Page 584 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Shock Syndromes 571

because 75% of the fluid distributes into the extravascular ultrasonic. The oscillometric method (e.g., Dinamap,
space. An increase in CVP is expected following fluid GE Healthcare Systems, Waukesha, Wis.; Cardell, CAS
therapy. However, if the CVP subsequently drops more Medical Systems Inc., Branford, Conn.) involves place­
than 3 cmH 2 O and the animal appears less stable follow­ ment of an appropriate-sized blood pressure cuff over a
ing redistribution, further therapy with colloids or blood peripheral artery. The mechanism of blood pressure mea­
products may be indicated. Additionally, sources of surement is to determine the oscillation of the artery at
intravascular volume loss or bleeding should be sought. systolic and mean arterial pressures and convert this mea­
surement to a numerical blood pressure. The diastolic
ARTERIAL BLOOD PRESSURE pressure is the pressure at which the maximal oscillation
Arterial blood pressure is defined as the force that is has decreased by 80%. Therefore diastolic pressure
exerted by the blood on the arterial wall. Arterial blood measurements are least accurate. The animal should be
pressure is not cardiac output, and it should not be placed in lateral recumbency to ensure that the limb is
assumed that adequate blood pressure is synonymous with near the level of the heart. Appropriate cuff size is critical
adequate cardiac output. In fact, cardiac output is a deter­ to obtain adequate readings. The width of the cuff should
minant of mean arterial blood pressure (i.e., mean arterial be approximately 40% of the circumference of the limb.
pressure ¼ cardiac output systemic vascular resistance). A cuff that is too large results in falsely decreased values,
If systemic vascular resistance is increased secondary to and a cuff that is too small results in falsely increased
vasoconstriction, the result is increased blood pressure. values. The oscillometric method provides systolic, dia­
However, cardiac output can decrease during hyperten­ stolic, and mean arterial pressures, as well as heart rate.
sion. An animal in pain can have hypertension yet lower The primary disadvantages of the oscillometric method
than normal cardiac output. The animal with poor include the cost of the equipment and inaccurate or
myocardial performance because of SIRS and vasoconstric­ unobtainable readings in animals weighing 5 to 10 kg.
tion caused by pain or hypothermia can have very poor car­ The Doppler ultrasonic method uses the Doppler
diac output. Therefore blood pressure monitoring should effect to detect movement of red blood cells past a crystal
be used in addition to other monitoring techniques to pro­ that emits Doppler waves. Each pulse of blood is
vide the most accurate assessment of cardiovascular status. converted to a sound that can easily be heard. The crystal
Arterial blood pressure can be measured by direct or is placed over a peripheral artery with an appropriate-
indirect methods. Direct measurement of arterial blood sized cuff placed proximal to the crystal. A sphygmoma­
pressure requires a catheter placed in a peripheral artery nometer is attached to the cuff and inflated until no
(usually dorsal pedal or femoral), a pressure transducer, sound is detected. The pressure is slowly reduced until
and a monitor. Accurate measurement of systolic, dia­ the first audible pulse is detected. Only systolic blood
stolic, and mean arterial pressures can be obtained with pressure is measured on a reliable basis, but diastolic pres­
proper positioning of the transducer (i.e., at the level of sure also can be obtained. The first audible pulse is the
the heart) and adequate calibration of equipment. The systolic blood pressure as indicated on the sphygmoma­
arterial waveform may be used to detect early deteriora­ nometer. The pressure continues to be slowly removed
tion of the cardiovascular system (i.e., flattening of the from the cuff until the audible signal changes tone. The
waveform). Placement of an arterial catheter is a chal­ change in tone occurs at the diastolic blood pressure.
lenge, especially in patients weighing less than 10 kg, Advantages of the Doppler method include detection
and the equipment is expensive, which may deter many of an audible pulse, reasonable cost, and reliable use in
clinicians from measuring arterial blood pressure directly. very small patients.
Nonetheless it is the gold standard, becomes much easier
to perform with practice, and also enables easy sampling URINE OUTPUT
and analysis of arterial blood. Urine output can be used as an indirect measurement of
Indirect measurement of arterial blood pressure is renal blood flow. It is easily measured by placing a urethral
most feasible in clinical practice. The most important catheter and collection system. Urine output of less than
factor to remember with indirect methods is that the 1 mL/kg/hr is abnormal in any animal that is volume
values obtained are not necessarily accurate, especially resuscitated, especially those receiving fluid therapy, and
in smaller animals (<10 kg), severely hypothermic possible causes should be explored (e.g., inadequate renal
animals, or those with extreme vasoconstriction. How­ perfusion, acute renal failure, or inappropriate fluid
ever, the trend of values obtained is extremely important retention).
and should be considered more important than the actual
BLOOD GAS ANALYSIS
values. Techniques for indirectly measuring blood
pressure have been described elsewhere 59 and are briefly Arterial and venous blood gas analysis can provide
reviewed below. valuable information about the shock patient. These
The two available methods of indirect arterial blood values are readily obtained using a portable “point of
pressure monitoring are oscillometric and Doppler care” blood gas analyzer. Laboratory blood gas analyzers