The concept of osmolality: its use in the evaluation of "dehydration" in the horse.

Osmolality is an indication of the osmotic pressure of plasma and depends on the amount of solute and solvent (water) present. The mean (+sd) plasma osmolality of 100 clinically normal animals was 282 (+6) mOsm/kg using lithium heparin as anticoagulant. The equation, osmolality=1.86 (sodium + potassium) +glucose +blood urea nitrogen + 9, was found to predict only crudely plasma osmolality. The plasma sodium: osmolality ratio was 0.49. Water and electrolyte disorders are classified into 3 types based on the measurement of electrolytes and osmolality: (1) Hypertonic dehydration (true dehydration desiccation), osmolality greater than 300 mOsm/kg, associated with water deprivation, some gastrointestinal emergencies and some types of diarrhoea; (2) hypotonic dehydration (acute desalting water loss), osmolalities less than 260 mOsm/kg, associated with acute diarrhoea, particularly salmonellosis; (3) isotonic dehydration (normal electrolyte and osmolality levels), in horses losing electrolytes and water in almost equal proportions. The importance of these observations and their significance in rational clinical management are discussed.