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Critical care for sick cattle-practical IV therapy (Proceedings)
Dehydration and electrolyte abnormalities occur commonly in cattle with gastrointestinal tract disease. Severe fluid and electrolyte abnormalities can result in death if left untreated.
Dehydration and electrolyte abnormalities occur commonly in cattle with gastrointestinal tract disease. Severe fluid and electrolyte abnormalities can result in death if left untreated. Five important questions must always be asked and answered regarding fluid and electrolyte therapy in cattle; (1) is therapy needed? (2) what route of administration? (3) what type? (4) how much should be administered?, and (5) how fast should the solution be administered? All calves ≥8% dehydrated (≥4 mm eyeball recession) or decreased/absent suckle should receive IV fluids. Oral fluids are very effectve in mild dehydration, particularly in calves with a suckling reflex.
There are 4 routes for parenteral fluid administration; (1) intravenous, (2) intraperitoneal, (3) subcutaneous, and (4) intraosseous. Crystalloid solutions can be administered all 4 routes; however, the contents must be accurately known and ideally the solution should be sterile and nonpyrogenic. Colloids can be administered by the intravenous and intraperitoneal routes. Isotonic solutions can be administered by the intravenous, intraperitoneal, subcutaneous, and intraosseous routes, whereas hypertonic solutions should only be administered intravenously or subcutaneously.
Intravenous administration of large volumes of isotonic crystalloid solutions requires intravenous catheterization and periodic monitoring of the flow rate and fluid container. It is often difficult to maintain a patent catheter and a constant flow rate using gravity-flow systems because of changes in position from recumbency to standing (thereby altering the hydrostatic pressure gradient) and because movement can lead to tangling of the fluid line. An additional problem in goats and occasionally sheep and calves is nibbling or suckling the fluid administration line with subsequent disruption. Catheter-related problems are easily identified and rectified in a veterinary hospital but usually go undetected and uncorrected when intravenous fluids are administered in on-farm situations.
The effectiveness of intraperitoneal fluid administration does not appear to have been extensively evaluated in dehydrated calves. The route is not currently recommended for neonatal calves because of altered peritoneal absorption in dehydrated calves and the risk of peritoneal adhesions, although neither claim appears to have been substantiated. Plasma has been administered safely to neonatal calves through a catheter in the paralumbar fossa. The effectiveness of subcutaneous or intraosseous fluid administration does not appear to have been evaluated in dehydrated calves.
A thorough understanding of osmolarity is central to successful parenteral and oral fluid therapy. Osmolality is "the number of dissolved particles per kilogram of water" and is measured. The normal value in cattle is approximately 285 mOsm/kg. Osmolarity is "the number of dissolved particles per liter of water" and is calculated. When applied to plasma or serum, note that the denominator of osmolarity is L (which is liters of plasma). 1 kg of plasma has 2 components: 1) plasma protein (normal concentration, 7 g/dl = 70 g protein / kg of plasma); 2) plasma water (1 kg of plasma - 70 g of plasma protein = 930 g = 0.93 L). The osmolarity of plasma water in ruminants is therefore 285/0.93 = 306 mOsm/L and this is the approximate osmolarity of isotonic solutions.
Erythrocytes are resistant to increases in plasma osmolarity, whereas they are susceptible to mild decreases in osmolarity; this is the basis of the red blood cell fragility test whereby red blood cell suspensions are placed in solutions of decreasing osmolarity. As the osmolarity of the solution decreases, water moves across the semipermeable red blood cell membrane into the erythrocyte, resulting in swelling of erythrocytes, fracture of the cell membrane, and hemolysis. This is why the rapid IV administration of hypotonic solutions causes hemolysis and hemoglobinuria. In contrast, intravenous administration of hypertonic solutions does not cause hemolysis, because increasing plasma osmolarity causes water to move out of erythrocytes, which are resistant to shrinking. Because of hypotonic-induced hemolysis, parenterally administered fluids should be isotonic or hypertonic.
The total amount of infused sodium ion is the single most important determinant of the success of fluid resuscitation. Sodium is the scaffold on which extracellular fluid volume is dependent; No sodium = No extracellular fluid volume = No animal. Practical fluid resuscitation should therefore focus on administering enough sodium ions at a fast enough rate, without inducing any deleterious effects.
A crystalloid is "a substance that forms a true solution and is capable of being crystallized". As a group, crystalloids are excellent at rapid but transient plasma volume expansion because they distribute throughout the extracellular fluid space and are not confined to the intravascular space. For instance, if you administer 5 L IV, within 30 minutes 1 L is intravascular & 4 L is in interstitial space. The usual treatment goal is to increase or maintain ECF volume, in which case the crystalloid solution should contain sodium at approximately 140 mEq/L. An isotonic crystalloid solution for ruminants has an approximate osmolarity of 295-315 mOsm/L. Based on this osmolarity range, Lactated Ringers and 5% dextrose are slightly hypotonic. A hypertonic crystalloid solution for ruminants has an osmolarity exceeding 315 mOsm/L. Commonly administered hypertonic solutions are 50% dextrose, 7.2% sodium chloride, and 8.4% sodium bicarbonate solutions.
Since the first report of the effects of hypertonic saline administration in cattle by Constable and colleagues in 1991, a number of studies have shown that rapid small volume IV administration of hypertonic saline coupled with oral administration of an electrolyte solution or water is as effective as traditional slow large volume intravenous administration of isotonic solutions in treating dehydrated calves and cows. Small volume hypertonic saline (HS) solution (2400 mOsm/L NaCl, 4mL/kg, IV over 4 minutes) has been successful in treating endotoxic shock in cattle and pigs, dehydration in calves, and hemorrhagic shock in dogs, cats, sheep, pigs, and horses. Hypertonic saline solution induces a rapid increase in plasma volume, cardiac output, and mean arterial pressure. The volume expansion that occurs due to osmotically drawing the intracellular water into the vascular space is approximately 3 mL for every 1 mL of hypertonic saline infused. The beneficial effect of HS has been previously attributed to one or more of the following: an increase in preload, a decrease in afterload, activation of a pulmonary vagal reflex, or an increase in cardiac contractility. Recent studies clearly indicate that HS: 1) rapidly but transiently (30 minutes) increases preload; 2) transiently (5 minutes) decreases afterload; 3) does not activate a pulmonary vagal reflex; and 4) transiently (5 minutes) decreases cardiac contractility
Ruminants have evolved over time to adapt to a variety of environments. One of the major adaptations has been the development of a large water reservoir, the rumen, that enables the animal to go without water for days and then rapidly rehydrate without any negative effects. This characteristic is more pronounced in species that have adapted to the desert, but is true of ruminants in general. It has also been proposed that the rumen plays a major role in the water economy of lactating ruminants. The ruminal epithelium is capable of absorbing large volumes of water, the mechanisms governing the movement of water across the ruminal epithelium operate to minimize insults to the osmotic balance between plasma, interstitial fluid, and intracellular fluid. The main force for water movement across the rumen wall is the gradient of osmolality between ruminal fluid (which is normally isotonic to plasma) and blood perfusing ruminal epithelium. We can clinically take advantage of this physiologic phenomenon in adult cattle by increasing plasma osmolality through administering hypertonic saline, while simultaneously maintaining or decreasing rumen osmolality through oral administration of an isotonic or hypotonic electrolyte solution containing sodium. The increase in osmolar gradient across the rumen wall following this combined treatment will cause a rapid and large movement of water from the rumen into the extracellular space, thereby rapidly expanding the plasma volume and correcting dehydration.
Another function of the rumen is absorption of large quantities of sodium. Cattle produce about 180 liters per day of saliva when fed alfalfa, and saliva usually contains 126 mEq/L of sodium. Approximately half of the sodium secreted with saliva can be reabsorbed by the forestomach, primarily through active transport mechanisms. This process is also responsible for the passive movement of some water from the rumen into blood. Chloride ion absorption is thought to occur primarily passively, following active sodium absorption. The vast ruminal capacity for sodium (and chloride) absorption can be taken advantage of when administering oral electrolytes to dehydrated cattle. Provided that the osmolality of the orally administered solution remains hypotonic to plasma, there will be net absorption of both water and electrolytes from the ruminal fluid. This absorption will be slow but sustained, because of the reservoir function of the rumen. Initial administration of intravenous hypertonic saline will provide a rapid resuscitation that will complement the slow but sustained resuscitation obtained from orally administered electrolytes.
Dehydrated adult cattle are administered 2 L of hypertonic saline (approximately 2400 mOsm/L; 8 times normal tonicity) into the jugular vein over 4 to 10 minutes. Allow cow to drink fresh water. This will require a 12 g needle or long extension set to obtain an adequate flow rate. If the cow doesn't drink within 5 minutes of hypertonic saline administration, tube cow with at least 5 gallons of water or hypotonic sodium containing electrolyte solutions. Hypertonic saline injection is a one time deal, it should not be repeated or hypernatremia may result. Hypertonic saline (7.2%) NaCl) should not be administered faster than 1 ml/kg/min; if administered at too fast a rate there is hemodynamic collapse due to vasodilation and decreased cardiac contractility. If hypertonic saline is administered at too slow a rate then the osmotically driven free water shift does not occur and we might as well administer an isotonic crystalloid solution.
Hypertonic saline solutions can be commercially purchased. Their duration of plasma volume expansion is shorter than hypertonic saline-dextran solutions. As with any hypertonic solution, (tonicity of solution is similar to 50% dextrose), perivascular damage will occur if the HSD solution is not administered intravenously.
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