Page 326 - Clinical Small Animal Internal Medicine

294 Section 4 Respiratory Disease

Echocardiography may show a right‐sided thrombus or a As thoracic cavity dimensions are increased in response
VetBooks.ir thrombus in the main pulmonary artery, or may also to respiratory muscle action, the lungs expand because
of the constant negative intrapleural pressure.
demonstrate changes suggestive of pulmonary hyperten-
Common thoracic wall injuries include blunt trauma,
sion. A normal echocardiogram does not rule out PTE.
Measurement of D‐dimers may have diagnostic utility in penetrating trauma, and flail chest. Blunt trauma may
patients with PTE. A 2003 study determined that D‐ cause crush and shear injuries to both soft tissues and
dimer concentrations >500 ng/mL were 100% sensitive skeletal structures; advantageously, the skeletal struc-
for predicting thromboembolic disease but specificity tures of the chest wall are relatively resistant to blunt
was 70%; conversely, D‐dimer concentrations greater force trauma. However, muscle is uniquely sensitive to
than 1000 ng/mL had 80% sensitivity and 94% specificity. crushing injury, and when damaged in low‐velocity acci-
The sensitivity of D‐dimer in the diagnosis of PTE is dents may become edematous and inelastic, and may
more important than specificity, as false negatives may lose the ability to efficiently and effectively contract.
have fatal consequences. Tests for hypercoagulability, Accordingly, thoracic wall compliance decreases and
such as antithrombin, fibrinogen, and thromboelastog- work of breathing subsequently increases. This may
raphy, should also be considered. Advanced imaging manifest as tachypnea but breaths may be shallow or
such as V–Q scans, spiral computed tomography, and/or deep; subsequent hypoxemia and/or hypoventilation are
pulmonary angiography might ultimately be required for relatively common. High‐velocity accidents are also
definitive diagnosis of PTE. associated with shearing injury. While soft tissue dam-
Pulmonary thromboemboli dissolution may begin age to the thoracic wall rarely contributes to patient
without treatment within hours of formation and com- morbidity, energy may be readily transmitted to intratho-
pletely dissolve within days. Nevertheless, a prothrom- racic organs to induce both crush and shearing injuries
botic tendency persists to promote continued formation at these sites. Additionally, a blunt force applied laterally
of pulmonary thromboemboli. Goals of treatment are to the thoracic wall may cause rib fractures. However,
reversal of the prothrombotic state and correction of pressure applied dorsoventrally to the thoracic cage
hemodynamic and pulmonary changes responsible for infrequently results in rib and/or sternebrae fractures
morbidity and mortality. Treatment usually consists of but such injuries may contribute to reduced thoracic
supportive care, oxygen supplementation, and anticoag- compliance.
ulant therapy; systemic or locally delivered thrombolytic Penetrating injuries to the thoracic wall are relatively
agents may be used, but the benefits must be weighed common in domestic dogs and cats, and common exam-
against potential serious risks. Oxygen administration ples include bite injuries and missile (i.e., bullet, arrow)
aims to correct hypoxemia caused by V–Q mismatch, trauma. These wounds readily induce stretching and
alveolar hypoventilation, and/or diffusion impairment. crushing of tissues in the direct path of penetration.
However, the response to oxygen depends on the degree While the actual chest wall penetration is frequently a
of vascular obstruction; pulmonary shunt occurs when minor issue in affected patients, one must be acutely
more than 50% of the circulatory bed surface area is aware of the potential underlying pleural and intratho-
occluded, yielding venous admixture and decreased oxy- racic injuries caused by the penetrating trauma. The type
gen responsiveness. Resolution of the underlying disease and severity of injury directly influence how a patient is
process should always be the primary goal. presented, and the medical team must be prepared to
triage a patient in profound respiratory distress that is in
need of immediate orotracheal intubation and intratho-
Chest Wall racic stabilization.
Flail chest results from the segmental fracture and/or
The chest wall is composed of bone and soft tissue struc- dislocation of two or more adjacent ribs. This type of
tures, including skeletal muscles, external fascia, costal injury is uncommon in dogs and cats because of the
parietal pleural nerves and vasculature; all play struc- inherent compliance and anatomic shape of the thoracic
tural and functional roles in respiration. The craniodor- cage. Blunt trauma and bite injuries are the most com-
sal external intercostals and parasternal intercostals mon causes of flail chest, and concurrent pulmonary
promote inspiration by pulling ribs in a cranial direction. damage, most notably pulmonary contusions and direct
The interosseous portion of the internal intercostals and injury from costal fractures, is a major contributor to
the caudalmost external intercostals contribute to morbidity associated with this condition; these contu-
expiration by pulling the ribs caudally. Given the normal sions may lead to decreased pulmonary compliance,
negative pressure within the pleural space, it is potential hypoventilation, and shunting, all of which cause
in nature with the parietal pleura held in constant con- hypoxemia. The pathognomonic respiratory pattern is
tact with the visceral pleura in the absence of disease. paradoxical movement of the unstable ribs (flail