Hyponatremia

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Nội dung Text: Hyponatremia

Hyponatremia Definition: Hyponatremia is defined as a plama [Na+] < 135 mEq/L. Etiology: Any process that limits the elimination of water or expands the volume around a fixed Na+content may lead to a decrease in Na+ concentration. -Expansion of the space surrounding the Na+ content: +Pseudohyponatremia: a high content of plasma proteins and lipids expands the nonaqueous portion of the plasma sample, leading to an errant report of a low ECF [Na+]. +Hyperosmolar hyponatremia: an osmotically active solute other than Na accumulates in the ECF, drawing water into the ECF and diluting the Na+ content. Common causes: hyperglycemia (resulting in a fall in plasma [Na+] of 1.6 to 2.4 mEq/L for every 100 mg/dL rise in plasma glucose), post-transurethral resection of the prostate (post-TURP) syndrome (glycine, mannitol, or sorbitol, can be absorbed into the ECF during bladder irrigation)
+Rarely, the ECF water content rises simply because the ingested quantity of water exceeds the physiologic capacity of water excretion in the kidney as in psychogenic polydipsia, water intoxication from poorly conceived drinking games, beer potomania, and the so-called “tea and toast” diet. -“Appropriate” ADH secretion occurs with a fall in effective circulating volume. In these conditions, thirst and water retention is stimulated, protecting volume status at the cost of the osmolar status. This category is classically subdivided based on the associated assessment of ECF status. +Hypovolemic hyponatremia may result from any causes of net Na+ loss. +Hypervolemic hyponatremia occurs in edematous states such as congestive heart failure (CHF), hepatic cirrhosis, and severe nephrotic syndrome. Despite the expanded interstitial space, the circulating volume is reduced. Alterations in Starling forces contribute to this apparent paradox, shifting fluid from the intravascular to interstitial space. -“Inappropriate” secretion of ADH is characterized by the activation of waterconserving mechanisms despite the absence of osmotic or volume-related stimuli. +Syndrome of Inappropriate ADH (SIADH)
Common causes: neuropsychiatric disorders (e.g., meningitis, encephalitis, acute psychosis, cerebrovascular accident, head trauma), pulmonary diseases (e.g., pneumonia, tuberculosis, positive pressure ventilation, acute respiratory failure), and malignant tumors (most commonly small cell lung cancer). Diagnose: o Hypo-osmotic hyponatremia  Urine osmolality > 100 mOsm/L  Euvolemia  The absence of conditions that stimulate ADH secretion,  including volume contraction, nausea, adrenal dysfunction, and hypothyroidism . -Pharmacologic agents may also stimulate inappropriate ADH secretion. Common culprits include antidepressants (particularly selective serotonin reuptake inhibitors [SSRIs]), narcotics, antipsychotic agents, chlorpropamide, and nonsteroidal anti-inflammatory drugs (NSAIDs). -Reset osmostat: the set point for plasma osmolality is reduced, occurs in almost all pregnant women and occasionally in those with a chronic decreased effective circulating volume.
Diagnosis Clinical Presentation -The symptoms are primarily neurologic +Acute hyponatremia (i.e., developing in 3 days' duration): symptoms and the increase in ICF volume is minimized. Diagnostic Testing:
aUrine [Na+] may be