Page 363 - Feline Cardiology

378 Section L: Comorbidities

mentioned previously, a normal CVP measurement lines the volume that is to be administered in a time
does not exclude left-sided congestive heart failure, frame that considers the rate of loss and patient’s hemo-
which can cause pleural effusion in the cat (see Chapter dynamic stability.
19). An important limitation is that pleural effusion The fluid type is chosen based on the patient’s require-
itself may alter CVP readings: removal of effusion via ment: either replacement or maintenance. Replacement
thoracocentesis decreases CVP by 0–7 (mean 4.5) cm fluids, such as lactated Ringer’s solution (LRS), Plasmalyte
H 2 O in cats with naturally occurring pleural effusion of 146, Normosol R, and 0.9% sodium chloride, are iso-
various causes (Gookin and Atkins 1999). Therefore, tonic crystalloid solutions that are indicated for replac-
measuring CVP on a cat with pleural effusion prior ing fluid deficits. Their sodium content approximates
to thoracocentesis may produce a falsely elevated the sodium content of blood (140–150 mEg/l). Examples
reading and an erroneous diagnosis of cardiogenic of indications include acute blood loss and dehydration.
effusion and right heart failure. To avoid this pitfall, Once a patient is euvolemic/hydrated, however, ongoing
CVP measurements should be made after thoracocente- losses are sodium-poor because sodium conservation is
sis in cats. one of the most important functions of the kidney.
Indeed, daily losses in urine, breath, and other normal
routes contain a total sodium concentration of only
CRITICAL CARE REQUIRING FLUID THERAPY IN 30–40 mEq/l. Therefore, when a patient is euvolemic,
THE CAT WITH HEART DISEASE
administration of replacement-type fluids like LRS con-
A patient that has sustained large fluid losses may require stitutes an excessive administration of sodium. When
immediate intravenous fluid resuscitation. Examples cats have a normal cardiovascular system, this excess
include cats that are hit by cars and suffer massive blood (and resultant expansion of intravascular volume) goes
loss, cats with severe fluid losses due to gastrointestinal unnoticed while renal natriuresis increases and a normal
disease, and various others. When these patients also state is restored. However, in cats with cardiovascular
have underlying heart disease, a dilemma exists because disease such as cardiomyopathy or systemic hyperten-
fluid administration should be sufficient to benefit the sion, these high concentrations of sodium may be poorly
patient, but not excessive such that it triggers congestive tolerated, and iatrogenic congestive heart failure is pos-
heart failure. Some guidelines for optimizing acute fluid sible. Therefore, in any cat (with or without cardiovas-
therapy in the feline cardiac patient are as follows: cular disease) that is euvolemic, parenteral fluid therapy
The time frame of fluid loss is important, because it should be changed when euvolemia is reached, not only
is correlated to the appropriate time frame of fluid in terms of rate of fluid administration, but also fluid
type, to minimize the risk of causing decompensation
replacement. For example, a large volume of blood lost
Comorbidities over the span of two hours can be replaced with blood of an otherwise compensated cardiac disorder.
(or crystalloids or colloids) over an equally rapid period
Maintenance-type fluids are also isotonic crystalloid
solutions, but they are lower in sodium concentration
of time, with minimal likelihood of fluid overload. By
contrast, a patient with debilitating illness lasting several
include Normosol M + 5% dextrose (sodium concentra-
weeks that presents severely dehydrated likely has acti- and contain other solutes to reach isotonicity. Examples
vated several mechanisms, including renal sodium tion is 40 mEq/l), Plasmalyte 56 + 5% dextrose (56 refers
retention and translocation of intracellular fluid to the to the concentration of sodium in mEq/l), and 0.45%
extracellular compartment, to compensate for such sodium chloride in 2.5% dextrose. Many of these solu-
losses. In this case, a gradual (24–48 hour) replacement tions have high levels of potassium (e.g., 10–20 mEq/l),
of fluid is indicated. Under either circumstance, an accu- which means they are ideal for maintaining anorexic
rate calculation of fluid deficits is essential for approxi- and/or hypokalemic patients but must never be bolused.
mating the proper fluid dose. Administration of fluids Individual cats have highly variable tolerances for
consists of replacing deficits, replacing ongoing overt intravascular volume expansion, and certain essential
losses, and offsetting insensible losses. Calculations are factors such as ventricular stiffness in the face of increas-
based on the simple principle that 1 liter of water (or ing diastolic ventricular volumes are difficult or impos-
body tissues) weighs 1 kg. Therefore, 10% dehydration sible to assess accurately in a clinical setting. Therefore,
of a 5 kg cat means 10% of 5 liters, or 1/2 liter (500 ml), monitoring the response to fluid administration is
must be administered to correct the deficit. Added to essential for preventing or minimizing negative out-
this are ongoing overt losses (e.g., loss of 50 ml in comes in cats with cardiovascular disease.
vomitus or diarrhea), and daily insensible losses (approx- Assessment of the respiratory rate and character are
imately 45 ml/kg/day normally lost in breath, urine, important and should be done in a regular and consistent
feces, etc.). The sum total of these three categories out- fashion during fluid administration. An increase in either,