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Pulmonary Thromboembolism
Robert Goggs, BVSc, PhD, DACVECC, DECVECC

Cornell University College of Veterinary Medicine Companion Animal Hospital, Ithaca, NY, USA

Pulmonary thromboembolism (PTE) refers to obstruc­ due to interstitial and alveolar edema. The V/Q mis­
tion of a pulmonary vessel or vessels by a thrombus and matches occur due to small airway constriction, reduced
encompasses both in situ thrombus formation and surfactant production, and development of pulmonary
embolization from elsewhere in the vasculature. In small edema and atelectasis. Small airway constriction occurs
animals, PTE is the preferred term because these mecha­ in both nonperfused and nonembolized areas of lung,
nisms are difficult to differentiate clinically and both which may lead to airway closure and alveolar collapse.
share a common pathophysiology. PTE is associated Surfactant production is reduced in dogs with experi­
with numerous diseases and disorders in small animals mental PTE, leading to fluid transudation into alveoli
(Table 32.1) and many patients with PTE have more than (pulmonary edema). Edema may also develop in nonem­
one predisposing condition. These underlying condi­ bolized regions due to increased hydrostatic pressure
tions can be categorized by how they affect Virchow’s combined with increased microvascular permeability
triad, although formation of thrombi in vivo typically resulting from neutrophil activation. Rarely, pulmonary
involves more than one such effect. infarction and pleural effusion may result from complete
The true incidence of PTE in dogs and cats is occlusion of distal pulmonary vascular branches.
unknown but it is probably more common than the lit­ The cardiovascular consequences of PTE are depend­
erature suggests. Difficulties definitively diagnosing ent upon the extent of vessel occlusion. There is substan­
PTE antemortem, limited numbers of postmortem tial reserve capacity in the pulmonary vasculature, which
examinations being performed, and rapid postmortem likely accounts for the subclinical nature of many PTE
fibrinolysis mean some cases are missed. In a retro­ events. In healthy dogs, >60% of the pulmonary vascula­
spective case series of 29 dogs with confirmed PTE, ture must be occluded before alterations in pulmonary
there was antemortem suspicion in less than 40% of vascular resistance (PVR) reduce pulmonary arterial flow.
dogs in which it was subsequently identified at nec­ Reflex vasoconstriction secondary to alveolar hypoxia,
ropsy. This study suggested a prevalence of PTE in dogs humoral factors such as serotonin released from acti­
of 0.9% over a 10‐year period. In cats, PTE was sus­ vated platelets, and neurogenic reflexes may also contrib­
pected in ~25% cats with respiratory signs and in only ute. Significant pulmonary vascular occlusion leads to
14% of cats in which it was subsequently diagnosed pulmonary hypertension and increased right ventricle
postmortem. Subsequently, a 24‐year prevalence of (RV) afterload. Severe, acute changes in RV afterload
0.06% was reported for PTE in cats. result in dilation and dysfunction. As the RV dilates, the
interventricular septum shifts leftward, impairing filling
and reducing diastolic distensibility, a concept known as
Pathophysiology ventricular interdependence. Consequent reductions in
left ventricular filling decrease cardiac output and may
Pulmonary thromboembolism frequently results in lead to signs of forward failure (hypotension, cardiogenic
hypoxemia, hyperventilation, and dyspnea. Arterial shock). If the patient survives an acute crisis but has
hypoxemia in patients with PTE is secondary to abnor­ residual pulmonary hypertension, then clinical signs of
mal ventilation/perfusion (V/Q) ratios within affected backward failure (hepatomegaly, ascites, pleural effusion)
lungs, potentially complicated by diffusion impairment may develop over the medium to long term.

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical