Recognition of an ineffective circulating volume
"Preload" parameters are those which address whether or not the heart is receiving a venous return sufficient to expect reasonable
forward blood flow. "Forward flow" parameters are those which address the issues of stroke volume, cardiac output, arterial
blood pressure and tissue perfusion. It is the forward flow parameters that are commonly and distinctly affected by aberrations
in circulating blood volume and constitute the usual tip-off that there is a problem. The column titles "hypodynamic" are
typical findings in hypovolemia, but may also be associated with any version of heart failure (congestive heart failure, mitral
insufficiency, aortic stenosis, pericardial tamponade) or end-stage sepsis. The column titled "hyperdynamic" might represent
the findings in hypervolemia, but are also typical of early systemic inflammatory response syndrome and hyperthermia.
The usual compensatory response to hypovolemia is an increase in heart rate and peripheral vasoconstriction (pale color, prolonged
capillary refill time, cool appendages, and oliguria). Diminished pulse quality (height and width of the pulse pressure wave
form) is the consequence of reduced stroke volumes secondary to the reduced venous return and tachycardia. Cardiac output
would be proportionately reduced, but blood pressure is usually maintained until the compensatory mechanisms can no longer
compensate. Metabolic markers of inadequate tissue oxygenation include an increasing base deficit, lactate, oxygen extraction,
and venous-arterial PCO2 difference, and a decreasing central venous PO2. For all parameters, the greater the deviation from normal, the greater the underlying circulatory deficit. As for all
cardiovascular or metabolic parameters, no one parameter defines a circulatory deficit; collect as many as possible in order
to be the most secure in your assessment.
Poor forward flow parameters may be associated with hypovolemia but they may also be associated with poor heart function.
In such cases it is possible to overload the heart (and cause pulmonary or systemic edema) prior to normalizing the forward
flow parameters. When the forward flow parameters have not responded to a reasonable dose of fluids or if there is reason
to suspect poor heart performance, fluids should be titrated to preload parameter end-points rather than forward flow parameter
end-points (Table 8-1). When the preload parameters have been normalized and the forward flow parameters are still unacceptable,
heart-specific therapy is indicated, such as sympathomimetic therapy if poor contractile function is suspected or pericardiocentesis
if pericardial fluid is identified.
Goals of blood volume restoration therapy
It is always the ultimate goal of fluid therapy to restore the circulatory signs to normal. The immediate goal of fluid therapy,
however, is acceptable (not necessarily normal) cardiovascular performance. The immediate goal of fluid therapy is to move
the patient away from the "death line" so as to minimize chances that the animal might die as a consequence of the hypovolemia.
The remaining fluid therapy to normalize the patient can be accomplished in a more leisurely (safe) manner. Previous attempts
to immediately and completely normalize cardiovascular performance has resulted in edema and hemorrhagic complications. The
goal of blood volume restoration therapy is to achieve what you want - restoration of acceptable cardiovascular performance,
without causing what you don't want - edema or hemorrhage. Shock fluid therapy must be cautiously aggressive; large volumes
are often required initially, but always with a eye on the preload parameters and the problems associated with the particular
fluid being administered. If the preload parameters are high or a fluid-specific problem develops before acceptable forward
flow parameters are achieved, a new plan must be developed.
General principles of fluid therapy
While there is a bewildering variety of fluids from which to choose, they can be largely be grouped into a few categories
with similar uses and indications.
General fluid categories
While circulating volume can be augmented by any fluid, there are vast differences between fluids with regard to their ability
to accomplish this without harming the patient. This is partly determined by their redistribution characteristics after their
administration and it is imperative to understand the physiology of fluid distribution across the vascular and cell membranes.
Fluid choice is also partly determined by their solute content compared to normal blood; electrolytes, albumin, and hemoglobin
concentrations cannot be allowed to get too high nor too low.
Isotonic crystalloids are commonly used to replace interstitial deficits in dehydrated patients and support circulating blood
volume. Their use should be restricted in patients with interstitial edema. Artificial colloids are primarily used for blood
volume and colloid osmotic pressure support and restoration. Plasma could be used for colloidal blood volume support, but
this use is usually precluded by its expense. Plasma could also be used for albumin replacement, but in the face of ongoing
albumin loss, this use is also limited by its expense. Plasma is, therefore, most often relegated to coagulation factor replacement.
Hemoblobin products are primarily used for hemoglobin replacement in the anemic patient.