Page 585 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

572 FLUID THERAPY

are prohibitively expensive unless high caseload allows oxygenation in many clinical shock syndromes. 24,63,68
nearly constant usage, whereas portable blood gas However, there are other clinical syndromes that can lead
analyzers can easily pay for themselves in just a few years. to hyperlactatemia, including diabetic ketoacidosis, neo­
One disadvantage of blood gas analysis (arterial or plasia, drugs and toxins, hepatic insufficiency, gastroin­
venous) is that the results obtained represent a single testinal disease, or metabolic disorders. Normal
moment in time, although the status of the patient may reference intervals vary with the equipment used, but
change minute by minute. The partial pressure of oxygen changes in lactate concentration in combination with
reflects the amount of oxygen dissolved in plasma and the other clinical indicators of shock are useful for monitor­
saturation of hemoglobin with oxygen can be determined ing effectiveness of therapy. Lactate concentration should
from the oxyhemoglobin dissociation curve. decrease over time if successful cardiovascular resuscita­
Arterial blood gas analysis provides information tion from shock has occurred. Occasionally, lactate
regarding gas exchange in the lung and arterial acid-base concentration will increase transiently after initiation of
balance. Arterial blood samples are most commonly therapy because improved perfusion results in a “wash­
collected from the femoral or dorsal pedal artery into a out” of waste products that did not previously enter
preheparinized syringe. The partial pressure of oxygen the systemic circulation. Lactate measurement has been
in arterial blood (PaO 2 ) represents the adequacy of gas shown to be an effective predictor of gastric necrosis in
exchange in the lung. dogs with GDV and thus serves as a useful predictor of
A “mixed venous” blood gas sample must be obtained prognosis and survival. 24
from the pulmonary artery, which requires placement of a
specialized catheter. The partial pressure of oxygen in ADDITIONAL THERAPIES
mixed venous blood (PvO 2 ) is a reflection of perfusion
of tissues on a global basis. Normal PvO 2 values range Various forms of shock can all lead to hypotension despite
from 35 to 45 mm Hg. Values less than 30 mm Hg indi­ intravascular volume resuscitation, therefore necessitating
cate poor perfusion and oxygen delivery to the peripheral the use of vasopressor and/or inotrope therapy
tissues. If a thermodilution catheter is not placed in the (Table 23-7). Since both cardiac output and systemic vas­
pulmonary artery to collect blood for PvO 2 determina­ cular resistance affect oxygen delivery to the tissues, ther­
tion, a jugular catheter placed to monitor CVP can be apy for hypotensive patients includes maximizing cardiac
used to collect a venous blood sample that may approxi­ function with fluid therapy and inotropic drugs and/or
mate a true mixed venous sample. modifying vascular tone with vasopressor agents. The
Acid-base analysis from a blood gas sample is reviewed most commonly used vasopressors are exogenous
in Chapter 9. catecholamines (epinephrine, norepinephrine, dopamine,
and phenylephrine). Vasopressin, a nonadrenergic vaso­
PULSE OXIMETRY pressor agent, has also been used for the treatment of
Pulse oximetry measures the saturation of hemoglobin catecholamine-refractory vasodilatory shock.
with oxygen (SaO 2 ) and can be monitored continuously Different sympathomimetics cause various changes in
and noninvasively. The PaO 2 provides information about the cardiovascular system, depending on the specific
oxygen dissolved in plasma, whereas SaO 2 provides infor­ receptor stimulation caused by the drug. 133 Convention­
mation concerning the oxygenation of red blood cells. To ally, adrenergic receptor location and function involves
be of value, pulse oximetry requires pulsatile flow of the a 1 -and b 2 -receptors located on the vascular smooth
blood to the extremities (interdigital web, digit) where muscle cells that lead to vasoconstriction and vasodilata­
it is measured. Many patients with shock have decreased tion, respectively, while b 1 -receptors in the myocardium
blood flow, especially to the extremities, which limits the primarily modulate inotropic and chronotropic activity.
effectiveness of pulse oximetry. The device may also be In addition, there are dopaminergic-1 receptors in the
applied to the tongue for accurate readings, but this tech­ renal, coronary, and mesenteric microvasculature that
nique is difficult in the conscious patient. Other common mediate vasodilatation and dopaminergic-2 receptors in
areas for probe placement are the ear, axilla, vulva, and the synaptic nerve terminals that inhibit the release of
prepuce. A rectal probe may be of value in the conscious, norepinephrine.
recumbent patient. Dopamine has various potential actions on adrenergic
and dopaminergic receptors. 133 Primarily dopaminergic
LACTATE effects are seen at low intravenous doses (1 to
The clinical use of lactate measurement has gained popu­ 5 mg/kg/min), mainly b-adrenergic effects are seen at
larity and acceptance in veterinary practice over the past moderate doses (5 to 10 mg/kg/min), mixed a- and
10 years due to the accessibility and reasonable cost of b-adrenergic effects are present at high doses (10 to
portable lactate analyzers. An elevated blood lactate con­ 15 mg/kg/min), and primarily a-adrenergic effects are
centration is frequently a marker of anaerobic metabolism seen at very high doses (15 to 20 mg/kg/min). The actual
and has been correlated with inadequate tissue dose response relationship is unpredictable in a given