IV Fluids: Short Term Resuscitation vs. Maintenance Fluid Therapy

Large fluid and electrolyte, colloid and energy deficits are especially relevant in patients after trauma with blood losses or extensive burns and in horses suffering from gastrointestinal problems like colitis and colic. In the longer term, even small deficits and excesses can be clinically relevant and must be corrected. Type and dosage of IV fluids must be oriented on key clinical and laboratory parameters. In the following, two main indications for IV fluid therapy, short-term resuscitation vs. long-term maintenance are discussed. They each demand a specific approach:

Short-term resuscitation

Initial resuscitation aims to restore volume deficits in order to compensate for dehydration and shock (acute fluid loss and/or centralization). Based on clinical signs and laboratory values, we must assess how dehydrated the patient is. Then, we can estimate the deficit to be replaced and choose an adequate route, dosage (volume and rate) and type of fluid: 

Cristalloid fluids
Balanced (electrolyte concentration similar to serum) isotonic solutions such as Lactated Ringer's Solution (LRS) are used for resuscitation. In severely dehydrated patients it can be difficult to administer isotonic solutions at the required fast rate. Special large gauge catheters or – less ideally - putting catheters in both jugular veins can help meet the shock rate of 90 ml/kg: 45L in a 500 kg horse given < 1 hour). Normal Saline (NaCl 0.9%) is unbalanced, isotonic and tends to produce an acidosis (excess of Cl-; excess Na). Indications are metabolic alkalosis, hyperkalemia (e.g. with uroperitonneaum; renal failure; HYPP) and hypercalcemia. Hypertonic saline (NaCl 7.2-7.5%) solution is unbalanced and hypertonic (about 8X normal saline). By elevating intravascular Na+ concentration, it draws water out of the cell (ICF and initially also from the interstitium) to rapidly expand the intravascular (and extracellular; ECF) fluid volume. Thus, the patient can be quickly resuscitated with little volume, which is a significant advantage in the field and for pre- operative stabilisation of patients in clinical shock. Since the expanding of the ECF happens directly at the expense of decreasing ICF, the total fluid deficit is not restored and this deficit must be addressed within 1-2 hours by high-volume isotonic crystalloid infusions, if indicated complemented by colloids. Hypertonic saline is contraindicated in patients with after severe sweating, hyperosmolar conditions (salt intoxication, ...) and renal disease.

Colloidal fluids

Crystalloids have very short effective half-lifes. Especially when TS are below normal, IV fluids containing colloids that help increase intravascular oncotic pressure should be used. Since the effective half-life increases with molecular weight, high-molecular weight colloids (Hetastarch; HAES as 10% solution at 6-10 (-20) ml/kg BW/day) are preferable to low to medium molecular weight dextrans. Colloids also decrease blood viscosity and coagulation. Plasma is an excellent method to deliver oncotic pressure. Albumin alone contributes approximately 70% of the plasma oncotic pressure, but other protein components in plasma can be beneficial in coagulation and wound healing. Whole Blood (cross-matching if possible) is indicated with significant blood loss based on estimated blood volume lost, clinical signs and – only in chronic blood loss - PCV. [...]