Figure 131

Physiologic approach to polyuric disorders. Among euvolemic hyper-natremic patients, those affected by polyuric disorders are an important subcategory. Polyuria is arbitrarily defined as urine output of more than 3 L/d. Urine volume can be conceived of as having two components: the volume needed to excrete solutes at the concentration of solutes in plasma (called the osmolar clearance) and the other being the free water clearance, which is the volume of solute-free water that has been added to (positive free water clearance [CH2O]) or subtracted (negative CH2O) from the isotonic portion of the urine osmolar clearance (Cosm) to create either a hypotonic or hypertonic urine.

Consumption of an average American diet requires the kidneys to excrete 600 to 800 mOsm of solute each day. The urine volume in which this solute is excreted is determined by fluid intake. If the urine is maximally diluted to 60 mOsm/kg of water, the 600 mOsm will need 10 L of urine for effective osmotic clearance. If the concentrating mechanism is maximally stimulated to 1200 mOsm/kg of water, osmotic clearance will occur in a minimum of 500 mL of urine. This flexibility is affected when drugs or diseases alter the renal concentrating mechanism.

Polyuric disorders can be secondary to an increase in solute clearance, free water clearance, or a combination of both. ADH—antidi-uretic hormone.

WATER DEPRIVATION TEST

CLINICAL FEATURES OF

DIABETES INSIPIDUS

Urine Osmolality with

Plasma Arginine

Increase in Urine

Water Deprivation

Vasopressin (AVP)

Osmolality with

Abrupt onset

Diagnosis

(mOsm/kg H2O)

after Dehydration

Exogenous AVP

Equal frequency in both sexes

Normal

> 800

> 2 pg/mL

Little or none

Rare in infancy, usual in second decade of life

Complete central

< 300

Indetectable

Substantial

Predilection for cold water

diabetes insipidus

Polydipsia

Partial central

300-800

< 1.5 pg/mL

> 10% of urine osmolality

Urine output of 3 to 15 L/d

diabetes insipidus

after water deprivation

Marked nocturia but no diurnal variation

Nephrogenic

< 300-500

> 5 pg/mL

Little or none

Sleep deprivation leads to fatigue and irritability

diabetes insipidus

Severe life-threatening hypernatremia can be associat-

Primary polydipsia

> 500

< 5 pg/mL

Little or none

ed with illness or water deprivation

* Water intake is restricted until the patient loses 3%-5% of weight or until three consecutive hourly determinations of urinary osmolality are within 10% of each other (Caution must be exercised to ensure that the patient does not become excessively dehydrated.) Aqueous AVP (5 U subcutaneous) is given, and urine osmolality is measured after 60 minutes. The expected responses are given above.

* Water intake is restricted until the patient loses 3%-5% of weight or until three consecutive hourly determinations of urinary osmolality are within 10% of each other (Caution must be exercised to ensure that the patient does not become excessively dehydrated.) Aqueous AVP (5 U subcutaneous) is given, and urine osmolality is measured after 60 minutes. The expected responses are given above.

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