Page 345 - Small Animal Internal Medicine, 6th Edition

CHAPTER 20 Diagnostic Tests for the Lower Respiratory Tract 317

increased ventilation cannot correct the hypoxemia because normally exchanged between the alveoli and the blood by
blood flowing by ventilated alveoli is already maximally diffusion across the respiratory membrane. This membrane
VetBooks.ir saturated. ̇ ̇ consists of fluid lining the alveolus, alveolar epithelium,
alveolar basement membrane, interstitium, capillary base­
Except where shunts are present, the PaO 2 can be improved
in dogs and cats with lung regions with low V/Q by providing
supplemental oxygen therapy administered by face mask, ment membrane, and capillary endothelium. Gases must
also diffuse through plasma and red blood cell membranes.
oxygen cage, or nasal catheter. Positive­pressure ventilation Functional and structural adaptations that facilitate diffu­
may be necessary to combat atelectasis (see Chapter 25). sion between alveoli and red blood cells provide an efficient
Ventilation of areas of lung with decreased circulation system for this process, which is rarely affected significantly
̇ ̇
(high V/Q) occurs in dogs and cats with thromboembolism. by disease.
Initially there may be little effect on arterial blood gas values
because blood flow is shifted to unaffected regions of the A-a Gradient
̇ ̇
lung. However, blood flow in normal regions of the lungs Hypoventilation is differentiated from V/Q abnormalities by
̇ ̇
increases with increasing severity of disease, and V/Q is evaluation of the PaCO 2 in conjunction with the PaO 2 . Quali­
decreased enough in those regions that a decreased PaO 2 and tative differences are described in the preceding paragraphs.
a normal or decreased PaCO 2 may occur, as described previ­ Hypoventilation is associated with hypoxemia and hypercap­
̇ ̇
ously. Both hypoxemia and hypercapnia are seen in the nia, and V/Q abnormalities are generally associated with
setting of extremely severe embolization. hypoxemia and normocapnia or hypocapnia. It is possible to
Diffusion abnormalities alone do not result in clinically quantify this relationship by calculating the alveolar­arterial
significant hypoxemia but can occur in conjunction with oxygen gradient (A-a gradient), which factors out the effects
̇ ̇
V/Q mismatching in diseases such as idiopathic pulmo­ of ventilation and the inspired oxygen concentration on PaO 2
nary fibrosis and noncardiogenic pulmonary edema. Gas is (Table 20.6).

TABLE 20.6

Relationships of Arterial Blood Gas Measurements
FORMULA DISCUSSION

Relationship is defined by sigmoid oxyhemoglobin dissociation curve. Curve plateaus at
Pao 2 ∝ Sao 2
greater than 90% Sao 2 with Pao 2 values greater than 80 mm Hg. Curve is steep at Pao 2
values of between 20 and 60 mm Hg (assuming normal hemoglobin, pH, temperature,
and 2,3-diphosphoglycerate concentrations).
Cao 2 = (Sao 2 × Hgb × 1.34) Total oxygen content of blood is greatly influenced by Sao 2 and hemoglobin concentration.
+ (0.003 × Pao 2 ) In health, more than 60 times more oxygen is delivered by hemoglobin than is dissolved
in plasma (Pao 2 ).
These values are increased with hypoventilation at alveolar level and are decreased with
Paco 2 = PAco 2
hypoventilation.
PAo 2 = FIo 2 (P B − PH 2O) Partial pressure of oxygen in alveolar air available for exchange with blood changes
− Paco 2/R directly with inspired oxygen concentration and inversely with Paco 2 . R is assumed to be
̇
̇
On room air at sea level: 0.8 for fasting animals. With normally functioning lungs (minimal V/Q mismatch),
PAo 2 = 150 mm Hg − alveolar hyperventilation results in increased PAo 2 and subsequently increased Pao 2,
Paco 2 /0.8 whereas hypoventilation results in decreased PAo 2 and decreased Pao 2.
̇
̇
A-a gradient quantitatively assesses V/Q mismatch by eliminating contribution of alveolar
A-a = PAo 2 − Pao 2
ventilation and inspired oxygen concentration to measured Pao 2 . Low Pao 2, with a
normal A-a gradient (10 mm Hg in room air) indicates hypoventilation alone. Low Pao 2
̇
̇
with a wide A-a gradient (>15 mm Hg in room air) indicates a component of V/Q
mismatch.
Paco 2 ∝ 1/pH Increased Paco 2 causes respiratory acidosis; decreased Paco 2 causes respiratory alkalosis.
Actual pH depends on metabolic (HCO 3 ) status as well.
A-a, Alveolar-arterial oxygen gradient (mm Hg); Cao 2 , oxygen content of arterial blood (mL of O 2/dL); FIo 2, fraction of oxygen in inspired air
(%); Hgb, hemoglobin concentration (g/dL); Paco 2, partial pressure of CO 2 in arterial blood (mm Hg); PAco 2, partial pressure of O 2 in
alveolar air (mm Hg); Pao 2, partial pressure of O 2 in arterial blood (mm Hg); PAo 2, partial pressure of O 2 in alveolar air (mm Hg); P B,
barometric (atmospheric) pressure (mm Hg); PH 2 O, partial pressure of water in alveolar air (100% humidified) (mm Hg); pH, negative
logarithm of H concentration (decreases with increased H ); R, respiratory exchange quotient (ratio of O 2 uptake per CO 2 produced); Sao 2 ,
+
+
̇
̇
amount of hemoglobin saturated with oxygen (%); V/Q, ratio of ventilation to perfusion of alveoli.