Page 526 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 21

Fluid and Diuretic Therapy in

Heart Failure

John D. Bonagura, Linda B. Lehmkuhl, and Helio Autran de Morais

Congestive heart failure (CHF) is a clinical syndrome THE NORMAL CIRCULATION
characterized by cardiac dysfunction, abnormal hemody-
namics, neurohormonal activation, release of cytokines, The central circulation is regulated largely by a need to
and renal retention of sodium and water. A cardiac or vas- maintain plasma volume, mean ABP, and tissue perfusion.
cular lesion that limits cardiac output and decreases arte- Of prime importance is the maintenance of normal effec-
rial blood pressure (ABP) triggers heart failure, leading to tive plasma volume and ABP in the central circula-
141,150
a homeostatic state that is characterized by vasoconstric- tion. These two variables depend on cardiac
tion and renal sodium retention. The stereotypical com- output, systemic vascular impedance, and renal regula-
pensatory responses to heart failure support ABP but also tion of sodium and water excretion. The reflexes that con-
promote a maladaptive state that leads to substantial mor- trol the circulation have evolved so that blood pressure
bidity and mortality. and plasma volume are maintained within a narrow range
Advanced CHF, as well as the therapy of this syndrome, even in the presence of sudden physiologic stresses, such
often is associated with alterations in renal function and a as exercise, hypotension, or hemorrhage. Blood pressure
variety of fluid, electrolyte, and serum biochemical and plasma volume are monitored by different mechanor-
abnormalities. Some of these disturbances are mild and eceptors and osmoreceptors located in the arteries, veins,
seemingly well tolerated, but others, such as hyponatremia heart, kidney, and central nervous system. Ultimately,
and acute renal failure, indicate severe circulatory dysfunc- two factors—cardiac output and systemic vascular resis-
tion and a need for urgent therapy. 75 There are tance (more precisely, vascular resistance and arterial
circumstances in which cardiac patients actually require impedance)—determine ABP (Figure 21-1). A change
fluidtherapytomaintainoptimalventricular fillingandpre- in either one of these two variables causes a parallel
vent deterioration of renal function. However, it is more change in blood pressure. Numerous physiologic
common for fluid therapy to produce edema or effusions variables can affect cardiac output and vascular impedance
inapreviouslycompensatedcardiacpatient.Saferestoration (Box 21-1), and many of these factors are perturbed in
of fluidandelectrolytebalanceinthepatientwithcardiovas- CHF. Of particular relevance in this chapter are
cular disease is challenging. To orchestrate such treatment, determinants of plasma volume in health and disease
the clinician must appreciate the pathophysiology of heart (Box 21-2). Plasma volume is a major contributor to
failure and the compensatory changes that develop. This venous pressure and cardiac filling. The serum sodium
chapter addresses some of the clinically relevant pathophys- concentration, as described more fully in Chapter 3, plays
iologic and therapeutic aspects of heart failure. a central role in determining plasma volume. Renal tubu-
Much of our understanding of hemodynamics, renal lar activity, vascular dynamics, hormones, and other vaso-
function, and neurohumoral activity in heart failure stems active factors regulate sodium balance. Abnormalities of
from many experimental studies in dogs and from a lim- sodium excretion are pivotal to the development of CHF.
ited number of clinical investigations of dogs and cats Attention also must be directed to the microcircula-
with spontaneous heart disease. However, studies of fluid tion and factors controlling fluid movement across
therapy in spontaneous CHF in dogs and cats are largely capillaries. Tissue perfusion is crucial for organ functions
unavailable. Accordingly, the recommendations offered such as the formation of urine, muscle contraction, and
here represent our interpretation of relevant animal stud- exchange of oxygen and carbon dioxide. Assuming the
ies and personal experience with the treatment of dogs maintenance of adequate mean ABP, regional vascular
and cats with CHF. resistance largely governs tissue perfusion across the

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