Page 370 - Feline Cardiology

386 Section M: Pulmonary Arterial Disorders

to detect thromboemboli. Thoracic radiographs are
routinely performed when respiratory signs or severe
debility are present. In cats with pulmonary thrombo-
embolism, radiographic abnormalities may include
pleural effusion (n = 11/20), alveolar opacification
(n = 8/20), conspicuous lucency of circumscribed areas
of the lungs suggesting hypoperfusion (n = 5/20), cir-
cumscribed mass (n = 3/20), and peribronchial and
interstitial markings (n = 2/20) (Schermerhorn et al.
2004; Norris et al. 1999; Sottiaux and Franck 1999).
Vascular changes may coexist or may occur independent
of pulmonary parenchymal lesions, and the vascular
changes may include asymmetrical enlargement in vas-
cular size or distribution (n = 6/15), proximal vascular
enlargement coupled with disproportionate tapering of
central or peripheral pulmonary arteries (“pruning”)
(n = 5/15), or both (Norris et al. 1999; Sottiaux and
Franck 1999). Some cats with pulmonary thromboem-
bolism have normal thoracic radiographic findings,
despite clinically evident dyspnea (Norris et al. 1999). Figure 25.1. Thoracic radiograph of an adult cat with pulmo-
The presence or absence of thoracic radiographic abnor- nary thromboembolism, dorsoventral view. Note the markedly
malities is both poorly sensitive (as evidenced by the lack enlarged left caudal pulmonary artery (arrows). Paradoxically, the
of suspicion of thromboembolism antemortem) and thromboembolus was best seen in the right pulmonary artery on
nonspecific (since no single radiographic finding pin- echocardiography.
points pulmonary thromboembolism). However, severe
dyspnea that is not seemingly related to upper airway normal value is <15 mm Hg, indicating adequate lung
obstruction, in a cat with normal thoracic radiographs, perfusion and diffusion of oxygen into blood. Values
should prompt a strong suspicion of pulmonary >15 mm Hg may occur with many pulmonary disorders;
thromboembolism. in the presence of radiographic pulmonary abnormali-
Echocardiography may rarely reveal a thrombus that ties, an elevated A-a gradient adds little diagnostic infor-
extends to the pulmonary trunk and pulmonic valve.
mation. However, in the absence of radiographic
Pulmonary Arterial Disorders such as respiratory distress (Pouchelon et al. 1997) or as gestive of pulmonary thromboembolism. The A-a gradi-
Such a finding may occur in cats with overt clinical signs
abnormalities, an elevated A-a gradient is strongly sug-
an incidental finding, as in the 6-year-old female spayed
ent is calculated as follows:
domestic shorthaired cat whose thoracic radiograph is
−
(
A a gradient mm Hg)
shown in Figure 25.1. This cat had a history of
FIO P b −
P H O −
−
occult/“asymptomatic” hypertrophic cardiomyopathy,
=
(
)
(PaCO /R) PaO 22
2
2
which was treated daily with atenolol 12.5 mg PO; on
routine echocardiographic follow-up 1 year later, a large
for room air, or 0.4 for 40% oxygen), P b is barometric
thrombus occupied the entire right pulmonary artery to
pressure (627–643 mm Hg; instantaneous values for any
the level of the right and left main pulmonary artery where FIO 2 is the fraction of inhaled oxygen (e.g., 0.21
bifurcation without producing any associated clinical location in the U.S. may be found at www.weather.com),
manifestations of illness (Etienne Côté, unpublished P H2O = 47 mm Hg, R= 0.8, and PaO 2 and PaCO 2 are
observation, 2004). obtained from the arterial blood gas measurement.
Arterial blood gas measurement may provide useful
information that helps increase the suspicion of pulmo- Advanced Testing
nary thromboembolism. Hypoxemia and hypocapnia Confirmatory tests for pulmonary thromboembolism
have been documented in some cases in other species, in humans include radiographic or computed tomo-
with hypercapnia in severe cases. An elevated alveolar- graphic angiography, and nuclear scintigraphy. Given
arterial oxygen difference can occur with pulmonary the limited availability of such modalities and hemody-
thromboembolism. The alveolar-arterial oxygen differ- namically fragile state of many cats with pulmonary
ence (A-a gradient) is the step in oxygen content between thromboembolism, confirmatory testing is undertaken
the alveoli of the lungs and the arterial circulation. A in very few suspected cases. One cat that underwent