[04.04] CMD - Sodium and Potassium Disorders V2.pdf

Disorders of water balance

What underlying physiological disturbance do sodium disorders actually reflect?

Aquaporins

What water channels facilitate the passive movement of water across renal tubules?

~50% of body weight

What is the estimated Total Body Water (TBW) percentage in adult females?

45% of body weight

What is the estimated Total Body Water (TBW) percentage in elderly females?

~60% of body weight

What is the estimated Total Body Water (TBW) percentage in adult males?

55% of body weight

What is the estimated Total Body Water (TBW) percentage in elderly males?

60% of TBW

What percentage of Total Body Water is contained within the Intracellular Fluid (ICF) compartment?

40% of TBW

What percentage of Total Body Water is contained within the Extracellular Fluid (ECF) compartment?

33% of ECF

What proportion of the Extracellular Fluid is intravascular fluid or plasma water?

66% of ECF

What proportion of the Extracellular Fluid is extravascular or interstitial fluid?

Across the capillary wall

Where does fluid movement between intravascular and interstitial spaces occur?

Capillary pressure (Pc), Plasma osmotic pressure (𝚷p), Interstitial fluid hydraulic pressure (Pif), and Interstitial fluid osmotic pressure (𝚷if)

What are the four opposing Starling forces that determine transcapillary fluid movement?

The solute or particle concentration of a fluid per kilogram of water

What is the physiological definition of osmolality?

Sodium (and its accompanying salts)

Which solute is the strongest determinant of osmolality in the extracellular fluid?

Shifts in water across membranes

By what mechanism do ECF and ICF osmolality maintain osmotic equilibrium?

3 Na+ ions out of the cell for 2 K+ ions into the cell per molecule of ATP

What is the stoichometry and direction of ion transport by the Na+-K+-ATPase pump?

Sodium (Na+)

What is the major extracellular cation responsible for maintaining ECF solute composition?

Chloride and Bicarbonate

What are the primary accompanying anions for extracellular sodium?

Potassium (K+)

What is the major intracellular cation responsible for maintaining ICF solute composition?

ATP, Creatine Phosphate, and Phospholipids

What organic phosphate esters accompany intracellular potassium?

2(Na + K) +

glucose (mg/dL) / 18

+

BUN (mg/dL) / 2.8

What is the clinical formula for calculating plasma osmolality?

285 to 295 mOsm/kg

What is the normal range of body fluid osmolality?

Vasopressin (AVP) secretion, water ingestion, and renal water transport

What three primary physiological mechanisms regulate plasma osmolality?

Plasma osmolality less than 285 mOsm/kg

What defines a hypoosmolar state?

Plasma osmolality greater than 295 mOsm/kg

What defines a hyperosmolar state?

Magnocellular neurons within the hypothalamus

Where is Vasopressin (AVP) synthesized in the brain?

Posterior pituitary (neurohypophysis)

From which anatomical structure is Vasopressin (AVP) released into the circulation?

V2 receptors

Which renal tubular receptor does Vasopressin bind to in order to induce aquaporin synthesis?

Plasma osmolality rising above 285 mOsm/kg

What is the primary osmotic trigger for AVP release?

Decrease in blood pressure or intravascular volume

What hemodynamic factor triggers AVP release?

Nausea, intracerebral Angiotensin II, serotonin, and various drugs

What non-osmotic stimuli can trigger AVP secretion?

Leftward shift

How does hypovolemia alter the plasma osmolality versus AVP secretion curve?

Rightward shift with reduced sensitivity

How does hypervolemia alter the plasma osmolality versus AVP secretion curve?

Proximal tubule

In which renal tubule segment is ~60% of filtered sodium reabsorbed?

Thick ascending limb of the loop of Henle (TALH)

In which nephron segment is ~25-30% of filtered sodium reabsorbed?

Distal nephron (DCT, CNT, and collecting duct)

Where does fine-tuning of 5-10% of Na-Cl excretion occur?

Angiotensin II

Which circulating hormone directly activates proximal tubule NaCl reabsorption?

Dopamine

Which endogenous agent exerts a natriuretic effect by inhibiting tubular NaCl reabsorption?

Aldosterone

Which mineralocorticoid activates NaCl reabsorption via ENaC in principal cells?

Epithelial Sodium Channels (ENaC)

Which channel in principal cells is activated by aldosterone to absorb sodium?

Sympathetic (alpha and beta adrenergic) nerve activity

Which neural pathway increases renal NaCl reabsorption during hypotension?

Increased sympathetic tone, activation of RAAS, and increased AVP release

What neurohumoral responses occur in underfilled arterial circulation / hypovolemia?

Decreased sympathetic tone, suppression of RAAS, and decreased AVP release

What neurohumoral responses occur in volume overload / overfilled circulation?

A combined salt and water loss leading to contraction of extracellular fluid volume

What is the physiological definition of hypovolemia?

Hyperglycemic glucosuria, excessive urea, and mannitol therapy

What factors cause osmotic diuresis leading to renal salt and water loss?

Gastrointestinal fluid loss from vomiting or diarrhea

What is the most common nonrenal source of hypovolemia?

500 to 650 mL per day

What is the normal daily volume of insensible water evaporation from skin and lungs in healthy adults?

Urine Na

What urine sodium and osmolality findings characterize hypovolemia from nonrenal losses?

Urine Na >20 mEq/L

What urine sodium level is expected in hypovolemia resulting from renal losses?

Organ perfusion and maintenance of circulatory integrity

What is the ultimate physiological objective of sodium and water homeostatic mechanisms?

135 to 145 mEq/L

What is the normal serum sodium concentration range?

Plasma Na+ <135 mEq/L

What serum concentration defines hyponatremia?

Increased circulating AVP (or increased AVP sensitivity) combined with free water intake

What is the dual mechanism responsible for almost all cases of hyponatremia?

Hyponatremia

Which electrolyte disorder occurs in up to 22% of hospitalized patients?

Fatigue, nausea, vomiting, dizziness, gait ataxia, headache, confusion, and muscle cramps

What are mild to moderate clinical symptoms of hyponatremia?

Lethargy, obtundation, seizures, coma, and respiratory arrest

What severe features indicate central nervous system dysfunction in hyponatremia?

Low serum osmolality (<280 mOsm/kg)

What laboratory finding confirms true hypoosmolar hyponatremia?

Normal or elevated serum osmolality (>280 mOsm/kg) in the presence of low serum sodium

What laboratory finding defines pseudohyponatremia?

Marked hyperproteinemia, severe hyperlipidemia, and uncontrolled hyperglycemia

What three metabolic conditions cause pseudohyponatremia?

Loss of both water and sodium, with sodium loss exceeding water loss

What characterizes hypovolemic hyponatremia?

Increased body water with normal total body sodium, insufficient to produce edema

What characterizes euvolemic hyponatremia?

Increased total body sodium and water, with a proportionally greater increase in total body water

What characterizes hypervolemic hyponatremia?

V1 receptors

Which vasopressin receptor mediates arterial vasoconstriction outside the kidney?

V2 receptors

Which vasopressin receptor mediates renal collecting tubule aquaporin insertion?

Vaptans (such as Tolvaptan and Conivaptan)

Which drug class acts as selective V2 receptor antagonists?

Dilute urine, increased free water clearance, and elevation of serum sodium

What are the urinary and serum effects of blocking V2 receptors with Tolvaptan?

Orthostatic hypotension, orthostatic tachycardia, low JVP, poor skin turgor, and dry mucous membranes

What physical examination signs indicate hypovolemic hyponatremia?

BUN-to-creatinine ratio >20:1

What serum blood urea nitrogen to creatinine ratio points toward hypovolemic azotemia?

Isotonic 0.9% Normal Saline hydration

What is the initial intravenous fluid treatment for hypovolemic hyponatremia from extrarenal loss?

Syndrome of Inappropriate Antidiuresis (SIAD / SIADH)

What is the single most common cause of euvolemic hyponatremia?

Small cell lung carcinoma

Which malignancy is most classically associated with ectopic AVP secretion and SIAD?

SSRIs (Selective Serotonin Reuptake Inhibitors)

Which commonly prescribed antidepressant class is known to induce SIAD?

Low solute intake (e.g., beer potomania or starvation diet)

Which etiology of euvolemic hyponatremia presents with urine osmolality <100-200 mOsm/kg and UNa <10-20 mM?

Fluid restriction

What is the primary non-pharmacologic treatment for euvolemic hyponatremia/SIAD?

Congestive heart failure, liver cirrhosis, and nephrotic syndrome

What three sodium-avid edematous disorders cause hypervolemic hyponatremia with Urine Na <20 mM?

Acute or chronic renal failure

Which hypervolemic state presents with hyponatremia and Urine Na >20 mM due to impaired urinary diluting/concentrating mechanisms?

Hypotonic blood relative to brain parenchyma causes water entry, leading to cerebral edema

What produces acute brain swelling during rapid drops in serum sodium?

Rapid displacement of excess sodium, potassium, chloride, and water out of brain cells

What constitutes the rapid phase of brain adaptation to acute hyponatremia?

Loss of organic osmolytes from brain cells over approximately 48 hours

What constitutes the slow phase of brain adaptation to chronic hyponatremia?

Overly rapid correction of chronic hypotonic hyponatremia

What medical error precipitates Osmotic Demyelination Syndrome (ODS)?

Duration of hyponatremia for 2–3 days or more with serum sodium ≤123 mEq/L

What is the primary risk factor for developing Osmotic Demyelination Syndrome?

Dysarthria, dysphagia, paraparesis, quadriparesis, and locked-in syndrome

What severe neurological symptoms signal Osmotic Demyelination Syndrome?

Magnetic Resonance Imaging (MRI) showing pontine hyperintensity

What is the diagnostic imaging modality of choice for confirming Central Pontine Myelinosis / ODS?

Maximum sodium rise of 8 to 10 mEq/L in 24 hours (or 18 mEq/L in 48 hours)

What is the maximum safe rate of serum sodium correction in chronic hyponatremia?

0.6 × Weight in kg

What is the formula for calculating Total Body Water in men?

0.5 × Weight in kg

What is the formula for calculating Total Body Water in women?

Sodium deficit = TBW × (Target Na - Plasma Na)

What is the mathematical equation for calculating total body sodium deficit?

154 mEq/L

How many milliequivalents of sodium are contained in 1 Liter of 0.9% Normal Saline?

Every 4 to 6 hours

How frequently should serum sodium be monitored during active correction of chronic hyponatremia?

Bolus of 100 mL Hypertonic 3% Saline

What urgent intravenous intervention is indicated for acute symptomatic hyponatremia with severe CNS features?

513 mEq/L

What is the sodium concentration of Hypertonic 3% Normal Saline?

Plasma Na+ >145 mEq/L

What serum concentration defines hypernatremia?

Hypernatremia

Which sodium disorder is always hyperosmolar (>280-285 mOsm/kg)?

Reduced thirst perception and diminished access to free water

Why are elderly patients uniquely predisposed to hypernatremia?

Polyuria with hypernatremia and inappropriately low urine osmolality

What clinical picture characterizes Diabetes Insipidus?

Central Diabetes Insipidus

Which disorder is caused by a complete failure of neurohypophyseal AVP secretion?

Nephrogenic Diabetes Insipidus

Which disorder is caused by renal tubular V2 receptor resistance to circulating AVP?

Free water deficit =

(Serum Na - 140) / 140