Fluid, Electrolyte Balance

Zehra Eren,M.D.

LEARNING OBJECTIVES explain general principles of disorders of water

balance explain general principles of disorders of

sodium balance explain general principles of disorders of

potassium balance recognize hyponatremia, hypernatremia recognize hyperkalemia, hypokalemia

Assesment of the patient

careful historyphysical examination and assessment

of total body water and its distributionserum electrolyte concentrationsurine electrolyte concentrationsserum osmolality

Definitions

Total body water Extracellular fluid volume Intracellular fluid volume Effective arterial blood volume: part of the intravascular volume that is in the arterial system and effectively perfusing tissues (700ml/70kg, men)

Definitions

Total body water Extracellular fluid volume Intracellular fluid volume Effective arterial blood volume: part of the intravascular volume that is in the arterial system and effectively perfusing tissues (700ml/70kg, men)

Solute Composition of Body Water Predominant solutes in ECF:

Sodium (Na+)Chloride (Cl−)Bicarbonate (HCO3−)

Predominant solutes in ICF: Potassium (K+)Protein−

Phosphate−

Plasma osmolality

Posm= 2 x [Na] + [glucose]/18 + blood urea nitrogen/2.8

Normal ECF osmolality:275-290mOsm/kgH2O

Plasma tonicity also called the effective plasma osmolality reflects the concentration of solutes that do not

easily cross cell membranes (mostly sodium salts) and therefore affect the distribution of water between the cells and the ECF

Plasma tonicity= 2 x[Na] + [glucose]/18 (if glucose is measured in mg/dL)

270-285 mosm/kg ECF and ICF are in osmotic equilibrium, at

steady state

Dehydration reduction in TBW below the normal level

without a proportional reduction in sodium and potassium, resulting in a rise in the plasma sodium concentration

primary loss of free water (as with unreplaced insensible losses or water loss in diabetes insipidus)

the major biochemical manifestation is

hypernatremia

Regulatıon of water and sodium balance

The kidney regulates water and sodium balance independently since water can be taken in without salt and salt can be taken in without water

Regulation of plasma tonicity and of the effective arterial blood volume involve different hormones

areas of overlap, such as the hypovolemic stimulus to the release of antidiuretic hormone (ADH)

Dısorders of water and sodium balance

Hyponatremia (too much water)Hypernatremia (too little water)Hypovolemia (too little sodium, the

main extracellular solute)Edema (too much sodium with

associated water retention)

Hyponatremia 

Serum Na <135 mEq/L

Hyponatremia almost always due to the oral or

intravenous intake of water that cannot be completely excreted

impaired water excretion that is most often due to an inability to suppress the release of antidiuretic hormone (ADH) or to advanced renal failure

Hyponatremia major causes of persistent ADH

secretion:

-syndrome of inappropriate ADH secretion

(SIADH)

-reduced effective arterial blood volume

Manifestations of Hyponatremia

The symptoms reflect neurologic dysfunction induced by cerebral edema and possible adaptive responses of brain cels to osmotic swelling

Nausea, malaise, headache, lethargyseizures, coma,respiratory arrest

Hypernatremia 

Serum Na>145 mEq/L

Causes of HypernatremiaHypovolemic hypernatremia 

1.Extrarenal losses (urine Na <20 mEq/L):

-insensible and perspiratory-gastrointestinal

2.Renal losses(urine Na >20 mEq/L)-osmotic diuresis

Ovolemic hypernatremiaDiabetes insipidus (dilute urine, urine Na variable)

Causes of HypernatremiaHypervolemic hypernatremia

-Hypertonic infusion (eg, NaHCO3)-Tube feeding

Manifestation of Hypernatremia

Rise in plasma Na and osmolality→water movement out of the brain→rupture of the cerebral veins→focal intracerebral and subarachnoidal hemorrages→possible ireversible neurologic damage

Lethargy, weaknees, irritability, twitching, seuzures, coma,

Osmotic demyelination

DISORDERS OF POTASSIUM (K)

Total body K determined by internal and external K balance

Internal balance

DISORDERS OF POTASSIUM (K)

Total body K determined by internal and external K balance

Internal balance

External balance-K freely filtered-Filtered K reabsorbed in proximal tubule -K secretion mediated by Na reabsorption-K secretion regulated by aldosterone secretion

Hypokalemia 

Serum K+ less than 3.5 mEq/L (mmol/L)

Clinical manifestations  Cardiovascular:

-Arrhythmias-Digitalis toxicity

Neuromuscular: 1.Smooth muscle:

-Ileus2.Skeletal muscle:

-Weakness-Paralysis-Rhabdomyolysis

Clinical manifestations Endocrine:

-Glucose intolerance Renal/electrolyte:

-Vasopressin resistance-Increased ammonia production-Metabolic alkalosis

Structural changes: Renal cystsInterstitial changesPT dilation, vacuolization

Hyperkalemia 

Serum K ≥5.0 mEq/L (mmol/L)

Psodohyperkalemia 

ThrombocytosisLeukocytosis Ischemic blood draw

Causes of Hyperkalemia

GFR <20 mL/min-Endogenous or exogenous K-Drugs that impair K excretion

Clinical manifestations Cardiovascular

-T-wave abnormalities-Bradyarrhythmias

Neuromuscular-Ileus-Paresthesias-Weakness-Paralysis

Clinical manifestations Renal/electrolyte

-Decreased ammonia production-Metabolic acidosis

SUGGESTED READING Goldman's Cecile Medicine, Goldman L, Schafer AI

Case files Internal Medicine, Toy Patlan

Current Medical Diagnosis and Treatment, Maxine A. Papadakis, Stephen J. McPhee, Eds. Michael W. Rabow, Associate Ed.

Current Diagnosis & Treatment: Nephrology & Hypertension Edgar V. Lerma, Jeffrey S. Berns, Allen R. Nissenson