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Transcript of 06 and 07 Hyponatremia

S. Faubel and J. Topf

6 Hyponatremia: The Pathophysiology

Hyponatremia: 6 The Pathophysiology

6

The Empire Strikes Back

109

The Fluid, Electrolyte and Acid-Base Companion

Introduction molality.

Hyponatremia is usually associated with hypoos-

ALWAYS EQUALS

USUALLY EQUALS

HYPOOSMOLALITY

HYPONATREMIA

Hyponatremia is a plasma sodium concentration less than 135 mEq/L. Since sodium is the major contributor to plasma osmolality, a low sodium concentration is usually associated with hypoosmolality. Hyponatremia is important because it can indicate a low plasma osmolality. Hyponatremia not associated with a low plasma osmolality is not worrisome and is not treated. The relationship between hyponatremia and hypoosmolality is similar to the relationship between fever and infection. In general, fever is not a problem in and of itself, but is an alert to the possible presence of infection. Likewise, hyponatremia in and of itself is not worrisome, but it is an alert to the possible presence of hypoosmolality. Hyponatremia associated with hypoosmolality is a concern because of the neurologic consequences of osmotic water shifts in the brain. (See Chapter 5, Osmoregulation.)

osmolality < 285 mmol/L

sodium < 135 mEq/L

Hyponatremia is a plasma sodium less than ______ mEq/L.

Hyponatremia is only a problem if it associated with ________ plasma osmolality.

decreased

135

Hyponatremia associated with ________ or elevated plasma osmolality is not dangerous.

normal

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S. Faubel and J. Topf

6 Hyponatremia: The Pathophysiology

Introduction Pseudohyponatremia Hyponatremia not associated with hypoosmolality is pseudohyponatremia.True hyponatremialow osmolality

Pseudohyponatremianormal osmolality high osmolality

True hyponatremia is hyponatremia associated with a low plasma osmolality. Pseudohyponatremia is hyponatremia associated with a normal or high plasma osmolality. The two types of pseudohyponatremia are: Hyponatremia with a normal plasma osmolality is due to a lab artifact from hyperlipidemia or hyperproteinemia. Hyponatremia with a high plasma osmolality is due to an increased concentration of osmotically active particles, such as glucose or mannitol. When hyponatremia is discovered, the presence of true hyponatremia should be assessed by measuring plasma osmolality. The causes of pseudohyponatremia are discussed in detail on the following pages.As with all abnormal labs, when hyponatremia is found, the level should be repeated to rule out a lab or blood-drawing error . When repeating the sodium level, the blood should be drawn through the skin (not from an IV line) and from a vein that does not have IV fluids flowing through it. Blood diluted by IV fluid can cause erroneous sodium measurements._________ is the major osmotically active solute in the ____________ compartment. Sodium extracellular

True hyponatremia is associated with __________, while _________ is associated with a normal or high plasma osmolality.

hypoosmolality pseudohyponatremia

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The Fluid, Electrolyte and Acid-Base Companion

Introduction Pseudohyponatremia Hyperproteinemia and hyperlipidemia cause pseudohyponatremia with a normal plasma osmolality.NORMAL PSEUDOHYPONATREMIA

protein, lipid volume plasma water plasma volume

protein or lipid volume plasma water plasma volume

cell volume

cell volume

The lab reports [Na+] as this Na plasma water+

but the lab actually measures this: Na+ plasma volume

With solute, this is normal: Na+ plasma water

but this is low: Na+ plasma volume

Pseudohyponatremia associated with a normal plasma osmolality is due to a lab error caused by either hyperproteinemia (e.g., multiple myeloma, infusion of immunoglobulin) or hyperlipidemia. With this type of pseudohyponatremia, the true plasma sodium concentration is normal. Hospital labs report plasma sodium concentration as milligrams of sodium per deciliter of plasma water; however, hospital labs measure the milligrams of sodium per deciliter of plasma volume. Therefore, an assumption is made that plasma water equals plasma volume. Remember from Chapter 1 that plasma is composed of water and solutes. With hyperproteinemia and hyperlipidemia, solute volume is increased, decreasing the amount of plasma water in a sample. Therefore, when the sodium content of plasma water is divided by plasma volume, the denominator is too big, causing the sodium concentration to be deceptively low. If the sodium content were divided by plasma water, the normal sodium concentration would be revealed. Because proteins and lipids are large molecules, increased levels add little to plasma osmolality. Therefore, with hyperproteinemia and hyperlipidemia, plasma osmolality is normal.

See Chapter 1, page 18 to understand the small contribution of large molecules to plasma osmolality .hyperlipidemia normal

In pseudohyponatremia associated with _____________ or hyperproteinemia, the sodium concentration is actually ___________.

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S. Faubel and J. Topf

6 Hyponatremia: The Pathophysiology

Introduction Pseudohyponatremia Increased glucose and mannitol cause pseudohyponatremia with a high plasma osmolality.

glucose or mannitol

Osmotically active particles draw water into the plasma compartment and dilute plasma sodium.

Hyponatremia in the face of elevated plasma osmolality is due to increased levels of osmotically active solutes in the plasma. Increased glucose (hyperglycemia) and mannitol administration are the most common causes of this type of pseudohyponatremia. An increase in these osmotically active particles draws intracellular water into the plasma which dilutes plasma sodium and causes hyponatremia. In this type of pseudohyponatremia, the sodium concentration is truly low (it is not a lab error), but plasma osmolality is increased from the glucose or mannitol.

In pseudohyponatremia due to hyperglycemia, the drop in sodium can be predicted from the glucose concentration. For every 100 mg/dL that the glucose is above 100 mg/dL, the sodium falls 1.6 mEq/L. For example, with a glucose of 800 mg/dL, sodium decreases by: (800 100) 100 1.6 = 11.2. Thus, the normal sodium in this setting is 1401.2 1 = 128.8. Remember the constant 1.6 by remembering sixteen. sweet glucose 1.6

Hyponatremia with a high plasma osmolality is secondary to increased concentration of another _________ active particle.

Increased ________ concentration is the most common cause of hyponatremia with a high plasma osmolality.

aaa osmotically glucose

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The Fluid, Electrolyte and Acid-Base Companion

Introduction True hyponatremia is due to a relative excess of water and can occur with hypovolemia, euvolemia and hypervolemia.

Na+ H2Ohypovolemia

Na+ H20euvolemia

Na+ H20hypervolemia

The measurement of plasma sodium is a concentration: the ratio of sodium to water. If hyponatremia is present, the amount of water relative to the amount of sodium is increased. In all cases, true hyponatremia is due to a relative excess of water, not a lack of sodium. The relative excess of water in hyponatremia can occur with all three types of volume status: hypovolemia, euvolemia and hypervolemia. As will be discussed, the assessment of volume status is an important component in the evaluation of hyponatremia.From this point forward, hyponatremia refers to true hyponatremia.

A solid understanding of both volume regulation and osmoregulation is important to understand the pathophysiology of the disorders associated with hyponatremia. W e recommend reading Chapter 4, Volume Regulation Chapter 5,Osmoregulation and prior to this chapter .Hyponatremia is due to excess __________ relative to ________. Sodium concentration is not a marker of volume status; hyponatremia can be associated with ____-volemia, ___-volemia and _____-volemia. water; sodium hypo; eu hyper

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S. Faubel and J. Topf

6 Hyponatremia: The Pathophysiology

Introduction Hyponatremia is always due to the ingestion of water in excess of the amount the kidney can excrete.

excretion

ingestion > excretion = hyponatremia ingestion

The relative excess of water in hyponatremia is always due to the ingestion of water that exceeds what the kidney is able to excrete. This occurs in three situations: ingestion of too much water which overwhelms normal kidneys ingestion of normal amounts of water in renal failure ingestion of normal amounts of water in the presence of ADH

The development of hyponatremia is often a confusing subject, despite the fact that it is simply the relationship between how much water is ingested (too much) and how much water is excreted (too little). Thus, one key to understanding the causes of hyponatremia is to understand the factors which affect water excretion. A brief overview of the pathophysiology of water excretion is presented on the following page.

Hyponatremia occurs when the __________ of water exceeds the ________ of water by the kidney.

ingestion excretion

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The Fluid, Electrolyte and Acid-Base Companion

Overview: The maximum daily urine volume depends on the daily solute load and minimum urinary concentration.14maximum daily urine volume (L)

12 10 8 6 4 2 050 100 200 400 600 800 minimum urinary concentration (mmol/L) 1200

The graph shows the maximum daily urine volume for several values of the minimum urinary concentration (50 to 1200 mmol/L), assuming a daily solute load of 600 mmol/day.

In the absence of renal failure, hyponatremia occurs when