Page 353 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Introduction to Fluid Therapy 343

INTRAOSSEOUS (INTRAMEDULLARY) approach. The shock dosage of synthetic colloids is
20 mL/kg for dogs and 10 to 15 mL/kg for cats. The
The intraosseous, or intramedullary, route is useful in
very young or small animals in which venous access is dif- shock dosage of isotonic crystalloids is 80 to 90 mL/kg
ficult. The procedure has been available for many years 6 for dogs and 40 to 60 mL/kg for cats. In experimental
and has received renewed attention. 14,16,29 This route studies, crystalloid fluids administered at 90 mL/kg/hr
did not cause pulmonary edema in normal dogs
provides rapid vascular access via bone marrow sinusoids 4,8
and cats.
and medullary venous channels and allows rapid disper-
Anesthetized cats receiving lactated Ringer’s solution
sion of fluid. The bone marrow does not collapse when
at a rate of 225 mL/kg for 1 hour developed serous nasal
the patient is hypovolemic, and access to the marrow is
simple. For some clinicians, this technique may be accom- discharge, chemosis, ascites, diarrhea, and fluid exudation
plished more rapidly than performing a venous cutdown. from catheter sites. At necropsy, these cats had ascites,
Sites that can be used for intraosseous administration of pancreatic edema, and accumulation of free fluid in the
fluid include the tibial tuberosity, trochanteric fossa of trachea. Body temperature decreased and CVP and left
atrial pressure increased in cats receiving 225 mL/kg/
the femur, wing of the ilium, and greater tubercle of
hr, whereas hematocrit, total protein concentration,
the humerus. The periosteum should be anesthetized
and colloidal osmotic pressure decreased in cats receiving
by infiltration with 1% lidocaine solution to avoid pain 4
both 90 and 225 mL/kg/hr.
during needle placement. The potential risks include
Lactated Ringer’s solution was administered to
osteomyelitis and pain on administration of fluid. How-
ever, pain was not observed clinically in two studies. 16,29 unanesthetized, dehydrated dogs at rates of 90, 225,
8
and 360 mL/kg for 1 hour. At rates of 90 and
225 mL/kg/hr, some dogs had serous nasal discharge,
HOW RAPIDLY MAY FLUIDS mild coughing, and slight chemosis. At 360 mL/kg/hr,
BE GIVEN? marked serous nasal discharge, restlessness, coughing,
dyspnea, pulmonary crackles, ascites, polyuria, chemosis,
Poiseuille’s law governs the flow of fluids through a protrusion of eyes, and diarrhea were observed. These
catheter: signs resolved when fluid administration was discontinued.
Hematocrit, TPP, and serum potassium concentration
pðP 1 P 2 Þr 4 decreased during fluid administration. In this study, body
Flow ¼
8ZL temperature decreased despite the fact that fluids were
warmed to 37 C. Serum sodium concentration remained

where P 1 P 2 represents the pressure differential on the unchanged, but pulse rate, respiratory rate, and systemic
fluid, Z is the viscosity of the fluid, r is the radius of arterial pressure increased slightly. Pulmonary capillary
the catheter, and L is the length of the catheter. Thus wedge pressure (PWP) and CVP increased, and these
the diameter of the catheter is of primary importance in measurements correlated well with one another. It was
establishing a rapid rate of flow. The choice of catheter concluded that lactated Ringer’s solution at 90 mL/kg/
length sometimes is affected by factors other than flow hr was tolerated safely. CVP should be monitored if fluids
rate (e.g., use of jugular catheters to monitor CVP). must be administered at rates in excess of 90 mL/kg/hr.
In a study of gravity flow of lactated Ringer’s solution, Contemporary losses must also be considered when
in vivo flow rates averaged 7% less than in vitro flow rates, adjusting the rate of fluid administration. Severe ongoing
presumably because of tissue pressure. 15 Fluid flow rate losses (e.g., vomiting and diarrhea in a patient with acute
increased by 50% when the pressure differential was gastroenteritis) may necessitate rapid administration to
increased by raising the fluid bag from 0.91 to 1.75 m. keep pace with contemporary fluid loss. When fluids
Flow rate increased linearly with increasing catheter radius are given rapidly, it is necessary to monitor cardiovascular
rather than geometrically as predicted by Poiseuille’s law. and renal function.
The rate of fluid administration is dictated by the mag- It usually is not necessary or desirable to replace the
nitude and rapidity of the fluid loss. The patient with hydration deficit rapidly in chronic disease states. Instead,
fluid-responsive shock syndrome requires aggressive fluid the hydration deficit may be calculated, the daily mainte-
administration. Fluid administration rates may vary, nance requirement of fluid added to this amount, and the
depending on the type of fluid or combination of types total volume administered over 24 hours. 35 Ongoing or
that has been chosen. One approach is to calculate a contemporary losses also must be considered and taken
“shock fluid dose” and administer it as rapidly as possible into consideration when estimating the patient’s fluid
in divided aliquots until a stable and sustainable cardio- requirements for a 24-hour period. This approach allows
vascular endpoint has been achieved (see Chapter 23). adequate time for equilibration of fluid and electrolytes
Clinical evaluation of the patient should occur after with the intracellular compartment and avoids potential
administration of each aliquot using a “titrate to effect” complications (e.g., edema or effusion related to