Page 394 - Small Animal Internal Medicine, 6th Edition

366 PART II Respiratory System Disorders

often be observed in the cranial mediastinum, especially on Other considerations in animals with increased perihi-
lateral views. lar opacity on radiographs include atrial enlargement and
VetBooks.ir in the caudal mediastinum. The most common caudal medi- heartbase tumors.
The caudal vena cava and the aorta are normally visible
Evaluation of the heart is described in Chapters 1 and 2.
astinal abnormalities are megaesophagus and diaphragmatic
pleural fluid accumulation.
hernia. Megaesophagus is an important consideration in Right-sided heart failure and pericardial effusion can cause
animals with respiratory signs because it is a common cause
of aspiration pneumonia. ULTRASONOGRAPHY
The mediastinum is normally located in the center of Ultrasonography is indicated in the diagnostic evaluation of
the thoracic cavity. An abnormal shift of the mediastinum dogs and cats with pleural effusion to search for masses,
is identified by a lateral change in the position of the diaphragmatic hernia, lung lobe torsion, and cardiac disease.
heart on ventrodorsal or dorsoventral views. Atelecta- Mediastinal masses, masses involving the pulmonary paren-
sis (i.e., lung lobe collapse), lobectomy, and adhesions of chyma adjacent to the body wall, and masses extending into
the mediastinum to the chest wall can cause the media- the thorax from the body wall may be identified and their
stinum to shift toward the abnormality. Space-occupying echogenicity evaluated. Ultrasonography can also be used to
lesions can cause the mediastinum to shift in the opposite guide aspiration needles or biopsy instruments to the lesion,
direction. although biopsies can be done safely only on solid masses.
The lymph nodes and the heart are mediastinal structures Ultrasonography is also useful for directing needle place-
but are considered separately to ensure a careful evalua- ment during thoracocentesis in animals with localized accu-
tion. The sternal nodes are located immediately dorsal to mulations of pleural fluid. Air interferes with the sound
the sternum near the thoracic inlet at the level of the first to waves, so structures surrounded by aerated lung cannot be
third sternebrae (Fig. 23.4). Enlargement is seen on lateral examined.
views and has the appearance of a discrete mass lesion. The In the emergency setting, ultrasonography can be used
hilar nodes are located at the heartbase around the carina. for rapid, minimally stressful assessment of the pleural
Enlargement is seen as a generalized increased soft tissue space for fluid or air. The acronym for the examination is
opacity in the perihilar region and is most easily seen on TFAST, which stands for thoracic focused assessment with
the lateral view. Common differential diagnoses for hilar sonography for trauma. See the reference by Lisciandro
lymphadenopathy include lymphoma and fungal infec- (2011) for probe positions and characteristic sonographic
tion (especially histoplasmosis). Other differential diagno- signs.
ses include metastatic neoplasia, eosinophilic pulmonary
granulomatosis, and mycobacterial infection. Any inflam- COMPUTED TOMOGRAPHY
matory disease can potentially cause lymphadenopathy. As discussed in Chapter 20, CT is more sensitive than radi-
ography in evaluating the thorax. CT is useful for identifying
abnormalities (e.g., masses, lung lobe torsion, thromboem-
bolism), determining the extent of mass lesions before thora-
cotomy, and increasing the likelihood of identifying cavitary
lesions in patients with spontaneous pneumothorax.

THORACOCENTESIS
Thoracocentesis is indicated for the collection of diagnostic
specimens in dogs and cats with pleural effusion, for removal
of pleural fluid or air to stabilize the condition of dogs and
cats with impaired ventilation, and before radiographic eval-
uation of intrathoracic structures in dogs and cats with
pleural fluid or air. Possible complications of thoracocentesis
are pneumothorax caused by lung laceration, hemothorax,
and iatrogenic pyothorax. Complications are rare if careful
technique is used.
Thoracocentesis is performed with the animal in lateral
or sternal recumbency, depending on which position is
FIG 23.4 less stressful. Fluid or air is usually present bilaterally
Lateral thoracic radiograph obtained in a dog with throughout the pleural space and can be retrieved from
pulmonary neoplasia and sternal and hilar the seventh intercostal space (ICS) by placing the needle
lymphadenopathy. The sternal node is the soft tissue opacity
resting on the caudal half of the second sternebra. The hilar approximately two thirds of the distance from the costo-
nodes are identified by increased soft tissue opacity around chondral junction toward the spine. If initial attempts are
the carina. Several discrete pulmonary nodules are also unsuccessful, other sites are tried or the animal’s position
present. is changed. Fluid may be more successfully retrieved from