As mentioned at the start of this chapter, we are mostly water, actually salt water with some other chemicals thrown in for good measure. The intravascular compartment, replete with cells and proteins, differs from the rest of the body. In fact the blood volume also differs with age and sex (Table 3.5). We may lose fluid in a number of ways, from the obvious – hemorrhage, urine, vomiting – to the less obvious – sweat, evaporation from exposed viscera or trachea, transudation between compartments. While fluid escapes from anywhere, replacement occurs only through the intravascular compartment.
Many types of fluids are available for intravascular administration (Table 3.6).
· Hypotonic solutions With an osmolality less than that of serum (285–295
· mOsm/kg), these are rarely used in anesthesia (except pediatrics), because very little of the infused fluid remains intravascularly (<10% of D5W), elec-trolytes are diluted, and cells swell.
· Isotonic solutions Preferred, though still only about 25% of the infused volumeremains intravascularly, with the rest seeping into the interstitial space; representatives include 0.9% sodium chloride (also known as normal saline) and lactated Ringer’s (which also contains potassium and calcium).
· Hypertonic solutions Available in solutions from 1.8% to 10% NaCl; 3% is themost common. While almost 65% of the infused volume remains intravas-cularly, these solutions may cause cellular dehydration, hypernatremia, and hyperchloremic metabolic acidosis.
· Colloid Containing large molecules, these solutions tend to remain intravascu-larly (assuming capillary integrity).
· Hespan® (hetastarch, hydroxyethyl starch) Associated with coagulationabnormalities with infusions of >1 L.
· Pentastarch Hetastarch’s younger brother, allegedly with less effect on coagu-lation.
· Albumin Very expensive; often refused by Jehovah’s Witnesses.
· Blood or blood components Associated with many risks and expense (see below). Blood substitutes We need solutions capable of carrying oxygen, without therisks and expense of blood transfusions. Unfortunately, as of this writing, these solutions – including perfluorochemical emulsions, stroma-free haemoglobin and synthetic hemoglobin – remain in clinical trials.