Introduction

TFAST stands for thoracic focused assessment with sonography for trauma and was developed as a sequela of abdominal FAST (AFAST) scanning as it was noticed that the sub-xiphoid window used in the AFAST scan provided an excellent view of the pleural space and pericardium in people. In dogs, the sub-xiphoid view is less reliable to assess the pericardium since the head is at a variable distance from the diaphragm depending on size, breed and conformation of the thoracic cavity. TFAST was subsequently developed to assess the thorax in dogs for pneumothorax and pleural effusion.

Method

Patients can be positioned either in lateral or in sternal recumbence for this examination. Sternal recumbency is often advantageous in patients with respiratory compromise and allows access to both thoracic walls. Dorsal recumbency is not recommended in patient in respiratory distress. Five different sonographic windows are used and the haircoat is parted in the location of the probe contact and the skin is prepared with alcohol. Spraying alcohol on the haircoat adjacent to the scan window often helps keeping it flattened and away from the scan window. The probe is placed in a left and right "chest tube site" or the highest outward point in the seventh or eighth intercostal space where a chest tube would be placed. The probe is held in place in a horizontal direction to assess the lungs for presence of pneumothorax. Following examination of the left and right chest tube sites, the left and right pericardial sites are evaluated. The ultrasound transducer is placed on the cranioventral thoracic wall behind the elbow, at the point where a heartbeat can be felt through the thoracic wall. From this point, the transducer is fanned back and forth to determine presence of pleural effusion or pericardial effusion. Finally, the transducer is placed caudal to the xiphoid and angled cranially to visualize the caudoventral pleural space and apex of the heart. In some dogs this view provides better visualization of the pericardium and pleural space as the liver can be used as a sonographic window with less gas interference.

Findings

The normal lung surface is not penetrated by the ultrasound beam as all sound waves are reflected back to the transduced due to a large difference in density and speed of sound (called acoustic impedance) in air versus soft tissues. Some of the sound waves are bounced back and forth between the transducer surface and the lung surface, creating a reverberation artifact that is visible as multiple echogenic lines parallel to the lung surface that decrease in intensity as they extend deeper and deeper on the ultrasound image. When observing this gas interface in real-time, a gliding motion can be seen of the lung surface relative to the thoracic wall and ribs during respiration. Free gas in the pleural space results in the same reverberation artifact and has the exact same appearance as the lung surface, but since it is not moving with lung expansion and collapse, it will remain static relative to the thoracic wall without a visible gliding motion. The ribs in transverse are identified as round structures with a hyperechoic surface and a complete distal shadow. Pulmonary disease can be recognized in TFAST examination as interruption of the normally smooth, hyperechoic surface with areas of consolidation seen as hypoechoic tissue and disruption of the normal reverberation artifact.

Pleural effusion is best seen in the pericardial sites. Fluid is hypo- or anechoic and due to the cardiac motion, strands of mediastinal fat or fibrous tissue can often be seen moving within the fluid. Fanning the ultrasound transducer back and forth between the heart and thoracic inlet allows differentiation between pericardial fluid and free fluid in the pleural space. Occasionally, mediastinal masses are seen in this window as well. Most mediastinal masses are hypoechoic or complex in echogenicity but have convex outer margins. Some mediastinal masses can be cystic as well. The volume of pleural effusion present is difficult to measure, but by placing the patient in sternal recumbency an estimation of how far dorsal the fluid extends can be made. The ventral aspect of the lungs is typically collapsed in presence of pleural effusion and presents as small triangular hyperechoic structures that move in and out of the scan field and the pleural effusion with respiratory motion.

Pericardial effusion is recognizable as fluid which directly adjacent to and surrounding the heart muscle. It is confined by a thin, curvilinear, hyperechoic capsule and does not extend between lung lobes. Cardiac tamponade is present if there is collapse of the right heart due to a large volume or an acute onset of pericardial effusion.

Pitfalls

Rapid respiratory motion makes it difficult to determine if the lung surface is gliding along the thoracic wall or if there is pneumothorax present due to the associated rapid movement of the chest wall. Depending on the clinical presentation of the patient and the level of suspicion for pneumothorax, sedation, radiography or thoracocentesis are possible alternatives. Pleural effusion can be missed if only a small amount is present that may not be accumulated in the evaluated windows. Mediastinal cysts in cats or cystic thymoma can mimic pleural effusion since chronic pleural effusion can have a loculated appearance as well with thick tissue strands separating multiple fluid caverns. Pericardial effusion may be mistaken for pleural effusion, or vice versa, if the pericardial sac is not clearly identified. Frequent practice under guidance of an experienced ultrasonographer is recommended to avoid these mistakes.

References

1.  Boysen SR, Lisciandro GR. The use of ultrasound for dogs and cats in the emergency room, AFAST and TFAST. Vet Clin North Am Small Anim Pract. 2013;43:773–797.

2.  Lisciandro GR. Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. J Vet Emerg Crit Care. 2011;21(2):104–122.

3.  McMurray J, Boysen S, Chalhoub S. Focused assessment with sonography in nontraumatized dogs and cats in the emergency and critical care setting. J Vet Emerg Crit Care. 2016;26(1):64–73.