Chapter: Lung

Posterior Screen CVA and Effusion Mid-Axillary Screen Lung Anatomy

Lungs

In the outpatient setting, the spectrum of pulmonary disease and characteristics of the ultrasound exam are notably different than that seen in the acute inpatient and intensive care unit settings.

The important findings with lung ultrasound are VPPI (lung) sliding, B-lines, consolidation, and effusion. Different probes and settings are needed to optimize these findings because sliding and B-lines are artifacts while consolidation and effusion are actual tissue/fluid (Matthias et. al., J of Ultrasound Med 2020:9999:1-8). We use two probes and three presets with Venue to optimally cover various patient presentations. These will be covered in more detail at the end of this chapter.

The spectrum and severity of pulmonary disease is different in the primary care clinic than in the emergency department or critical care unit where lung ultrasound was originally developed. Clinic patients are usually sub-acutely or chronically ill with cough, mild shortness of breath, or localized chest discomfort. These patients need a careful but efficient evaluation of all lung lobes. The supine approach is not optimal for the clinic.

Clinic patients can always be examined in the upright position, which has two advantages. The first is that the lungs are more fully expanded in the upright position in all but the most obese patients. The second is that pleural effusions are better detected and quantified in this position. As with the sinus exam, the standard patient position for the lung exam in clinic can be standing. This makes for a quicker and easier exam, if a patient can stand for a short while. The sitting patient position on the exam table is acceptable but less efficient and more awkward for the physician in a few places without improvement in imaging.

Except for the pneumothorax exam, the curvilinear probe is used with Venue for lung exams because more VPPI can be seen with each probe placement, allowing a quicker exam. The modestly higher frequency of the curvilinear probe is an additional advantage.

With the phased array probe, it doesn’t make much difference whether the probe orientation on the lungs is parasagittal or transverse because the probe is close to square. However, with the rectangular curvilinear probe there is an advantage to using a transverse, intercostal view because more VPPI is seen with each probe placement.

The intercostal spaces are mostly horizontal at the cephalad chest but become more obliquely angled in the caudad chest so the probe needs to be subtly rotated to stay intercostal as it is moved on the chest. If rib shadow begins to show, the probe is not optimally positioned.

The clinic physicians using Venue and the curvilinear probe begin with the posterior chest, where abnormalities are more frequent. Except for very large patients, a first vertical line of ultrasound gel is applied to the right chest from diaphragm to lung apex in between the medial scapula and the paraspinous muscles and a second line is applied from diaphragm to the caudad border of the mid-scapula. A pass along these two lines with the curvilinear probe usually covers the posterior lung. The probe should stop moving briefly in each intercostal space while the patient breathes.

The right posterior chest is cleared of gel and the patient is turned to position the right lateral chest in good exam position. A mid-axillary line of gel is applied for a single probe pass. The gel is wiped off, the patient is turned back, and the process is repeated for the left lung. Finally, the patient is turned for the anterior exam and one midclavicular line of gel with one curvilinear probe pass is usually enough on each side.

Below is an image that shows the approximate surface projections of the main lobes of the lung. We document localized findings at COC with “lobe terminology”, such as “posterior, superior, RLL” or “lateral RML” to correlate with radiology reports of the lung. This approach also helps communicate better to non-IMBUS physicians.

 

PNEUMOTHORAX AND VPPI (LUNG) SLIDING: (Linear probe; PNEUMO preset)

The supine patient position is better for a suspected pneumothorax because free intrapleural air moves to the most anterior chest (usually third or fourth anterior intercostal spaces). The critical finding is VPPI sliding, the presence of which virtually excludes pneumothorax in the area, so the exam should be optimized for sliding.

We use the linear probe and a preset called PNEUMO that has RES frequency, 6 cm depth, a focus position on the VPPI, and ARTIFACT-ENHANCING settings. In B-mode, “ants marching” on the VPPI is sought as shown in the following clip.

 

M-mode has a higher frame rate than B-mode, so it is better at detecting more subtle sliding. The PNEUMO preset gives excellent M-mode tracings to identify the normal “seashore” pattern of a breathing patient in contrast to the “bar code” pattern with pneumothorax. Here is the normal pattern with PNEUMO.

If a patient has an intact VPPI (no pneumothorax) but a non-ventilated lung (e.g., mucous plugging, obstructing tumor, or a misplaced endotracheal tube), B-mode sliding and the seashore pattern with M-mode can be absent. However, pneumothorax can still be excluded if a “lung pulse” is seen, as demonstrated in the following image from a healthy subject who held his breath.

The lung pulse is seen because cardiac motion is transmitted through a non-ventilated lung if the VPPI is intact. You can feel the carotid with your left hand while M-mode is running to confirm that the brief motion in the tracing corresponds with the heartbeat.
If sliding and both M-mode signs of an intact VPPI are absent, pneumothorax can be strongly suspected, but a “lung point” should be sought. Move the probe laterally in the intercostal space searching for where sliding of the more posterior lung comes in and out of view. A lung point may be very posterior or even absent with a large pneumothorax.

 

VERTICAL LINE ARTIFACTS

When device settings are optimized for artifacts, three vertical lines are important (Lee et. al., Med Ultrason 20:379-384 2018).

Lung comets are a reverberation artifact seen almost only with the higher frequency linear probe. They are not associated with disease, arise at the VPPI, move with intact sliding, and fade quickly so are only 1-2 cm long (too short to be confused with B-lines). The following non-Venue clip shows a lung comet.

 

B-lines are a “ring down” artifact that is produced by a specific arrangement of air and fluid at an intact VPPI. Depending on the device and settings, B-lines can appear continuous or as a stack of successive short horizontal bands. They can look like a ray of light and go to the bottom of the screen without fading. B-lines cover up A-lines and move with sliding. Sparse B-lines may be seen at the lung bases in young, healthy people and the number of B-lines and the number of chest areas positive for B-lines increase with age without necessarily indicating pathology. The following Venue clip shows diffusely increased B-lines in a clinic patient with decompensated HFrEF.

 

Vertical lines originating below consolidation or atelectatic lung are usually called B-lines, but they are technically different and don’t necessarily indicate an interstitial syndrome in the area.

Z-lines are regularly seen, particularly in thin individuals, and have been confused with B-lines. However, they do not originate at the VPPI, do not move with sliding, and blend in with, rather than covering, A lines. Z-lines are weaker in appearance and, in contrast to B-lines, fade with increasing depth. Here is a clip with the linear probe and the PNEUMO preset showing 2-3 stationary Z-lines.

 

DYSPNEA AND B-LINES: (Curvilinear probe; LUNGS_BLINES preset)
When acute or subacute dyspnea is the dominant symptom, a patient should have pneumothorax excluded in the supine position and then be upright for a curvilinear probe exam. When these patients have clues that suggest infection or PE, the LUNGS_CONSOL preset described in the next section should be used first.
Other dyspneic patients with clues more consistent with heart failure, obstructive lung disease, or interstitial lung disease need an exam optimized for B-lines. The LUNGS_BLINES preset has a depth of 12 cm, focus position at the VPPI, and ARTIFACT-ENHANCING settings.
The B-line count should be the maximum number in an ultrasound frame, not the estimated cumulative number seen during respiratory motion. When potential B-lines are seen with Venue, we freeze the screen, scroll back with a finger swipe, and count the frame with the most B-lines. B-lines that are wide at the VPPI are considered fused and should be counted as more than one. More than 6 B-lines in a frame is abnormal with a full intercostal curvilinear probe view. The following frame shows 4 B-lines in the caudad right lower lobe of a healthy older patient.


INFECTION/PE and CONSOLIDATION/EFFUSION: (Curvilinear probe; LUNGS_CONSOL preset)
Effusions and consolidation are the most important findings when infection or PE are suspected. The LUNGS_CONSOL preset has a depth of 8 cm, focus position on the VPPI, and parameters that SUPPRESS artifacts and ENHANCE tissue definition. The depth can be increased when large effusions or consolidations are seen. B-lines may be seen with this preset, but many are suppressed, so the count is falsely low. If a survey with LUNGS_CONSOL is negative for consolidation and effusion, but hints of B-lines are seen, we switch to LUNGS_BLINES for a quick exam of the posterior lower lobes, which should detect most conditions associated with abnormal B-lines.

Free-flowing pleural effusions are best seen with an upright patient and a parasagittal probe position at the medial, posterior lung border. Here is a very small pleural effusion in this location (the anechoic triangle at the top with expiration).

The earliest ultrasound sign of infection/inflammation should be thickening of the VPPI with a “lumpy bumpy” appearance. As sub-pleural consolidation progresses, small hypoechoic spaces appear in the VPPI and any vertical lines begin below these hypoechoic areas. The hypoechoic areas can be small, moderate, or large. Here is an example of a small sub-pleural consolidation with a wide vertical line beginning below the consolidation.

Viral pneumonias should have diffuse small subpleural consolidation, sometimes with skip areas, with or without vertical lines below the consolidations. With bacterial pneumonia, the process is almost always focal and the dividing line between extensive focal sub-pleural consolidation and full consolidation is a judgment call that depends on how deep the process extends below the VPPI and the presence of air bronchograms. It is not necessary to see B-lines with bacterial pneumonia.

Moderate-sized hypoechoic gaps, with or without B-lines, were described in a series of pulmonary embolism patients and large gaps were seen with lung metastases. This finding is probably only 40-50% sensitive for PE but it can be a good clue in the right patient. We had a clinic patient who was 4 days after an uncomplicated lumbar micro-discectomy. Bilateral, migratory chest and shoulder pain had been present without dyspnea. Early on there was a sharp inspiratory discomfort in the back, but the pain was not pleuritic at the time of the clinic visit and was thought to be from the surgical site. The PCP thought that PE was lower likelihood, but the IMBUS exam showed multiple moderate-sized sub-pleural lower lobe consolidations. CT showed multiple, small, bilateral PEs. Here is the Venue clip obtained with a modestly different preset from our current LUNGS_CONSOL.