement: 1 to 2 mEq/kg/day

• Daily K requirement: 0.5 to 1 mEq/kg/day

• AHA Recommended Na intake: 4 to 6 grams per day

Maintenance Fluids

To Replace Ongoing Losses,

NOT Pre-existing Deficits

Maintenance Fluids

D5 0.45NS + 20 mEq KCl/L at 125 ml/H

How much Sodium is Enough???

» NS» 0.9% = 9 grams Na per liter

» 0.45 NS = 4.5 grams per liter» 125 ml/hour = 3000 ml in 24 hours» 3 liters X 4.5 grams Na = 13.5 GRAMS Na!

(If 0.2 NS: 3 liters X 2 grams Na = 6 grams Na)

Assessment of Disorders of Volume

and Electrolytes• Effects are variable and complex

• Simplified treatment algorithms cannot address the variable and complex nature of these disorders

• Acid - Base balance is integral with these disorders

Hyponatremia• Na loss

– True loss of Na– Dilutional (water excess)– Inadequate Na intake

• Classified by extracellular volume– Hyovolemic (hyponatremia)

• Diuretics, renal, NG, burns

– Isotonic (hyponatremia)• Liver failure, heart failure, excessive hypotonic IVF

– Hypervolemic (hyponatremia)• Glucocorticoid deficiency, hypothyroidism

Na Volume

Check Ur Na

< 10 mmol/L

VomitingDiarrhea3rd spaceHepatorenal

Adrenal InsufficiencyDiureticsSalt-Wasting SyndromeSIADH

> 20 mmol/L

SIADH• Causes

– Cancers (pancreas, oat cell)– CNS (trauma, stroke)– Pulmonary (tumors, asthma, COPD)– Surgical stress– Medications

• Anticonvulsants, antineoplastics, antipsychotics, sedatives (morphine)

SIADHToo much ADH

Affects renal tubule permeability

Increases water retention (ECF volume) Increased plasma volume, dilutional hyponatremia,

decreases aldosterone, increased GFR

Increased Na excretion (Ur Na >40mEq/L)

Fluid shifts into cellsSymptoms: thirst, dyspnea, vomiting, abdominal cramps,

confusion, lethargy

SIADH Treatment

• Fluid restriction– Will not responded to fluid challenge

(distinguishes from pre-renal cause)

• Possibly diuretics

Hypovolemia and Metabolic Abnormality

• Acidosis– May result from decreased perfusion

• Alkalosis– Complex physiologic response to more chronic

volume depletion

Paradoxical Aciduria

Na

Cl

Na

H

K

Loop of Henle

HypochloremicHypovolemia

Hypernatremia

Relatively too little H2O

– Free water loss (burns, fever)– Diabetes insipidus (head trauma, surgery,

infections, neoplasm)• Dilute urine

– Nephrogenic DI• Kidney cannot respond to ADH

Hypernatremia

• Hypovolemic– GI loss, osmotic diuresis– Increased Na load (usually iatrogenic)

[0.6 X wt (kg)] X [Serum Na/140 - 1]

Free water deficit:

Hypernatremia Volume Replacement

• Example:

• Na 153, 75 kg person

• (0.6 X 75) X [(153/140) - 1]

• 45 X [1.093 -1]

• 45 X 0.093 = 4.2 Liters

Potassium

• 98% intracellular• 20 to 40 mEq/L of urine

– Kidneys cannot retain K

• Dietary sources– Chocolate, dried fruits, nuts– Fruits: oranges, bananas, apricots– Meats– Potatoes, mushrooms, tomatoes, carrots

Potassium and Ph

• Acidosis– Extracellular H+ increases, moves intracellular

forcing K+ extracellular

• Alkalosis– Intracellular H+ decreases, to keep intracellular

fluid neutral, K+ moves into cells

Hyperkalemia

• Associated medications– ACE inhibitors, beta-blockers, antibiotics,

chemotherapy, NSAIDS, spironolactone

• Treatment– Mild: dietary restriction, assess medications– Moderate: Kayexalate

• Do NOT use sorbitol enema in renal failure patients

Hyperkalemia

• Emergency (> 6 mEq/l)

• Treatment– Monitor ECG, VS– Calcium gluconate IV– Insulin and glucose IV– Kayexalate, Lasix + IVF, dialysis