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Ancillary
Notes for
Fluid
Therapy
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Fluid, electrolyte and acid-base abnormalities occur
commonly in small animal patients so treatment of these abnormalities is a daily
occurrence in small animal practice. Although treatment can be life-saving, in most
circumstances, therapy is symptomatic -
An attempt should be made to OBTAIN URINE AND BLOOD SAMPLES
FOR DIAGNOSTIC PURPOSES PRIOR TO FLUID THERAPY as fluid therapy may make test results
difficult in interpret. If fluid therapy is broadly defined to include many types
of fluids, then the indications for fluid therapy are quite diverse and include:
Total body water
accounts for ~ 60 to 70% of body weight, less in old or obese animals and more (up to 80%)
in young animals. Total body water is distributed across interconnected compartments. 2/3
of total body water is located within cells and 1/3 in extracellular locations including
plasma and interstitial locations. Various solutes hold water within the various
compartments, although shifts of both water and solutes may occur between compartments.
Water shifts more readily than solute across compartments. For example, the vascular
(plasma) compartment is defined by the amount of albumin in circulation. If albumin is
decreased, less water can be held in the vascular compartment. Intracellular fluid
is rich in potassium; 95% of total body potassium is located within cells. Interstitial fluid includes fluids: Surrounding cells Water loss occurs
by several routes in the normal animal: Insensible routes are those that are not
readily observed and include the loss of fluid through the respiratory tract during
respiration and loss of fluid by sweating. Dogs and cats sweat minimally through there
foot pads so most insensible losses are through the respiratory tract. The fluid lost
during breathing is close to pure water and does not contain many solutes ( a hypotonic
loss). The basal loss of water through breathing is about: 10-15 ml/lb/day. High
environmental temperatures, fever and activity result in increased insensible losses. Sensible losses are those that can be more
easily detected and measured. Sensible losses can occur through the urinary and
gastrointestinal tracts. The loss of fluid through the GI tract is negligible in healthy
animals but can be substantial through vomiting or diarrhea. In the healthy animal,
sensible loss is through urine and is about 12-20 ml/lb/day. Loss of water through the
urinary and GI tracts is usually accompanied by loss of electrolytes (in both health and
disease) (an isotonic loss). Adding the insensible and sensible fluid loss together for
a normal animal, the net basal loss is about 20-30 ml/lb/day. Abnormal or accelerated loss of fluid includes: High environmental temperature (increases insensible
losses) In order to maintain fluid balance, the animal must intake
or generate as much fluid as is being lost in the urine and by insensible routes. The
average adult dog or cat must generate or intake 20 to 30 mls of fluid per pound of body
weight in order to meet losses and maintain balance. Young animals must intake larger
fluid volumes to maintain balance as their losses are greater. Of the required fluid need,
~20% can be generated as a by-product of metabolic reactions, the other 80% must be
ingested as water in the food or as free water. Canned foods are comprised of as much as
70% water so animals ingesting canned foods may not drink much free water. The patient's history may provide information that will
suggest the animal has under gone increased fluid losses (vomiting, diarrhea, polyuria)
and is at risk for dehydration. Physical parameters which are used to assess hydration
status are crude and can be influenced by body type and other factors. The skin turgor
(elasticity) is most consistent when checked by picking up a fold of skin over the back.
The skin should snap back into place immediately in animals with normal hydration. The
skin over the cervical region can be very loose in certain breeds (e.g. bloodhound) and is
not a good location to check hydration. If an animal has ascites (abdominal fluid) the
weight of the abdominal contents can pull against the skin on the back, pulling it back
into place after a fold of skin is pinched up and giving a false impression of normal skin
elasticity and hydration. Placing the animal in sternal recumbancy before checking skin
turgor removes the influence of gravity on the abdominal contents. Emaciated animals lose elasticity in the skin resulting in
a false impression of being dehydrated even when normally hydrated. Conversely obese
animals may be dehydrated but have normal skin elasticity as the weight of the fat in the
subcutaneous tissues causes the skin to snap back into place when a fold is lifted. The oral membranes are normally wet and slick. As animals
dehydrate, the saliva thickens and the membranes feel sticky or tacky when you run your
finger over them. If the animal is panting due to fear or respiratory disease the oral
membranes may dry out and feel sticky even if the animal is not dehydrated. The tissues behind the orbit are high in water content. As
the animal dehydrates the eyes sink into the orbits. Eye position must be interpreted in
light of normal eye position for the breed. Long nosed (dolichocephalic) breeds such as
Doberman pinchers normally have deep set eyes. Severely dehydrated animals (loss of 12 to 15 % of body
weight as water loss) show signs of hypovolemic shock including pale membranes, weak
pulses and tachycardia. Clinical signs of dehydration are more severe if the animal
loses fluid abruptly compared to a gradual loss that allows for some degree of
acclimation. Animals can be subclinically dehydrated. The minimum amount
of fluid loss that will be reflected as changes in physical parameters is ~5 to 7 % of
body weight as water.
Clinical signs of dehydration (these are
based on % body weight lost as fluid, NOT the % of total body water deficit) < 5% not detectable 5-6% slight loss of skin elasticity 6-8% definite loss of skin elasticity 10-12% tented skin stands in place 12-15% signs of hypovolemic shock, death
Some laboratory parameters are influenced by hydration
status including:
If primary disease affecting these parameters in NOT
present, then increases in PCV, TP, USG and BUN are all indicative of dehydration. The
anemic animal may falsely appear to have a normal PCV if dehydration concentrates the red
blood cells. Animals with inflammatory disease may have elevated TP with normal hydration
status. Inflammatory disease will increase globulins whereas dehydration will increase
albumin. Elevations in BUN due to dehydration are accompanied by a concentrated USG
(>1.025 in dogs and >1.035 in cats). Elevations in BUN with low USG are indicative
of primary renal disease. 10% dehydrated = 5 lb deficit requires 2,500 ml fluid to rehydrate Types of Dehydration
The majority (probably >90%) of dogs and cats that become dehydrated have sustained isotonic losses of fluids. Gastrointestinal and urinary losses of fluids result in the loss of water AND electrolytes including sodium, potassium, chloride and bicarbonate. The amount and type of electrolytes lost depend on the disease that is causing the abnormal loss. The fluid and electrolytes which are lost originate initially from the ECF compartment. Although the concentration of electrolytes in these abnormal fluids is variable, the fact that the fluid lost is NOT pure water often leaves the ECF compartment deficit in volume but WITHOUT a change in osmolality. If the fluid lost through the GI tract contains sodium in proportions similar to ECF, then serum sodium CONCENTRATION will NOT change. As long as both ECF and ICF compartment osmolality remain similar to one another, additional fluid shifts between the two compartments are unlikely to occur. Isotonic Dehydration
If the loss outside the body contains water and solute in proportions similar to ECF, then the proportion of water to solute in the remaining ECF will not change. The osmolality of ECF will remain normal despite the reduction in the size of the compartment. Hypotonic Dehydration
The fluid lost in the urine when aldosterone is reduced is very high in sodium, leaving the ECF with a deficit of sodium and reduced in osmolality compared to normal Hypotonic dehydration is named for the reduced osmolality of ECF which remains after a hypertonic fluid is lost from the body. Some references will call this type of dehydration, hypertonic dehydration, taking the name from the hypertonic fluid that is lost. Regardless of what it is called, the best example of this type of dehydration is hypoadrenocorticism (Addison's Disease). Addison's disease is a deficiency of glucocorticoids and mineralocorticoids (aldosterone). Aldosterone is responsible for resorption of sodium and elimination of potassium in the renal tubules so deficiency results in sodium loss and potassium retention. The high sodium containing urine is hypertonic to ECF. The remaining ECF is hypotonic compared to normal ECF. In order to reestablish equal tonicity between ICF and ECF either sodium can moves from ICF to ECF or water can move from ECF to ICF. Water movement is passive, electrolyte movement is usually active so water moves into the cells. The end result is that ECF suffers an even greater water loss resulting in more severe signs of hypovolemia compared to a patient that is vomiting for reasons other than Addison's disease. Additionally patients with Addison's disease may have GI losses of fluids Hypertonic Dehydration
The fluid loss to the outside is close to pure water Hypertonic dehydration is a rare type of dehydration. Like the discussion in the previous , this may be called either hypotonic or hypertonic dehydration by different authors depending upon the frame of reference for the name. This condition occurs in patients with hypotonic fluid losses such as those patients with diabetes insipidus (ADH deficiency) whose urine is close to pure water. Usually animals with DI will drink adequate amounts of water to maintain normal hydration but if they are prevented from drinking (e.g. sustained water deprivation test) the loss of more water than electrolytes in the urine will leave the ECF relatively hypertonic compared to ICF. Water moves from the cells into the ECF in attempt to equalize osmolality. The cells which most often manifest signs of intracellular water loss are neurons. The signs are those of dullness and dementia. Basal Electrolyte Requirements (Na+, Cl- and K+ ) The normal dog and cat loses electrolytes, including sodium, potassium, and chloride in the urine, stool, and minimally in sweat. The ongoing basal loss of each of these electrolytes is approximately 0.02 mEq of each electrolyte/kcal of energy expended by the animal per day. 3-9% body content of each of these electrolytes is lost daily . This energy expenditure may be in the form of calories from ingested food or may be energy expenditure from the animal’s own tissue stores. Electrolyte losses in urine and stool will be accelerated in the face of disease. Dogs and cats have very little loss of electrolytes in sweat because they sweat minimally. A minimal amount of fluid is lost through the foot pads. Respiratory (insensible) losses are close to pure water and contain very few electrolytes.
There are three major components to fluid therapy, including rehydration, which is the phase of fluid therapy in which the dehydrated animal’s fluid volume is returned to normal. Maintenance is the component of fluid therapy in which the basal losses of fluid from the urinary tract and insensible routes are replaced if the animal is unable to drink water to compensate for these basal fluid losses. The losses of fluid to urine contain electrolytes and are approximately isotonic losses, whereas losses through the respiratory tract, insensible losses, are poor in electrolytes and are close to free water or pure water. The third component of fluid therapy is to determine the amount of fluid lost from ongoing disease such as vomiting, diarrhea, or polyuria, pathologic gastrointestinal and urinary tract losses contain electrolytes and water, and are therefore considered isotonic losses of fluid. When designing a fluid therapy plan one needs to decide for each of the three constituents, which fluid to use, how much fluid to use, by which route should the fluid be administered, and how fast should the fluid be administered. Terminology When isotonic solutions are given intravascular, they have the same tonicity as extracellular water and will not cause any change in the red blood cells in circulation. Hypotonic and hypertonic solutions administered IV can cause hemolysis, although the red cells are surprisingly tolerant of a fairly wide range of tonicity of solutions and will tolerate the administration of hypo and hypertonic solutions. Be aware that at extremes of tonicity, both hyper and hypo, red cell damage can occur. For example, if the hypotonic solution is pure water, administering pure water IV will reduce the osmolality of extracellular fluid, causing a shift of fluid into intracellular water of the cells, including red blood cells, which will swell and subsequently rupture, resulting in the release of free hemoglobin, which is hemolysis. The IV administration of severely hypertonic solutions will result in an increase in osmolality of the extracellular fluid and plasma, which will result in a movement of water out of cells, including red blood cells in an effort to equalize osmolality across the two fluid compartments, which will result in a crenation or shrinking or red blood cells, which is also detrimental to the integrity of the red cell membrane, resulting in a leakage of hemoglobin into the plasma, (hemolysis). The most frequently used crystalloid solutions in small animal practice include lactated Ringer’s solution, Ringer’s solution and various concentrations of dextrose and saline. Additionally, there are commercial mixtures of these four fluid types, and home-made mixtures of crystalloid fluids can be created. Lactated Ringer’s solution is one of the most common crystalloid solutions used in a small animal practice. It has an osmolality comparable to extracellular fluid, so it is considered an isotonic solution. Most lactated Ringer’s solution that are commercially available are marketed for people, therefore the actual osmolality of lactated Ringer’s solution is slightly less than the osmolality of canine and feline extracellular fluid/plasma. The sodium, potassium, and chloride concentrations of lactated Ringer’s solution are comparable to extracellular fluid, although the sodium concentration is iso ionic to human extracellular fluid/plasma and is slightly lower than the sodium concentration of canine and feline extracellular fluid/plasma. Lactated Ringer’s solution contains lactate, which is converted by the liver to bicarbonate. Animals with severe liver pathology may fail to convert lactate to bicarbonate. There is some fairly recent data suggesting that patients with lymphoma have a degree of carbohydrate intolerance and an elevation of lactate. Many oncologists recommend against the use of lactated Ringer’s solution in patients with lymphoma because of the concern of adding to the lactate burden, resulting in acidosis. Lactated Ringer’s solution contains very few calories at 9 kcal/l. The average caloric requirement of an adult dog and cat is approximately 20-30 kcal/lb/day, therefore it is impossible to meet an animal’s caloric requirement by the administration of lactated Ringer’s solution. Lactated Ringer’s solution is predominantly a rehydration solution. There are potential consequences of using lactated Ringer’s solution on a long-term basis as a maintenance solution.
Ringer’s solution does not contain lactate. It contains a higher concentration of chloride compared to lactated Ringer’s solution. Ringer’s solution is considered an acidifying solution. This does not mean that it can cause metabolic acidosis, but it can assist a patient who has a metabolic alkalosis correct their own acid base abnormality. Most dogs and cats who are vomiting, vomit both gastric contents containing hydrochloric acid and duodenal contents containing bicarbonate. Therefore, their acid/base status can be normal if there are equivalent losses of acid and base. If the animal has pyloric destruction, such as a foreign body or a mass lesion, they may vomit predominantly gastric contents and not duodenal contents. This will leave the animal in a relative state of alkalosis because of the loss of acid. The yellow rectangle signifies fluid in the tubules of the kidney. A large amount of blood sodium is filtered and subsequently reabsorbed by the kidney tubules. Positively charged sodium ions must be reabsorbed in conjunction with a negatively charged ion and this is usually chloride. If the animal is chloride deficient because of the loss of chloride from vomiting, sodium will be reabsorbed with bicarbonate. If the animal is already alkalotic because of the gastric loss of acid, the reabsorption of bicarbonate from kidney tubular fluid back into the blood stream will worsen the degree of alkalosis. If the animal receives a fluid that is high in chloride to replace the chloride deficit, then sodium can again be reabsorbed in conjunction with chloride, allowing for renal elimination of bicarbonate, and allowing to correct its own alkalosis 5% dextrose is hypotonic relative to extracellular fluid. It contains only dextrose, no electrolytes. 5% dextrose is a constituent of most maintenance solutions. It can be used a sole fluid for rehydration for animals who cannot tolerate the administration of sodium, such as those with cardiac disease. More commonly, it is used as part of the rehydration solution in these sodium intolerant animals, used in combination with lactated Ringer’s or sodium chloride. 0.9% sodium chloride solution is isotonic. It contains only sodium and chloride. Sodium chloride is used when blood sodium is decreased; such as in Addison’s disease because it has a higher concentration of sodium than does lactated Ringer’s. It may be used in lieu of lactated Ringer’s for rehydration in patients with lymphoma. It is used as a rehydration solution when a patient has an increased potassium or calcium, and the administration of additional calcium or potassium are contraindicated. These diseases include Addison's disease in which the patient has an elevated potassium, may have a mild increase in calcium, and has a low blood sodium. Oliguric acute renal failure is acute renal failure characterized by minimal urine output. These patients have an elevated potassium. Urine retention due to either bladder rupture or the presence of an obstruction in the urinary tract, such as due to a stone may result in an elevation in potassium. There are numerous disease states resulting in hypercalcemia. The most common cause of hypercalcemia is lymphoma, mediated via the paraneoplastic condition of pseudohyperparathyroidism. Half strength saline is a hypotonic solution. It is infrequently used by itself in small animal practice. Its primary indication is in severely hyper-osmolar animals. A disease state which can cause hyper-osmolarity is diabetes mellitus when the blood glucose is extremely elevated at ~800-1000 mg/dl. Most diabetic patients, although they are hyper-osmolar, are not hyper-osmolar to the extent of causing neurologic signs, which is the indication for the use of half strength saline. Half strength saline does form a constituent of many maintenance solutions. The other indication for use of half strength saline is in treatment of severely hypernatremic animals...an uncommon condition. Hypertonic dextrose is any percentage of dextrose which has an osmolality in excess of extracellular fluid. Dextrose is available in several commercial strengths. Any concentration of dextrose can be created by adding additional dextrose to a commercial solution. The predominant use of hypertonic dextrose solutions is as a calorie source in total parenteral nutrition solutions. There is also an indication to promote urine flow in patients with oliguric acute renal failure. TPN solutions containing 50% dextrose are extremely hypertonic and can cause an irritation to the blood vessel if given in a small peripheral vein, such as cephalic or saphenous. Hypertonic solutions are typically given in larger veins such as the jugular vein in which blood flow is faster to dilute the hypertonicity of the infused solution and reduce irritation to the lining of the blood vessel. Maintenance solutions can be purchased commercially or can be made from mixtures of rehydration solutions. Maintenance solutions are hypotonic, referable to extracellular fluid. They have more potassium than extracellular fluid and less sodium than extracellular fluid. Examples include: Talbot's solution Maintenance solutions can also be created in large dogs who are receiving several liters of fluid on a daily basis by simply alternating on a liter by liter basis 5% dextrose with either lactated Ringer’s or sodium chloride, with the addition of potassium chloride to each liter of fluid. Example: why use maintenance solutions? Take for example a patient who presents with a history of vomiting and is dehydrated. Once the patient is rehydrated and still has no desire to drink, fluids must be administered to maintain a normal level of hydration. Say for example, antiemetic drugs are being administered so that vomiting is no longer occurring. If the animal is given lactated Ringer’s on an ongoing basis, hypernatremia and hypokalemia are potential consequences. If the dog weighs 50 pounds, its maintenance losses of fluid are approximately 20-30 ml/lb, for an approximate loss of 1 liter of fluids (approximately half of this loss is electrolyte containing isotonic loss through the urinary tract and approximately half is a hypotonic, essentially electrolyte free loss of fluids through the respiratory tract, insensible losses). This animal will be expending approximately 20-30 kcal/lb on a daily basis from its own energy stores if not eating. For each kcal of energy expended, the dog will lose 0.02 mEq of sodium and potassium through the urine and stool, for a total loss of each of these electrolytes of 20 mEq. Lactated Ringer’s contains 130 mEq/l of sodium and 4 mEq/l of potassium. If the dog is receiving one liter of LRS daily to meet maintenance fluid losses, he is receiving about 6 times as much sodium on a daily basis as he is losing, and about 1/5 the amount of potassium that it is losing. If the animal is relatively healthy, for a period of time, it can compensate and the kidneys can excrete the extra sodium load and conserve potassium to prevent the development of hypokalemia. Most animals treated with fluid therapy are not healthy or may not be able to adequately compensate, resulting in hypernatremia and hypokalemia. This is the basis on which the composition of maintenance solution is based, why they are lower in sodium, higher in potassium, lower in osmolality, than are rehydration solutions. If this same patient continued to vomit or have diarrhea, then it would have ongoing losses which contained electrolytes and water, and then sustained administration or repetitive administration of lactated Ringer’s solution or other rehydration solutions would be indicated. Hypertonic saline is used in states of hypovolemic shock. Historically, hypertonic saline was carried by soldiers on battlefields. A very small volume of hypertonic saline administered IV to a hypovolemic patient will result in the mobilization of fluid from the cells into the extracellular fluid compartments, causing an expansion of blood volume and maintenance of blood flow to vital organs. Hypertonic saline solutions must subsequently be followed by the administration of isotonic solutions to reestablish fluid balance in intracellular and extracellular fluid locations. Hypertonic saline infusions are also used in pre-hospital situations, such as by paramedics in ambulances. Hypertonic saline is also used in some emergency hospital environments because small volumes can be administered over a much shorter period of time than large volumes of fluid. Small volumes of hypertonic saline solution can rapidly restore blood pressure, cardiac output, and tissue perfusion, and then be followed by larger volumes of isotonic solutions. Colloidal solutions are solutions that contain large particles. These large particles are either proteins or complex sugars, such as dextrans or hetastarches. Colloidal solutions because they are physically large molecules, will stay in the bloodstream and hold water in the bloodstream by their oncotic effect. Colloidal solutions are indicated in patients who have a disease resulting in a loss of albumin. Take for example an animal with hypoalbuminemia due to bowel loss of protein who is presented with ascites, subcutaneous edema, and you intend to perform an exploratory surgery and a bowel biopsy. It is albumin within the bloodstream which maintains the oncotic effect to keep crystalloid solutions intravascular. If blood protein, specifically, albumin, is decreased, the administration of crystalloid solutions will result in the leakage of these crystalloid solutions out of the bloodstream, worsening the ascites and the subcutaneous edema. Solutions containing large molecules, (colloidal solutions) will stay intravascular and may mobilize the abnormal fluid out of the abdominal cavity or subcutaneous tissues and return it to the vascular spaces. Dextrans are an example of colloidal solutions. Dextrans are complex sugars. There are two sizes of dextrans that are commercially available, the 40,000 molecular weight unit dextrans, which are called low molecular weight dextrans, and 70,000 molecular weight dextrans, which are called high molecular weight dextrans. The dextrans are renally eliminated, and thus have a duration of effect lasting hours to days. In humans, 40% of low molecular weight dextrans are still present in circulation 12 hours after administration and 70% of high molecular weight dextrans are still in circulation 12 hours after administration. It is not know whether the duration of effect is the same in dogs and cats, but it is know that dextrans continue to exert an oncotic effect only for hours to days. Dextrans can interfere with the function of platelets and coagulation factors. They should not be administered to patients who have preexistent coagulaopathies. Routes of fluid administration: There are numerous routes by which fluids can be administered:
The oral route should be use whenever possible if the intent of fluid therapy is to provide calories. Fluids can be administered via the oral route either via a syringe or by a tube, which is either passed or implanted. Syringe administration of fluids is most easily accomplished by pulling the lip away from the teeth creating a pouch, slightly tilting back the animal’s head, and pouring the fluid from the syringe into the pouch, allowing it to flow between the teeth into the pharynx. Dogs and cats resent having the syringe placed between the teeth. Pharyngostomy tubes are tubes which are placed at the angle of the jaw, through the skin and subcutaneous tissues, enter the oropharynx, and then passed down the esophagus into the stomach. Pharyngostomy tubes are not well tolerated, as the tube may contact the arytenoid cartilages around the opening to the trachea, may cause swelling of the arytenoid cartilage and respiratory distress, especially in cats. The location of the tube in the back of the pharynx is poorly tolerated by dogs and cats and they will often use their tongue to grasp the pharyngostomy tube in the pharynx and flip the tube out of the esophagus and out the oral cavity. Gastrostomy tubes are tubes that are placed in the stomach, exiting through the abdominal wall. Gastrotomy tubes can be placed surgically, using an endoscope or by non-endoscopic blind techniques. The early use of gastrostomy feeding tubes in cats with hepatic lipidosis has substantially improved the prognosis for this disease in cats. Greater than 50% of cats with hepatic lipidosis died before the widespread acceptance of the use of gastrostomy tubes. The most important component of treating cats with hepatic lipidosis is to provide alimentation. Early implantation and use of a gastrostomy tube has reduced the mortality of cats with hepatic lipidosis to less than 20%.
Nasogastric tubes are small diameter tubes that are passed through the nasal cavity into the nasopharynx and into the esophagus and stomach. Nasogastric tubes are more commonly used in cats than dogs because of the short length of the nasal passages. Nasogastric tubes are not well tolerated. They can be used for the administration of fluid, electrolytes, and calories to a cat for several days to improve the stability of the patient before it is anesthetized for a more substantial tube placement. Given the small diameter of the nasal cavity of the cat, a small diameter tube is required, so the fluid must be quite liquefied before it can be placed through a nasogastric tube. The largest tube that can usually be placed in a nose of a cat are 8 French tubes. French size divided by 3 is equal to the outer diameter of the catheter in millimeters, so a 9 French tube has an outer diameter of 3 mm. The tube should be lubricated well with a lubricant. Topically anesthetic drops such as those used to desensitize the cornea of the eye can be instilled in the nose to allow passage of the nasogastric tube. There are two major channels weaving through the nasal turbinates. The dorsal meatus terminates at the cribriform plate. The ventral meatus follows along the dorsal surface of the hard palpate into the nasopharynx. When you are inserting the nasogastric tube, keep the tip of the tube directed ventrally and medially in an attempt to cause the tube to enter the ventral meatus. If the tube is introduced and meets resistance after passing a short distance, it has likely passed into the dorsal meatus and is abutting on the cribriform plate. Although unlikely to happen with a soft feeding tube, the cribriform plate can be damaged or even penetrated if the feeding tube is forced. Jejunostomy tubes are tubes, which are placed in the jejunum, usually surgically, although Jejunostomy tubes can be placed through an endoscopic procedure. If the intent of fluid therapy is to provide calories, the cliché’ "if the gastrointestinal tract works, use it" is appropriate. It is the most economical and physiologic method of providing calories. A procedure that is starting to be more commonly used as the placement of an esophagostomy tube, a tube that is placed directly through the wall of the cervical esophagus. Esophagostomy tubes are much better tolerated than pharyngostomy tubes and are easier to place than gastrostomy tubes. Percutaneous endoscopically placed gastric tubes (PEG) tubes are placed with endoscopic visualization. All that is left in the animal is a piece of tubing which passes from the inside of the stomach to the outside of the body wall. This technique will be discussed further in the gastrointestinal section of the small animal medicine course The oral route of delivery is the easiest and least expensive method of delivering calories, as long as the animal can tolerate oral administration. The absorption of fluids, electrolytes and foods from the stomach is slow. Obviously the oral route of delivery of fluids is contraindicated if the animal needs rapid fluid loading, such as in states of shock. Hypertonic solution, such as 50% dextrose should not be given orally, as the osmotic draw of these hypertonic solutions will bring water into the stomach, causing gastric distension. Gastric distension is one stimulus for vomiting. Whenever the oral route of administration is used, there is always the danger of aspiration of fluid and food into the airways. If you are passing a stomach tube its is important to have several parameters that you check to ensure that the stomach tube is actually being placed in the stomach and not in the lungs. If the animal has a short hair coat you may actually see the tubing ripple below the skin in the cervical region as it passes through the cervical esophagus. You may palpate the stomach tube in the cervical region as a distinct tubular structures adjacent to the trachea. When the stomach is placed to a level of the gastric lumen you may smell gastric contents or aspirate gastric contents. You can blow into the tube and put the stethoscope over the left cranial abdomen, listening for air reverberating in the stomach. You could put the open end of the stomach tube into a container of water. If the tube is in the lung you will see bubbles in the water that coincide with each breath. Maximum gastric capacity is determined by the size of the animal. Cats have a wide range of gastric capacities, with smaller cats having a greater gastric capacity per unit of body weight.) The volumes that are force fed are always much less than maximum capacity. Absolute maximum gastric capacity = 90 ml/kg - dog Cat:
When initiating forced feeding in an animal that has previously been anorexic, I begin with a very small volume (5 to 10 ml per feeding for cats and small dogs and 20 to 50 ml per feeding for larger dogs. Prior to the next infusion of food or fluid through the feeding tube, gastric contents should be aspirated to confirm gastric emptying is taking place, rather than placing additional food in the stomach that may not have emptied. If the stomach is emptying between feedings, then the volume per feeding can be increased. Intravenous fluids must be sterile which increases the cost of administration by the IV route. The advantages of intravenous fluids include an immediate effect, and exact delivery of dose of medication. There is no question as to the magnitude of absorption when the drug is given by the IV route. It is more time consuming to deliver IV fluids than fluids by other routes. The animal has to be monitored more closely for risk of fluid overload. Catheter infections can lead to systemic complication. There are many types of catheters that are commercially available. You are encouraged to practice with as many catheter types as possible during your training. Jugular catheters are through- the- needle catheters therefore, when the puncture is made and the catheter is threaded through the needle, the hole in the vein wall is slightly larger than the catheter, and there will be some hemorrhaging around the catheter at the puncture site.
Short catheters sold for placement in peripheral veins such as (a) and (b), have the catheter on the outside of the needle so the hole in the vein wall made by the needle is smaller than the catheter. The catheter fills the hole in the vein wall and there is minimal leakage of blood around the catheter. c) is a butterfly catheter (no, butterflies are not free, they cost ~$2.00). The "catheter" portion that goes in the vein is a rigid needle. Butterfly catheters are used for short term, small volume, infusions such as the administration of CaparsolateTM used to treat heartworm disease, or the administration of a bolus of chemotherapeutic agent, such as vincristine. Because of its rigidity, it is not useful for long term fluid administration as the needle may lacerate the vein. (d) is a catheter that has the conformation of a butterfly catheter but the catheter is flexible teflon rather than metal. The catheter has a wire stylet that is removed after placement. This style of catheter is very useful for placement in peripheral veins in small dogs and cats and breeds of dogs with short, crooked legs. It can also be used to drain fluid or air from body cavities. The flexibility of the catheter after the stylet is removed, reduces the chance of organ laceration. See the Diagnostic Techniques site for additional information on placing jugular or cephalic catheters.
Simple venipuncture models are available in the psychomotor skills lab on the second floor of McCoy Hall. This room can be accessed any time of day by obtaining a numerical code from Teri Olsen.. Catheters should be placed after aseptic surgical preparation of the skin puncture site. The site at which the skin is punctured by the catheter is usually dressed with an antiseptic or antibiotic ointment. Although it is recommended to change this dressing daily, in our ICU, we do not usually change the bandage on catheters unless they get wet. Peripheral IV catheters should be removed every 48-72 hours. A new catheter should be in place and functional before the old catheter is removed, in case the animal has a cardiovascular crisis during catheter placement. Catheters which are left in place long term can cause permanent damage to the vein, resulting in fibrosis and occlusion. If the patient is receiving very slow rates of fluid administration every few hours, a few milliliters of saline, or heparinized saline, should be bolused rapidly through the catheter to reduce the chance of the formation of clots in the catheter. If the animal objects to infusion through the catheter, or if there is redness, pain, or swelling at the catheter site, the IV catheter should be removed. Drugs may be acidic, alkaline, or mechanically irritating, and should be diluted with the appropriate diluent prior to administration through an IV catheter. The package insert will provide information on what diluent is appropriate for each drug. Local infections at the catheter site are called phlebitis. This can be infectious or inflammatory. If infectious, bacterial organisms can enter systemic circulation resulting in septicemia. Circulating bacteria can establish other foci of infection in joints or on heart valves. Blood clots forming at the catheter site may enter systemic circulation resulting in thromboembolic disease. Most of the thrombi are filtered by the pulmonary system without consequence but severe thromboembolic disease can be a sequel of IV catheterization Hypodermoclysis. The administration of fluids subcutaneously is a rapid, easy method of providing fluids. Fluids for subcutaneous administration should be sterile and relatively close in tonicity to extracellular fluid. Use 18-22 gauge needles. Place about 10-20 ml/kg per site. Theoretically, avoid the administration of dextrose-containing solutions. When one administers subcutaneous fluids, the fur is usually not shaved; therefore, small numbers of bacteria are introduced with the subcutaneous fluid. 5% dextrose is a good bacterial growth medium and may result in subcutaneous abscessation. The absorption of fluids from the subcutaneous space is relatively slow, occurring over 6-8 hours. If the animal is severely dehydrated, blood is shunted away from the subcutaneous tissues to more vital structures, and absorption of fluids will be delayed even longer. Subcutaneous administration of fluids is indicated for animals with mild dehydration
Sterile , isotonic fluids including blood can be administered in the peritoneal cavity. The absorption from the peritoneal cavity is fairly rapid. The intraperitoneal route of administration can be used in neonatal animals which are too small to perform venipuncture or to place an IV catheter. There is a risk of damage to organs. The introduction of bacteria into the peritoneal cavity can result in serious peritonitis which may be difficult to treat. My preferred route of administration to neonatal animals is via the bone marrow cavity. This is called intramedullary or intraosseous administration. Bones which are most readily accessible for intramedullary administration are the proximal shaft of the femur or humerus. Neonatal animals have soft enough bones that you can place a standard hypodermic needle in either of these locations. One can use the intramedullary route in mature animals, as well, using a standard diagnostic bone marrow needle. There is a special marrow needle, called a Cook Needle, that is marketed for implantation into the bone marrow for sustained administration of fluids, but the bone marrow needles used for diagnostic collection of bone marrow work equally well. The absorption of drugs and fluids administered in the medullary cavity is extremely rapid because the marrow cavity is comprised of a fibrous stroma supporting bone marrow elements, interspersed with an extensive collection of venous sinuses. There is a small risk of creating bone infection by the administration of fluids by this route. In neonatal animals, one has to be careful not to introduce the needle into the proximal portion of the bone, and exit the distal aspect of the bone into the joint cavity.
Cranial is to your right. In this radiograph, contrast media was injected through a bone marrow needle in the proximal humerus and a radiograph immediately obtained. The large blood vessels filled with contrast media, are leaving the marrow cavity almost immediately upon injection.
When a bone marrow needle is placed in the femur, it is placed in the inter-trochanteric fossa which is located between the greater and third trochanter of the femur. This site is much more accessible in the cat than it is in the dog. Bone marrow needles placed for interosseous administration of fluids and drugs can be left in place for the same duration as IV catheters. They are excellent methods of administering drugs during cardiopulmonary resuscitation, as it may be very difficult to raise a vein for catheter placement. Although drugs are not typically administered by the rectal route, it is the preferred route for administering drugs to patients with hepatic encephalopathy that are too demented or depressed to tolerate drugs by the oral route. Administration of lactulose and neomycin is indicated reduce the generation or uptake of ammonia, which is contributing to signs of hepatic encephalopathy. Enemas administered rectally may be retained by some animals, especially cats. Fleet enemas are comprised of phosphates. If the enema solution is not passed, phosphate will be systemically absorbed, which will complex calcium resulting in hypocalcemic tetany. Valium can be administered to seizing animals when it is difficult to access a vein. The rate of fluid administration is determined by the route, by the fluid type and the purpose of treatment. If the fluid contains potassium, the fluid should not be administered any more rapidly than a rate of 0.5 mEq/kg of potassium per hour. If the animal is in shock, fluid should be administered as fast as possible, which is usually equivalent to a blood volume given over approximately one hour. Routine fluid administration in a non-shocky animal is extremely variable. General guidelines are approximately 3-5 mls per pound per hour.
IV infusion pumps allow for a more constant rate of infusion of fluids. If the animal bends its limb, it may occlude an IV catheter, precluding gravity flow of fluids. A fluid pump may be able to overcome this partial occlusion of a catheter, but even if the fluid pump cannot overcome catheter occlusion, most are equipped with audible alarms, so you will be informed that the catheter is not functioning. A strip of tape can be placed on a bottle or bag of IV fluids with times designated. Someone in the practice can periodically check on animals on IV fluids and confirm that the fluid volume in the bag or bottle is declining as anticipated. Buretrols are small plastic cylinders that are filled from a stock bottle, or bag, of fluids. It is much easier to see the residual volume of fluid in a rigid sided container than a flexible container. Monitoring Fluid
Therapy Central venous pressure measures the ability of the right heart to accommodate the fluid load presented to it. Normal animals have central venous pressures of less than 5 cm of water. Assuming there are no clots in the IV catheter, central venous pressures repeatedly above 10 cm of water are indicative of impending onset of edema and fluid overload. Measurement of central venous pressure requires a central catheter (jugular catheter) be in place. See the Diagnostic Technique Web site for details on CVP. Clinical signs of increased and decreased sodium, potassium or calcium are usually reflected in excitable tissues, including cardiac muscle which may manifest as cardiac arrhythmia; a smooth muscle, sometimes manifested as ileus, resulting in diarrhea or vomiting; and skeletal muscle, resulting in skeletal muscle weakness. Electrolyte aberrations may result in depression or CNS hyperexcitibility. Hypokalemia. Low potassium is a very common sequel of many diseases and treatments. Animals who are eating normally rarely become hypokalemic, but animals who are anorexic become rapidly hypokalemic if they are vomiting, have diarrhea, or increased urine outputs. Signs of hypokalemia are skeletal muscle weakness and smooth muscle weakness resulting in ileus. Humans develop a low urine specific gravity as a consequence of hypokalemia. It is unknown whether this occurs in dogs and cats. Severely low potassium can result in cardiac arrhythmia's. Although dogs become weak with hypokalemia, cats may develop a skeletal muscle myopathy, which is painful. They will have an elevated creatinine kinase reflecting muscle damage. Treatment of hypokalemia can take place by oral, subcutaneous or IV routes. Several commercial potassium supplements are available for veterinary patients as pills, powders, or pastes, usually liver flavored.
Hyperkalemia. Elevated potassium occurs less commonly than low potassium. Elevated potassium occurs if the animal develops low output renal failure, Addison's disease, or has an obstruction to urine output. Acidosis causes a mild hyperkalemia not usually severe enough to cause clinical signs of hyperkalemia, but the hyperkalemia associated with acidosis is additive with other causes of hyperkalemia. Tissue trauma, especially that induced by heat stroke, will cause massive disruption of cells located in all tissues with the release of potassium into extracellular fluid and plasma. Potassium sparing diuretics can cause elevated blood potassium, but are not frequently used. Potassium penicillin in therapeutic doses will not cause hyperkalemia. Pseudohyperkalemia is an in vitro manifestation of marked increases in white cells greater than 100,000/ul, or platelets greater than 1 million/ul. Elevated potassium will cause skeletal muscle weakness. Hyperkalemia is more likely to cause cardiac arrhythmia than is low blood potassium. ECG changes correlate with the magnitude of elevation of blood potassium. An early ECG change is elevation in the amplitude of the T-wave followed by a prolongation of the PR interval, with a gradual loss of the P-wave. Hyperkalemia is usually fatal when blood potassium approaches 10-11 mEq/L. Treatment of hyperkalemia is to dilute out potassium with non-potassium containing solutions. Sodium chloride is the rehydration solution of choice. There are three treatments which can be administered to either drive potassium intracellularly, or antagonize the effect of potassium on the myocardium:
The myocardial cells at rest have a resting membrane potential of -90 millivolts before they undergo depolarization and generate an action potential, which is demonstrated by the blue line in this graph. Animals with an elevated blood potassium have a less negative resting membrane potential, which results in an action potential that is weak and spreads weakly and inefficiently from one myocardial cell to the next. The administration of calcium will antagonize the effects of potassium on myocardial cells, reestablishing the resting membrane potential of the myocardial cells back to their normal -90 millivolts. Calcium can be administered as either calcium gluconate or calcium chloride. Calcium chloride is about 10 times as potent as calcium gluconate, and the dose must be appropriate for the salt of calcium which is used. Calcium itself can cause cardiac arrhythmia's and should always be administered with electrocardiographic monitoring. As glucose enters cells under the influence of insulin, potassium tags along. Animals with high blood potassium will shift potassium intracellular if they are given either glucose or glucose plus regular (crystalline) insulin. The onset of action of this treatment is within minutes, duration of effect is 30-60 minutes. Bicarbonate is administered IV. It will tie up hydrogen ion in extracellular fluid, creating a gradient for hydrogen ion to move from the intracellular fluid to the extracellular fluid in exchange for potassium, which moves into the cell. This reestablishes a balance between the amount of potassium in the cell and in the extracellular fluid. The onset of effect is within minutes. The duration of activity is about 30-60 minutes. Hypocalcemia. Low blood calcium may occur in female dogs and cats at the end of gestation, or more commonly during early lactation. This is most common in small breed dogs and is called eclampsia. Ethylene glycol is metabolized to oxalate, which ties up calcium. Free roaming animals, or any animal who has potential exposure to antifreeze, presenting with extremely low blood calcium should be strongly considered to have ethylene glycol poisoning. Hypoparathyroidism is reduced production of parathyroid hormone by the parathyroid glands and occurs infrequently. Chronic renal failure is rarely associated with low blood calcium. Phosphate enemas, if they are not passed, will be systemically absorbed and suppress blood calcium. Hyperthyroid cats, undergoing bilateral thyroid excision, can have damage or removal of the parathyroid glands, resulting in hypocalcemia. The signs of low blood calcium in dogs and cats are weakness characterized by muscle rigidity or tetany. The treatment of low blood calcium is the administration of calcium salts and/or vitamin D. Hypercalcemia. High blood calcium occurs more commonly than low blood calcium. The most common cause is the production of a biochemical substance by tumors, resulting in the retention of calcium. This is called pseudohyperparathyroidism, most commonly occurring in patients with lymphoma or tumors of the apocrine anal sac. Pseudohyperparathyroidism has been reported associated with other tumor types. Primary hyperparathyroidism occurs uncommonly. Pseudo and primary hyperparathyroidism result in marked increases in calcium, usually in excess of 17 mg/dl. Tertiary hyperparathyroidism is a relatively uncommon event occurring in chronic renal failure patients, which may result in a moderate increase in calcium. Bone remodeling, due to infection, growth, or neoplasia, can result in a mild increase in blood calcium, usually no more than 13 mg/dl. Vitamin D intoxication can be associated with the iatrogenic supplementation of vitamin D or calcium, or may be associated with the ingestion of a rodenticide called Rat-Be-Gone. Signs associated with hypercalcemia are similar to other electrolyte abnormalities. The animals are weak with some degree of muscle twitching. Hypercalcemia can cause cardiac arrhythmia's, infrequently. Hypercalcemia interferes with the ability of antidiuretic hormone to interact with ADH receptors on renal tumor epithelial cells, leading to polyuria. (nephrogenic diabetes insipidus). Hypercalcemia less commonly causes renal failure resulting in azotemia. The treatment for high blood calcium is to give a calcium free, high sodium, solution. The rehydration solution of choice is 0.9% sodium chloride. Steroids will decrease blood calcium in patients with lymphoma, but will also impair the ability to make a diagnosis of lymphoma. Steroids should not be used in hypercalcemic patients until lymphosarcoma is either diagnosed or ruled out. Lasix® will promote the loss of sodium. The loss of sodium is accompanied by the renal loss of calcium. Diphosphanates and calcitonin are effective measures to decrease blood calcium regardless of the cause of hypercalcemia. Hyponatremia. Low blood sodium is most commonly due to Addison's disease or iatrogenic from the administration of electrolyte free solution, such as 5% dextrose. Other causes of hyponatremia are relatively uncommon. The treatment of hyponatremia is the administration of either 0.9% sodium chloride, or 3% sodium chloride, depending upon the severity of hyponatremia. Normal serum sodium balance levels should be achieved gradually over 24-48 hours. Hypernatremia is uncommon in dogs and cats, most often it is iatrogenic from the repetitive administration of high sodium containing solutions. Animals who have severe Hypernatremia in excess of 170 meq/L show nervous system signs of depression progressing to coma. Treatment is the administration of either 5% dextrose or 0.45% sodium chloride. Blood sodium levels should decline slowly at a rate no faster than 1 mEq/L/hour. Rapid reduction in serum sodium levels will result in worsening of neurologic signs. Acid base status is best evaluated by blood gas analysis, although total CO2 is reflective of total bicarbonate, and will provide information on the presence of metabolic alkalosis or acidosis. Urine pH can be reflective of general acid base status, but urine pH is also influenced by diet and by the presence of bacterial infections. When bicarbonate administration is being contemplated, use the lower formula, where body weight is multiplied by 0.3 times the bicarbonate deficit, to determine the number of mEq of bicarbonate to administer. The value .6 is based on the bicarbonate being distributed to total body water, which is 60% of total body weight. It takes approximately 24 hours for IV administered bicarbonate to equilibrate across total body water. CO2 + H2O --- H2CO3
--- H+ + HCO3 BW (kg) (0.6) x HCO3 deficit = mEq HCO3 My preference is to be conservative and use the second formula, and provide half of the calculated dose of bicarbonate as an IV bolus, and the other half over the next 24 hours. One can always deliver additional bicarbonate, but cannot take it back. There are numerous bicarbonate precursors that are converted by the liver to bicarbonate, which can be administered in patients who may be sodium intolerant, such as those patients with cardiac disease.
Paradoxical CSF acidosis is a condition caused by the administration of a large amount of bicarbonate to an acidotic patient, which will result in neurologic dysfunction. When one gives bicarbonate, the equation is shifted towards the left of the screen, towards the generation of CO2. CO2 is very diffusible, and will diffuse across the blood brain barrier and then shift the equation in the cerebral spinal fluid towards the right with a generation of hydrogen ions. Even though additional bicarbonate is being generated in the CNS, the neurons perceive the increase in hydrogen ions and malfunction. The bicarbonate that was administered IV cannot cross the blood brain barrier. Renal Failure. Patients with renal failure typically have metabolic acidosis. Patients with oliguric acute renal failure will often have an elevation in potassium, so they should be rehydrated with 0.9% sodium chloride until their urine output is known, or until potassium values are measured. Conversely, patients with chronic renal failure have high urine outputs. They are polyuric. Potassium is either normal or decreased. Rehydration solution of choice is lactated ringers with additional potassium. Bicarbonate may be indicated in renal failure patients if they are severely acidotic. Patients with diabetes mellitus often have metabolic acidosis, low potassium from urinary losses of potassium, and have elevated blood glucose. The fluid of choice is lactated Ringer's with additional potassium. As the patients are treated with insulin, the initial low potassium values will decrease even further. Some diabetic patients will appear to have a normal blood potassium because they are concurrently acidotic and the acidosis has shifted potassium from an intracellular to an extracellular location. If a diabetic patient is showing neurologic signs and has a marked increase in blood glucose in excess of approximately 800 mg/dl, they may be a hyper-osmolar diabetic. The fluid of choice for rehydration is 0.45% sodium chloride with additional potassium. Patients with cardiac disease may be sodium intolerant and may have development or worsening of pulmonary edema and/or ascites when sodium containing solutions are administered. Patients with cardiac disease still may be dehydrated and require fluid therapy. The rehydration solution of choice is either D5W or a mixture of D5W with half strength lactated Ringer's or half strength saline. Cardiac patients who are acidotic should be cautiously monitored if sodium bicarbonate is administered because of the additional sodium load. Patients with gastrointestinal disease have variable acid base and electrolyte status. If they have a pyloric obstruction, they will vomit only gastric contents and may develop metabolic alkalosis. The rehydration of choice is Ringer's solution with or without potassium, depending upon the duration of vomiting and the magnitude of potassium loss. Most animals who vomit will vomit duodenal contents containing bicarbonate and stomach contents containing hydrochloric acid. Therefore their acid base is very often normal. Fluid of choice for rehydration is lactated Ringer's with or without bicarbonate, with or without potassium. Patients with diarrhea have a loss of electrolytes in water. The rehydration solution of choice is usually lactated Ringer's with or without bicarbonate and/or potassium. Notes Menu | Glossary | Search | VM 551 Home | Courses Home | External WebThis page was last edited on
December 31, 2003 by CRD |
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you still must treat underlying cause of
disease.
