Shock Pathophysiology
The blood volume in the rabbit is 50-60 ml/kg in contrast to 90 ml/kg in the dog. When intravascular volume deficits result in poor perfusion, it has been recommended in the past that crystalloids be administered fast in volumes equivalent to the animal's blood volume. However, resuscitation with crystalloids alone can result in significant pulmonary and pleural fluid accumulation. The resultant hypoxaemia contributes to the shock pathophysiology.
Rabbits are difficult to resuscitate from hypotensive episodes. In the rabbit, when baroreceptors have detected inadequate arterial stretch, vagal fibres are stimulated simultaneously with sympathetic fibres. As a result, the heart rate may be normal or slow, instead of the typical tachycardia demonstrated by the dog. In the author's experience, normal rabbits have heart rates between 180 and 240 bpm (beats per minute), systolic blood pressure of 90-120 mmHg and temperatures of 37.7-38.8°C). Most rabbits presenting for hypovolaemic shock demonstrate heart rates <200 bpm, hypotension (systolic blood pressure <90 mmHg), and hypothermia (temperature <36.6°C).
Because cardiac output is a function of contractility and rate, the compensatory response to shock normally seen in dogs and birds is most likely blunted in rabbits. The hyperdynamic signs of shock seen in the dog and birds are not typically seen in the rabbit. Shock in the rabbit is most commonly decompensatory, manifested by normal or slow heart rate, severe hypothermia (<36.6°C), weak or non-palpable pulses and profound mental depression. The mucous membranes are grey or white and capillary refill is not evident. The bradycardia and low cardiac output contribute to hypothermia, and hypothermia accentuates the bradycardia.
The hypothermia most likely plays a significant role in the poor compensatory response and adds to the difficulty in providing adequate fluid resuscitation without causing pulmonary oedema. The theory is that as the rectal temperature falls, the adrenergic receptors become refractory to catecholamines. This leads to the normal or slow heart rate and most likely impaired compensatory vasoconstriction, in spite of the presence of norepinephrine and epinephrine. Part of the resuscitation plan in the rabbit must include rewarming. Once the rectal temperature approaches 37.8°C, it appears that the adrenergic receptors begin to respond to catecholamines. Temperatures during this rewarming phase must be checked frequently in rabbits to prevent hyperthermia.
Types of Fluids
Individual characteristics of fluids influence the dose, type and volume of fluid administered. Crystalloid solutions are commonly used together with colloids in the resuscitation phase. The four basic groups (i.e., crystalloids, synthetic colloids, haemoglobin-based oxygen carriers and blood products) of fluids will be discussed. Blood transfusions will be discussed separately after fluid therapy.
Crystalloids
Crystalloid solutions can be used together with colloids during the resuscitation phase. Crystalloids are the mainstay of the rehydration and maintenance phases of fluid therapy. Crystalloids (also called replacement fluids) are fluids containing sodium chloride and other solutes that are capable of distributing to all body fluid compartments. The most commonly used replacement fluids are 0.9% saline, lactated Ringer's solution and Normosol-R (CEVA Laboratories, Overland Park, KS) or Plasmalyte-A (Baxter, Deerfield, IL) although the latter two are not commonly used or widely available in the UK.
Hypertonic saline is a hyperosmolar crystalloid fluid used for resuscitation of hypovolaemia. It is usually given as a 7.5% solution (2600 mOsm/l). The hyperosmolarity leads to rapid intravascular volume expansion by drawing fluids from the interstitial and intracellular space into the intravascular space. It is administered in small volumes, at 3 ml/ kg, over 5-10 minutes. Its major benefit is that it can produce an equivalent intravascular volume expansion to colloids but at one quarter of the volume. Due to the osmotic diuresis and rapid redistribution of the sodium cations that ensue following administration of hypertonic saline, the intravascular volume is transient (30 minutes) and additional fluid therapy with a colloid must be used with hypertonic saline (Hetastarch at 3 ml/kg). Potential side effects include hypernatraemia, hyperchloraemia, hypokalaemia and dehydration. Hypertonic saline should be avoided in dehydrated patients since the extravascular fluid compartment is volume depleted before therapy.
Colloids
Colloids are fluids containing large molecular weight substances that generally are not able to pass through capillary membranes. Colloids can be considered intravascular volume expanders. Two types of colloids will be discussed. Natural colloids are blood products as whole blood, plasma or albumin, and synthetic colloids including Hetastarch and Oxyglobin®.
Treatment of Hypovolaemic Shock
Resuscitation from hypovolaemic shock can be safely accomplished with a combination of crystalloids and colloids and rewarming procedures. The animal must be handled as little as possible. Placement of an intravenous or intraosseous catheter will facilitate fluid administration. Initial blood work is done to determine the packed cell volume (PCV), total protein (TP), glucose and azostick. In the hypovolaemic rabbit a rapid infusion of isotonic crystalloids is administered at 10-15 ml/kg. Hetastarch is administered at 5 ml/kg over 5-10 minutes. The blood pressure is checked and once it is above 40 mmHg systolic, then only maintenance crystalloids are given while the patient is aggressively warmed. The warming procedures should be done within the next 30-60 minutes with warm water bottles and warming the intravenous fluids. Active rewarming is required for patients with moderate to severe hypothermia and core temperatures below 35°C. Active external rewarming can be accomplished by hot water bottles, heated blankets, a warmed incubator or a forced warm air blanket for small animals. Forced air enclosed around the patient has proved effective in core-rewarming in humans. Temperatures increased from 33.3°C F to 37.2°C in 1 hour at the Animal Emergency Center using the forced air system (Thermacare®), which is a system of disposable plastic and paper covers and a heat source that directs warm air across the skin. This system simultaneously provides convective heat transfer and shielding against radiant heat loss.
Fluid Therapy for Dehydration Deficits in Rabbits
Once immediate life-threatening fluid deficits are replaced, provide additional fluid based on estimated percentage of dehydration and maintenance needs. The percentage of dehydration can be subjectively estimated based on the presence and degree of loss of body weight, mucous membrane dryness, decreased skin turgor, sunken eyes and altered mentation. These parameters are largely subjective because they can also be affected by decreased body fat and increased age. 4-6% dehydration is estimated based on increased skin tenting, dry oral mucous membranes and normal pulses; 10% dehydration is shown by severe skin tenting, very dry mucous membranes and dry eyes; >10% dehydration is also accompanied by signs of hypovolaemic shock.
To determine the volume of fluid required for rehydration, use the formula:
Volume (l) = hydration deficit x body weight (kg) x 1000
For example, a 2 kg rabbit that is 10% dehydrated will require 2 x 10/100 l, or 200 ml of fluids for rehydration. Dehydration deficits are added to daily maintenance fluid requirements and estimate ongoing losses. Maintenance requirements for the small mammal and bird are higher than those required for dogs and cats and are estimated at 3-4 ml/kg. The small mammal and bird require larger maintenance volumes due to their high metabolic rate. 80% of the calculated fluid deficit can be replaced in the first 24 hours. Usually acute losses are given within 6-8 hours. After successfully treating hypovolaemic shock and replacing fluid deficits estimated on the percentage of dehydration, you can administer maintenance fluids alone until the rabbit can maintain hydration on its own, provided no ongoing losses are present. The adage 'as soon as the gut works, use it' is recommended for early enteral feeding which is discussed below.
Gastric Stasis and Nutrition
Gastrointestinal stasis is a common cause of anorexia in the rabbit but anorexia or decreased fibre intake will also cause gastrointestinal stasis. The underlying cause of gastrointestinal stasis can be multiple and sometimes vague. Rabbits with gastrointestinal stasis often have a history of inappropriate diet, or stress in the household. The primary clinical signs are decreased appetite, abdominal discomfort and reduction in stool pellet size and amount to absence of stool production. Low-fibre diets lead to decreased intestinal motility, abnormal caecal fermentation, changes in the pH and microbial dysbiosis. A firm, doughy mass palpable in the cranial abdomen (the stomach with food, hair and decreased amount of fluid) is consistent with gastrointestinal stasis. A complete blood count (CBC), chemistry profile and urinalysis are performed to rule out an underlying aetiology (i.e., renal disease).
When using Oxbow Critical Care®, divide the illness energy requirement by 2.69 since this is the kcal/g of dry powder. When using RockSolid Herpetoculture enteral product, divide the requirement by 4. The total daily grams of dry powder are divided into several small meals spaced evenly over 24 hours. Weighing and feeding based on dry product is more accurate because it is independent of the amount of water being added, which can vary depending on the accuracy of the person mixing the food. More or less water can be added to facilitate passage through a feeding tube.
The most suitable enteral diets for syringe feeding rabbits have a high percentage of non-digestible fibre, low fat and relatively low carbohydrates. Herbivore enteral diets are commercially available from Oxbow Pet Products (http://www.oxbowhay.com). The Oxbow products have been specifically formulated for nutritional support of herbivorous small mammals. The kcal is given on a dry matter basis with 2.69 kcal/g of dry weight of the powder. When mixed as directed, Critical Care Enteral powder:water 1:1.5 v/v provides approximately 1.9 kcal/ml and the slurry can be used for syringe feeding in stable patients in hospital or for home feeding by the owner. The syringe is placed into the diastema; the large space between the incisors and premolars. Syringe feeding needs to be done slowly to avoid aspiration. Unfortunately, this formulation will clog a 5 or 8 French nasogastric tube even when liquidised and is not suitable for nasogastric tube feeding.
When a rabbit has had anorexia for more than 24 hours and is weak and dehydrated, a nasogastric tube is part of the treatment plan to deliver calories and rehydrate the stomach contents. The rabbit is rehydrated as discussed above. A 5 or 8 French soft flexible paediatric feeding tube (Argyl feeding tube) is used (these are softer than a red rubber tube). The length necessary to reach the stomach is determined by measuring from the nose to the last rib. A stylet should not be used, since the oesophagus of the rabbit can be perforated with any additional force. A local anaesthetic (2% lidocaine gel) is placed into the rabbit's nostril. The rabbit must be properly restrained while protecting its back, and the head is ventrally flexed but with the neck straight (to avoid compression of the trachea) by an assistant. The tube is passed ventrally and medially into the ventral nasal meatus. The end of the tube is advanced until it reaches the stomach. Verification of placement is determined with a radiograph and/or aspiration of gastric contents. The Advanced Nutrition Support Enterals for herbivores from RockSolid Herpetoculture (http://www.rocksolidherpetoculture.com/ ) is an enteral diet for herbivorous reptiles. The powder diet has 8% fibre concentration and although this is lower than rabbits require, it is suitable for temporary nasogastric tube nutrition for 2-3 days (personal communication Susan Donaghue). The nasogastric formulation using the RockSolid enteral herbivore diet is prepared by mixing 13 ml/kg of powder with 20 ml/kg of water to provide approximately 1.2 kcal/ml when mixed. The slurry is delivered by syringe into the nasogastric tube every 6 hours, and it is followed by 5-6 ml of water after each feeding to keep the tube patent and provide additional fluid. Most rabbits with uncomplicated gastric stasis will start to eat and produce slightly soft stools after 12-36 hours. The tube can remain in place until the rabbit eats on its own and starts to produce stool. Complications that have been observed are nasal discharge from tube irritation. The tube should be removed and antibiotics included in the treatment protocol.