غدير الماء
01-05-2011, 08:09 PM
Why is water so important? It comprises 55% – 65% of an average adult's body weight.1 A newborn is about 95% water; an elderly or obese person, around 40%.1 A loss of body water, whether acute or chronic, can cause a range of problems from mild lightheadedness to convulsions, coma, and in some cases, death.2
That's why many hospitalized patients have some type of intravenous fluid therapy as part of their care. Some need fluid to maintain water balance; others require it as replacement or restorative therapy.
Though fluid therapy can be a lifesaver, it's not innocuous. Giving the wrong fluid can be deadly. Since there are more than a hundred different types of IV fluids that vary in effect, you need a simple way to keep them straight. Here, we'll review the principles that govern fluid therapy and give you an update on the latest evidence regarding its use.
How the body's water is distributed
Water is found in two major body compartments: inside (intracellular) and outside (extracellular) the cells. While cells hold two-thirds of the body's water, the remaining one-third is divided between the vascular and interstitial spaces; one-quarter of it is found in the vessels, and the remainder is in the interstitium.1,2
Because the walls that separate these compartments are porous, water moves freely between them.2 The tiny pores that line the walls of the cells and capillaries also let small particles like sodium and chloride pass through easily.
Large molecules like proteins and starches usually stay put. But a "two-pore theory" suggests that there are some larger pores along the venous capillaries that let big particles pass into the interstitial space.3 This theory is used to explain why some of our efforts to expand blood volume following trauma tend to fail.
Forces that move water
Hydrostatic and osmotic pressures, along with hormones, tightly regulate the body's water. Intravenous fluid therapy primarily manipulates these two types of pressure. Hydrostatic pressure simply reflects the weight and volume of water: The more volume, the higher the blood pressure. That's why fluids are given to patients in shock;4 increasing the intravascular volume raises blood pressure, which helps keep vital organs perfused.
But this strategy has a downside: Think of a balloon partly filled with water. Make it semipermeable with tiny pinpricks and watch it drip. Then add more water to the balloon and watch the rise in hydrostatic pressure cause the drip to become a spray.
Giving a large volume of IV fluid has the same effect on circulation. The weight stretches the vascular pores, allowing fluid and all sizes of particles to quickly escape from the vessels.3 In fact, about 75% of a normal saline (NS) bolus leaves the vascular bed instantly.5 The edema that results from aggressive saline administration is linked to poor outcomes in trauma patients.4
Fluid therapy also affects osmotic pressure. Osmosis is often defined as "the diffusion of water across a semipermeable membrane from an area of high concentration to an area of low concentration."6 But it may be easier to understand when put this way: Water moves into the compartment with the higher concentration of particles, or solute. Water is actually pulled into the compartment in the same way that a sponge soaks up a spill. This pull is called osmotic pressure.
While the size of particles distinguishes the two major types of fluid—crystalloid (small) or colloid (large)—it's the number of particles in each compartment that keeps water where it's supposed to be.6 Nurses give fluids with more (or fewer) particles than blood plasma to pull fluid into the compartment that needs it most.
So how do you know where the water is needed? To assess water balance, you'll measure the osmolality of blood plasma. Osmolality is the number of particles (osmoles) in a kilogram of fluid; osmolarity is the number of particles in a liter of fluid. These terms are often used interchangeably because the density of water is 1 kg/L. Normal serum osmolality is around 300 mOsm/L.1,4
Crystalloids come in three tonicities
Crystalloids are so named because they are made of substances that form crystals. Salt is a perfect example. Its crystals readily form out of particles of sodium and chloride, and then dissociate in water. Because the particles are small, weighing around 30 kilodaltons (kDa),3 they can easily pass in and out of the pores between compartments.
Crystalloids are categorized by their tonicity, a synonym for osmolality. A fluid that's isotonic has the same number of particles—the same osmolality—as plasma. Therefore, an isotonic solution won't promote the shift of fluids into or out of the cells, causing them to swell or shrink. Isotonic crystalloids shouldn't cause edema, either, and usually don't when given in moderation.
Normal saline (0.9%) and lactated Ringer's (LR) solution are two of the most commonly used isotonic fluids. They're currently the mainstay of resuscitation therapy,4,7 and are often used for electrolyte replacement and for perioperative fluid administration.8,9
New evidence suggests that these fluids may do more harm than originally thought.9,10 (For a summary of fluid research, see "The fluid controversy heats up" at the end of this article.) While fluid overload has always been a concern when giving IV fluids, research now shows that isotonic crystalloids are proinflammatory.10 Lactated Ringer's, in particular, activates neutrophils, which destroy surrounding tissue by way of oxidative burst—the process whereby a neutrophil undergoes apoptosis, or programmed cell death, and spews its contents, including hydrogen peroxide, into surrounding tissue.10,11 This process may be the trigger for acute respiratory distress syndrome (ARDS).11,12
Dextrose 5% in water (D5W) is another isotonic crystalloid. However, it's not used for resuscitation because, as its glucose is metabolized, this fluid quickly becomes hypotonic. In fact, D5W is a good source of free water.1 As with other hypotonic fluids, such as 0.45% NS, the water quickly shifts out of the vascular bed and into the cells, by way of osmosis.
Nurses frequently give hypotonic fluids to correct cellular dehydration and hypernatremia.1 Give them with caution, however, because as they shift water out of the vascular bed, hypotonic fluids can worsen hypotension in a patient with low blood pressure.1
Hypertonic fluids, on the other hand, have more particles than the body's water. They pull water back into circulation from the cells and interstitial spaces, which can shrink the cells.12,13 These fluids are also used to correct electrolyte imbalances. Hypertonic saline has an additional benefit: It suppresses inflammation. That's one reason it's gaining respect as a resuscitation fluid of first choice.10,12
Colloids 101: The other fluids
Unlike in crystalloids, the particles suspended in colloids don't break down into smaller pieces in water. Most of them are larger than 30 kDa, so they won't fit through most capillary pores. Therefore, colloids tend to stay in the vascular bed,3 which is why they are used for volume expansion.
The advantage of administering colloids is that you can give smaller amounts of fluid (about 250 ml) and achieve the same effect you would with four liters of crystalloids.10 However, the downside is that, as hydrostatic pressure rises in the capillaries, the pores stretch and let colloids pass through.3 The edema that results takes longer to resolve than that produced by crystalloids.
Commonly used colloids include human albumin, a natural protein that's separated from plasma; hetastarch (HES), a synthetic starch derived from hydroxyethyl glucose; mannitol, an alcohol sugar; and dextran, a polysaccharide. These and other frequently used IV solutions are listed in the table at the bottom of this article, along with their actions, uses, and nursing considerations.
Like crystalloids, colloids are linked to a number of complications. Fluid overload is common to both types of fluid. Specifically, albumin is linked to anaphylaxis and pulmonary edema; HES and dextran are associated with a range of hypersensitivity reactions and bleeding. What's more, mannitol, traditionally used to treat increased intracranial pressure (ICP), can actually raise ICP and increase cerebral edema.
Monitoring therapy: What to watch for
Before you hang any IV fluid, know what you're hanging, why it's been ordered, and what complications may occur. Since fluid overload is common to all IV solutions, be alert for its signs: neck vein distention, increased blood pressure, adventitious lung sounds, and respiratory distress.
Monitor fluid balance by checking intake and output at least every shift to make sure the kidneys are functioning properly. Weigh your patient daily and assess vital signs regularly, watching for trends such as weight gain, increasing blood pressure, or a rise in heart or respiratory rate.
If you suspect volume overload, discontinue the fluid immediately, and give supplemental oxygen and diuretics as ordered. If the patient is having trouble breathing, elevating the head of the bed may help.
When infusing LR, be sure to monitor the patient's electrolyte levels. Watch especially for a rise in potassium, which can lead to cardiac dysrhythmias. If the potassium is seriously high, you may need to give calcium chloride. Another antidote is insulin, given with 50% dextrose. As insulin drives glucose into the cells, it activates the sodium-potassium pump, which moves potassium back into the cells as well.
Likewise, you should monitor serum sodium levels in patients receiving NS or hypertonic saline solutions.1,2 Because an excess of serum sodium causes brain cells to shrink, most of the early signs of hypernatremia are neurological: muscle weakness, twitching, personality changes, agitation, and hallucinations. Hypotonic or isotonic fluids are the treatment of choice.
On the flip side, hyponatremia can result from disease or hypotonic saline or dextrose solutions. Here, brain cells swell. Signs include headache, weakness, nervousness, vomiting, tremor, convulsions, and coma. The patient's pupils may be dilated, and you may also note Babinski's sign. Most cases of hyponatremia occur in postop women and small children.2 The condition is usually treated with hypertonic saline.
Other complications of fluid therapy include phlebitis, infiltration, and extravasation. Many IV fluids are irritating to the veins, so if you note redness and swelling at or along the IV site, discontinue the fluid and remove the IV immediately. Then apply warm compresses, and restart the infusion at another site.
Infiltration occurs when IV fluid leaks into the tissue surrounding the IV site. It's caused by improper catheter placement, or by dislodgement of the catheter—typically, from patient movement. Most of the time, infiltration of a nonirritating fluid won't cause any harm, though a large amount of any fluid can cause tissue damage.
Extravasation is a more serious type of infiltration. It's caused by a vesicant—an IV fluid (or drug) that can irritate the vein walls, trigger vasoconstriction, or cause the vein to rupture. Examples include IV potassium, calcium, magnesium, or 20% – 50% dextrose. Pain, infection, and severe tissue necrosis may result. If you suspect extravasation, stop the infusion and call the physician immediately to determine the next step.
Most of the time, IV therapy does more good than harm. Knowing the different types of solutions, as well as their uses and adverse effects, will help ensure that you administer these fluids safely and appropriately.
________________________________________
________________________________________
The fluid controversy heats up
There are tons of studies on fluids, none of them conclusive. We need to standardize our research efforts or the fluid controversy will never come to a close. Here's a brief look at the evidence we've got to date:
Fluids for resuscitation
The standard amount of crystalloids recommended for trauma victims—three to eight times the estimated blood loss—came under scrutiny once researchers found a higher mortality rate among patients who received fluids than those who did not. Limiting fluids, called permissive hypotension, is now the trend.
"Less is more" works except in cases of traumatic brain injury, where hypotension can cause brain damage.
Nevertheless, studies show that aggressive and excessive fluid resuscitation with isotonic crystalloids reduces oxygenation by diluting hemoglobin and stimulating rebleeding. It also promotes coagulopathy, abdominal compartment syndrome, and hypochloremic acidosis.
Crystalloids, particularly lactated Ringer's solution, can accelerate systemic inflammation in trauma patients by activating neutrophils. Research shows that it's the body's neutrophils that spark the diffuse cellular injury that leads to acute respiratory distress syndrome and multiple organ failure (MOF).
Because of these dangers, there's a renewed interest in hypertonic saline (HS) with or without dextran as a resuscitation fluid of first choice. The major advantage of HS is that it can expand volume using as little as 250 ml of fluid. But the best part is the finding that HS decreases neutrophil activation, blunts inflammation, and helps prevent lung and bowel injury.
The future may bring gene therapy to stop inflammation, improved blood substitutes to support oxygenation, and designer fluids with antioxidants to decrease reperfusion injury. You may also see splanchnic-directed therapy to prevent MOF.
Perioperative fluid therapy
The current recommendations for administering perioperative fluid therapy are based on studies of critically ill patients. New evidence suggests that this practice may be detrimental, and that both the type and amount of fluid affect surgical outcomes.
Because stress causes the body to hang onto water and increases capillary permeability, surgical patients are prone to weight gain. Those undergoing major surgery have a marked stress response. Giving these patients lots of fluid (>5 L) promotes significant third-spacing. Those who received a large volume of fluid had more cardiac and pulmonary complications from fluid overload. The most common were pulmonary edema, atelectasis, and pneumonia. They also experienced decreased gastric motility and prolonged ileus.
While the amount of fluid had no effect on wound healing in these patients, limiting fluids proved to significantly shorten postop ileus and improve tissue oxygenation. The goal for fluid therapy in major surgery should be to optimize cardiac and pulmonary function, shorten postop ileus, and avoid renal problems.
Patients undergoing minor and moderate procedures benefited from receiving one to three liters of fluid. Those who were restricted to one liter or less had more nausea and vomiting and dizziness. Bottom line: less is more, some is better than none.
That's why many hospitalized patients have some type of intravenous fluid therapy as part of their care. Some need fluid to maintain water balance; others require it as replacement or restorative therapy.
Though fluid therapy can be a lifesaver, it's not innocuous. Giving the wrong fluid can be deadly. Since there are more than a hundred different types of IV fluids that vary in effect, you need a simple way to keep them straight. Here, we'll review the principles that govern fluid therapy and give you an update on the latest evidence regarding its use.
How the body's water is distributed
Water is found in two major body compartments: inside (intracellular) and outside (extracellular) the cells. While cells hold two-thirds of the body's water, the remaining one-third is divided between the vascular and interstitial spaces; one-quarter of it is found in the vessels, and the remainder is in the interstitium.1,2
Because the walls that separate these compartments are porous, water moves freely between them.2 The tiny pores that line the walls of the cells and capillaries also let small particles like sodium and chloride pass through easily.
Large molecules like proteins and starches usually stay put. But a "two-pore theory" suggests that there are some larger pores along the venous capillaries that let big particles pass into the interstitial space.3 This theory is used to explain why some of our efforts to expand blood volume following trauma tend to fail.
Forces that move water
Hydrostatic and osmotic pressures, along with hormones, tightly regulate the body's water. Intravenous fluid therapy primarily manipulates these two types of pressure. Hydrostatic pressure simply reflects the weight and volume of water: The more volume, the higher the blood pressure. That's why fluids are given to patients in shock;4 increasing the intravascular volume raises blood pressure, which helps keep vital organs perfused.
But this strategy has a downside: Think of a balloon partly filled with water. Make it semipermeable with tiny pinpricks and watch it drip. Then add more water to the balloon and watch the rise in hydrostatic pressure cause the drip to become a spray.
Giving a large volume of IV fluid has the same effect on circulation. The weight stretches the vascular pores, allowing fluid and all sizes of particles to quickly escape from the vessels.3 In fact, about 75% of a normal saline (NS) bolus leaves the vascular bed instantly.5 The edema that results from aggressive saline administration is linked to poor outcomes in trauma patients.4
Fluid therapy also affects osmotic pressure. Osmosis is often defined as "the diffusion of water across a semipermeable membrane from an area of high concentration to an area of low concentration."6 But it may be easier to understand when put this way: Water moves into the compartment with the higher concentration of particles, or solute. Water is actually pulled into the compartment in the same way that a sponge soaks up a spill. This pull is called osmotic pressure.
While the size of particles distinguishes the two major types of fluid—crystalloid (small) or colloid (large)—it's the number of particles in each compartment that keeps water where it's supposed to be.6 Nurses give fluids with more (or fewer) particles than blood plasma to pull fluid into the compartment that needs it most.
So how do you know where the water is needed? To assess water balance, you'll measure the osmolality of blood plasma. Osmolality is the number of particles (osmoles) in a kilogram of fluid; osmolarity is the number of particles in a liter of fluid. These terms are often used interchangeably because the density of water is 1 kg/L. Normal serum osmolality is around 300 mOsm/L.1,4
Crystalloids come in three tonicities
Crystalloids are so named because they are made of substances that form crystals. Salt is a perfect example. Its crystals readily form out of particles of sodium and chloride, and then dissociate in water. Because the particles are small, weighing around 30 kilodaltons (kDa),3 they can easily pass in and out of the pores between compartments.
Crystalloids are categorized by their tonicity, a synonym for osmolality. A fluid that's isotonic has the same number of particles—the same osmolality—as plasma. Therefore, an isotonic solution won't promote the shift of fluids into or out of the cells, causing them to swell or shrink. Isotonic crystalloids shouldn't cause edema, either, and usually don't when given in moderation.
Normal saline (0.9%) and lactated Ringer's (LR) solution are two of the most commonly used isotonic fluids. They're currently the mainstay of resuscitation therapy,4,7 and are often used for electrolyte replacement and for perioperative fluid administration.8,9
New evidence suggests that these fluids may do more harm than originally thought.9,10 (For a summary of fluid research, see "The fluid controversy heats up" at the end of this article.) While fluid overload has always been a concern when giving IV fluids, research now shows that isotonic crystalloids are proinflammatory.10 Lactated Ringer's, in particular, activates neutrophils, which destroy surrounding tissue by way of oxidative burst—the process whereby a neutrophil undergoes apoptosis, or programmed cell death, and spews its contents, including hydrogen peroxide, into surrounding tissue.10,11 This process may be the trigger for acute respiratory distress syndrome (ARDS).11,12
Dextrose 5% in water (D5W) is another isotonic crystalloid. However, it's not used for resuscitation because, as its glucose is metabolized, this fluid quickly becomes hypotonic. In fact, D5W is a good source of free water.1 As with other hypotonic fluids, such as 0.45% NS, the water quickly shifts out of the vascular bed and into the cells, by way of osmosis.
Nurses frequently give hypotonic fluids to correct cellular dehydration and hypernatremia.1 Give them with caution, however, because as they shift water out of the vascular bed, hypotonic fluids can worsen hypotension in a patient with low blood pressure.1
Hypertonic fluids, on the other hand, have more particles than the body's water. They pull water back into circulation from the cells and interstitial spaces, which can shrink the cells.12,13 These fluids are also used to correct electrolyte imbalances. Hypertonic saline has an additional benefit: It suppresses inflammation. That's one reason it's gaining respect as a resuscitation fluid of first choice.10,12
Colloids 101: The other fluids
Unlike in crystalloids, the particles suspended in colloids don't break down into smaller pieces in water. Most of them are larger than 30 kDa, so they won't fit through most capillary pores. Therefore, colloids tend to stay in the vascular bed,3 which is why they are used for volume expansion.
The advantage of administering colloids is that you can give smaller amounts of fluid (about 250 ml) and achieve the same effect you would with four liters of crystalloids.10 However, the downside is that, as hydrostatic pressure rises in the capillaries, the pores stretch and let colloids pass through.3 The edema that results takes longer to resolve than that produced by crystalloids.
Commonly used colloids include human albumin, a natural protein that's separated from plasma; hetastarch (HES), a synthetic starch derived from hydroxyethyl glucose; mannitol, an alcohol sugar; and dextran, a polysaccharide. These and other frequently used IV solutions are listed in the table at the bottom of this article, along with their actions, uses, and nursing considerations.
Like crystalloids, colloids are linked to a number of complications. Fluid overload is common to both types of fluid. Specifically, albumin is linked to anaphylaxis and pulmonary edema; HES and dextran are associated with a range of hypersensitivity reactions and bleeding. What's more, mannitol, traditionally used to treat increased intracranial pressure (ICP), can actually raise ICP and increase cerebral edema.
Monitoring therapy: What to watch for
Before you hang any IV fluid, know what you're hanging, why it's been ordered, and what complications may occur. Since fluid overload is common to all IV solutions, be alert for its signs: neck vein distention, increased blood pressure, adventitious lung sounds, and respiratory distress.
Monitor fluid balance by checking intake and output at least every shift to make sure the kidneys are functioning properly. Weigh your patient daily and assess vital signs regularly, watching for trends such as weight gain, increasing blood pressure, or a rise in heart or respiratory rate.
If you suspect volume overload, discontinue the fluid immediately, and give supplemental oxygen and diuretics as ordered. If the patient is having trouble breathing, elevating the head of the bed may help.
When infusing LR, be sure to monitor the patient's electrolyte levels. Watch especially for a rise in potassium, which can lead to cardiac dysrhythmias. If the potassium is seriously high, you may need to give calcium chloride. Another antidote is insulin, given with 50% dextrose. As insulin drives glucose into the cells, it activates the sodium-potassium pump, which moves potassium back into the cells as well.
Likewise, you should monitor serum sodium levels in patients receiving NS or hypertonic saline solutions.1,2 Because an excess of serum sodium causes brain cells to shrink, most of the early signs of hypernatremia are neurological: muscle weakness, twitching, personality changes, agitation, and hallucinations. Hypotonic or isotonic fluids are the treatment of choice.
On the flip side, hyponatremia can result from disease or hypotonic saline or dextrose solutions. Here, brain cells swell. Signs include headache, weakness, nervousness, vomiting, tremor, convulsions, and coma. The patient's pupils may be dilated, and you may also note Babinski's sign. Most cases of hyponatremia occur in postop women and small children.2 The condition is usually treated with hypertonic saline.
Other complications of fluid therapy include phlebitis, infiltration, and extravasation. Many IV fluids are irritating to the veins, so if you note redness and swelling at or along the IV site, discontinue the fluid and remove the IV immediately. Then apply warm compresses, and restart the infusion at another site.
Infiltration occurs when IV fluid leaks into the tissue surrounding the IV site. It's caused by improper catheter placement, or by dislodgement of the catheter—typically, from patient movement. Most of the time, infiltration of a nonirritating fluid won't cause any harm, though a large amount of any fluid can cause tissue damage.
Extravasation is a more serious type of infiltration. It's caused by a vesicant—an IV fluid (or drug) that can irritate the vein walls, trigger vasoconstriction, or cause the vein to rupture. Examples include IV potassium, calcium, magnesium, or 20% – 50% dextrose. Pain, infection, and severe tissue necrosis may result. If you suspect extravasation, stop the infusion and call the physician immediately to determine the next step.
Most of the time, IV therapy does more good than harm. Knowing the different types of solutions, as well as their uses and adverse effects, will help ensure that you administer these fluids safely and appropriately.
________________________________________
________________________________________
The fluid controversy heats up
There are tons of studies on fluids, none of them conclusive. We need to standardize our research efforts or the fluid controversy will never come to a close. Here's a brief look at the evidence we've got to date:
Fluids for resuscitation
The standard amount of crystalloids recommended for trauma victims—three to eight times the estimated blood loss—came under scrutiny once researchers found a higher mortality rate among patients who received fluids than those who did not. Limiting fluids, called permissive hypotension, is now the trend.
"Less is more" works except in cases of traumatic brain injury, where hypotension can cause brain damage.
Nevertheless, studies show that aggressive and excessive fluid resuscitation with isotonic crystalloids reduces oxygenation by diluting hemoglobin and stimulating rebleeding. It also promotes coagulopathy, abdominal compartment syndrome, and hypochloremic acidosis.
Crystalloids, particularly lactated Ringer's solution, can accelerate systemic inflammation in trauma patients by activating neutrophils. Research shows that it's the body's neutrophils that spark the diffuse cellular injury that leads to acute respiratory distress syndrome and multiple organ failure (MOF).
Because of these dangers, there's a renewed interest in hypertonic saline (HS) with or without dextran as a resuscitation fluid of first choice. The major advantage of HS is that it can expand volume using as little as 250 ml of fluid. But the best part is the finding that HS decreases neutrophil activation, blunts inflammation, and helps prevent lung and bowel injury.
The future may bring gene therapy to stop inflammation, improved blood substitutes to support oxygenation, and designer fluids with antioxidants to decrease reperfusion injury. You may also see splanchnic-directed therapy to prevent MOF.
Perioperative fluid therapy
The current recommendations for administering perioperative fluid therapy are based on studies of critically ill patients. New evidence suggests that this practice may be detrimental, and that both the type and amount of fluid affect surgical outcomes.
Because stress causes the body to hang onto water and increases capillary permeability, surgical patients are prone to weight gain. Those undergoing major surgery have a marked stress response. Giving these patients lots of fluid (>5 L) promotes significant third-spacing. Those who received a large volume of fluid had more cardiac and pulmonary complications from fluid overload. The most common were pulmonary edema, atelectasis, and pneumonia. They also experienced decreased gastric motility and prolonged ileus.
While the amount of fluid had no effect on wound healing in these patients, limiting fluids proved to significantly shorten postop ileus and improve tissue oxygenation. The goal for fluid therapy in major surgery should be to optimize cardiac and pulmonary function, shorten postop ileus, and avoid renal problems.
Patients undergoing minor and moderate procedures benefited from receiving one to three liters of fluid. Those who were restricted to one liter or less had more nausea and vomiting and dizziness. Bottom line: less is more, some is better than none.