Page 428 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

418 FLUID THERAPY

of hypovolemia, heart failure, vasoconstriction, or port in the pulmonary artery and taking the
endotoxemia. This clinical sign has been examined care- measurements with sophisticated and expensive equip-
fully in humans and was found to be a poor predictor ment. Placement of these catheters in small patients
of volume status. 162 CRT is significantly affected by body (<5 kg) is particularly difficult and makes it virtually
temperature and ambient temperature. 10,74 CRT also can impossible to obtain such readings in a clinical setting.
appear normal immediately after cardiac arrest. In dogs A newer, simpler technique uses access to a vein and an
and cats, it is usual to use the mucous membranes of artery with injection of lithium into a vein and withdrawal
the mouth for testing capillary refill, and this technique of blood from the artery while measuring lithium concen-
may avoid some of the changes occurring in people as a tration (LiDCO, Cambridge, UK). This method can pro-
result of alterations in ambient temperature because the vide a limited number of cardiac output measurements in
temperature of the mouth remains relatively constant. medium- to large-sized dogs. A method for providing
The ability to assess CRT accurately is affected by the continuous cardiac output measurements based on pulse
presence of pigment in the mucous membranes of some contour analysis (PulseCO, Cambridge, UK) also has
animals, making it impossible to obtain a result in these been developed, but it does not respond well to rapid
individuals. changes in cardiac output. 41,53 In humans,
Heart rate increases in response to hypovolemia but is transesophageal echocardiography has been used to esti-
a nonspecific sign. In anesthetized animals that develop mate cardiac output and to set goals for stroke volume to
unexplained tachycardia, I often give a fluid bolus to improve fluid therapy intraoperatively. 70,190 An ideal
determine whether the animal is hypovolemic. stroke volume for a particular patient is established by giv-
A decreased heart rate after fluid infusion without ing fluid boluses and assessing the stroke volume
resumption of tachycardia is indicative of preexisting response. If stroke volume does not increase after a fluid
hypovolemia. bolus, additional fluid therapy would not likely be help-
Low CVP, PCWP, and systemic blood pressure all can ful. An echo/Doppler machine has been used in cats to
imply low circulating volume but also can change for measure cardiac output. 148 Measurements correlated well
other reasons. The CVP and PCWP probably are better with cardiac output determined by thermodilution but
measurements of volume status because they are affected consistently underestimated cardiac output. 148
by cardiac preload, which is largely dependent on blood Urine output decreases with hypovolemia but also
volume. Static pressures such as these, however, still are decreases with hypotension or low cardiac output. If
not very good predictors of overall volume status (see urine output remains relatively normal, it is unlikely that
Chapters 15 and 16 for more information on CVP mea- the animal is hypovolemic. Measurement of urine volume
surement and interpretation). In dogs and cats receiving requires time, and it is difficult to obtain accurate
IPPV and direct arterial pressure monitoring, systolic measurements at shorter time intervals than every hour.
pressure may vary because of the effect of intrathoracic Consequently, measurement of urine volume cannot be
pressure changes on venous return. Although not totally used to monitor acute changes in circulating volume.
predictable, significant decreases in systolic pressures The only available method for the measurement of urine
associated with ventilation are indicative of hypovolemia output involves the insertion of a urinary catheter, and
(assuming ventilation pressure is 10 to 20 cm H 2 O). this involves some risk of introducing a urinary tract infec-
In one study, the systolic pressure variation was approxi- tion (UTI). 133,146,173,194 The risk of UTI with catheteri-
mately 6%, with a 5% loss of blood volume, and it zation is greater in female than in male dogs. 15
increased linearly to approximately 11% with 30% loss If monitoring urine output is necessary, a sterile urinary
of blood volume. 136 Systolic pressure variation was much catheter should be inserted aseptically and immediately
less in hypotension without hypovolemia. 142 Plethysmo- connected to a closed drainage system. 112 The reservoir
graphic techniques are being developed to monitor this of the urinary collection system should be maintained
parameter noninvasively, but the results have not been below the level of the patient. If the animal is being
very promising to date. 31 The PCWP was a better predic- moved, it is best to clamp the drainage system so that
tor of responders to a fluid bolus than was the systolic urine cannot reflux up the tubing into the bladder. The
pressure variation in one study in human cardiac urinary catheter should be left in the patient for the
patients. 12 shortest duration possible because the risk of a UTI
Cardiac output tends to decrease with hypovolemia, increases with every day the catheter is left in place. Ide-
but this is a relatively nonspecific change because cardiac ally, the animal should not receive antibiotics while the
output also decreases with increased systemic vascular catheter is in place (unless the UTI already has been
resistance or myocardial failure. Evaluation of cardiac diagnosed) because use of antibiotics increases the likeli-
output in conjunction with pressure measurements hood of antibiotic-resistant UTI. Withholding antibiotics
allows the clinician to interpret volume status more read- may not be feasible in a surgical setting, and it is impor-
ily. Determination of cardiac output and PCWP can be tant to monitor for development of UTI using urinalysis
carried out by placement of a thermistor and pressure and urine culture.