Thoracic imaging in emergency situations (Proceedings)

We experience veterinary emergencies on a weekly, if not daily, basis. Rapid and accurate patient assessment, diagnostic imaging interpretation and treatment can be the difference between patient survival and death. The goal of this talk is to provide a solid foundation for image interpretation in emergency situations. The emergency situations addressed in this talk are: pleural effusion, diaphragmatic hernia, pericardial effusion, heart failure, non-cardiogenic pulmonary edema, pulmonary thromboembolism, pneumothorax, pneumomediastinum, airway obstruction and hypvolemia.

Pleural effusion

Identifying pleural effusion begins with recognizing the radiographic findings associated with pleural effusion. Typically pleural effusion is characterized by rounding of lung lobe tips, soft tissue opacity between the lung border and the internal chest wall, pleural fissue lines and poor visualization of the ventral cardiac margin. The presence of pleural effusion alone can also result in the appearance of a pulmonary interstitial pattern. Once pleural effusion is identified the next step is to try and identify a cause. Pleural effusion can be divided into blood, pus, water (transudate or modified transudate) and chyle. Radiographic findings can assist in differentiating these fluids but ultimately the diagnosis requires a thoracentesis with fluid analysis and cytology. Knowing the type of fluid present is important in ranking the differentials of the fluid. Also the presence of pleural effusion does not limit the clinician from evaluating the thorax for clues to the underlying cause such as caudal displacement to the carina associated with a cranial mediastinal mass or widening to the cranial mediastinum associated with a coagulopathy. In conjunction with radiographs, thoracic ultrasound is useful in locating fluid pockets, evaluating intra-thoracic structures and guiding needle placement into fluid or masses. Thoracic ultrasound is also used for cardiac evaluation.

Diaphragmatic hernia

Confirming the presence of a diaphragmatic hernia can be difficult due to concurrent pleural effusion. However both ultrasound and iodinated contrast can be used to further evaluate a suspected diaphragmatic hernia. A peritoneogram can be performed to evaluate for a hernia. A peritoneogram is performed by injecting 1.1 ml/kg of iodinated contrast into the abdomen (2.2 ml/kg with severe pleural effusion). After injecting contrast the patient should be wheel barrowed to distribute contrast to the cranial abdomen, and thorax if a diaphragmatic tear is present. Non-ionic iodinated contrast is preferred since it will pull in less fluid thereby resulting in less contrast dilution and better contrast visualization.

Pericardial effusion

The presence of pericardial effusion (PE) can severely debilitate a patient. Right atrial tamponade associated with PE can result in decreased venous return and diminished cardiac output. Acute pericardial effusion can be difficult to identify radiographically since the cardiac silhouette has not had time to expand. However secondary changes associated with pericardial effusion can be more consistent. When there is a suspicion of pericardial effusion the clinician should evaluate the radiographs for caudal vena caval distention, hepatomegaly, ascites and pleural effusion. The presence of pulmonary venous congestion can be used to help distinguish DCM from pericardial effusion. Ultrasound is a more sensitive and specific diagnostic to identify pericardial effusion, evaluate for a right atrial or heart base mass and guide the pericardiocentesis. Once pericardial effusion is confirmed the abdomen should be evaluated for evidence of neoplasia and chest three view chest radiographs should be obtained prior to pericardiocentesis, if possible, to evaluate for pulmonary metastases.

Left heart failure

In dogs left heart failure typically results in perihilar edema. Occasionally subpleural edema may also be present. Both radiographs and ultrasound are used in combination to confirm the presence of left heart failure. Radiographic evidence of left heart failure consist of perihilar edema, pulmonary venous congestion and typically cardiomegaly. Ultrasonographically, left atrial enlargement is used to confirm the presence of left heart failure. The left atrium to aorta ratio is used to evaluate for left atrial enlargement.

Left heart failure in cats can be more challenging. Cats do not always have perihilar edema but rather can have patchy pulmonary infiltrates or even have a bronchial pattern associated with heart failure. Pulmonary venous congestion on radiographs and left atrial enlargement on ultrasound are more consistent indicators of heart failure in the cat.

Non-cardiogenic edema or High permeability edema (NCE)

NCE is characterized by perihilar and caudodorsal edema without underlying heart disease. Underlying causes include vasculitis, seizures, asphyxiation, upper airway obstruction, electric shock and respiratory distress syndrome. Pulmonary lymphoma, although not a true non-cardiogenic edema, can mimic the appearance of NCE. NCE does not respond to lasix therapy like left heart failure.

Pulmonary thromoembolism (PTE)

PTE has many different appearances. Radiographically PTE can have no changes, have an interstitial or alveolar infiltrate or have a lucent appearance associated with oligemia (small vessels). On cardiac ultrasound there may be right sided enlargement, pleural effusion and evidence of pulmonary hypertension. Pulmonary hypertension can be identified by a tricuspid regurgitant jet > 2.75 m/sec (> 30 mmHg) or a pulmonic insufficiency greater than 1.9 m./sec. Other indirect indicators of pulmonary hypertension include RV hypertrophy, IVS flattening during systole and pulmonary artery enlargement.

Pneumothorax

Free air in the thorax can be due to trauma, bulla rupture or lung pathology. Pneumothorax is best identified radiographically by visualizing sharp lung margins. Dorsal elevation of the heart is another radiographic finding but the reason the heart appears elevated is not due to air under the heart but rather due to collapse of the lung which normally elevates the heart away from the chest wall when the patient is in lateral recumbency. Deep chested dogs or hypovolemic dogs can have a pseudopneumothorax appearance. Thoracentesis with air removal is typically the first treatment option. If the pneumothorax persists then constant suction may be required. Thoracic CT is typically used to try and identify a bulla if no underlying cause, such as trauma, is suspected.

Pneumomediastinum

Pneumomediastinum is characterized by gas around the trachea, mediastinal vessels and occasionally around the heart. Typically gas is also present in the subcutaneous tissues and sometimes tracking into the retroperitoneal space. Identification of the pneumomediastinum may be relatively easy but identifying the underlying cause can be more difficult and potentially more life threatening. Gas in the mediastinum may come from a tracheal tear, esophageal tear or potentially from lung pathology that is adhered to the mediastinum and leaking. Esophageal tears can be extremely serious since esophageal contents can leak into the mediastinum resulting in a mediastinits. Iodinated contrast should be used to evaluate esophageal integrity.

Upper Airway Obstruction

Dogs and cats can differ in their radiographic appearance of this condition. Dogs typically will have hypoinflated lungs. Tenting of the diaphragm may also be present. In contrast, cats will more commonly have pulmonary hyperinflation and caudal retraction of the diaphragm. In severe cases the cat's sternum may be dorsally elevated. In both cases, radiographs of the neck should be obtained to evaluate for caudal retraction of the larynx and gas dilation of the naso and oropharynx. Also the radiographs should be evaluated for a foreign body. Ultrasound can be used to evaluate for laryngeal disease.

Hypovolemia

Radiographically this is characterized by a small heart (microcardia), small caudal vena cava and small pulmonary vessels. When the heart is small it can lift away from the sternum and have the false appearance of a pneumothorax. Underlying causes for hypovolemia include hemorrhage (abdominal), shock or Addison's disease.