Fluids isotonic/hypertonic/hypotonic

Commonly administered LVPs include such products as Lactated Ringers Injection USP, Sodium Chloride Injection USP (0.9%), which replenish fluids and electrolytes, and Dextrose Injection USP (5%), which provides fluid plus nutrition (calories), or various combinations of dextrose and saline. In addition, numerous other nutrient and ionic solutions are available for clinical use, the most popular of which are solutions of essential amino acids or lipid emulsions. These solutions are modified to be hypertonic, isotonic, or hypotonic to aid in maintaining both fluid, nutritional, and electrolyte balance in a particular patient according to need. Indwelling needles or catheters are required in LVP administration. Care must be taken to avoid local or systemic infections or thrombophlebitis owing to faulty injection or administration technique. 

Crystalloid solutions consist of electrolytes in water. Crystalloid solutions may be isotonic, hypertonic or hypotonic. Isotonic solutions have approximately the same osmolality as plasma and, therefore, may be given rapidly in large volumes into peripheral veins. Hypertonic solutions act to draw water into the extracellular fluid (ECF) from the intracellular fluid and represent a method of rapidly restoring circulating volume at the expense of tissue hydration. Hypotonic solutions are usually only used to correct plasma hypertonicity. Because true hypotonic solutions (e.g. sterile water) cause erythrolysis (Krumbhaar 1914), they can only be given slowly via a central vein (Worthley 1986). For this reason, isotonic solutions containing a metabolizable substrate, such as dextrose, and no electrolytes are usually used. 

Fluids can be classified further according to their tonicity. Isotonic solutions (i.e., normal saline or 0.9% sodium chloride [NaCl]) have a tonicity equal to that of the ICF (approximately 310 mEq/L or 310 mmol/L) and do not shift the distribution of water between the ECF and the ICF. Because hypertonic solutions (i.e., hypertonic saline or 3% NaCl) have greater tonicity than the ICF (greater than 376 mEq/L or 376 mmol/L), they draw water from the ICF into the ECF. In contrast, hypotonic solutions (i.e., 0.45% NaCl) have less tonicity than the ICF (less than 250 mEq/L or 250 mmol/L) leading to an osmotic pressure gradient that pulls water from the ECF into the ICF. The tonicity, electrolyte content, and glucose content of selected fluids are shown in Table 24—3. 

Adding an isotonic solution to the extracellular fluid (ECF) does not change intracellular volume. Adding a hypertonic solution to the ECF decreases cell volume, whereas adding a hypotonic solution increases it (Table 78-1). 

Body fluids, including blood and tears, have the same osmotic pressure as that of a 0.9% w/v sodium chloride solution. Solutions having the same osmotic pressure as that of 0.9% w/v NaCl solution are said to be isotonic with blood. Solutions with a higher osmotic pressure than body fluids are called hypertonic and those with a lower osmotic pressure are called hypotonic. 

Solutions that have identical osmotic pressures are said to be isotonic solutions. Fluids administered intravenously must be isotonic with body fluids. For example, if cells are bathed in a hypertonic solution, which is a solution having an osmotic pressure higher than that of the cell fluids, the cells will shrivel because of a net transfer of water out of the cells. This phenomenon is called crenation. The opposite phenomenon, called lysis, occurs when cells are bathed in a hypotonic solution, a solution with an osmotic pressure lower than that of the cell fluids. In this case the cells rupture because of the flow of water into the cells. 

Eye drops are often formulated to be isotonic with tear fluid but deviations from tonicity do not cause problems, although hypertonicity may cause stinging of the eye and hypotonicity may increase the permeability of the cornea. 

Nonionic contrast media in either hypertonic or hypotonic solution, when injected in-tracerebrally or into the subarachnoid space of rats, will cause distinct depression of the central nervous system (CNS) and associated brain functions but no excitation (675). Such depressive action can obscure the excitatory action caused by ionicity or chemotoxicity of myelographic agent. When an isotonic contrast medium is injected and mixed with the normally produced cerebrospinal fluid (CSF) in the subarachnoid space of the rat, the resultant mixture is hypertonic with higher levels of sodium and chloride. This movement of sodium and chloride into CSF without accompaniment of water is unexpected. Mennini et al. (672) noted that contrast enhancement of brain parenchyma is never achieved by direct intracarotid or intravenous injection of nonionic contrast media, unless the BBB is spontaneously or experimentally broken ( 6). 

Fluids and electrolytes are stored in two compartments intracellular (inside the cell) and extracellular (outside the cell). The amount of electrolytes in fluid is called a concentration. There are three types of fluid concentrations iso-osmolar (same concentration), hypo-osmolar (low concentration), and h) er-osmolar (high concentration). These concentrations are used to describe IV solutions as isotonic (iso-osmolar), hypotonic (hypo-osmolar), and hypertonic (hyper-osmolar). 

Osmosis plays an important role in living systems. The membranes of red blood cells, for example, are semipermeable. Placing a red blood cell in a solution that is hypertonic relative to the intracellular solution (the solution inside the cells) causes water to move out of the cell ( FIGURE 13.26). This causes the cell to shrivel, a process called crenation. Placing the cell in a solution that is hypotonic relative to the intracellular fluid causes water to move into the cell. This may cause the cell to rupture, a process called hemolysis. People who need body fluids or nutrients replaced but cannot be fed orally are given solutions by intravenous (IV) infusion, which feeds nutrients directly into the veins. To prevent crenation or hemolysis of red blood cells, the IV solutions must be isotonic with the intracellular fluids of the blood cells. 

A 0.90% (0.15 M) sodium chloride solution is known as a physiological saline solution because it is isotonic with blood plasma that is, it has the same concentration of NaCl as blood plasma. Because each mole of NaCl yields about 2 mol of ions when in solution, the solute particle concentration in physiological saline solution is nearly 0.30 M. Five-percent-glucose solution (0.28 M) is also approximately isotonic with blood plasma. Blood cells neither swell nor shrink in an isotonic solution. The cells described in the preceding paragraph swell in water because water is hypotonic to cell plasma. The cells shrink in 5%-urea solution because the urea solution is hypertonic to the cell plasma. To prevent possible injury to blood cells by osmosis, fluids for intravenous use are usually made up at approximately isotonic concentration. 

Some of the best examples of osmosis are those associated with living organisms. For instance, if red blood cells are placed in pure water, the cells expand and eventually burst as a result of water that enters through osmosis. The osmotic pressure associated with the fluid inside the cell is equivalent to that of 0.92% (mass/volume) NaCl(aq). Thus, if cells are placed in a sodium chloride (saline) solution of this concentration, there is no net flow of water through the cell membrane, and the cell remains stable. Such a solution is said to be isotonic. If cells are placed in a solution with a concentration greater than 0.92% NaCl, water flows out of the cells, and the cells shrink. Such a solution is said to be hypertonic. If the NaCl concentration is less than 0.92%, the solution is hypotonic, and water flow s into the cells. Fluids that are intravenously injected into patients to combat dehydration or to supply nutrients must be adjusted so that they are isotonic with blood. The osmotic pressure of the fluids must be the same as that of 0.92% (mass/volume) NaCl. 

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