Page 429 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 419

MONITORING CHANGES IN both on the rate of change and on the actual serum con-
COMPOSITION centration of the electrolyte.
Determination of venous saturation with oxygen
Blood samples must be obtained to monitor changes in allows assessment of a combination of volume and com-
the composition of the blood. The results of sodium, position. The assumption with this method is that blood
potassium, chloride, calcium, bicarbonate, pH, carbon entering the tissue must have sufficient oxygen content
dioxide tension (PCO 2 ), PO 2 , osmolality, colloid osmotic (Hb saturation %) and be flowing rapidly enough to
pressure, hematocrit, protein, glucose, urea, and creati- provide adequate oxygen delivery to the tissue. The lower
nine determinations may affect fluid therapy decisions. the supply, the greater the depletion in oxygen content
When a patient requires monitoring of the composition and the lower the venous oxygen saturation. Determina-
of blood, it is important to determine how blood samples tion of mixed venous oxygen saturation would be the
are to be obtained intraoperatively. It often is difficult to ideal measurement from a whole-animal perspective,
obtain samples from peripheral venous catheters (partic- but would require placement of a pulmonary arterial
ularly in small patients), and other sites must be used. catheter. The principle, however, can be applied to indi-
Samples can be obtained from the jugular vein with rela- vidual organs. For example, does the venous saturation
tive ease, and a jugular catheter should be placed if several of blood leaving the kidneys, heart, or the brain suggest
samples are likely to be required. If it is not necessary to adequate blood supply? Fluid therapy in a pig model
measure CVP, a short intravenous catheter can be used designed to provide mixed venous saturation (SO 2 )of
(1.5 to 2 inches). Also useful in the anesthetized patient 60% showed that HES provided better tissue
are the lingual veins. These vessels usually are readily oxygenation than did LRS. 85 Samples are easier to obtain
accessible during anesthesia and can be sampled several from a central venous site than the pulmonary artery,
times without the insertion of a catheter. All of these and the measurement of central venous saturation
measurements can be obtained using such samples, but (ScvO 2 ) may provide a good estimate of mixed venous
147
care must be taken with interpretation of PO 2 . 134 Single saturation. Studies in humans suggest a target ScvO 2
arterial samples can be obtained from the lingual, femo- of 70%.
ral, ulnar, auricular, coccygeal, or dorsal pedal arteries.
If several samples will be required and it is advisable to
know the PaO 2 , an arterial catheter should be placed. In MONITORING CHANGES
most dogs and cats, the most accessible vessel for this pur- IN DISTRIBUTION
pose is the dorsal pedal artery over the metatarsal area. If
this vessel is inaccessible (e.g., bilateral tibial fractures) or Dehydration is monitored using the clinical signs
cannot be catheterized, it is feasible to use the other described earlier. The presence or absence of peripheral
vessels mentioned. If a femoral arterial catheter is placed, edema and ascites should be readily apparent. In some
great care is needed because it is relatively easy for such cases, it may be helpful to measure limb or abdominal cir-
catheters to pull out of the vessel while still attached to cumference to determine whether the fluid accumulation
the skin. Unless a long stiff catheter has been placed in is increasing or decreasing. Measuring the size of the
the femoral artery, it is not advisable to allow the animal abdomen is particularly difficult but still may be of use
to recover with the catheter still in place. The ulnar artery in individual patients. An indelible marker can be
is difficult to catheterize because the shape of the limb used to identify the site of measurement for future refer-
makes it difficult to approach the site at a sufficiently nar- ence and thus improve accuracy. Pleural fluid accumula-
row angle. The auricular arteries are useful in dogs and tion can be monitored only by thoracic radiography or
can be used into the postoperative period, although there by draining the fluid on an intermittent or continuous
is some risk of ischemia with prolonged catheterization. basis.
A catheter can be placed in the lingual artery after induc- ICP can be measured and can play a crucial role in the
tion of anesthesia, but it must be removed before the end management of patients with increased ICP. The catheter
of surgery and the vessel held off for 15 minutes after the is inserted into the cranial vault and attached to a measur-
catheter has been removed to prevent the formation of a ing device. The simplest approach is to use a fluid-filled
sublingual hematoma. Care must be taken when flushing catheter, which can provide sensitive measurements of
auricular and lingual arterial catheters to prevent the ICP and also allow measurement of intracranial compli-
injection of air because air could be introduced into the ance. The latter can be helpful because it can provide
carotid arteries, resulting in air embolism of the cerebral an estimate of the risk of brain herniation. A fiberoptic
arteries. catheter that measures pressure indirectly can be inserted
The electrocardiogram is used to presumptively iden- directly into the brain. The objective measurement of
tify changes in serum electrolyte concentrations. The intraocular pressure with a Schi tz or applanation tonom-
electrocardiogram is useful in this regard because the eter may help guide fluid therapy in patients with high
magnitude of electrocardiographic changes is dependent intraocular pressure.