Page 576 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

564 FLUID THERAPY

hemorrhagic shock or sepsis. 10 The role of gender and but a jugular catheter may prove beneficial once the
genetic predisposition in the response to injury and patient is more stable. The cephalic, lateral saphenous
outcome of veterinary patients is currently unknown. (dogs), or medial saphenous (cats) veins are most
commonly used for initial placement of one or two
CLINICAL MANAGEMENT intravenous catheters. The catheter(s) should be an
over-the-needle catheter that is as large and short as pos­
The initial approach to the animal with clinical evidence sible to maximize flow rates through the catheter because
of cardiovascular shock should focus on resuscitation of the rate of flow is proportional to the radius to the fourth
the “ABCs,” or airway, breathing, and circulation. power and inversely proportional to the length. If venous
If the animal is not breathing at least eight times per catheter placement is not possible, either a venous
minute or the gag reflex is absent, an endotracheal tube cutdown or intraosseous catheter placement should be
should be placed and positive pressure ventilation performed. Intraosseous catheter placement is further
initiated, if needed. Supplemental oxygen should be discussed in Chapter 15.
supplied to all patients via mask, flow-by, or into the Once an intravenous catheter is placed, the clinician
endotracheal tube, if applicable. If the animal has no must decide what type and how much fluid to administer
detectable heartbeat and is pulseless, chest compressions for the treatment of shock. There are basically four types
should be performed and an electrocardiogram moni­ of fluids that are typically used for the management of
tored. The administration of atropine, epinephrine, shock: crystalloids (isotonic and hypertonic), synthetic
and/or vasopressin may be required to obtain return of colloids, natural blood products (red blood cells, plasma,
spontaneous circulation. If the animal has received any albumin), and oxygen carrying solutions. The various
reversible drugs (benzodiazepines, opioids, a 2 -agonists, types and doses are listed in Table 23-3. Although the
etc.) before assessment, reversal of these drugs is specific type of shock may help dictate the best therapeu­
recommended to minimize adverse cardiovascular effects tic approach, it is important that the clinician understand
of the drugs. Postoperative patients should be evaluated the constituents of and potential side effects of each fluid
from a risk-benefit perspective before reversal of all opioid type.
analgesics because rapid reversal following major surgery
could lead to excessive pain. The use of an agonist-antag­ ISOTONIC CRYSTALLOIDS
onist opioid drug (e.g., butorphanol) may decrease the Isotonic crystalloids, also known as replacement fluids,
severity of respiratory depression while allowing for are electrolyte-containing fluids with a composition sim­
continued analgesia following administration of pure ilar to that of the extracellular fluid. They have a similar
m-agonist drugs in painful patients. osmolarity as plasma and the electrolytes are small in size
Restoration of adequate circulation requires identifica­ (i.e., sodium has a molecular weight of 23 Da compared
tion and control of any internal or external hemorrhage. with glucose at 180 Da). Examples include 0.9% sodium
Fluid therapy is the cornerstone of treatment for shock. chloride, lactated Ringer’s solution, Normosol-R, and
Although fluid therapy is frequently contraindicated in Plasmalyte 148. Although decades of investigation have
patients with cardiogenic shock or hypervolemia, most not defined the ideal fluid for the treatment of shock,
other types of shock will be at least partially responsive the initial resuscitation fluid for the treatment of patients
to intravascular volume augmentation. Aggressive, yet in shock is most commonly isotonic crystalloids. A dose
judicious use of fluids will serve to increase tissue perfu­ up to approximately one blood volume is typically used:
sion, decrease tissue hypoxemia, reduce secondary 90 mL/kg in the dog and 50 mL/kg in the cat. Isotonic
cytokine injuries, and maximize a successful outcome. crystalloids rapidly distribute into the extracellular fluid
A prospective study looking at people who died in the compartment following administration, and only approx­
hospital after admission for treatment of injuries found imately 25% of the delivered volume remains in the intra­
that inadequate fluid resuscitation was the most common vascular space by 30 minutes postinfusion. 131 Although
mismanagement recorded. 25 Adequate intravascular theoretically this increase in interstitial fluid volume
volume replacement is crucial to restore perfusion to the might predispose to interstitial edema and deranged oxy­
major organs, thus reducing morbidity and mortality gen transfer to the cells, this has not been shown in a
9
associated with hypovolemia. Approximately 50% of hypo­ canine hemorrhagic shock model. However, it is impor­
tensive, septic humans will have normalization of cardio­ tant that the veterinarian avoid overzealous use of iso­
vascular hemodynamics with fluid therapy alone. 119 tonic crystalloids to prevent volume overload and
Access to the venous circulation is vital for rapid vol­ interstitial edema, pulmonary edema, or cerebral edema.
ume resuscitation. The intravenous or intraosseous Patients with a low colloid osmotic pressure, pulmonary
routes are preferred because absorption from the subcu­ contusions, cerebral trauma, fluid nonresponsive renal
taneous or peritoneal space is slow and unpredictable, disease, or cardiac disease are at highest risk for
especially in the face of systemic vasoconstriction. Periph­ complications. In addition, substantial hemodilution of
eral veins are preferred for the initial resuscitation efforts, red blood cells, plasma proteins, clotting factors, and