Page 549 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid and Diuretic Therapy in Heart Failure 537

R
P
T

Systole
Expiration Inspiration

Diastole

Inflate
balloon
Figure 21-7 Pressure tracing recorded from the pulmonary artery of a dog undergoing diuretic treatment
of congestive heart failure. The pulmonary artery pulsatile pressure and occlusion (wedge) pressure are
indicated. Balloon inflation is complete at the arrow. Notice that the pulmonary artery diastolic pressure is
closely related to the wedge pressure. The pulmonary artery diastolic pressure is often used to track changes
in the wedge pressure (provided there is no pulmonary vascular disease). Variations in the pressure recording
baseline are related to ventilation. Measurements are generally made during expiration to avoid the “dips”
associated with negative intrapulmonary pressures.

Special end-hole, balloon-tipped catheters (Swan-Ganz) Cardiac output also can be measured when the cath-
can be used to occlude pulmonary arterial flow temporar- eter is equipped with a thermistor near the catheter tip
ily, permitting measurement of the damped left atrial and a cardiac output computer is available (see
pressure waveform, which is transmitted through the Figure 21-6, B). With this information, four potential
44
valveless pulmonary venous and capillary beds. 44,45,78,163 hemodynamic subsets may be encountered :
The mean value of such a determination is called the • Normal cardiac output and normal pulmonary capil-
pulmonary capillary wedge pressure and is equivalent lary wedge pressure (the normal situation)
to the mean left ventricular filling pressure (but not • Normal cardiac output with high pulmonary capillary
equivalent to the end-diastolic pressure in some patients) wedge pressure predisposing to edema (left-sided
(Figure 21-7). Pulmonary edema generally is associated CHF with volume expansion)
with pulmonary capillary wedge pressures greater than • Low cardiac output and low pulmonary capillary
20 to 25 mm Hg. These values are guidelines, and even wedge pressure (volume depletion, as with excessive
higher values may not be associated with edema in chronic diuresis)
left-sided heart failure. The clinician can measure the • Low cardiac output and high pulmonary capillary
pressure filling the left ventricle and estimate the tendency wedge pressure (severe left-sided heart failure, cardio-
to form pulmonary edema by determining whether low genic shock)
(<7 mm Hg), optimal (12 to 18 mm Hg), or high TheCVPalsocanbemeasuredthroughdual-portSwan-
(>20 mm Hg) venous pressures are present in the cardiac Ganz catheters (see Figure 21-6, A). In biventricular CHF,
patient (see Figures 21-6 and 21-7). 45 The rate of fluid both the wedge pressure and CVP are abnormally high (see
administration, diuretic dosage, and cardiac therapy are Figures 21-7 and 21-8). With excessive diuresis of the
guided by these measurements. Marked reductions in pul- patient, both pressures are reduced. A relatively common
monary capillary wedge pressure can be observed in some situationafterdiuresisinthosewithprimary left-sidedheart
patients after administration of furosemide, hydralazine, failureispersistentlyhighwedgepressurewithrelativelylow
or sodium nitroprusside. Noninvasive estimation of left (<5 mm Hg) CVP and normal jugular venous pressure.
atrial pressure may be possible in the future by application This situation can lead to reduced right-sided filling and
of advanced Doppler echocardiographic methods, in par- prerenal azotemia. Reducing the diuretic dosage improves
ticular, the ratio of early ventricular filling velocity cardiac output but may exacerbate pulmonary edema.
(E-wave) to the tissue Doppler velocity (Ea-wave). Noninvasive determination of cardiac output is feasible