Fluids, Electrolytes and Acid-Base Disorders

How is water distributed across the ECF, ICF and their sub-parts in the body?

TBW is 60% of body weight

ICF is 40% of body weight and 2/3 of TBW

ECF is 20% of body weight and 1/3 of TBW

Plasma is 1/3 of ECF

Interstitial fluids are 2/3 of ECF

What are the normal daily outputs for urine, stool and insensible losses for the average person?

Urine: 800-1500mL/day

Stool: 250mL/day

Insensible: 600-900mL/day

What are the typical indications for normal saline, D5 1/2 NS, D5W, and lactated ringer's solution?

NS: increase intravascular volume if dehydrated/blood loss

D5 1/2NS: standard maintenance fluid (often with KCl)

D5W: correcting hypernatremia

Lactated Ringer's: replace intravascular volume after trauma (don't use if hyperkalemic)

What BUN/Cr and FeNa are indicative of prerenal azotemia (hypovolemia, kidney hypoperfusion)?

FeNa <1%

BUN/Cr >20

How may hypovolemia, generally, best be treated?

Bolus of lactated ringer's to achieve euvolemia

Maintain urine output at 0.5-1mg/kg/hr

If blood loss, replace with crystalloid

D5 1/2NS with 20mEq KCl/L for maintenance

How might you calculate the rate of maintenance fluid administration?

4/2/1 Rule for Hourly rate:

4mL/kg for first 10 kg 2 for next 10kg 1 for each kg over 20

What are the three classifications of hyponatremia, and what are the major causes of each?

Hypotonic: Na <280. multiple causes discussed elsewhere

Isotonic: pseudohyponatremia. Due to increased plasma solids decreasing relative [Na], but the actual amount is the same. Often ~high protein, lipids

Hypertonic: osmotic substances shift water out of cells, causing dilutional drop in [Na]. ~hyperglycemia (every 100 BG rise~3 Na drop), Mannitol, radiocontrast

What are the three subcategories of hypotonic hyponatremia, and the major causes of each?

Hypovolemic: extrarenal Na losses (vomiting, diarrhea, bleeding) if low urine sodium. If high urine sodium, excess diuretics or low aldosterone or ATN

Euvolemic: SIADH, psychogenic polydipsia, hypothyroidism, haloperidol, cyclophosphamide, pure water after exercise

Hypervolemic: water retention from CHF, liver disease or renal failure

At what level of hyponatremia do symptoms usually begin?

<120

What are the main clinical features of hyponatremia?

Neurologic: headache, delirium, muscle twitch, hyperactive DTRs, inc ICP, seizures, coma

GI: nausea, vomiting, ileus, diarrhea

CV: HTN from increased ICP

Oliguria to anuria

How is hyponatremia typically dx?

Low plasma osm if true hyponatremia

High urine osm if inc ADH, low urine osm if kidneys responding right

Urine Na should be low if hyponatremic. If >20, ~salt-wasting nephropathy, hyperaldosteronism, diuretics. If <40, consider SIADH.

How are the various forms of hyponatremia tx?

Isotonic/hypertonic: tx the underlying d/o

Hypotonic hyponatremia: if mild, withhold free water until spontaneous re-equilibration

If moderate (110-120) give loop diuretics with saline

If severe (<110): hypertonic saline to inc Na by 1-2/hr, no more than 8/24 hours

What are the major causes hyper, iso and hypovolemic hypernatremia?

Hypovolemic hypernatremia: renal loss from osmotic diuresis (diabetes), renal failure, diarrhea

Iso: diabetes insipidus (water loss)

Hyper: iatrogenic (TPN, NaHCO3), glucocorticoids, Cushing's, primary hyperaldosteronism

What are the typical symptoms of hypernatremia, and how is it dx?

Sxs: neurological (AMS, restless, weak, focal deficits, confusion, seizures, coma)

Tissues and mucous membranes dry

Dx: If kidneys are responding appropriately, urine volume should be low with osm >800.

Give desmopresin to differentiate central from peripheral diabetes insipidus

How should hypo, iso and hypervolemic hypernatremia be treated?

Hypo: isotonic NaCl to restore hemodynamics, then replace free water deficit

Iso: vasopresin if DI, oral fluids or D5W

Hyper: diuretics (furosemide) and D5W, dialyze if renal failure

What are the two forms of calcium in the body, and how do hypoalbuminemia and pH affect the calcium level?

Protein-bound form (majority)

Free ionized form (physiologically active)

Hypoalbuminemia: total Ca low, normal ionized

Alkaline pH: increased Ca bound to albumin, leading to hypocalcemic state

How do PTH, calcitonin and vitamin D all control calcium and phosphate levels via the bone, kidneys and gut?

PTH: inc plasma Ca, dec PO4

Calcitonin: dec plasma Ca and PO4

Vit D: inc plasma Ca and PO4

What are the most common causes of hypocalcemia?

Hypoparathyroidism (#1): often from thyroid surgery

Acute pancreatitis, renal failure

Hyperphosphatemia (binds Ca)

Pseudohypoparathyroidism (end-organ resistance, high PTH)

Hypomagnesemia, Vit D defic

Citrated blood transfusion

DeGeorge Syndrome

What are the clinical sxs of hypocalcemia?

Rickets, osteomalacia

Numbness/tingling, hyperactive DTRs

Chvostek's Sign (facial nerve twitch on tap)

Trousseau's (carpal spasms with BP cuff inflation for 3min)

Grand mal seizures

Basal ganglia calcifications

Arrhythmia, prolonged QT

What is the diagnostic approach to hypocalcemia?

Get BUN, Cr, Mg, albumin, ionized Ca

PO4 high in renal insuff/hypopara, low in vit D def

PTH low in hypopara, high in pseudohypo and vit D def

How can one best treat hypocalcemia?

Acute: IV calcium gluconate

Long-term: oral Ca and vit D supplements

For PTH def: vit D with Ca, thiazides (lower urine Ca, prevent urolithiasis)

Correct hypomagnesemia

What are the major causes of hypercalcemia?

Hyperparathyroidism, renal failure, Paget's, Acromegaly, Addison's

Malignancy, Multiple Myeloma

Vit D intox, Milk-Alkali syndrome, Thiazides, Li

Sarcoidosis

Familial Hypocalciuric hypercalcemia (low urine Ca excretion)

What are the key clinical features of hypercalcemia?

Stones, Bones, Groans and psychic Moans!

Nephrolithiasis/calcinosis

Bone aches/pains, osteitis fibrosis cystic

Muscle pain/weakness, pancreatitis, gout, constipation

Depression, fatigue, anxiety

Shortened QT interval

What is the urinary cAMP level in primary hyperparathyroidism?

Very high

What is the treatment approach to hyperparathyroidism?

NS with furosemide to increase excretion and inhibit reabsorption

Bisphosphonates w/ calcitonin if osteoclastic disease (malignancy, Paget's)

Glucocorticoids if Vit D associated or Multiple myeloma

Hemodialysis in renal failure

What effect do pH, insulin and cell lysis have on potassium levels?

Hypokalemia ~ alkalosis and insulin

Hyperkalemia ~ acidosis and cell lysis

What are the most common causes of hypokalemia?

GI loss (diarrhea, vomiting)

Renal loss (prim/sec hyperaldosteronism, Mg def, diuretics)

Bartter's Syndrome

Bactrim or Amphotericin B

Epinephrine

What are the main sxs of hypokalemia?

Arrhythmia (U-wave, flat/inverted T wave)

Muscle weak, paralysis, cramps

Decreased DTRs

Ileus, nausea/vomiting

Exacerbates digitalis toxicity

How is hypokalemia best tx?

Oral KCl

IV KCl if severe (<2.5) or with arrhythmias

Always correct low Mg first

What are the major causes of hyperkalemia?

Renal failure, Addison's

ACEi, spironolactone (K-sparing)

Acidosis

cell breakdown

GI bleed

insulin deficiency

Rapid B-blocker admin

What are the main sxs of hyperkalemia?

Arrhythmia (peaked T wave, QRS widening, merging with T, loss of P waves to sine pattern)

Muscle weakness, flaccid paralysis

Decreased DTRs

Resp failure, nausea, vomiting, int colid, diarrhea

How is hyperkalemia tx?

IV calcium to stabilize cardiac membranes

Glucose insulin to promote Na/K, push K into cells

Sodium bicarb to inc pH, shift K into cells

Kayexelate (binds K in colon)

Furosemide

Hemodialysis for renal failure pts

Which organ has the greatest effect on magnesium regulation?

Kidney

What are the major causes of low magnesium?

Malabsorption/steatorrhea

Alcoholism

TPN w/o Mg

Renal (SIADH, diuretics, Bartter's)

Gentamicin, Amphotericin B, Cisplatin

DKA

What are the major features of hypomagnesemia?

Increased DTR, seizures, weakness, AMS

Coexisting hypocalcemia and hypokalemia

Prolonged QT interval, T-wave flattening, torsade de pointes

How is hypomagnesemia tx?

Mild: oral magnesium oxide

Severe: parenteral Mg (Mg-sulfate)

What are the major causes of hypermagnesemia?

Renal Failure (#1)

Excessive laxatives or antacids

Iatrogenic in pre/eclampsia

Rhabdomylosysis

Adrenal insufficiency

What are the main symptoms of hypermagnesemia?

Progressive loss of DTRs

Nausea, weakness

Facial parasthesias

ECG: widened QRS, peaked T, inc PR interval

Somnolence/coma

How is hypermagnesemia typically treated?

IV calcium gluconate for cardioprotection

Saline/furosemide

Intubate if respiratory depression

How is phosphate typically regulated in the body?

Mostly stored in the bone

Vitamin D controls PO4 absorption in GI

PTH enhances PO4 excretion in kidney

What are the main causes of hypophosphatemia, and how does it present? Tx?

Causes: EtOH abuse, increased renal excretion, hypokalemia/Mg, resp acidosis

Sxs: none usually, but nonspecific if severe

Tx: K-phos capsules, milk. Parenteral supplement if NPO

What are the main causes of hyperphosphatemia? Sxs? Tx?

Causes: renal insufficiency, bisphosphonates, tumor calcinosis

Sxs: metastatic calcification leading to low Ca w/ neuro changes

Tx: phosphate-binding antacids, hemodialysis if renal failure

What are the main causes of increased anion gap acidosis?

MUDPILES

Methanol

Uremia

Diabetic ketoacidosis

Propylene glycol or paraldehyde

Infection, Iron, Isoniazid

Lactic Acidosis

Ethylene Glycol

Salicylates

What are the most common causes of non-anion gap acidosis?

Diarrhea (#1)

Proximal or distal renal tubular acidosis

Carbonic anhydrase inhibition (acetazolamide)

Ureterosigmoidostomy

What are the main clinical features of metabolic acidosis?

Hyperventilation

Decreased CO

Decreased tissue perfusion

What is Winter's formula and how is it interpreted?

Expected PaCO2 = 1.5(HCO3-) 8

If within predicted range, ~simple met acid with secondary hypocapnea

If actual higher than calculated, then both met and resp acidosis

What are the most common causes of saline-sensitive and resistant metabolic alkalosis?

Saline sensitive (ECF contraction): vomiting, NG suction, diuretics

Resistant (ECF expansion): increased mineralocorticoids from adrenal d/o, Cushing's, K deficiency