Fluid & Electrolyte Disorders

Diagnosis of Fluid & Electrolyte Disorders

·  Serum osmolality (normal 285–295 mosm /kg) can be calculated from the

following formula:

Osmolality = 2(Na+ meq/L) + Glucose (mg/dL)/18 + BUN (mg/dL)/2.8

 

·  One mmol of NaCl, however, forms two ions in water (one Na+ and one

Cl–) and has an osmole concentration of roughly 2mosm/kg H2O

·  Osmolality:

o Osmole per kilogram of water

·  Osmolarity:

o Osmole per liter of solution

Normal Plasma Values

Na+                         135-145 meq/L

K+                           3.5-5.0 meq/L

Cl                             98-107 meq/L

HCO3                      22-28 meq/L

Ca2+                        8.5-10.5 mg/dL

Phosphorus               2.5-4.5 mg/dL

Mg2+                       1.8-3.0 mg/dL

Osmolality                 280-295 mosm/kg

Treatment of Specific fluid, Electrolyte & Acid-base Disorders

Disorders of Sodium Concentration

·  Hyponatremia

Ø Serum sodium concentration < 130meq/L

Ø Most common electrolyte abnormality observed in a general

hospitalized population

 

·  Urine sodium > 20meq/L: Renal salt wasting

Ø Salt-losing nephropathy

Ø Diuretics

Ø ACE inhibitors

Ø Mineralocorticoid deficiency

·  Urine sodium < 10meq/L or FE(Na) < 1%:

Ø Sodium retention by the kidney to compensate for extrarenal fluid

losses:

§ Vomiting, diarrhea, sweating, or third-spacing like ascitis

·  Fractional excretion of Sodium:

FE(NA) =(U/P)/Na divided by (U/P)Cr x 100

Ø < 1% indicates extrarenal source

Ø 1% indicates intrarenal cause

Isotonic Hyponatremia:

·  Pseudohyponatremia (decreased volume of H2O)

Ø Hyperlipidemia

Ø Hyperproteinemia

Ø Plasma osmolatity remains normal as its measurement is

unaffected by lipids or proteins

Hypertonic hyponatremia:

·  Hypertonic hyponatremia is most commonly seen with

hyperglycemia. When blood glucose becomes acutely elevated,

water is drawn from the cells into the extracellular space, diluting the

serum sodium.

·  The plasma sodium level falls 2 mEq/L for every 100 mg/ dL rise

when the glucose concentration is between 200 and 400 mg/dL.

·  This “dilutional or translocational hyponatremia” is not

“pseudohyponatremia”, since the sodium concentration does indeed

fall.

·  Infusion of hypertonic solutions containing osmotically active

osmoles (eg, mannitol) may also cause hypertonic hyponatremia by

drawing water to the extracellular space

Hypotonic Hyponatremia:

·  Capacity of kidney to excrete electrolyte-free water is potentially great –

upto 20-30 L/day - in presence of a normal GFR

·  Electrolyte-free water intake must > 30 L/day for hyponatremia to develop

·  Cause: Impairment of electrolyte-free water excretion –Renal failure,

Inappropriate ADH excess

Hypotonic Hyponatremia

Hypovolemic Euvolemic Hypervolemic

Ur. Na+ < 10 mg/L

(extra renal losses)

1. Dehydration

2. Diarrhea

3. Vomiting

Ur. Na+ > 20 mg/L

(Renal salt loss)

1. Diuretics

2. Ace inhibitors

3. Nephropathies

4. Mineral corticoid ¯

Cerebral Na+ wasting syndrome

 

Hypotonic Hyponatremia

Euvolemic

·  SIADH

·  Post op hyponatremia

·  Hypothyroidism

·  Psychogenic polydipsia

·  Thiazide diuretic Rx

·  ACE inhibitor Rx

·  Endurance exercise

Hypervolemic

·  Edematous states

·  CHF

·  Liver disease

·  Nephrotic syndrome

·  Adv. Renal failure

·  Treatment:

Ø Replacement of lost volume with isotonic or half-normal (0.45%)

saline or lactated Ringer’s infusion

Ø Corticosteroids – if hypocortisolism is considered in DD

Euvolemic Hypotonic Hyponatremia

Syndrome of Inappropriate ADH Secretion (SIADH)

·  Hypovolemia physiologically stimulates ADH secretion

·  Diagnosis of SIADH is made only if the patient is euvolemic

 

Causes of Syndrome of Inappropriate Secretion of ADH (SIADH)

CNS disorders

·  Head trauma

·  Stroke

·  Subarachnoid hemorrhage

·  Hydrocephalus

·  Brain tumor

·  Encephalitis

·  Meningitis

·  Acute psychosis

·  Acute intermittent porphyria

Pulmonary lesions

·  Tuberculosis

·  Bacterial pneumonia

·  Aspergillosis

·  Bronchiectasis

·  Neoplasms

·  Positive pressure ventilation

Causes of Syndrome of Inappropriate Secretion of ADH (SIADH)

Malignancies

·  Bronchogenic carcinoma

·  Pancreatic carcinoma

·  Prostatic carcinoma

·  Renal cell carcinoma

·  Adenocarcinoma of colon

·  Thymoma

·  Osteosarcoma

·  Malignant lymphoma

·  Leukemia

Drugs

Increased ADH production

·  Antidepressants: amitriptyline, desipramine, imipramine, MAO inhibitors,

fluoxetine (SSRI)

·  Antineoplastics: cyclophosphamide, vincristine

·  Carbamazepine

·  Clofibrate

·  Neuroleptics: thiothixene, thioridazine, fluphenazine, haloperidol,

trifluoperazine

Potentiated ADH action

·  Carbamazepine

·  Chlorpropamide, tolbutamide

·  Cyclophosphamide

·  NSAIDs

·  Somatostatin analogs

Causes of Syndrome of Inappropriate Secretion of ADH (SIADH)

Others

·  Postoperative

·  Pain

·  Stress

·  AIDS

·  Pregnancy (physiologic)

Clinical Features:

·  SIADH is characterized by:

Ø Hyponatremia

Ø Decreased osmolality (< 280 mosm/kg)

Ø Increased urine osmolality (>150 mosm /kg)

Ø Absence of cardiac, renal, or liver disease

Ø Normal thyroid and adrenal function

Ø Urine sodium usually > 20 meq/L

·  The expansion of extracellular volume is not large enough to cause clinical

hypervolemia, hypertension, or edema.

·  Low blood urea nitrogen (BUN) (< 10mg/dL)

·  Hypouricemia (< 4mg/dL) not only dilutional

·  A high BUN suggests a volume-contracted state – precludes a diagnosis

of SIADH.

 

Postoperative Hyponatremia

·  Hyponatremia may result from direct absorption of hypotonic irrigating

fluids during:

Ø Endometrial ablation

Ø Transurethral prostate resection

·  Symptomatic intraoperatively

Psychogenic polydipsia

·  Marked excess free water intake (generally > 10 L/d) may produce

hyponatremia.

·  Euvolemia is maintained through the renal excretion of sodium. Urine

sodium is therefore generally elevated (> 20 mEq/L), but unlike SIADH,

levels of ADH are suppressed.

·  Urine osmolality is appropriately low (< 300 mosm/kg) as the increased

free water is excreted.

·  Hyponatremia from bursts of ADH occurs in manic-depressive patients

with excess free water intake.

·  Psychogenic polydipsia is observed in patients with psychological

problems, and these patients frequently take drugs interfering with water

excretion.

·  Similarly, excessive intake of beer, which contains very small amounts of

sodium (< 5 mEq/L), can cause severe hyponatremia in cirrhotic patients,

who have elevated ADH and often a decreased GFR

Treatment

Symptomatic hyponatremia:

·  Symptomatic hyponatremia is usually seen in patients with SIADH

·  Serum sodium levels are generally under 120meq/L.

Rate and Degree of correction

·  Central pontine myelinolysis

·  Osmotically induced demyelination

·  Overly rapid correction of serum sodium

Ø To levels > 135meq/L within initial 48 hours or an increase of more

than 25meq/L within the first 24 hours of therapy