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Questions and Answers
What is the primary role of the kidneys in fluid and electrolyte balance?
Which ions are predominant in the extracellular fluid compartment?
What is the normal range for plasma osmolality?
Antidiuretic hormone (ADH) is primarily responsible for what function in fluid management?
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Under what circumstances is fluid resuscitation indicated?
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What is the primary goal of fluid resuscitation?
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Which of the following is NOT a sign of intravascular volume depletion?
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Which fluid type remains primarily in the intravascular space?
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After administering crystalloid for fluid resuscitation, when should colloids be considered?
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For patients with clinically significant edema, what fluid may be considered along with diuretics?
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What is the recommended fluid for resuscitation in hypovolemia?
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Which replacement fluid should be used for patients who are hypernatremic?
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Maintenance intravenous fluids are indicated for which group of patients?
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What is the primary goal of maintenance intravenous fluids?
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Which of the following represents the correct method to estimate daily fluid volume in children weighing more than 20 kg?
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Which type of hyponatremia is characterized by low sodium levels combined with fluid overload?
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What is the mainstay of therapy for hypervolemic hypotonic hyponatremia?
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Which condition does NOT typically cause hyponatremia?
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What is a treatment option for normovolemic hypotonic hyponatremia?
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Which of the following describes hypernatremia?
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What type of diuretics is considered the most potent?
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Study Notes
Fluid Compartments and Electrolyte Composition
- Two thirds of the body's total water is inside cells (intracellular water).
- The remaining third of the body's total water is outside cells (extracellular water), further divided into interstitial fluid and plasma.
- The kidneys regulate water and electrolyte excretion, maintaining a stable extracellular environment.
- Potassium, magnesium, and phosphate are the primary ions within cells.
- Sodium, chloride, and bicarbonate are the main ions in the extracellular space.
- Water flows across cell membranes from areas of low osmolality to high osmolality.
Plasma Osmolality
- Plasma osmolality signifies the concentration of particles in solution.
- It reflects the overall osmolality of the body's water.
- The normal osmolality of body fluids ranges between 280 and 295 mOsm/kg.
Volume Regulation
- Antidiuretic hormone (ADH) plays a key role in regulating fluid balance.
- Aldosterone is the primary hormone governing sodium homeostasis.
Fluid Resuscitation
- Implemented in patients experiencing signs and symptoms of intravascular volume depletion.
- Its objective is to replenish intravascular volume and prevent organ hypoperfusion.
- Prompt resuscitation is crucial, as intravascular volume depletion can lead to organ dysfunction and death.
Signs and Symptoms of Intravascular Volume Depletion
- Tachycardia (heart rate greater than 100 beats/min)
- Hypotension (systolic blood pressure less than 80 mmHg)
- Orthostatic changes in heart rate or blood pressure
- Elevated BUN/SCr ratio exceeding 20:1
- Dry mucous membranes
- Decreased skin turgor
- Reduced urine output
- Dizziness
- Improvement in heart rate and blood pressure after a 500-1000 mL fluid bolus
Crystalloids vs. Colloids
- Crystalloids, like normal saline, Ringer's lactate, and dextrose in water, are commonly used.
- Crystalloids, particularly sodium and chloride, do not readily cross cell membranes but distribute evenly in the extracellular space.
- D5W metabolizes into water and carbon dioxide.
- Water freely permeates any membrane in the body, resulting in even distribution throughout the total body fluid.
- Colloids, like packed red blood cells, pooled human plasma, semisynthetic glucose polymers, and hetastarch, remain primarily in the intravascular space due to their large size, preventing them from crossing the capillary membrane.
Distribution of IV Fluids
- Crystalloids (0.9% sodium chloride or lactated Ringer's solution) are recommended for treating hypovolemia.
- Colloids may be considered after crystalloid resuscitation (typically 4-6 L) fails to achieve hemodynamic goals or when significant edema limits further crystalloid administration.
Albumin Administration
- 25% albumin may be used in conjunction with diuretics for patients with clinically significant edema or low albumin levels (below 2.5 g/dL).
Replacement Fluid Selection
- The sodium status determines the choice of replacement fluid for replenishing extracellular volume.
- The treatment objective is to achieve a positive fluid balance.
- Normal saline is appropriate for patients who are neither hypo- nor hypernatremic.
- Half-isotonic saline or dextrose solutions are recommended for hypernatremic patients.
Maintenance Intravenous Fluids
- Administered to patients who cannot tolerate oral fluids.
- The goal is to prevent dehydration and maintain normal fluid and electrolyte balance.
- Typically delivered as a continuous infusion through a peripheral or central intravenous catheter.
Daily Volume Estimation (Children and Adults)
- For children, administer 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10-20 kg (totaling 1500 mL), and 20 mL/kg for each kilogram exceeding 20 kg (approximately 2500 mL/day).
- For adults, administer 20-40 mL/kg/day.
Disorders in Osmoregulation
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Hyponatremia: A patient can have hypotonic, isotonic, or hypertonic hyponatremia based on plasma osmolality. Hyponatremia is further categorized based on volume status:
- Hypovolemic Hypotonic Hyponatremia: Occurs due to volume depletion and extracellular fluid loss. Treatment involves sodium replacement.
- Hypervolemic Hypotonic Hyponatremia: Caused by excessive water intake relative to sodium and commonly observed in patients with heart failure, liver or renal failure, and nephrotic syndrome. Management involves water restriction.
- Normovolemic Hypotonic Hyponatremia: May be caused by syndrome of inappropriate antidiuretic hormone (SIADH). Persistent ADH secretion and water ingestion lead to hyponatremia. Fluid restriction and diuretics (furosemide or demeclocycline) are treatment options.
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Hypernatremia: Can occur with:
- Normal total body sodium and pure water loss
- Low total body sodium with hypotonic fluid loss
- High total body sodium due to salt gain
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Management includes correcting the underlying cause, replacing water deficits, and providing sufficient water to compensate for ongoing losses.
Clinical Use of Diuretics
- Diuretics reduce sodium and chloride reabsorption in renal tubules, increasing urine volume.
- Diuretics are categorized based on their action site within the renal tubules.
Loop Diuretics
- Most potent diuretics.
- Examples include furosemide, bumetanide, torsemide, and ethacrynic acid.
Thiazide Diuretics and Other Sulfonamide Diuretics
- Less potent than loop diuretics.
- Examples include chlorthalidone.
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