Critical care for sick cattle—practical IV therapy (Proceedings)
Dehydration and electrolyte abnormalities occur commonly in cattle with gastrointestinal tract disease. Severe fluid and electrolyte abnormalities can result in death if left untreated. Five important questions must always be asked and answered regarding fluid and electrolyte therapy in cattle; (1) is therapy needed? (2) what route of administration? (3) what type? (4) how much should be administered?, and (5) how fast should the solution be administered? All calves ≥8% dehydrated (≥4 mm eyeball recession) or decreased/absent suckle should receive IV fluids. Oral fluids are very effectve in mild dehydration, particularly in calves with a suckling reflex.
There are 4 routes for parenteral fluid administration; (1) intravenous, (2) intraperitoneal, (3) subcutaneous, and (4) intraosseous. Crystalloid solutions can be administered all 4 routes; however, the contents must be accurately known and ideally the solution should be sterile and nonpyrogenic. Colloids can be administered by the intravenous and intraperitoneal routes. Isotonic solutions can be administered by the intravenous, intraperitoneal, subcutaneous, and intraosseous routes, whereas hypertonic solutions should only be administered intravenously or subcutaneously.
Intravenous administration of large volumes of isotonic crystalloid solutions requires intravenous catheterization and periodic monitoring of the flow rate and fluid container. It is often difficult to maintain a patent catheter and a constant flow rate using gravity-flow systems because of changes in position from recumbency to standing (thereby altering the hydrostatic pressure gradient) and because movement can lead to tangling of the fluid line. An additional problem in goats and occasionally sheep and calves is nibbling or suckling the fluid administration line with subsequent disruption. Catheter-related problems are easily identified and rectified in a veterinary hospital but usually go undetected and uncorrected when intravenous fluids are administered in on-farm situations.The effectiveness of intraperitoneal fluid administration does not appear to have been extensively evaluated in dehydrated calves. The route is not currently recommended for neonatal calves because of altered peritoneal absorption in dehydrated calves and the risk of peritoneal adhesions, although neither claim appears to have been substantiated. Plasma has been administered safely to neonatal calves through a catheter in the paralumbar fossa. The effectiveness of subcutaneous or intraosseous fluid administration does not appear to have been evaluated in dehydrated calves.
A thorough understanding of osmolarity is central to successful parenteral and oral fluid therapy. Osmolality is "the number of dissolved particles per kilogram of water" and is measured. The normal value in cattle is approximately 285 mOsm/kg. Osmolarity is "the number of dissolved particles per liter of water" and is calculated. When applied to plasma or serum, note that the denominator of osmolarity is L (which is liters of plasma). 1 kg of plasma has 2 components: 1) plasma protein (normal concentration, 7 g/dl = 70 g protein / kg of plasma); 2) plasma water (1 kg of plasma - 70 g of plasma protein = 930 g = 0.93 L). The osmolarity of plasma water in ruminants is therefore 285/0.93 = 306 mOsm/L and this is the approximate osmolarity of isotonic solutions.
Erythrocytes are resistant to increases in plasma osmolarity, whereas they are susceptible to mild decreases in osmolarity; this is the basis of the red blood cell fragility test whereby red blood cell suspensions are placed in solutions of decreasing osmolarity. As the osmolarity of the solution decreases, water moves across the semipermeable red blood cell membrane into the erythrocyte, resulting in swelling of erythrocytes, fracture of the cell membrane, and hemolysis. This is why the rapid IV administration of hypotonic solutions causes hemolysis and hemoglobinuria. In contrast, intravenous administration of hypertonic solutions does not cause hemolysis, because increasing plasma osmolarity causes water to move out of erythrocytes, which are resistant to shrinking. Because of hypotonic-induced hemolysis, parenterally administered fluids should be isotonic or hypertonic.
The total amount of infused sodium ion is the single most important determinant of the success of fluid resuscitation. Sodium is the scaffold on which extracellular fluid volume is dependent; No sodium = No extracellular fluid volume = No animal. Practical fluid resuscitation should therefore focus on administering enough sodium ions at a fast enough rate, without inducing any deleterious effects.