Page 398 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

388 FLUID THERAPY

1.0 mL/kg/hr also is a useful assessment of adequate vol- pressure within this system, such as CVP, MAP, or SBP,
ume, and 1.0 to 2.0 mL/kg is considered optimal with are used as indirect assessments of blood volume. How-
normal renal function, providing that the urine specific ever, various physiologic or pathophysiologic conditions
gravity is within a normal range of concentration may lead to an increase or decrease in pressure with or
(1.020 to 1.030). Lower urine specific gravity values with without loss or gain of fluid.
small volumes of urine may suggest a lack of
concentrating ability, resulting in the urine produced
rather than adequate volume and glomerular filtration CENTRAL VENOUS PRESSURE
rate. A higher urine specific gravity indicates the require- The CVP is a measure of the hydrostatic pressure within
ment for continuing fluid therapy in a patient with nor- the intrathoracic vena cavae. 1,30 The CVP is slightly
mal concentrating ability. When administering fluids to higher than the right atrial pressure (RAP), and RAP is
critically ill animals, measuring preload, stroke volume, quantitatively similar to right ventricular pressure at
or CO (targeted to specific values) is superior to measur- end diastole 30 or preload. 12 However, CVP does not reli-
ing systemic blood pressure. Central venous and PCWP ably predict right ventricular end-diastolic volume. 64
are surrogate markers of preload, but use of the pulmo- Accurate placement of the catheter and consistency in
nary artery catheter (PAC) to measure the PCWP has positioning of the animal are extremely important in
been shown to be inconsistent, of questionable value, interpretation of results and determination of trends. 30
and associated with increased morbidity in human medi- CVP measurements may be obtained from the caudal
cine and is being used less frequently. 39 This technique is vena cava in cats. 48 Keeping in mind potential pitfalls,
also difficult to perform in general practice and therefore measurement of the CVP during a fluid challenge, such
not recommended. The CVP and SABP measurements as would be administered in hypovolemia or acute renal
frequently are used in veterinary medicine to guide fluid failure, can be valuable in assessing the effect of therapy.
resuscitation. Measurement of CVP and SABP are In the hypovolemic patient, for example, if no appreciable
associated with major pitfalls (see Arterial Blood Pressure increase in CVP 30 is observed after a fluid bolus, addi-
and Central Venous Pressure section discussed later), but tional fluid or colloid should be administered (refer to
are still of value when used in conjunction with the physi- Chapter 15 for an in-depth discussion of CVP ). It has
cal examination. Suggested measurements for conditions been my experience that a rapid infusion of 20 mL/kg
requiring optimal resuscitation include 5 to 8 mm Hg of a crystalloid may not be “tolerated” in some patients
(6.5 to 10.5 cm H 2 O) CVP, 80 to 100 mm Hg mean regardless of cardiovascular status (Table 16-4). Nausea,
arterial pressure (MAP), and 100 to 120 mm Hg systolic vomiting, shivering, and restlessness are frequently noted
blood pressure (SBP), and for patients in which the goal is in these individuals (Box 16-1). The reason for the
adequate resuscitation (i.e., those with ongoing noncom- observed signs may be associated with a vagally mediated
pressible hemorrhage), a MAP of 65 mm Hg and a SBP of baroreceptor reflex secondary to atrial stretch. Should
90 to 95 mm Hg are acceptable, physical findings are nor- CVP increase above an acceptable range after such a chal-
mal, until hemorrhage is controlled either spontaneously lenge in an animal with acute renal failure, fluid adminis-
or surgically. Pulmonary contusions and other pulmonary tration should be curtailed or stopped (Table 16-4).
conditions predisposing to capillary leak with increased These are general recommendations, and CVP does not
hydrostatic pressure are additional indications for cau- reliably predict whether administration of a fluid bolus
tious adequate resuscitation. The patient’s base deficit will or will not significantly increase CO under all
or blood lactate concentration also may be used to assess conditions. 66,67 Factors other than intravascular volume
perfusion. The goal should be to achieve an adjusted base that influence CVP measurements include cardiac func-
excess of 0 to þ4 mEq/L and a lactate concentration less tion (e.g., systolic or diastolic dysfunction), pulmonary
than 1.4 mmol/L in cats and 2.0 mmol/L in dogs. hypertension (e.g., pulmonary thromboembolic disease),
Where a central line (jugular catheter) is in place, a recent venous compliance (e.g., increased systemic vascular
study has identified venous saturation (ScvO 2 ) of 68% in resistance), and intrathoracic pressure (e.g., pleural effu-
dogs to reflect a minimal adequacy of perfusion, which is sion, pneumothorax, pericardial effusion, mechanical
similar to that reported in humans. 31 However, as mor- ventilation). Although mechanical ventilation affects
tality significantly decreased above this value, it is CVP, threshold values of CVP in ventilated patients still
recommended that fluid resuscitation with a goal of may be of value to predict hemodynamic instability when
ScvO 2 >70% be achieved. assessed in response to increasing airway pressure induced
by positive end-expiratory pressure (PEEP). 38 In this
study of patients with acute lung injury, subjects with
Intravascular Volume CVP less than 10 mm Hg usually had decreased CO when
Blood is composed of plasma and red cells and is challenged with increasing PEEP, whereas those with
separated from the interstitial and intracellular CVP greater than 10 mm Hg had increased, decreased,
compartments by the vascular walls. Measurements of or unchanged CO.