There are 5 key structures that can be identified during thoracic VPOCUS of the pleural space and lungs in healthy animals: bat sign, glide sign, A-lines, B-lines, curtain sign.
Patient Position, Probe Selection and Settings for Thoracic VPOCUS
Patients can be in sternal (preferred position for dyspneic patients), standing or in lateral recumbency (the latter is reserved for patients that are not experiencing respiratory distress). Dorsal recumbency should be avoided.
Similar to VPOCUS of the abdomen, shaving is not required, the fur is parted, and alcohol is used as the coupling agent. Depth for pleural and lung ultrasound is generally set at 4–6 cm in most cases. A 6–10 MHz (7 MHz) microconvex probe is generally used. Remember that the two key enemies of ultrasound, bone and air, are encountered when performing ultrasound of the pleural space and lung. This is advantageous as bone and the subsequent rib shadowing provides landmarks to work with, and artifacts are often present when the ultrasound beam encounters air. These artifacts change depending on the underlying status of the lung and pleural space. Assess the patient to decide which clinically important binary questions need answering first. Make sure a thorough evaluation for the specific underlying pathology is undertaken for each question asked.
Normal Findings on Thoracic VPOCUS
Bat sign/gator sign: When the ultrasound probe is placed over the lung and perpendicular to the ribs we can see the rib heads, rib shadowing, and the pleural line. The image obtained is called a “bat sign” or “gator sign” as the rib heads and pleural line resemble the wings and body of a bat, or a gator’s eyes peaking above the water line, respectively. Identifying the bat sign assists novice sonographers in locating the pleural line; the first white line below the rib heads. The pleural line is essential to identify as it is the interface between the parietal pleura of the thorax and the visceral pleura of the lung, and is the location we assess for most pleural and lung pathology.
The glide sign is visualized as a shimmering along the pleural line (pulmonary-parietal interface), represents normal to-and-fro motion of the lung sliding along the chest well during respiration. This is normal.
There are two key rules to remember when assessing the glide sign: 1) the lining of the lung (visceral pleura) must be in contact with the thoracic pleura (parietal pleura) to create the shimmer of the glide sign and 2) the patient must breathe to create the shimmering glide sign.
A-lines: A stands for air
Air is located below the pleural line when the lungs are filled with air and when there is air in the pleural space which occurs with pneumothorax. Therefore, A-lines are seen with normal lung and when a pneumothorax is present.
A-lines are horizontal white lines equidistant from the skin surface to the pleural line that project through the far field of the ultrasound image.
They are a type of reverberation artifact that occurs when ultrasound beams are reflected back and forth between the probe and pleural line due to the presence of air below the pleural line
Hyperechoic streaks originating from the lung surface of the pleural line, extending through the far field without fading, and swinging to-and-fro with the motion of the lung during respiration.
B-lines occur as the result of air and fluid in proximity to each other at the lung surface.
The presence of a small number of isolated B-lines may be normal in healthy dogs and cats (noted in 10–30% of patients). Up to 3 at a single site can still be normal.
Anything more than 3 B-lines at a single site is associated with pathology.
Key criteria to identify a B-line (all criteria must be present):
- Vertical white lines
- Originate at the lung surface
- Moves with the pleura
- Extends to the far field
Obscures A-lines if present.
The caudal border of the thorax is located by identifying the curtain sign; the transition between the thorax and abdomen, which is easily seen with sonography.
It is essential that patient positioning and the underlying pathology be considered when it comes to diagnosing pleural space pathology.
Air and fluid accumulate in different regions of the pleural space depending on the position in which the patient is evaluated.
Fluid tends to accumulate in the most gravity dependent areas while air tends to rise to the non-gravity dependent areas of the pleural space.
There are 3 key findings that help identify the presence of a pneumothorax, two are exclusion criteria, one is an inclusion criteria.
Pneumothorax appears as the absence of a glide sign. The presence of a glide sign rules out pneumothorax with confidence. Lack of a glide sign should prompt consideration of a pneumothorax but a glide sign is not always easy to identify, even in healthy patients.
The presence of B-lines excludes pneumothorax at those focal probe placement sites because B-lines originate from the lung surface.
Finding a lung point confirms a pneumothorax on that side of the thorax. If the glide sign is not seen and there is strong suspicion of a pneumothorax a search for the lung point should be undertaken as identification of the lung point is pathognomonic for a pneumothorax.
The presence of a glide sign excludes pneumothorax at the probe placement site, as the presence of a glide sign requires contact of the surface of the lung with the chest well (air or fluid in the pleural space will prevent the lung from contacting the chest wall and prevent shimmer of the glide sign from occurring).
Sonographically locating the most sensitive thoracic site to diagnose pneumothorax in a standardized manner: air will accumulate at the most caudal dorsal portion of the thorax when the patient is sternal recumbency or in the standing position.
Sternal is the preferred position in which to scan acutely dyspneic patients as it minimizes respiratory distress and subsequently the work of breathing associated with restraining the patient in lateral recumbency.
Identify the most caudal-dorsal site, which is the most sensitive site for air to accumulate with the patient in sternal, and also the region that has the most lung movement making it easier to identify a glide sign.
Defining the Lung Point
If the glide sign is identified with confidence it rules out pneumothorax. Unfortunately, it is not always easy to identify a glide sign with confidence. If this is the case, a pneumothorax can be confirmed by identifying the lung point.
The lung point is defined as the site within the thorax where the lung recontacts the parietal pleura and creates an intermittent glide sign within half the ultrasound beam when the patient breathes. It is the exact point within the thorax where there is a return of the glide sign: movement of the probe from an area where there is no perceived glide sign, to an area where the glide sign reappears intermittently within a region of the ultrasound image. To find the lung point slide the probe cranially and ventrally until you note a point of lung reconnecting with the thorax wall or you see a glide again.
In patients with extensive pneumothorax, there will not be a lung point if the lung does not recontact the parietal pleura on that side of the thorax. Most of these patients are sufficiently dyspneic to justify thoracentesis without the need to confirm a lung point.
The presence of a glide sign excludes pleural effusion at the site of probe placement, as the presence of a glide sign requires contact of the surface of the lung with the chest well (air or fluid in the chest cavity prevent the lung from contacting the chest wall).
Pleural effusion appears as the absence of a glide sign with anechoic fluid between the chest wall and the hypoechoic lung, or as anechoic triangles adjacent to the heart and outlining the diaphragm (outside the pericardial sac).
The two pleural VPOCUS techniques used to identify pleural effusion include: 1) subxiphoid window and 2) the transthoracic windows in the ventral regions of the thorax.
Patient positioning is important to consider when searching for pleural effusion and different techniques are required to identify small quantities of fluid with patients in lateral vs. sternal/standing positions.
In lateral recumbency, fluid accumulates at the widest gravity dependent sites of the thorax, generally at the pericardial window.
In sternal recumbency (preferred position to scan acutely dyspneic patients), effusion will accumulate ventrally.