Page 396 - Clinical Small Animal Internal Medicine

364 Section 5 Critical Care Medicine

importance of body weight monitoring is especially true patient but the use of positive end‐expiratory pressure
VetBooks.ir in the patient in which the potential for third space loss (PEEP) will increase CVP measurements.
Although serum lactate is used commonly as an indi-
is a concern.
Quantification of urine output allows assessment of
adequacy of urine output, especially if oligoanuria from cator of hypoperfusion, its usefulness as an indicator of
dehydration is limited since dehydration must be severe
renal injury or polyuria from underlying disease pro- enough to result in decreased oxygen delivery to result in
cesses is of concern. In addition, serial measurements of hyperlactatemia. Elevations in serum lactate can also
USG may provide insight into fluid balance. For example, occur due to severe liver dysfunction and spurious eleva-
consider the patient on crystalloid fluid therapy who has tions in lactate can be seen when difficulty is encoun-
a decrease in urine output. Once technical complications tered with sample acquisition.
such as catheter patency and placement have been con-
sidered and it has been determined that perfusion is
adequate, USG will help to differentiate between physi- Conclusion
ologic and pathophysiologic oliguria. Recall that the use
of hydroxyethyl starches will invalidate the use of specific Fluid therapy is an integral part of small animal practice.
gravity measurement in the urine. Approaching a patient’s fluid needs in a logical fashion
The venous side of the circulation is a capacitance sys- where the needs for rapid volume expansion versus cor-
tem. As such, it has a relatively low pressure until fully rection of dehydration or electrolyte abnormalities are
replete and distended. Once the venous side of the circu- identified and then targeted will maximize the benefits
lation has no further ability to distend, pressure within of fluid therapy. Each patient is unique. Each of the com-
the vasculature will rise. This can be correlated to the monly available fluid types has properties that can be
volumes and pressures within the heart. Based on leveraged to the advantage of our patients. This chapter
Starling’s law, it is known that initial expansion of ven- illustrates how perfusion, hydration, and water and elec-
tricular volume will increase cardiac output and cardiac trolyte balance affect not only the type of fluid therapy
contractility. Overdistension of the ventricles proves that is indicated but also the rate and route of fluid
maladaptive and results in decreased cardiac contractil- administration. By understanding the physiology of fluid
ity as well as increased intravascular pressures. Central balance in the body, clinicians can tailor their fluid pre-
venous pressure is generally reflective of venous volume scription to address interstitial dehydration, correct free
status and can be a useful invasive monitoring technique water and albumin deficits, and manage electrolyte
to guide fluid therapy. When measuring central venous abnormalities safely. This knowledge also allows fluid
pressure (CVP), a catheter is placed in the jugular vein in choices to be altered during rapid volume expansion to
close proximity to but not within the right atrium of the meet the special challenges of each case.
heart. When measured in the absence of pulmonic The decision to use a specific fluid should be based on
obstruction or pulmonary hypertension, CVP approxi- the goals of therapy and how effectively that fluid choice
mates right atrial pressure. CVP can be measured manu- will achieve that goal, instead of a knee‐jerk response.
ally using a water manometer or continuously using a Veterinary patients are dynamic and the unique needs of
pressure transducer. Normal CVP is considered to be each patient vary. The need to identify our patients’
0–10 cmH 2 0 (1 mmHg is roughly 1.36 cmH 2 O). CVP fluid needs and to address them appropriately, even if
should ideally be measured at the end of expiration in the their clinical presentation is unusual, cannot be
spontaneously breathing or mechanically ventilated overestimated.

Further Reading

Boldt J. Modern rapidly degradable hydroxyethyl starches: critically ill dogs and cats. J Vet Emerg Crit Care 2002;
current concepts. Anesth Analg 2009; 108: 1574–82. 12(4): 235–43.
Craft EM, Powell LL. The use of canine‐specific albumin in Valverde A, Gianotti G, Rioja‐Garcia E, Hathway A. Effects
dogs with septic peritonitis. J Vet Emerg Crit Care 2012; of high‐volume, rapid‐fluid therapy on cardiovascular
22(6): 631–9. function and hematological values during isoflurane‐
Hansen B, DeFrancesco T. Relationship between hydration induced hypotension in healthy dogs. Can J Vet Res
estimate and body weight change after fluid therapy in 2012; 76: 99–108.