Page 392 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

382 FLUID THERAPY

86 doses recommended above and administered rapidly
(< 5 minutes). The impact of this maneuver on CVP
ECG II 10
(both the magnitude of any increase and how quickly it
returns to its baseline value), heart rate, pulse profile,
mucous membrane color, capillary refill time, blood pres-
sure, and skin temperature can then be used to make
inferences about venous return relative to cardiac perfor-
mance. If the change seen in those parameters is modest
and the CVP returns to baseline rapidly, another fluid
challenge is administered, and the process is repeated.
a c
If necessary, repeated fluid boluses may be given until
v the CVP is elevated 2 to 4 cm water (2 to 3 mm Hg)
x above baseline and takes 10 to 15 minutes to fall back
to its baseline value. Once this condition is met, it is likely
y
that blood volume and venous return are nearly optimal
3
M relative to cardiac performance and that further increases
Figure 15-14 Tracing from a patient monitor screen with in CVP are unlikely to yield significant increases in cardiac
simultaneous display of the electrocardiogram (top) and right atrial output. In animals with normal right-sided heart function
pressure (RAP, bottom). The RAP trace is characterized by the and normal pleural and intraabdominal pressures, the
positive a, c, and v waves and by the two negative depressions CVP should not be pushed higher than about 15 cm of
termed the x and y descents. The a wave represents the increase in water (10 to 12 mm Hg). When that pressure is reached,
RAP during atrial contraction; the c wave represents the slight it is likely that pulmonary venous pressure is above 12 to
increase in atrial pressure as the tricuspid valve bulges into the right 15 mm Hg (assuming that the left and right ventricles are
atrium during early ventricular contraction; and the v wave functioning symmetrically), increasing the likelihood of
represents the increase in pressure that occurs as blood flows into pulmonary edema. When this limit has been reached, giv-
the atrium while the tricuspid valve is still closed. The x descent
corresponds to the period of ventricular ejection when blood is ing more fluids will not help cardiac output (because it
emptied from the heart. The y descent represents the decrease in will no longer significantly increase EDV) but will only
atrial pressure that follows opening of the tricuspid valve and rapid make the patient congested. If there is a need to further
blood flow into the ventricle. The mean RAP is 3 mm Hg, and the increase cardiac output or blood pressure, positive inotro-
electrocardiograph-derived heart rate is 86 beats/min. pic agents such as dobutamine and/or vasoactive drugs,
such as norepinephrine or vasopressin, are administered.
CVP monitoring is also useful in less critical situations.
connected to the catheter with a short, stiff tube is more
When administering fluids to an animal with oliguria or
accurate in these patients.
congestive heart failure, the CVP can be used to monitor
Measurement of CVP in animals during fluid challenge
therapy and help prevent inadvertent overadministration
yields important information about cardiovascular status.
of fluids. In that setting, the baseline CVP is measured
As intravenous fluids are administered and the intravascu-
before fluid therapy is begun and subsequently measured
lar blood volume expands, venous return and CVP begin
at intervals frequent enough to minimize the risk of fluid
to increase. A rapid infusion of 20 mL/kg of crystalloid
overload, usually every 2 to 8 hours. During chronic
or 5 mL/kg of colloid into a euvolemic animal with nor-
(slow) fluid administration, significant increases in CVP
mal cardiac function results in a modest increase in CVP
will not occur until the venous system’s volume capacity
(2 to 4 cm H 2 O) that returns to baseline within 15
has been reached. Since this typically occurs only after the
minutes. A minimal increase or no increase in CVP
onset of physical signs of edema or congestion, any
implies that the vascular volume is markedly reduced.
observed increase merits evaluation.
A CVP that increases and returns to baseline rapidly
(<5 minutes) implies that there is reduced vascular vol-
ume and that the initial volume load has been
accommodated by rapid changes in vasomotor tone. A REFERENCES
very prolonged return to baseline (>30 minutes) 1. Anter AM, Bondok RS. Peripheral venous pressure is an
suggests that the intravascular blood volume is elevated alternative to central venous pressure in paediatric surgery
relative to cardiac performance. A large increase in CVP patients. Acta Anaesthesiol Scand 2004;48:1101–4.
(>4cm H 2 Oor >3 mm Hg) implies reduced cardiac 2. Arinzon Z, Feldman J, Fidelman Z, et al. Hypodermoclysis
compliance or increased venous blood volume or both. (subcutaneous infusion) effective mode of treatment of
dehydration in long-term care patients. Arch Gerontol
When administering fluids to treat a dog or cat with
Geriatr 2004;38:167–73.
noncardiogenic shock, an immediate therapeutic goal 3. Bjornson HS, Colley R, Bower RH, et al. Association
may be to give an intravenous fluid challenge using the between microorganism growth at the catheter insertion