How to: Use Enteral Fluid Therapy in the Critical Care Setting

Water absorption and blood flow
Water absorption from the gastrointestinal tract in the normal horse primarily occurs in the large intestine, and more specifically the large colon. In an adult horse a volume of up to 100 l of fluid and associated secretions is absorbed during the course of the day (Argenzio et al. 1974; Argenzio and Stevens 1975; Argenzio 1975). In the hypovolaemic horse, in order to protect the vital organs, blood flow is diverted from the gastrointestinal tract. Once blood flow is reduced, so too are gastrointestinal motility and absorption. In addition, obtunded, hypovolaemic horses have a reduced thirst drive. It is for this reason that using oral fluid therapy in hypovolaemic animals is unsuccessful at best and detrimental in certain scenarios.

Assessment of hydration status
The majority of ‘sick’ horses that present to clinicians for treatment are hypovolaemic, but not dehydrated. Hypovolaemia is defined as a loss of circulating volume (both water and salt loss). Clinical signs of hypovolaemia include tachycardia, increased capillary refill time, haemoconcentration (increased packed cell volume and total solids), increased lactate concentrations, reduced urine output and concentrated urine (uring specific gravity [USG]>1.040). Dehydration is defined as a loss of total body water and is more challenging to quantify. Clinical signs of dehydration include decreased bodyweight (not helpful clinically, but used experimentally), sunken eyes, increased skin tent over the upper eyelid and tacky mucous membranes. Dehydrated animals are always hypernatraemic. [...]