Page 408 - Clinical Small Animal Internal Medicine

376 Section 5 Critical Care Medicine

improvement in respiratory status. Patients that are little or no improvement in clinical signs of respiratory
VetBooks.ir experiencing dyspnea are often excessively fractious and distress can be indicative of pulmonary thromboembo-
lism (PTE). Respiratory monitoring plans should be
resistant to physical restraint.
Respiratory monitoring in the critically ill veterinary
believed to be present.
patient should include initial and serial evaluation for the tailored to the individual patient and the pathology
presence of all the above physical exam findings. Serial
evaluation often allows the clinician to put quantitative
data (see later) into their proper context. The response to
specific or empiric therapies is best assessed based on a Assessment of the Adequacy
combination of these physical exam findings, imaging, of Alveolar Ventilation
and quantitative laboratory assay results.
The alveolar gas equation or “PCO 2 equation” (PACO 2 ~
VdotCO 2 /V A ) provides the rationale for all forms of ven-
Determination of Response tilatory status monitoring. The equation states that the
to Specific or Empiric Therapies partial pressure of carbon dioxide in alveolar gas varies
directly with CO 2 production and inversely with alveolar
Respiratory monitoring plans are frequently designed with ventilation. The adequacy of alveolar ventilation is
the goal of assessing response to therapy. Serial monitor- defined by the alveolar partial pressure of carbon diox-
ing of respiratory rate is perhaps the most widely used ide. If alveolar ventilation is adequate for the current
approach in most settings (e.g., monitoring response to level of CO 2 production then PACO 2 will be within the
diuretics in presumed congestive heart failure patients). normal range. If alveolar ventilation is inadequate or
Other examples might include the assessment of pulse oxi- excessive, the PACO 2 will be increased or decreased,
metry, respiratory effort, and breathing pattern in response respectively. The rate of production of carbon dioxide
to oxygen supplementation. Such minimally invasive (VdotCO 2 ) is largely irrelevant. Ventilatory adequacy is
approaches can be quite useful but in other cases, more defined relative to current carbon dioxide production no
invasive procedures such as intubation, positive pressure matter what that current level might be. In extreme cases
ventilation, and arterial blood gas sampling are required such as malignant hyperthermia, carbon dioxide pro-
both to correct the problem and to monitor progression. duction exceeds the level that any healthy respiratory
Marked improvement in respiratory status following system could remove from the system; however, those
intubation is expected for upper airway obstruction in patients are still defined as hypoventilating despite hav-
the absence of other co‐morbidities. Likewise, hypoxemia ing normal (or even exceptional) respiratory function.
due solely to hypoventilation should resolve with oxygen Alveolar gas composition is not uniform across all ave-
supplementation in all cases short of apnea or agonal oli and is exceedingly difficult to sample. Thus, the par-
breathing. Failure of hypoxemia to resolve with oxygen tial pressure of carbon dioxide from other sites serves as
supplementation typically indicates significant venous a surrogate in clinical practice. Clinically useful surro-
admixture and physiologic or anatomic right‐to‐left gate samples include arterial blood, venous blood (mixed,
shunting. Hypoxemia that resolves with oxygen supple- central, or peripheral), and end‐tidal exhalate. Typical
mentation in a patient that simultaneously demonstrates canine blood values for PCO 2 are shown in Table 38.1.

Table 38.1 Blood gas analysis reference intervals

Peripheral
Arterial Arterial Mixed venous Central venous venous
(cat) (dog) (dog) (dog) (dog)
pH 7.39 +/−0.08 7.40 +/−0.03 7.36 +/−0.02 7.35 +/−0.02 7.36 +/−0.02

PCO 2 31 +/−5 37 +/−3 43 +/−4 42 +/−5 43 +/−3
Base deficit −2 to +8 −4 to 0 −2 to 0 −4 to 0 −2 to 0
Bicarbonate 18 +/−4 21 +/−2 23 +/−2 22 +/−2 23 +/−1
Total CO 2 N.D. 22 +/−2 24 +/−2 23 +/−2 24 +/−2
PO 2 105 +/−10 102 +/−7 53 +/−10 55 +/−10 58 +/−9
Source: Adapted from Ilkiw et al. (1991). Values are presented as mean +/− standard deviation.
N.D., not determined.