LUNG ULTRASOUND AND CONFIRMING PERICARDIAL EFFUSION
Soren Boysen DVM, DACVECC; Serge Chalhoub DVM, DACVIM (SAIM);
Faculty of Veterinary Medicine, University of Calgary
Calgary, AB Canada
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. Patients can be in sternal (preferred position for dyspneic patients), standing or in lateral recumbency.
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.
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.
Glide sign visualized as a shimmering along the pleural line (pulmonary-parietal interface), which represents normal to-and-fro motion of the lung sliding along the chest well during respiration. This 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. These points are important to remember.
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.
Curtain sign: 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.
Alveolar interstitial syndrome (AIS)
AIS is diagnosed when there are an increased number of B-lines.
>3 B-lines at any single location, or multiple sites is indicative of AIS.
“Wet lungs” are present when there are increased B lines present (>3 B lines at a single site).
“Dry lungs” are present when there is a glide sign, A lines and 3 or fewer B lines at an occasional site evaluated.
B-lines can originate anywhere, which is why it is important to scan multiple portions of the lung.
The number of B lines correlates with the severity of AIS (the more B lines the “wetter” the lungs.
When AIS is identified on lung ultrasound, the same differential diagnosis should be considered as an interstitial alveolar pattern on thoracic radiographs.
It is important to note that lung ultrasound will only detect lung pathology if the pathology is at the periphery of the lung (outer 3mm) – fortunately most diseases that cause AIS (cardiogenic pulmonary edema, trauma induced contusions, aspiration pneumonia, etc.) will reach the lung surface.
Z-lines: these lines arise from the parietal pleura (thoracic wall side of the pleural line), not the lung surface. Therefore, they do not move with the glide sign and the do not erase A-lines. They are ill-defined and disappear after 2-5 cm. Significance unknown (not associated with known pathology). They are present in 80% of healthy humans. They can be seen in patients with pneumothorax.
E-lines come from subcutaneous emphysema and they do form a comet tail like b-lines. They are identified by the fact they originate proximal (superficial) to the pleural line and therefor pass through and obliterate the pleural line. Caused by accumulation of air in the subcutaneous tissues. They do extend to the bottom of the ultrasound screen, but do not move with respirations.
Placing the probe over the stomach or at the curtain sign and failing to realize the probe location can sometime lead to a false positive finding of B lines.
Sonographic Technique to Identify AIS
As a general rule of thumb, the authors’ scan 9 sites in a sliding fashion on each side of the chest, plus the subxiphoid site, to ensure adequate exploration of the lungs.
The thorax is generally divided into thirds from dorsal to ventral
The dorsal third of the thorax is scanned first. Start at the same caudal dorsal location as described for identification of pneumothorax (the most caudo-dorsal site of the thorax)
From this site, the probe is slid cranially between intercostal spaces, pausing as necessary to assess the presence of lung pathology.
Once the dorsal sites of the thorax have been examined, the probe is slid ventrally within the intercostal space just caudal to the scapula until the middle third of the thorax is reached (roughly the height of the heart base or peri-hilar region). The probe is then slid caudally, pausing as necessary to assess lung the presence of lung pathology, until the curtain sign is encountered.
Lastly, the probe is slid cranially and ventrally along the curtain sign until the cardio-diaphragmatic window is identified. The probe is then turned parallel to the ribs at this location and slid ventrally until the sternal muscles are seen. The prove is then slid cranially a rib space at a time until the cranial thoracic inlet is identified at roughly the third intercostal space.
The heart will be encountered using this technique, at which point the probe can be slide dorsally off from the ventral region until lung is encountered to look for the lung pathology. The probe remains parallel to the ribs while it is slid dorsally.
The probe is then returned to the ventral regions, remaining parallel to the ribs (to ensure pleural effusion is not missed while also looking for lung pathology).
One of the best places to identify pericardial effusion is the subxiphoid view.
By rocking the probe until it is parallel to the patient, the ventral region of the thoracic cavity is visible.
In dogs, the pericardium usually contacts the diaphragm (5% of dogs may not have the heart contact the diaphragm, and it is therefore not possible to find the heart at the subxiphoid location).
If the left ventricular free wall can be seen blending with the diaphragm or liver, then pericardial effusion is ruled out.
If the left ventricular free wall is separated from the diaphragm by anechoic fluid, it indicates one of two things; pericardial effusion or pleural effusion.
Identification of the heart in healthy cats via the subxiphoid view is difficult as the heart does not contact the diaphragm on most cats. However, in the case of pericardial effusion (most often seen in cats with heart failure) the pericardial sac may extend to the diaphragm allowing pericardial effusion to be diagnosed at this site in cats.
Pericardial effusion can also be detected in a right parasternal short axis view of the heart.
Sonographic Technique to Identify Pericardial Effusion
The patient can be standing or sternal and the probe placed just behind the right forelimb, or in right lateral recumbency.
It is often easiest to obtain a short axis view of the heart at the level of the papillary muscles, referred to as the “mushroom” view.
The probe is then slowly slid dorsally (a few mm at a time). The left ventricle and mitral valves will become more and more apparent, and this will lead to what we refer to as the “fish mouth view”.
If these structures are identified it then becomes apparent when pericardial effusion is present; a circular collection of fluid surrounding the heart contained within the pericardial sac.