Although most measures of solute concentration take into account the number and weight of the molecules dissolved in a solvent, osmolality measures only the number of particles in solution without regard to size, weight, or charge.1 In biological systems, the relative osmolality of the intracellular and extracellular spaces determines the fluid volume in each compartment.2,3 Therefore, it is extremely important that osmolality be closely regulated. In mammals, the release of vasopressin in response to an increase in plasma osmolality is one of the body's primary mechanisms for controlling osmolality.4,5 Abnormalities in this system can quickly lead to life-threatening fluid and electrolyte imbalances.6
Knowledge of the plasma osmolality of a particular species is of paramount importance when formulating fluid treatment for a patient. Commonly used prepackaged fluids, such as lactated Ringer's solution and physiologic saline (0.9% NaCl) solution, are specifically designed for human patients who have an osmolality of approximately 301 mOsm/L.7 Veterinary-specific solutions have a similar osmolality. These fluids may be isotonic, hypotonic, or hypertonic for other species, depending on the specific plasma osmolality of each species. The plasma osmolality of some commonly kept pets (eg, cats and dogs) have been determined8,9; however, to our knowledge, there is no published information regarding plasma osmolality in psittacine species.
Plasma osmolality is routinely measured in clinical laboratories by use of freezing-point osmometers. These automated devices rely on the principle that each mole of dissolved solute will decrease the freezing point of a liquid by 1.86°C.3 Osmolality can also be estimated by use of calculations.2 The difference between the measured and calculated osmolality is the osmolar gap and is of clinical importance when exposure to a toxin is suspected. Whereas an osmolar gap of up to 10 mOsm/kg is considered normal in humans and dogs, gaps with values higher than this suggest the presence of unmeasured solutes.1,10 Toxins such as ethanol, isopropanol, methanol, acetone, ethyl ether, mannitol, or ethylene glycol (as well as their metabolites) can attain high plasma concentrations with relatively low molecular weights, which causes high osmolar gaps.1,10
Several equations have been proposed for calculating plasma osmolaltiy.11–14 Nevertheless, because there are many plasma solutes in relatively small amounts (ie, glucose)2 or that move freely between the extracellular and intercellular space (ie, urea),5,15 equations for use in mammalian species are often reduced to (2 × Na+) + (BUN concentration/2.8) + (glucose concentration/18), (2 × Na+) + (glucose concentration/18), or (2 × Na+).2,5 Because osmolality values have not been determined for clinically normal psittacines, equations for calculating plasma osmolality in these birds have not been developed.
In the study reported here, plasma osmolality was measured in a population of healthy adult Hispaniolan Amazon parrots. We also evaluated the ability of various mathematic equations to accurately estimate osmolality in this population. Our hypothesis was that there would be good agreement between the calculated and measured osmolality of the parrots.
Osmette, Precision Systems, Natick, Mass.
Olympus AU640e chemistry analyzer, Olympus America Center, Valley, Pa.
Prism 5.0 for Mac, GraphPad Software, La Jolla, Calif.
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Feldman Z, Zachari S, Reichenthal E, et al. Brain edema and neurological status with rapid infusion of lactated Ringer's or 5% dextrose solution following head trauma. J Neurosurg 1995;83:1060–1066.