"The single most important factor in successful resuscitation from shock is time: rapid expeditious therapy in the early stages
may lead to good results, but adequate therapy that is delayed may be ineffective." (Shoemaker, W.C)
Small animals in crisis will present for a myriad of reasons: vomiting, diarrhea, bloat, urinary obstruction, dystocia, trauma...the
list is endless. A rapid assessment must determine whether the animal has decompensated or is likely to decompensate. Decompensation
will be directly related to Airway, Breathing and/or Circulatory failure.
Shock is a phenomenon of ineffective circulating volume. Significant loss of intravascular volume, or hypovolemia, can result from many causes, to include: trauma, loss of plasma water during vomiting and diarrhea, extreme venodilation
from systemic inflammation, and significant hemorrhage. A positive outcome is optimized by rapid and aggressive fluid resuscitation,
with hemostasis employed as required. The ability to create an effective fluid resuscitation plan depends on understanding
the pathophysiology of shock and the different body fluid compartments and the dynamics of fluid movement and distribution
between fluid compartments.
Body fluid compartments and fluid dynamics
There are 3 major fluid compartments; intravascular, interstitial and intracellular. Fluid movement from the intravascular
to interstitial and intracellular compartments occurs at the capillary. A capillary "membrane, consisting of capillary endothelial
cells and the subendothelial cell matrix separates the capillary intravascular space from the interstitial fluid compartment.
This capillary "membrane" is freely permeable to water and small molecular weight particles such as ions, glucose, acetate,
lactate, gluconate, and bicarbonate. Gases such as oxygen and carbon dioxide diffuse freely through the capillary endothelial
cell to enter or exit the intravascular compartment.
The interstitial compartment is that space between the capillaries and the cells. Fluids support the matrix and 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. Any particle movement between the
interstitium and the cell must occur through some transport mechanism (eg. channel, ion pump, carrier mechanism)
Fluids are in a constant state of flux across the capillary endothelial membrane, through the interstitium and into and out
of the cell. The amount of fluid that moves across the capillary "membrane" depends on a number of factors, to include: capillary
colloid oncotic pressure, hydrostatic pressure, and permeability.
The natural particles in blood that create COP are proteins—globulins, fibrinogen, and albumin. Albumin is the most numerous
and the smallest, approximately 69,000 daltons. The hydrostatic pressure within the capillary is the pressure forcing outward
on the capillary "membrane" generated by the blood pressure and cardiac output. Fluid moves into the interstitial space when
intravascular hydrostatic pressure is increased over COP, membrane pore size increases, or intravascular COP becomes lower
than interstitial COP.
Transmembrane ion pumps regulate intracellular water maintaining cellular and organelle integrity. The primary active pump
for solute transport across the cell membrane is the sodium-potassium pump. Membrane ion pumps also regulate intracellular
calcium, hydrogen, chloride, magnesium, glucose and amino acid concentration. These membrane transport systems occur in every
cell in every organ, and require energy to function.
Energy required to drive transmembrane ion exchange is supplied by cleavage of adenosine triphosphate (ATP). In contrast
to 38 ATP molecules that are produced during aerobic metabolism, anaerobic metabolism produces lactate and only 2 ATP molecules
per glucose molecule. Oxygen and glucose are transported from the intravascular space to the cell through a fluid medium.
Carried by hemoglobin to the capillaries, oxygen normally diffuses with great ease through the capillary membrane to the cells,
most of which are located less than 50 micrometers from a capillary.
The conduit for fluid transport is the vascular system. The heart serves as the pump of the conduit. The primary function
of the cardiovascular system is oxygen and glucose delivery to the tissues. Oxygen delivery is dependant on the product of
arterial flow and arterial oxygen content. Hemoglobin concentration and its dissociation curve are the prime components of
arterial oxygen content.
Arterial flow is a product of cardiac ouput and systemic vascular resistance. Cardiac output is a product of myocardial contraction
and heart rate. Sinus node stretch directly increases heart rate in an effort to promote volume ejection. Venous return
(preload), as defined by the Frank-Starling law of the heart, increases the stretch of the heart chambers resulting in increased
force of contraction. Factors influencing venous return to the heart include: mean circulatory filling pressures, right atrial
pressures, and resistance of the arteries.
Blood flow is also influenced by pressure differences and compliance within the vascular circuit as well as viscosity of the
fluid medium. Extrinsic and intrinsic regulation of the cardiovascular system also affect blood flow to the tissues. Intrinsic
metabolic autoregulation affects local organ blood flow, and is influenced by oxygen availability and removal of metabolic
Continuously produced and utilized, ATP production becomes heavily dependant on oxygen availability during high energy output
states. Optimum ATP production, therefore, depends on both oxygen delivery (DO2) to the cell and oxygen utilization (VO2) by the cell. Other than oxygen supplementation, reestablishing and maintaining intravascular fluid volume supports maximum