Disturbances in Tonicity Flashcards
What is serum Na concentration (osmolality) indicitave of?
total body WATER content
Hypernatremia
Too little water relative to salt
Hyponatremia
Too much water relative to salt
How do you calculate serum osmolality?
]Na]*2+[gluc]+[BUN]
it’s approximately 2*[Na]+10
Where is ADH made? secreted? in response to what?
Made in hypothalamus
Secreted from pituitary
It’s released in response to high plasma osmolality –> production of concentrated urine
What are the 4 requirements for maintaining water balance?
- Deliver H2O
- Diluting segment intact (i.e. not poisoned by diuretics)
- Functional medullary gradient (i.e. also not poisoned by diuretics)
- Vasopressin action intact (no mutations in vasopressin receptor or aquaporin receptor)
What are the 2 vasopressin receptors?
Antidiuretic hormone V2 receptor: collecting duct
Vasopressor hormone V1 receptor: vascular smooth muscle
What are the 2 main stimuli for vasopressin release?
- osmolality
- volume
they interplay with each other: if you’re hypovolemic, you’re less sensitive to changes in plasma osmolality
Response to ECF volume loss
Decreased ECF Volume —>
(1) Decreased RBF, hemodynamic changes to increase FF, increase prox tubular salt reabsorption, increase renin, AngII, increase aldosterone, increased sympathetic activity –> increased NaCl reabsorption
(2) Decreased renal BF and H2O delivery, increased vasopressin release, increased water reabsorption, increased thirst due to vasopressin and Ang II–> increased water reabsorption
What determines osmolality?
Oral and IV free H2O intake
vs.
Free H2O excretion (urine) and losses (respiratory, IV, GI)
What does it mean if the [Na] is high?
It is not Na overload!!!
It is hypernatremia: Free water loss >> Na loss
What can cause hypernatremia?
Most common cause: unreplaced water loss: sweat loss, GI loss, renal loss (diabetes insipidus, osmotic diuresis)
Defectiv thirst or osmoreceptor function, damaged osmoreceptor function
Administration of hypertonic Na solution
Treatment of hypernatremia
Replace water loss: PO, IV
Treatment is based on magnitude of water defifit
Water deficit = current TBW x (serum [Na]/140) - 1
Slow correction if chronic hypernatremia: reduce by 12 meq/24h
Example of marathon rummer that collapsed & has no volume depletion, weight gain, hyponatremia (low serum Na), hypoosmolarity, and more concentrated urine (suggesting ADH action)
What’s going on?
Run marathon –> start sweating –> ADH release
+
excess hypotonic fluid intake during marathon
+ Failure to make dilute urine due to vasopressin
=
Net free water gain in setting of Na loss –> hyponatremia
How do you treat acute hyponatremia?
If asymptomatic and the stimulus of vasopressin release is self-limited, no treatment
You can treat underlying volume depletion to suppress ADH: give normal saline
If CNS symptoms, treat with hypertonic saline or vasopressin antagonists
What can cause hypovolemic hyponatremia?
Extrarenal losses: diarrhea, vomiting, burns, pancreatitis, insensible fluid loss, 3rd spacing of fluids
Renal losses: diuretics, osmotic diuresis (hyperglycemia), mineralocorticoid deficiency
CHF patient with high serum Na (hyponatremia), low serum osmolarity (hypoosmolarity), low BP, weight gain, S3 (volume overload due to HF, based on history/exam), high urine osmolarity: more concentrated than serum
What’s going on with this pt?
CO decreased –> LVEDP increasese to compensate
Decreased effective arterial volume –> same response as volume loss: result is increased NaCl reabsorption, increased water reabsorption
Note that with CHF, hyponatremic have much worse outcome
What can cause hypervolemic hyponatremia?
CHF, cirrhosis, nephrotic syndrome, renal failure
Syndrome of inappropriate ADH
Endocrinopathies, hypothyroidism, glucocorticoid deficiency
How do you treat hypervolemic hyponatremia?
Treat underlying dz i.e. ionotropes to improve cardiac function, diuretics, reduce afterload
Fluid restriction: can be hard bc patient will feel thirsty due to ADH and AngII
Vasopressin antagonists can be used to produce aquaresis in severe hyponatremia but you have to be careful not to correct chronic hyponatremia too rapidly
What are possible subcategories of syndromes of inappropriate ADH (SIADH)?
Ectopic production: tumors - lung, pancreas
Pulm dz: pneumonia, lung abscess
Drugs: SSRIs, chloropropamide, thiazide, diuretics, carbamazepine, oxcarbazepine, cyclophosphamide, etc
CNS disturbances: stroke, hemorrhage, infection, trauma, psychosis
Surgery: abdominal or thoracic surgery, due to stimulation of pain afferents
Treatment of SIADH
Treat underlying etiology
Fluid restriction
Severe symptomatic hyponatremia: hypertonic saline, vasopressin antagonists, correct chronic hyponatremia slowly due to concern for central pontine myelinolysis
What are the subcategories of volumes in hypoosmolar hyponatremia?
- *Hypovolemia**: low BP, orthostasis = total body water decreased, total body Na very low
- extrarenal losses, renal losses
- *Euvolemia**: total body water up, total body Na normal
- hypothyroidism, glucocortiocoid deficiency, drugs, SIADH
- *Hypervolemia** = edema, total body water increased, total body Na increased
- CHF, cirrhosis, nephrotic syndrome, chronic renal failure
