Chapter 20 Fluid Therapy in Reptiles
Fluid imbalances are frequently seen in captive reptiles and are most often characterized by low circulatory fluid volumes. In many cases, suboptimal environmental conditions, such as low temperature and inadequate humidity, are major contributing factors to the development of anorexia, compromised immune status, and increased susceptibility to infectious agents. Most reptilian patients are presented in a state of chronic dehydration and volume depletion, often accompanied by severe electrolyte imbalances. In reptiles, trauma is a common cause of acute blood loss, whereas chronic blood loss is usually caused by conditions leading to decreased production or increased loss of red blood cells.
It is essential to identify the cause of dehydration and initiate corrective measures. Before treatment of other organ system abnormalities, a normovolemic state must be established. This state, along with optimal environmental conditions, will ensure optimal response to treatment, such as antimicrobial therapy and nutritional support. The general principles of fluid and transfusion therapy established in domestic animals also apply to reptiles.
Knowledge of species differences as well as the pathophysiology of reptilian diseases is essential for successful treatment of the reptilian patient presented with fluid imbalances or blood loss. Studies have shown that in reptiles the percentage of total body water (up to 75%) per unit of body weight is higher than in mammalian species. Compared with mammals, a higher portion of fluid is located within the intracellular space, although the interstitial fluid volume and plasma volume are lower in reptiles.8 Consequently, hypotonic fluids should be administered to reduce extracellular fluid osmolality and promote movement of water to the intracellular space. Hypertonic fluid solutions should be administered cautiously because they will cause fluid to move from the intracellular to the extracellular space.
The goal of effective fluid therapy is to restore normovolemia and maintain organ function, such as adequate perfusion of the kidneys. Fluid therapy should be initiated only when there is evidence of volume depletion. This mandates determination of the degree of dehydration and the electrolyte status of the patient. A thorough visual and physical examination will provide an estimate of the degree of dehydration. This information, combined with the history and the nature of the disease, may indicate the type of fluid and electrolyte imbalance.
Clinical signs of dehydration in reptiles include dry mucous membranes, eyes sunken into the orbit, decreased skin turgor, and a decrease in heart rate. Before initiating fluid therapy, it is mandatory to determine the degree of dehydration, and a venous blood sample should be submitted for hematologic and plasma biochemical determinations, including an electrolyte profile. These baseline values should be used to assess the degree of dehydration and monitor the success of fluid therapy. Although species differences are apparent, a packed cell volume (PCV) greater than 50%, increased plasma protein values (>6-8 g/dL), elevated levels of uric acid, and high creatinine values (>1 mg/dL) should be considered signs of dehydration.
Evaluation of a venous blood sample will provide the most accurate assessment of the health status of the reptile patient, including hydration status and organ function. As a general rule, a blood sample representing 1% of the total body weight of the reptile may be collected safely. Depending on the species, the total blood volume ranges between 5% and 8% of the reptile’s body weight.
Collection of a venous blood sample for a complete blood count (CBC) and biochemical evaluations may be challenging in some reptiles because of their small size or the risk involved in restraining potentially dangerous species, such as venomous snakes or large crocodilians. In very small patients, it may only be possible to collect a single or a few drops of blood; however, this is sufficient to prepare a blood smear and fill a hematocrit tube for determination of the PCV and total protein.
In reptiles the anticoagulant of choice is lithium heparin because ethylenediaminetetraacetic acid (EDTA) has been shown to result in hemolysis of chelonian red blood cells. It is important to establish a protocol for handling and submission of reptilian blood samples for biochemical determinations. Blood should always be collected and handled using the same technique, including sampling site, collection tubes (plastic containers are preferred), and shipment to a laboratory.3 It is best to establish a protocol with the same laboratory, which will also assist in establishing a database for reference values.
In chelonians the preferred site for collection of a venous blood sample is the jugular vein. Additional sampling sites include the brachial plexus, the dorsal cervical sinuses (especially in aquatic turtles), the dorsal and ventral tail veins, and subcarapacial veins. Collection of a blood sample from the brachial plexus carries the risk of lymph contamination, which may alter hematologic and plasma biochemical parameters.
In snakes a blood sample may be obtained from the ventral tail vein or by cardiocentesis. With the latter technique, the snake is placed in dorsal recumbency, and the heart is located either by palpation or by visualization of the moving scales from the beating ventricle. The heart is then immobilized between two fingers, and an appropriately sized needle is inserted in the midline between two scales at a 45-degree angle, aiming cranially at the apex of the heart.
In lizards the ventral tail vein is the collection site of choice. The ventral abdominal vein is an alternate site for collection of a venous blood sample in most lizard species; however, the patient needs to be effectively immobilized to prevent excessive struggling. Crocodilians are typically bled from the supravertebral vessel located caudal to the occiput. The ventral tail vein may also be used for collection of a blood sample.
In reptile patients, similar to domestic animals and avian species, the most effective, accurate, and predictable route of fluid therapy is through an intravenous (IV) or intraosseous (IO) catheter. However, catheterization of a peripheral vein may present challenges in reptiles because of their size and anatomy. For successful catheter placement, it is essential to be familiar with the normal anatomy and location of the vessel to be catheterized. Most approaches require a cutdown procedure, and local anesthesia should be applied with an effective agent such as lidocaine or bupivacaine. Care and maintenance of the catheter are similar to that for domestic small animal species.
In chelonians a venous catheter may be placed into the jugular vein and sutured or taped in place. The right jugular vein is better suited than the left vein and is usually readily visible underneath the skin. In lizards the large, ventral abdominal vein is my preferred site for placement of an IV catheter. With the lizard in dorsal recumbency, a 1- to 2-cm midline incision is made, and the vessel may be easily visualized. After placement of an appropriately sized catheter, it may be sutured, glued, or taped in place and connected to an extension set. In large lizards the cephalic vein may also be catheterized after a cutdown procedure. In snakes a cutdown procedure following aseptic preparation of the incision site is required to visualize and catheterize the right jugular vein. After catheterization the catheter should be sutured to the skin with the hub incorporated into the suture.
In lizards, crocodilians, and small chelonians an IO catheter may be placed as a useful alternative for IV access. The IO route of fluid and drug administration is almost equally effective as IV administration. After administration of a local anesthetic agent, an appropriately sized spinal needle is inserted into the tibia of lizards and crocodilians. In chelonians the tibia, femur, or the marrow of the shell may be used. Strict aseptic techniques should be followed to minimize complications such as development of osteomyelitis. IO catheters are most effective for constant-rate infusions because the bone marrow space is limited in reptiles and will not allow for large boluses of fluids. In green iguanas especially, care should be taken not to damage the bone excessively during catheter placement because many patients have metabolic bone disease.
Intracoelomic administration of fluids is more effective than subcutaneous (SC) or oral (PO) administration but less effective than the IV or IO route. In general, fluids are readily absorbed from the coelomic cavity; due to the ease of administration; however, care should be taken not to overload the animal with fluids. This is especially true for chelonians, and only calculated volumes of fluids should be administered. SC administration of fluids is recommended for maintenance requirements and is not effective in cases of severe dehydration. PO fluid administration requires a functional gastrointestinal (GI) tract and is beneficial to meet maintenance requirements. In severely dehydrated patients, it may become necessary to administer fluids through several routes.