CHELONIAN BLOOD OSMOLARITY AND IMPLICATIONS FOR FLUID THERAPY. Kevin Eatwell, BVSc (hons) Dipl. ZooMed (Reptilian) DECZM (Herp) MRCVS

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1 CHELONIAN BLOOD OSMOLARITY AND IMPLICATIONS FOR FLUID THERAPY Kevin Eatwell, BVSc (hons) Dipl. ZooMed (Reptilian) DECZM (Herp) MRCVS Exotic Animal and Wildlife Service, Division of Clinical Veterinary Services, Hospital for Small Animals, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG ABSTRACT: In order to maintain water in the circulations tortoises must maintain an osmotic gradient. They achieve this by altering blood osmolality. Elevations above normal can suggest subclinical dehydration, before there are changes in blood PCV, proteins or uric acid levels, which are commonly used to determine the hydration status of a tortoise. Plasma Urea and Sodium have an influence on osmolality. Sick chelonians do not appear to have elevated sodium levels compared to healthy individuals and therefore fluids designed for mammalian use are appropriate. KEY WORDS: tortoises, blood osmolarity, osmolality, sodium, urea, fluid therapy INTRODUCTION Sick chelonians often have bloodwork as part of their clinical evaluation. However evaluating dehydration can be difficult until cases are severe. Lymph dilution can also be present and alter a wide variety of parameters, further complicating interpretation (Wilkinson, 2004; Lopez-Olvera et al., 2003; Walters et al., 2001). Water is essential for life and the tortoises have a number of unique physiologic adaptations to survive harsh environments and hibernation. Protein breakdown and excretion of nitrogenous waste is one of the major factors influencing the amount of water lost and tortoises are able to modify the protein breakdown products they utilise. Species where fresh water is limited (such as terrestrial species) use uric acid. This forms insoluble urate salts in the bladder which exert no osmotic effect. As a tortoise dehydrates it uses water from the bladder to maintain its circulatory volume. The effects of this were observed by Darwin noting the bladder contents of the giant tortoises on their voyages (used as a water source for sailors) became less pure as they continued on their journey. The specific gravity of the urine, its ph and its rate of production also allow an assessment of the tortoise s water balance and is easy to check in a consultation. Body weight is also a good guide to the hydration status given the sheer size of a full bladder versus and empty one. In order to draw water into the circulation the tortoise must maintain an osmotic gradient and this is achieved by altering their blood osmolality, based on the availability of water. 109

2 Blood osmolality can suggest subclinical dehydration, before there are changes in blood PCV, proteins or uric acid levels, which are commonly used to determine the hydration status of a tortoise (Wilkinson, 2004). However measuring blood osmolality is unpractical for the clinician due to the need for an osmometer, which cost at least $1,500. However calculating blood osmolarity is possible. Plasma urea and sodium have an influence on osmolality. Chelonia selectively produce urea to elevate plasma osmolality. This enables more water to be retained in the circulation. Urea is a rapidly diffusible solute that can be excreted when water is available again. Retaining plasma sodium also allows for water to be kept in the circulation. Sodium also appears to be at similar levels in good quality and lymph diluted blood samples and so is a useful parameter to measure whichever venepuncture site is utilized (Eatwell, 2005). Sodium levels in stored blood is statistically significantly elevated from fresh blood, but this is minor and unlikely to be of any clinical significance (Eatwell, 2007). Other electrolytes such as chloride, potassium, and glucose influence osmolality. Thus a simple equation can be used to estimate blood osmolality, which is essentially calculating blood osmolarity. Under most conditions osmolarity is identical to osmolality. 2(Na + + K + ) + glucose + urea = osmolarity (Wilkinson, 2004). It is only when the plasma and the urine has reached an equilibrium and the capacity for increasing osmolality has been exceeded that clinical dehydration follows. Thus the classical signs noted in mammals do not apply to chelonia. Instead monitoring bodyweight, urine ph, urine specific gravity, and plasma urea, and sodium is utilized by tortoise clinicians. In practical terms a guide can be obtained by evaluating blood sodium, and urea. If elevated above the levels expected for a healthy tortoise (in a comparable physiologic state), appropriate fluid therapy is indicated. These parameters can help guide appropriate quantities and type of fluid therapy required by the tortoise. The purpose of this review was to identify the levels of sodium, and urea which are appropriate for a well hydrated tortoise, and compare this to commercially available fluids to allow for a more appropriate selection of fluid therapy for tortoises. PLASMA OSMOLARITY, SODIUM AND UREA LEVELS IN HEALTHY CHELONIANS There has been little work looking at blood osmolarity in tortoises. A detailed data set has been published, which showed a wide variation in blood osmolarity based on the season in hibernated spur thighed (Testudo graeca) and Hermann s (Testudo hermanii) tortoises (Gilles-Baillien and Schoffeniels, 1965; Table 1). In this paper there was marked seasonal variation in osmolarity, sodium and urea values. The mean plasma osmolarity of these tortoises ranged between 258 to 467 mosm/l. Mean sodium 110

3 levels reported were from 105 meql/l to 167 (105 to 167 mmol/l), in apparently healthy individuals. Mean urea values varied between 11.2 and mg/dl (3.1 and 103 mmol/l). These values are likely to reflect a poorly managed hibernation in these tortoises and more recent data is available on the sodium and urea values in healthy tortoises. Urea reference ranges quoted for terrestrial chelonians are generally below 5.9 mg/dl (2.1mmol/L). However this does not account for the huge influence of seasonality. More recent studies in hibernated Tortoises of the genus Testudo produced mean sodium values of 144 meq/l (144 mmol/l) in March (n = 38), 143 meq/l (143 mmol/l) in April (n = 38) and 136 meq/l (136 mmol/l) in August (n = 65), where hibernation was better managed with bathing before and after a hibernation period of 3 months. 4 Mean urea values recorded were 20.2 mg/dl (7.2 mmol/l) in March (n = 38) and, 6.7 mg/dl (2.4 mmol/l) in April (n = 38) (Eatwell, 2005). The August values correlate well with Gills-Baillien and Schoffeniels (1965) as their mean sodium value in August was 136 meq/l (136 mmol/l). PLASMA SODIUM AND UREA LEVELS IN SICK CHELONIANS Evaluating fluid balance in a sick chelonian is important and sodium and urea levels can give a guide to the osmolarity of a chelonian and guide fluid therapy. It is often assumed that sick tortoises after hibernation would have elevations in these values. One report suggests that sick chelonians have blood sodium levels between ( mmol/l), blood urea levels between mg/dl ( mmol/l), and blood osmolarities of mosm (Wilkinson, 2004). Analysis of our recent laboratory reports for ill Testudo tortoises (n = 16) presenting for treatment produced mean sodium levels of 135 (135 mmol/l). Individuals varied between 108 and 147 ( mmol/l). Mean urea levels found (n = 21) were 6.2 mg/dl (2.2mmol/l) and ranged between 1.1 to 29.4 mg/dl (0.4 to 10.5 mmol/l). It would appear that many clinically ill tortoises are not suffering from elevated blood sodium levels and increased blood osmolarity. OSMOLARITY AND SODIUM LEVELS IN PARENTERAL FLUIDS Fluid therapy for reptiles has been a subject for debate over the years. Initially debate has centred on the relatively larger size of the intercellular fluid compartment relevant to the extracellular fluid compartment relative to mammals (Thorson, 1968). Many authors have suggested that the osmolarity of replacement fluids should be lower than mammals and using mammalian fluids with the addition of 10% sterile water for injection is recommended. However the bladder acts as a large extracellular store which was not accounted for in the initial work. In addition sick chelonia have blood osmolarities and sodium levels similar to 111

4 commercially available mammalian fluids. Hypotonic fluids may only really benefit those suffering from hypertonic dehydration. Ideally fluid replacement should be geared to return the osmolality to that expected for the species at the particular time of year. This information is not fully known and so a general approach is adopted. Most commercially available fluids for rehydration therapy in mammals have values of mOsm (Table 2). Given the lack of current data on the osmolality of tortoises the sodium values in the plasma can be compared to the sodium content of the fluids and used as a guide. More detailed evaluation of fluids can be found from numerous sources (Mader and Rudloff, 2006). Given what is known about plasma osmolality and sodium concentrations it would follow that products such as lactated ringers or hartmanns solution are suitable for rehydration (replacement therapy) in sick tortoises without the need for dilution. REFERENCES Eatwell K Plasma total calcium, ionised calcium and 25-hydroxycholecalciferol levels in captive tortoises (Testudo spp.) maintained under natural light in the United Kingdom. RCVS Diploma dissertation. Eatwell K Effects of storage and sample type on ionized calcium, sodium and potassium levels in captive tortoises, Testudo spp. J Herpetol Med Surg, 17(3): Gilles-Baillien M, Schoffeniels E Variations saisonnieres dans la composition du sang de la torte greque Testudo hermanni. Ann Soc R Zool Belg, 95: Lopez-Olvera JR, Montane J, Marco I, Martinez-Silvestre A, Soler J, Lavin S Effect of venipuncture site on hematologic and biochemical parameters in marginated tortoise (Testudo marginata). J Wildl Dis, (39)4: Mader DR, Rudloff E Emergency and Critical Care. In Mader DR ed. Reptile Medicine and Surgery. Saunders Elsevier, MO, Thorson TB Body fluid partitioning in reptiles. Copeia, 3: Walters M., Lopez J., Brewer B., Steeves E Subcarapacial sampling in tortoises. Proc Brit Vet Zool Soc, pp 45. Wilkinson R Clinical pathology. In: McCarthur S, Wilkinson R, Meyer J, Innis JC, Hernandez-Divers S, eds. Medicine and Surgery of Tortoises and Turtles. Blackwell Publishing LTD, Oxford,

5 Table 1. Mean blood electrolyte and osmolarity values from healthy spur thighed and Hermann s tortoises (data from Gilles-Baillien and Schoffeniels, 1965). Month Na+ K+ Ca2+ Cl Urea mg/dl Osmolarity (mosm/l) Jan 156 (156) 3.7 (3.7) 2.4 (1.2) 124 (124) 86.8 (31) 349 Feb 161 (161) 3.0 (3.0) 5.4 (2.7) 123 (123) (38) 449 March 157 (157) 3.8 (3.8) 5.4 (2.7) 125 (125) 95.2 (34) 443 April 167 (167) 4.6 (4.6) 4.6 (2.3) 134 (134) (103) 467 May 129 (129) 4.9 (4.9) 5.0 (2.5) 86 (86) (37) 340 June 105 (105) 4.3 (4.3) 4.8 (2.4) 66 (66) 72.8 (26) 258 July 115 (115) 4.5 (4.5) 4.6 (2.4) 94 (94) 11.2 (4) 290 Aug 136 (136) 5.5 (5.5) 5.4 (2.7) 108 (108) 33.6 (12) 322 Sept 136 (136) 4.9 (4.9) 5.0 (2.5) 99 (99) 30.8 (11) 338 Oct 138 (138) 4.8 (4.8) 4.8 (2.4) 110 (110) 61.6 (22) 343 Nov 141 (141) 5.2 (5.2) 5.2 (2.6) 99 (99) 58.8 (21) 349 Dec 155 (155) 6.3 (6.3) 6.2 (3.1) 124 (124) 86.8 (31) 404 Table 2. Sodium concentrations and osmolality of commercially available parenteral fluids. Fluid type Sodium Osmolarity Sodium chloride 0.9% 154 mmol/l 308 mosm Lactated ringers 130 mmol/l 273 mosm Hartmanns 131 mmol/l 279 mosm Sodium chloride 0.9% and glucose 5% 154 mmol/l 560 mosm 113