How I Do It: Lung Ultrasound
LUS can be performed on the whole chest, just laying the probe in the intercostal spaces, avoiding the ribs. The probe can be positioned both longitudinally, perpendicular to the ribs, and obliquely, along the intercostal spaces (Figure 4). The longitudinal approach allows visualization of the so-called "bat-sign" (Figure 5). In a longitudinal view the bat sign identifies the upper and lower ribs (the wings of the bat) and, a little deeper, the pleural line (the back of the bat). The oblique approach allows visualizing a larger part of the pleural line, which is not interrupted by the rib shadows (Figures 1 and 5).
(Enlarge Image)
Figure 4.
Longitudinal and oblique approach to lung ultrasound.
(Enlarge Image)
Figure 5.
Longitudinal and oblique lung scanning. A. Longitudinal lung scanning: the upper rib, the pleural line and the lower rib draw an image that resembles a bat. B. Oblique lung scanning: the pleural line is not interrupted by the ribs, and appears as a horizontal line.
The diagnostic approach based on LUS can vary according to different settings and clinical situations, following the main principles of what is today known as "point-of-care ultrasound". Maximum effectiveness of the method is obtained through a clinically-driven and focused assessment. If properly driven and correctly interpreted, some sonographic signs become highly accurate for diagnosing specific pulmonary conditions. For example, in a stable patient with acute spontaneous pleuritic pain, ultrasound examination will start from the painful chest area, focusing on signs of focal pleural and parenchymal abnormality. If the pain is caused by a pulmonary condition with involvement of the parietal pleura, this will be easily detected by LUS. Indeed, LUS is a surface imaging technique, highly sensitive in detecting pleural abnormalities. The clinical suspicion and pre-test probability will guide the diagnostic process to rule in or rule out with high accuracy several conditions, such as PNX, pleuritis, pneumonia, lung peripheral infarction. In this setting, a highly specific sign is the lung point, which represents the transition point between the typical sonographic pattern of PNX (absence of lung sliding and of B-lines) into the normal pattern of lung sliding, and depicts the physical limit of PNX as mapped on the chest wall. However, the lung point can be employed to detect the extension of PNX, but not its volume. Up-to-now, LUS is not recognized as a method to differentiate between large and small PNX.
In a patient with acute dyspnea, if cardiogenic pulmonary edema is in the differential diagnosis, LUS will be used to examine the anterior and lateral chest to detect the diffuse signs of interstitial and alveolar edema, which usually respect three highly specific features: they are correlated with the severity of the respiratory failure, follow a regular and symmetric spatial distribution, and usually progress from the lateral and inferior (dependent zones) to the anterior upper chest areas. The scanning technique that should be employed in the emergency setting is the eight-zone examination, consisting of scanning four chest areas per side (Figure 6): areas 1 and 2 denote the upper anterior and lower anterior chest, whereas areas 3 and 4 denote the upper lateral and basal lateral chest, respectively. In the critically ill patient with acute respiratory failure, a more rapid anterior two-region scan may be sufficient to rule out the interstitial syndrome due to cardiogenic acute pulmonary edema. However, this focused anterior scanning, while still highly accurate in the critically ill, may not be sufficient in patients who are not severely dyspnoic, since presence of B-lines on the anterior chest usually denotes a more severe degree of pulmonary congestion in case of heart failure. Again, this is another example of adaptation of the LUS technique and signs to the specific setting and clinical condition.
(Enlarge Image)
Figure 6.
Eight-zone scanning scheme of the antero-lateral chest (according to Volpicelli G et al. See ref. [15]).
If the main clinical suspicion is the possibility of a PNX, the LUS examination is started from the non-dependent zones for air collection, corresponding to the anterior-inferior chest in the supine patient. In this case, examining only one hot zone per side will help rule out PNX with high sensitivity both in the extreme emergencies and in stable patients. The hot zone examination will also be enough to confirm PNX in unstable/cardiac arrest patients. Only in stable patients will examination be extended to the lateral chest to confirm PNX.
In the setting of chronic patients, with less time pressure and more borderline cases, the scanning technique should always be more comprehensive. It can include the anterior, lateral and dorsal chest. Different approaches have been proposed: a detailed scanning scheme has been used in many studies on patients with heart failure, on dialysis, and with pulmonary fibrosis, focused on the assessment of B-lines. These approaches allow accurate examination of the whole chest, which can be applied in several settings in chronic conditions. It is particularly useful for quantifying the extent of the LUS abnormalities, and for assessing intra-patient variations after therapeutic interventions, including dialysis. Ultrasound scanning of the anterior and lateral chest is obtained on the right and left hemithorax, from the second to the fourth (on the right side to the fifth) intercostal spaces, and from the parasternal line to the axillary line (Figure 7). The posterior chest is scanned along the paravertebral line, linea scapularis and posterior axillary lines (Figure 8). The sum of the B-lines found on each scanning site yields a score denoting the extent of the pulmonary interstitial syndrome. Zero is defined as a complete absence of B-lines in the investigated area. When B-lines are a few in a scanning site, they can be easily counted. When they are more numerous, they tend to be confluent and it is less easy to clearly enumerate them. To obtain a semiquantification of the sign, you can consider the percentage of the scanning site occupied by B-lines (i.e., the percentage of white screen compared to black screen below the pleural line) and then divide it by ten (i.e., 30% corresponds to about 3 B-lines, 70% corresponds to about 7 B-lines, and so on).
(Enlarge Image)
Figure 7.
Twenty-eight scanning site scheme of the antero-lateral chest (Modified from Jambrik et al. See ref. [46]).
(Enlarge Image)
Figure 8.
Scanning scheme for the posterior chest (Modified form Gargani L et al. See ref. [24]).
In the case of limited time, even in a chronic setting the examination can be more focused and should be clinically driven. In patients with heart failure, it is important to scan the dependent zones, i.e., lung posterior basis if we are evaluating an out-patient, or along the posterior and mid-axillary lines if we are scanning an in-patient who has been lying down for many hours. In patients with interstitial lung disease such as pulmonary fibrosis, it is mandatory to scan the posterior chest, because the disease generally starts in this region. It is advisable to always scan the costo-phrenic angles on both sides, and to assess the "curtain sign", that is the relative movement of the lung, which covers the subdiaphragmatic organ during inspiration (the liver on the right side, or the spleen on the left side) and the diaphragm (Figure 9 and Additional file 3 http://www.cardiovascularultrasound.com/content/12/1/25/additional). This is the region where free pleural effusion can be detected. Identification of pleural effusion is the most established application of LUS. The effusion should first be sought in dependent zones, i.e., lateral and posterior chest. By LUS it is possible to differentiate pleural effusion from atelectasis, consolidations, masses, or an elevated hemidiaphragm, that sometimes may hardly be distinguished on a chest X-ray. LUS images pleural effusion as an anechoic or hypoechoic space between the two pleural layers, with the lung appearing either aerated or consolidated. LUS for the diagnosis of pleural effusion is especially valuable in critically ill patients, showing better sensitivity and reliability than bedside chest X-ray. Ultrasound can detect the effusion, evaluate its volume, provide information on its nature, and indicate the appropriate area for thoracentesis. Moreover, LUS outperforms CT scan in the diagnosis of complex effusion due to its ability to distinguish septa and fibrin inside the collection.
LUS can be performed in any position (supine, lateral decubitus, or prone), since lung abnormality distribution does not change so rapidly that significant information would be missed (apart from pleural effusion) just by changing the patient's position. Moreover, in the case of pulmonary congestion, even if the position of B-lines changes, the overall distribution tends to remain the same, without clinically relevant differences. The supine position is perfect for scanning the anterior chest, whereas the lateral chest may be examined in the semi-supine position (on the left decubitus to scan the right axillary lines, and on the right decubitus to scan the left axillary lines). The ideal position for scanning the posterior chest is with the patient sitting on the bed, his/her back turned to the operator (Figure 10). Indeed, it is possible to also scan patients while they are standing, sitting or lying flat, without significant differences in results. The only real limitation is when LUS needs to be extended to the dorsal chest of a patient lying down who is intubated in the intensive care unit, or a patient who is unconscious and cannot be moved. In these situations, using small probes that may be better placed between the bed and the patient may allow the best result.
(Enlarge Image)
Figure 9.
Left costophrenic angle. During inspiration the lung moves downward and the lung air prevents the visualization of part of the spleen.
(Enlarge Image)
Figure 10.
Position of the patient to scan the posterior chest.
The LUS examination can be performed using any commercially available 2-D scanner. Different transducers have been used, such as phased array (cardiac), convex (abdominal), microconvex, and linear (vascular) probes. Higher frequencies and macro probes are useful for the evaluation of the pleural line and subpleural space, so should be preferred for assessing PNX. Phased-array probes can be successfully employed to detect pleural effusion, thanks to low frequency and consequent ability to provide a deeper view of the chest. However, these latter probes have limitations in detecting PNX and when a detailed examination of the sub-pleural space is needed. The convex and microconvex probes are the most universally used, all-purpose probes for LUS, thanks to their intermediate frequency values, which allow a reasonable visualization of the pleural line and subpleural space, without losing the overview of the chest. B-lines can be detected by all these different probes, but again low frequency probes are probably the best for this application. Although the number of B-lines may be slightly different when using different probes in a specific chest site, the overall clinical picture does not change by changing the transducer. The possibility of easily assessing B-lines with any kind of transducer is one of the advantages of this technique, so no one should give up on scanning a patient just because the "ideal" probe is not available.
Portable machines and pocket-sized devices have also been proposed for assessing B-lines, as well as pleural effusion. There is no need for a second harmonic or Doppler imaging mode, so even older ultrasound machines can be employed. Visualization of lung consolidations is possible with all probes as well. In the case of a small consolidation, a phased-array transducer may offer less detail, whereas a linear transducer would magnify it. In the case of large consolidations, a linear probe may be unsuitable for detecting the consolidations' borders precisely, whereas convex and microconvex, and even the phased-array transducers, would be more appropriate. The depth should be tailored to the patient: very thick ribcages, large muscles and obese patients need greater depths, even to visualize the pleural line. Very thin patients and children may require less depth. Depth should also be adjusted to the target of our examination: if we are looking for PNX, the depth should be lower, in order to better visualize the pleural line and assess the presence or absence of the sliding sign. If we are looking for a free pleural effusion, the depth should be greater, for a better overview of the costo-phrenic angles. Normally the focus should be positioned at the pleural line level, but it should be moved deeper when our main target is less superficial.
How I Do it: The Setting, the Scanning Technique, the Probe
LUS can be performed on the whole chest, just laying the probe in the intercostal spaces, avoiding the ribs. The probe can be positioned both longitudinally, perpendicular to the ribs, and obliquely, along the intercostal spaces (Figure 4). The longitudinal approach allows visualization of the so-called "bat-sign" (Figure 5). In a longitudinal view the bat sign identifies the upper and lower ribs (the wings of the bat) and, a little deeper, the pleural line (the back of the bat). The oblique approach allows visualizing a larger part of the pleural line, which is not interrupted by the rib shadows (Figures 1 and 5).
(Enlarge Image)
Figure 4.
Longitudinal and oblique approach to lung ultrasound.
(Enlarge Image)
Figure 5.
Longitudinal and oblique lung scanning. A. Longitudinal lung scanning: the upper rib, the pleural line and the lower rib draw an image that resembles a bat. B. Oblique lung scanning: the pleural line is not interrupted by the ribs, and appears as a horizontal line.
The diagnostic approach based on LUS can vary according to different settings and clinical situations, following the main principles of what is today known as "point-of-care ultrasound". Maximum effectiveness of the method is obtained through a clinically-driven and focused assessment. If properly driven and correctly interpreted, some sonographic signs become highly accurate for diagnosing specific pulmonary conditions. For example, in a stable patient with acute spontaneous pleuritic pain, ultrasound examination will start from the painful chest area, focusing on signs of focal pleural and parenchymal abnormality. If the pain is caused by a pulmonary condition with involvement of the parietal pleura, this will be easily detected by LUS. Indeed, LUS is a surface imaging technique, highly sensitive in detecting pleural abnormalities. The clinical suspicion and pre-test probability will guide the diagnostic process to rule in or rule out with high accuracy several conditions, such as PNX, pleuritis, pneumonia, lung peripheral infarction. In this setting, a highly specific sign is the lung point, which represents the transition point between the typical sonographic pattern of PNX (absence of lung sliding and of B-lines) into the normal pattern of lung sliding, and depicts the physical limit of PNX as mapped on the chest wall. However, the lung point can be employed to detect the extension of PNX, but not its volume. Up-to-now, LUS is not recognized as a method to differentiate between large and small PNX.
In a patient with acute dyspnea, if cardiogenic pulmonary edema is in the differential diagnosis, LUS will be used to examine the anterior and lateral chest to detect the diffuse signs of interstitial and alveolar edema, which usually respect three highly specific features: they are correlated with the severity of the respiratory failure, follow a regular and symmetric spatial distribution, and usually progress from the lateral and inferior (dependent zones) to the anterior upper chest areas. The scanning technique that should be employed in the emergency setting is the eight-zone examination, consisting of scanning four chest areas per side (Figure 6): areas 1 and 2 denote the upper anterior and lower anterior chest, whereas areas 3 and 4 denote the upper lateral and basal lateral chest, respectively. In the critically ill patient with acute respiratory failure, a more rapid anterior two-region scan may be sufficient to rule out the interstitial syndrome due to cardiogenic acute pulmonary edema. However, this focused anterior scanning, while still highly accurate in the critically ill, may not be sufficient in patients who are not severely dyspnoic, since presence of B-lines on the anterior chest usually denotes a more severe degree of pulmonary congestion in case of heart failure. Again, this is another example of adaptation of the LUS technique and signs to the specific setting and clinical condition.
(Enlarge Image)
Figure 6.
Eight-zone scanning scheme of the antero-lateral chest (according to Volpicelli G et al. See ref. [15]).
If the main clinical suspicion is the possibility of a PNX, the LUS examination is started from the non-dependent zones for air collection, corresponding to the anterior-inferior chest in the supine patient. In this case, examining only one hot zone per side will help rule out PNX with high sensitivity both in the extreme emergencies and in stable patients. The hot zone examination will also be enough to confirm PNX in unstable/cardiac arrest patients. Only in stable patients will examination be extended to the lateral chest to confirm PNX.
In the setting of chronic patients, with less time pressure and more borderline cases, the scanning technique should always be more comprehensive. It can include the anterior, lateral and dorsal chest. Different approaches have been proposed: a detailed scanning scheme has been used in many studies on patients with heart failure, on dialysis, and with pulmonary fibrosis, focused on the assessment of B-lines. These approaches allow accurate examination of the whole chest, which can be applied in several settings in chronic conditions. It is particularly useful for quantifying the extent of the LUS abnormalities, and for assessing intra-patient variations after therapeutic interventions, including dialysis. Ultrasound scanning of the anterior and lateral chest is obtained on the right and left hemithorax, from the second to the fourth (on the right side to the fifth) intercostal spaces, and from the parasternal line to the axillary line (Figure 7). The posterior chest is scanned along the paravertebral line, linea scapularis and posterior axillary lines (Figure 8). The sum of the B-lines found on each scanning site yields a score denoting the extent of the pulmonary interstitial syndrome. Zero is defined as a complete absence of B-lines in the investigated area. When B-lines are a few in a scanning site, they can be easily counted. When they are more numerous, they tend to be confluent and it is less easy to clearly enumerate them. To obtain a semiquantification of the sign, you can consider the percentage of the scanning site occupied by B-lines (i.e., the percentage of white screen compared to black screen below the pleural line) and then divide it by ten (i.e., 30% corresponds to about 3 B-lines, 70% corresponds to about 7 B-lines, and so on).
(Enlarge Image)
Figure 7.
Twenty-eight scanning site scheme of the antero-lateral chest (Modified from Jambrik et al. See ref. [46]).
(Enlarge Image)
Figure 8.
Scanning scheme for the posterior chest (Modified form Gargani L et al. See ref. [24]).
In the case of limited time, even in a chronic setting the examination can be more focused and should be clinically driven. In patients with heart failure, it is important to scan the dependent zones, i.e., lung posterior basis if we are evaluating an out-patient, or along the posterior and mid-axillary lines if we are scanning an in-patient who has been lying down for many hours. In patients with interstitial lung disease such as pulmonary fibrosis, it is mandatory to scan the posterior chest, because the disease generally starts in this region. It is advisable to always scan the costo-phrenic angles on both sides, and to assess the "curtain sign", that is the relative movement of the lung, which covers the subdiaphragmatic organ during inspiration (the liver on the right side, or the spleen on the left side) and the diaphragm (Figure 9 and Additional file 3 http://www.cardiovascularultrasound.com/content/12/1/25/additional). This is the region where free pleural effusion can be detected. Identification of pleural effusion is the most established application of LUS. The effusion should first be sought in dependent zones, i.e., lateral and posterior chest. By LUS it is possible to differentiate pleural effusion from atelectasis, consolidations, masses, or an elevated hemidiaphragm, that sometimes may hardly be distinguished on a chest X-ray. LUS images pleural effusion as an anechoic or hypoechoic space between the two pleural layers, with the lung appearing either aerated or consolidated. LUS for the diagnosis of pleural effusion is especially valuable in critically ill patients, showing better sensitivity and reliability than bedside chest X-ray. Ultrasound can detect the effusion, evaluate its volume, provide information on its nature, and indicate the appropriate area for thoracentesis. Moreover, LUS outperforms CT scan in the diagnosis of complex effusion due to its ability to distinguish septa and fibrin inside the collection.
LUS can be performed in any position (supine, lateral decubitus, or prone), since lung abnormality distribution does not change so rapidly that significant information would be missed (apart from pleural effusion) just by changing the patient's position. Moreover, in the case of pulmonary congestion, even if the position of B-lines changes, the overall distribution tends to remain the same, without clinically relevant differences. The supine position is perfect for scanning the anterior chest, whereas the lateral chest may be examined in the semi-supine position (on the left decubitus to scan the right axillary lines, and on the right decubitus to scan the left axillary lines). The ideal position for scanning the posterior chest is with the patient sitting on the bed, his/her back turned to the operator (Figure 10). Indeed, it is possible to also scan patients while they are standing, sitting or lying flat, without significant differences in results. The only real limitation is when LUS needs to be extended to the dorsal chest of a patient lying down who is intubated in the intensive care unit, or a patient who is unconscious and cannot be moved. In these situations, using small probes that may be better placed between the bed and the patient may allow the best result.
(Enlarge Image)
Figure 9.
Left costophrenic angle. During inspiration the lung moves downward and the lung air prevents the visualization of part of the spleen.
(Enlarge Image)
Figure 10.
Position of the patient to scan the posterior chest.
The LUS examination can be performed using any commercially available 2-D scanner. Different transducers have been used, such as phased array (cardiac), convex (abdominal), microconvex, and linear (vascular) probes. Higher frequencies and macro probes are useful for the evaluation of the pleural line and subpleural space, so should be preferred for assessing PNX. Phased-array probes can be successfully employed to detect pleural effusion, thanks to low frequency and consequent ability to provide a deeper view of the chest. However, these latter probes have limitations in detecting PNX and when a detailed examination of the sub-pleural space is needed. The convex and microconvex probes are the most universally used, all-purpose probes for LUS, thanks to their intermediate frequency values, which allow a reasonable visualization of the pleural line and subpleural space, without losing the overview of the chest. B-lines can be detected by all these different probes, but again low frequency probes are probably the best for this application. Although the number of B-lines may be slightly different when using different probes in a specific chest site, the overall clinical picture does not change by changing the transducer. The possibility of easily assessing B-lines with any kind of transducer is one of the advantages of this technique, so no one should give up on scanning a patient just because the "ideal" probe is not available.
Portable machines and pocket-sized devices have also been proposed for assessing B-lines, as well as pleural effusion. There is no need for a second harmonic or Doppler imaging mode, so even older ultrasound machines can be employed. Visualization of lung consolidations is possible with all probes as well. In the case of a small consolidation, a phased-array transducer may offer less detail, whereas a linear transducer would magnify it. In the case of large consolidations, a linear probe may be unsuitable for detecting the consolidations' borders precisely, whereas convex and microconvex, and even the phased-array transducers, would be more appropriate. The depth should be tailored to the patient: very thick ribcages, large muscles and obese patients need greater depths, even to visualize the pleural line. Very thin patients and children may require less depth. Depth should also be adjusted to the target of our examination: if we are looking for PNX, the depth should be lower, in order to better visualize the pleural line and assess the presence or absence of the sliding sign. If we are looking for a free pleural effusion, the depth should be greater, for a better overview of the costo-phrenic angles. Normally the focus should be positioned at the pleural line level, but it should be moved deeper when our main target is less superficial.
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