Page 52 - Feline Cardiology

46 Section B: Diagnostic Testing

Differential diagnoses for an alveolar pulmonary other (pleural fissure lines) as well as from the thoracic
pattern include diseases that lead to filling of the alveoli wall. It may be unilateral or bilateral (most are bilateral
with fluid or loss of air from the alveoli (atelectasis). since fluid may cross the mediastinum in most cats [Fox
Diseases that can lead to a generalized or diffuse increase et al. 1999]). When pleural effusion is present, the radio-
in lung radiopacity include pulmonary edema (cardio- graphic findings will depend on the quantity of effusate.
Diagnostic Testing hematogenous), pulmonary hemorrhage (trauma or ventral portion of the thorax will be evident with a scal-
genic or noncardiogenic), pneumonia (aspiration or
On the lateral projection, an increased radiopacity in the
coagulopathy), disseminated granulomatous disease,
loped appearance of the lungs when a large volume of
effusion is present. On the DV or VD projection, there
and neoplasia. Parasitic pneumonias (e.g., Toxoplasma
gondii) may occur in an acute infection or as a relapsing
will be retraction of the lung borders from the thoracic
disease resulting in multiple, patchy, coalescing alveolar
cardiac border may be partially or totally obliterated.
lesions. Possible causes of noncardiogenic edema include wall and blunting of the costophrenic angles, and the
diseases that alter capillary permeability (infection, toxic The cranioventral and caudoventral segments of the
inhalants, systemic toxins, allergy or anaphylaxis, trauma, mediastium usually appear widened because of the free
uremia), increased capillary pressure or obstruction fluid. In cases of pleural effusion, the VD projection
(fluid overload, lymphatic obstruction, venous obstruc- is preferred over the DV projection for better assessment
tion), decreased oncotic pressure (hypoalbuminemia), of cardiac size and shape, as long as the patient is
or neurogenic causes (head trauma, seizures, electric clinically stable enough to tolerate the restraint.
shock, brain disease). In cats with heartworm disease Thoracocentesis for therapeutic fluid drainage may be
(compared to dogs), lesions of pulmonary arteritis are necessary prior to radiography (see Chapter 3 for more
often less extensive; therefore the main caudal pulmo- information on pleural effusion). If a large volume of
nary arteries must be carefully inspected. Pulmonary effusion is suspected (by clinical signs), radiographic
arterial hypertension results in enlarged pulmonary exposure levels should be increased (often abdominal
arteries; however, the pulmonary veins remain a normal technique chart levels will result in good exposure).
size. Additional possible radiographic changes range Left atrial enlargement is a common finding in cats
from dilation of pulmonary arteries and right ventricu- with heart disease. A comparison of electrocardiography
lar enlargement with “pruning” of pulmonary arteries to (ECG), radiography (lateral projection) and echocar-
a diffuse pulmonary interstitial pattern consistent with diography for diagnostic accuracy at detecting left atrial
pneumonitis. See Chapter 23 for further discussion on enlargement in cats revealed good agreement (r = 0.70;
feline heartworm disease. In contrast, left to right shunt- p = 0.001) between radiography and echocardiography
ing lesions (e.g., most ventricular septal defects) cause (and radiography was superior to ECG) (Schober et al.
an increased volume of blood in the pulmonary vascular 2007). Radiographic indices of left atrial enlargement
system in proportion to the size of the defect and quan- have low sensitivity and high specificity for enlargement,
tity of blood shunting through it. This increased volume with the vertebral heart size method having the best
results in enlarged pulmonary veins and arteries, whereas specificity (0.95) and positive predictive value (0.88–
pulmonary arterial hypertension does not cause an 0.9) of all the variables tested (including subjective
increase in the size of the pulmonary veins. radiographic interpretation). The lower sensitivity of
Pleural effusion is a common sequela of heart failure radiography for detecting left atrial enlargement likely
in the cat, even when heart disease is primarily left- reflected cases of moderate or mild enlargement. Selected
sided. However, pleural effusion can also be noncardio- ECG variables such as P wave amplitude and duration
genic. At least 50 cc of effusion must be present for it to had a high specificity and positive predictive value but
be radiographically apparent in a cat (Owens 1982). a low sensitivity and negative predictive value for left
Potential differential diagnoses of pleural effusion in the atrial enlargement in cats; thus, P wave changes may
cat include hydrothorax (heart failure, uremia, hypopro- suggest left atrial enlargement but their absence does not
teinemia, fluid overload), pyothorax, hemothorax, chy- rule it out (Schober et al. 2007). When left atrial enlarge-
lothorax, neoplasia, and pleuritis. A complete database ment is present, the clinician may notice slight elevation
including CBC, serum biochemistry profile, urinalysis, of the distal trachea and carina in the lateral projection.
retroviral testing, fluid analysis, and thoracic ultraso- If there is left atrial enlargement, but the ventricle is not
nography with echocardiography is often required to also markedly enlarged, the cardiac silhouette has been
differentiate cardiogenic from noncardiogenic pleural described to have the appearance of a tilted ice cream
effusion. Pleural effusion is radiographically evident as cone on the lateral projection (Rishniw 2000) (Figure
focal areas of increased soft tissue opacity located within 6.9). On the VD or DV projection, an enlarged left auric-
the thoracic cavity, which separate lung lobes from each ular appendage, which extends beyond the heart border,