Page 390 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

380 FLUID THERAPY

quantitatively similar to the pressure in the right ventricle Amplified
at the end of diastole. As RAP and right ventricular end- 10
diastolic pressure (EDP) increase, right ventricular end-
diastolic volume (EDV) increases. The relationship 8 Normal
between ventricular EDP and EDV is not linear
(Figure 15-11). At low ventricular volumes, an increase 6
in EDV does not increase ventricular EDP significantly.
At high ventricular volumes when the limit of ventricular Cardiac index (L/min/m 2 ) Depressed
distention is reached, small increases in EDV increase 4
both ventricular EDP and atrial pressure substantially.
Clinically, if EDV is increased by administration of intra- 2
venous fluids, there is little initial increase in EDP (steep
portion of curve, Figure 15-11) until the limit of ventric-
ular expansion is reached. At that point, administration of –4 0 4 8 12
more fluids no longer increases EDV substantially, but
Right atrial pressure (mm Hg)
EDP and atrial pressure increase rapidly (plateau portion Figure 15-12 Family of Starling curves plotting the relationship of
of curve, Figure 15-11). Ventricular EDV is an important right atrial pressure (RAP) (or right ventricular end-diastolic
determinant of stroke volume and cardiac output. The pressure) with cardiac index (CI). In the normal heart, small
relationship between EDV and stroke volume is nearly increments in RAP within the normal range (0 to 4 mm Hg) yield
linear in normal animals; as EDV increases, stroke volume large increases in CI. This response is augmented in dogs with
and cardiac output increase according to the Frank-Star- endogenous sympathetic cardiac stimulation or in dogs receiving
ling law of the heart. In many situations (e.g., shock inotropic drugs (“amplified” curve) and is depressed in dogs with
states) it is desirable to increase both stroke volume and myocardial failure (“depressed” curve).
cardiac output maximally to optimize oxygen transport
to tissues. This is accomplished most effectively by
increasing ventricular EDV. output. At higher pressures, a large increase in RAP does
EDV is not easily measured clinically. However, CVP not substantially increase cardiac output (plateau phase of
can be monitored to make inferences about possible Frank-Starling curves in Figure 15-12).
changes in ventricular EDV or cardiac output. Just as The relationship between CVP, RAP, EDV, cardiac
the relationship between EDP and EDV is not linear, output, and the vascular system is complex and dynamic.
the relationship between CVP or RAP and cardiac output Consequently, a single measurement of CVP usually
is not linear (Figure 15-12). At low pressures, a small provides no information about cardiac output or vascular
increase in CVP/RAP generates a large increase in cardiac blood volume. Indeed, even when used to evaluate
hemodynamic response to fluid challenge, the CVP can
be misleading, particularly when the fluid challenge is
38,84
+50
Although there is a general
administered slowly.
Left ventricular volume change (mL) +30 and yet have a normal CVP. Nevertheless, repeated
trend for animals with reduced blood volume to have a
low CVP, a dog or cat could experience lethal blood loss
+40
measurements during fluid therapy can give important
clues about the relationship between blood volume and
cardiac function. When these results are interpreted in
light of other clinical findings, valuable information
+20
about hemodynamic status may be obtained.
MEASUREMENT
+10
A central venous catheter must be in place. The catheter
tip should ideally reside within the thorax just outside the
right atrium. A catheter placed in the lateral saphenous
5 10 15 20 25 30 35 40
vein and positioned so that the tip resides in the caudal
Left ventricular pressure (mm Hg) 1,44,83
Figure 15-11 Graph depicting the relationship between left vein cava may be an acceptable substitute. Materials
ventricular diastolic pressure and left ventricular volume gain in the necessary for CVP measurement using a water manome-
dog. As pressure increases from zero, diastolic volume increases ter include:
rapidly until the limit of the ventricular distention is reached. At that 1. Central venous catheter in place
point, even large increases in pressure will not increase ventricular 2. Water manometer (Medex manometer set, Smiths
volume substantially. Medical, London)