All anesthetic drugs act as cardiovascular and respiratory depressants on varying levels, and they can compromise a patient's
homeostasis at unpredictable times in unpredictable ways. A patient must maintain adequate blood pressure for the duration
of any surgical procedure because prolonged anesthetic hypotension can have devastating effects such as renal failure or brain
damage. Proper fluid therapy is imperative in maintaining blood pressure under anesthesia. There are many different fluid
options, and the goal of fluid therapy is similar to that of anesthesia, which is to maximize benefits while minimizing side
effects. A thorough knowledge of fluid products, benefits, and side effects is essential to accomplish this.
Physiology of fluid therapy
There are 3 major fluid compartments; intravascular, interstitial and intracellular. Fluid movement from the intravascular
to interstitial and intracellular compartments occurs at the capillary membrane. This membrane is freely permeable to water
and small molecular weight particles. The interstitial compartment is the space between the capillaries and the cells. Fluids
support the cells within the interstitial space. The intracellular compartment is separated from the interstitial space by
a cell membrane. This membrane is freely permeable to water but not small or large molecular weight particles.
Fluids to be administered must concentrate within the body fluid compartment where the volume deficit lies. Crystalloids such
as Lactated Ringers solution are water based solutions with small molecular weight particles that are freely permeable to
the capillary membrane. Colloids such as 6% Hetastarch are water based solutions that are of a larger molecular weight that
are too large to freely pass across the capillary membrane. Colloids are thought of as intravascular volume replacement fluids
and crystalloids as interstitial volume replacement solutions.
Sympathomimetics such as Dopamine, and Dobutamine are indicated when the patient is unresponsive to vigorous fluid therapy
so their blood pressure and, therefore, tissue perfusion has not returned to acceptable levels.
Intra-operative fluid status can be assessed with blood pressure monitoring. There are three values considered when measuring
blood pressure. They are as follows:
1. Systolic pressure, which is the pressure in the arteries produced by contraction of the ventricles and subsequent
propulsion of blood through the aorta and major arteries. It is the highest pressure exerted throughout the cardiac cycle,
and should be above 90mmHg during anesthesia.
2. Diastolic pressure, which is the pressure in the arteries when the heart is resting between contractions. It is the lowest pressure exerted throughout
the cardiac cycle.
3. Mean arterial pressure (MAP), which is the average pressure in the arteries over the cardiac cycle. It is considered
the most important of the blood pressure values because it is the best indicator of perfusion to vital organs. This value
is usually provided with an oscillometric monitor during a surgical procedure. The MAP during anesthesia should range between
70-90 mmHg, and a MAP below 60 indicates decreased perfusion to internal organs.
A Doppler ultrasonic flow detector does not provide an accurate MAP so decisions should be based on the systolic value.
Hypotension is defined as a blood pressure value that is below normal limits. Causes of anesthetic hypotension include excessive
anesthetic depth, excessive vasodilation, cardiac insufficiency, and excessive blood loss (hypovolemia). Adequate preoperative
fluid resuscitation should be considered for emergent surgical procedures such as a splenectomy secondary to a hemoabdomen,
Caesarian section, or gastric dilitation and volvulus (GDV). These patients are at more risk for anesthetic hypotension due
to hemodynamic changes such as massive blood loss in the case of a hemoabdomen, the rapid expansion of the aorta when compression
is relieved after the uterus is externalized during a Caesarian section, or the lack of circulation from the pressure abdominal
distension puts on the caudal vena cava from a GDV. The shock dose for crystalloids is 40-90 ml/kg in dogs and 20-60 ml/kg
in cats. The full shock dose is rarely necessary so fluids should be administered incrementally.