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32 Pulmonary Thromboembolism 319

becomes available, NT‐proBNP may become integral to normal CTPA study essentially rules out PTE as the
VetBooks.ir the evaluation of these patients. cause of the patient’s respiratory distress, unless the
index of suspicion is very high.
Single‐channel CTPA has been used for experimental
Electrocardiography
Electrocardiographic abnormalities are likely uncom­ PTE diagnosis and investigation of PTE following total
mon in acute PTE. The most likely alteration is sinus hip arthroplasty. No emboli were identified in the arthro­
tachycardia secondary to sympathetic stimulation asso­ plasty patients, but the authors stated that the CTPA
ciated with hypotension or hypoxemia. Myocardial protocol enabled consistent diagnostic quality acquisi­
hypoxia or cardiac hypertrophy may cause ST segment tion visualization down to fifth‐generation PA branches.
depression or slurring. Only with chronic pulmonary Multidetector row CT is also increasingly available in
hypertension will ECG morphology changes occur, veterinary medicine. These multislice CT scanners per­
reflecting right ventricular compromise (right axis devi­ mit imaging of the whole thorax in sedated patients
ation, increased amplitude of S‐, T‐ or P‐waves). without the need for breath‐hold techniques to eliminate
motion artifact. Multidetector CTPA has been reported
in dogs using 16‐slice and 64‐ slice scanners, and proto­
Definitive Diagnostic Imaging cols to maximize PA contrast enhancement have been
published. Although the majority of publications to date
Computed Tomography Pulmonary Angiography have focused on normal patients, multidetector CT angi­
Rapid, multislice helical CTPA is key to diagnosis of PTE ography has been successfully used to detect PTE in
in human medicine, and is now integral to PTE diagnosis experimental dogs, to identify a descending aortic
in small animals too. These studies are obtained by per­ thrombus secondary to spirocercosis in a dog and in
forming rapid, spiral thoracic CT concurrent with bolus dogs with PTE secondary to immune‐mediated hemo­
injection of contrast media. The contrast agent is infused lytic anemia (IMHA).
through a peripheral catheter using a pressure injector,
linked electronically to the scanner to ensure peak MRI and Clot‐Labeling Techniques
enhancement of the pulmonary arteries.
Diagnostic criteria for PTE using CTPA are failure to The use of magnetic resonance imaging (MRI) in the
enhance the entire arterial lumen due to occlusive filling diagnosis of PTE is still in its infancy, but recently this
defects, partial filling defects surrounded by contrast technology has been used for angiography in dogs. The
material, producing the “polo mint” or “railway track” safe, noninvasive nature of MRI offers potential advan­
signs, and peripheral intraluminal filling defects that tages over CT – lack of ionizing radiation and no need
form acute angles with the arterial wall (Figure 32.5). A for iodinated contrast material. However, the need for
cardiac gating and, more importantly, a requirement for
general anesthesia limit the utility of MRI‐based tech­
niques in unstable veterinary patients. Nevertheless, MR
angiography appears comparable to CTPA in experi­
mental studies in dogs.
Direct thrombus imaging techniques have also been
developed. Clots can be imaged by identifying the
thrombus degradation product methemoglobin via a
heavily T1‐weighted image. Clot‐labeling techniques
utilizing radiolabeled antibodies against P‐selectin, D‐
dimers or GPIIIa have been performed experimentally in
dogs. The feasibility and utility of these techniques
remain uncertain and they may be best suited to more
stable patients, patients in whom CTPA is equivocal or
those with specific contraindications to iodinated con­
trast material use.
Figure 32.5 A transverse CT angiographic image of the heart and
main pulmonary arteries from a cat with PTE. Right is on the left of
the image. There is a strongly attenuating signal from contrast Other Imaging Modalities
medium in the heart and in the left pulmonary artery (arrow), but Selective pulmonary angiography is now largely obso­
there is substantially lower attenuation by the right pulmonary
artery (arrowhead) due to an occlusive thrombus, which has lete in human medicine. Since selective pulmonary
prevented the entry of contrast material. angiography involves the risk of general anesthesia and