Page 421 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 411

amount of un-ionized drug. In most cases, management redistribution of blood to the pulmonary circulation)
of metabolic alkalosis requires the administration of chlo- hypervolemia, increased pulmonary venous pressure
ride-containing solutions. This normally is achieved using (e.g., left ventricular failure, mitral regurgitation), or
0.9% NaCl supplemented with KCl. Mild alkalosis may be increased pulmonary flow (e.g., left-to-right shunt, ane-
caused by hypoalbuminemia, and correction of serum mia). Volume overload should be treated with diuretics or
albumin concentration may be sufficient to correct the phlebotomy as described earlier (hypervolemia). In
alkalosis. animals with left ventricular failure or mitral regurgita-
tion, the aim of therapy is to promote forward flow by
CHANGES IN DISTRIBUTION using vasodilators or positive inotropes. In the acute
setting, dobutamine is a suitable positive inotrope
because it increases myocardial contractility while tending
DEHYDRATION to decrease systemic vascular resistance. Nitroprusside or
Dehydration reduces vascular volume and results in nitroglycerin can be used to decrease peripheral vascular
changes in the volume of the intracellular space. The type resistance and can be titrated to effect. Ideally, therapy
and extent of change in the various compartments should be monitored using a catheter that allows mea-
depend on the type of fluid lost. With pure water loss, vol- surement of pulmonary capillary wedge pressure
ume contraction occurs in the intracellular compartment, (PCWP).
whereas with hypotonic dehydration, an increase in the A decrease in colloid osmotic pressure rarely causes
volume of the intracellular compartment may occur. With pulmonary edema acutely in dogs and cats, but it is
pure water loss, it is relatively simple to replace the important to take low colloid osmotic pressure into
circulating volume, but it takes longer to replenish the account when designing an anesthetic regimen because
volume lost from the rest of the body. These concepts pulmonary edema may occur with smaller increases in
are discussed further in Chapter 3. pulmonary hydrostatic pressure. Both ketamine and large
doses of oxymorphone have been shown to increase pul-
PERIPHERAL EDEMA monary vascular pressures. 42,82 If it is thought that low
Peripheral edema usually is a reflection of poor circula- colloid osmotic pressure is contributing to pulmonary
tion, leaky capillaries, or low oncotic pressure. Peripheral edema, therapy should be instituted to increase
edema may have little impact on the course of anesthesia colloid osmotic pressure (e.g., plasma, dextrans,
and surgery, but edema in certain locations may make hetastarch [HES], polygelatins). In the case of pulmonary
induction and maintenance of anesthesia difficult for edema related to leaky membranes, therapy should
the anesthetist. If the limbs are edematous, it may be dif- be aimed at reducing pulmonary vascular pressure
ficult to achieve venous or arterial access. In such cases, it (e.g., nitroprusside, diuretics) and providing supportive
may be necessary to use the jugular vein to place an intra- care for the animal. Supportive care involves provision
venous catheter because the neck usually is less affected of oxygen, suction of froth from the airway, and institu-
than are the limbs. Occasionally, dogs suffer damage to tion of positive-pressure ventilation if necessary. Mechan-
or occlusion of the jugular veins that can be associated ical ventilation may improve gas exchange in patients with
with edema of the head and neck, potentially including pulmonary edema. Positive-pressure ventilation with the
the airway. Great care should be taken when performing addition of positive end-expiratory pressure (PEEP) or
endotracheal intubation in edematous animals because continuous positive airway pressure (CPAP) may not
the affected tissue often is very fragile. It may be necessary reduce lung water but may increase access to previously
to create a tracheostomy if the upper airway becomes collapsed regions of the lung and may increase the capac-
obstructed and there is no way to improve venous drain- ity of the interstitium to hold fluid.
age. Therapy aimed at improving local (e.g., hot packs,
massage) and general (e.g., positive inotropes) circulation PLEURAL FLUID
or increasing colloid osmotic pressure may reduce periph- Pleural fluid acts as a space-occupying lesion and impairs
eral edema. ventilation. In most cases, pleural fluid should be drained
before anesthetizing the animal. If there appears to be a
PULMONARY EDEMA continuous air leak from the lung, it is best to place a chest
Pulmonary edema is of great concern to the anesthetist drain before anesthesia or place a large-gauge catheter
because it impairs gas exchange in the lungs and poten- (e.g., 14 gauge) that can be aspirated rapidly to remove
tially reduces uptake of inhaled anesthetics. Formation any accumulated air. In cases of hemothorax, blood is
of edema in the pulmonary circulation is a result of defibrinated during its residence in the pleural space.
increased hydrostatic pressure, decreased colloid osmotic Accumulated blood can be aspirated from the pleural
pressure, or damage to the endothelium allowing leakage space and given back to the animal intravenously without
of fluid. Increased hydrostatic pressure may be caused by providing additional anticoagulants. Autotransfusion
absolute (e.g., volume overload) or relative (e.g., should only be performed if there is minimal risk of