Lung ultrasonography is easy to learn, simple to perform, and has strong clinical utility for the critical care clinician. Interestingly, radiologists have not been instrumental in developing critical care applications of lung ultrasonography. Perhaps because lung ultrasonography in the intensive care unit (ICU) is a purely bedside technique, it required a frontline ICU clinician to develop the field. Dr. Daniel Lichtenstein is responsible for developing critical care lung ultrasonography. In the 1990s, he established the principles of the field and developed the semiology of lung ultrasonography that is in current use.1 Based on his original and continued work, in the past few years there have been numerous published studies from other groups, which have served to validate and expand the field. This section will review critical care applications of lung ultrasonography.
Basic Principles of Lung Ultrasonography
Air is the enemy of the ultrasonographer. There is a large difference in the acoustic impedance and velocity of ultrasound between tissue and air. This leads to complete reflection of the ultrasound wave at the first air–tissue interface. When combined with the unfavorable attenuation coefficient of air, this leads to a pattern of repeating horizontal lines consistent with a reverberation artifact or a homogeneous amorphic grayness that occupies the ultrasound screen deep to the tissue–air interface. This frustrates any attempt to scan through air to deeper body structures.
The alveolar lung parenchyma is normally filled with air; so well-aerated lung is not visible as a discreet structural entity with ultrasonography, as the ultrasound waves are blocked and reflected by air. When a disease process reduces the amount of air within the lung, ultrasound findings change in a predictable fashion. Atelectatic lung is airless so it appears as a discrete structure with tissue echogenicity. Likewise, lung that is consolidated from pneumonia appears as a well-defined hyperechoic structure. Lung that is edematous, though still aerated, has ultrasonographic findings that are different from normally aerated lung. One of the limitations of lung ultrasonography is that abnormalities that do not involve the pleural surface cannot be visualized, such as focal lesions surrounded by aerated lung. Fortunately, most lung processes that are of interest to the intensivist (e.g., pneumonia, hydrostatic pulmonary edema, lesional edema) extend to the periphery of the lung.
Lung ultrasonography may be performed with a wide variety of ultrasound machines with two-dimensional (2D) scanning capability. It was fully described using a machine manufactured in 1990. A 3.5–5.0 MHz transducer of convex sector design works well. Vascular transducers of higher frequency may also yield serviceable images, although the examination may be limited by a lack of penetration in the larger patient. A microconvex transducer has the advantage that it fits well between rib interspaces. As lung ultrasonography will generally be performed in the context of a whole body approach, many groups use a phased array cardiac transducer for general critical care ultrasonography (lung, pleura, abdominal) to reduce cost. The small footprint of the cardiac transducer permits scanning between rib interspaces. Some machines allow the phased array cardiac transducer to be quickly configured with settings that are optimized for abdominal and thoracic imaging. Transducers of linear design may be used, but these are difficult to use in a longitudinal scanning orientation in the thin individual. Paradoxically, high-end, recent generation ultrasound machines may yield inferior lung ultrasound images compared with machines from the 1990s. Complex image smoothing technology that is appropriate for advanced cardiac imaging may provide suboptimal results for lung ultrasonography.
Performance of Lung Ultrasonography
By convention, lung ultrasonography is generally performed in a longitudinal scanning plane with the transducer held perpendicular to the skin surface. Multiple sites on the chest are scanned in sequence. It is advisable to scan the thorax using a standard section approach, as results can then be reported in reference to a particular area. Many patients who are critically ill are in a supine position. This presents a challenge to the ultrasonographer, as the posterior thorax may be difficult to image by virtue of being blocked by the surface of the bed. For the purpose of scanning an ICU patient, the chest may be divided into anterior, lateral, and posterior areas. The anterior area is bordered by the sternum and the anterior axillary line, while the posterior axillary line borders the lateral and posterior areas. The posterior thorax is an important area to image because the majority of pleural effusions and consolidations are found in the dependent thorax. To image these areas, the transducer must be pressed into the mattress with the probe face angled anteriorly. Alternatively, the patient may be rolled to a lateral decubitus position to fully expose the posterior thorax. Lung ultrasonography is then performed, as with the lateral and anterior exam, by applying the transducer at multiple interspaces on the back. The optimal manner of scanning the thorax is to move the transducer across the chest wall in a series of scan lines examining each interspace and underlying lung in sequence. This allows the examiner to construct a three-dimensional (3D) image of the thorax from multiple 2D images gathered in organized scan-line sequences.
A pitfall to avoid is the failure to place the probe posteriorly enough on a supine patient, thus missing dependant consolidations and pleural effusions not seen more anteriorly or laterally along the diaphragm. Thoracic ultrasonography of the patient who is able to sit up with arms abducted allows much easier scanning of the entire thorax with the multiple scan-line technique, but this is not usually feasible in the critically ill patient.
Key Findings of Lung Ultrasonography
Lung ultrasonography is superior to standard supine radiography and similar to chest computerized tomography (CT) in detecting pneumothorax, normal aeration patterns, alveolar–interstitial fluid accumulation, lung consolidation, and pleural fluid.2 Novice lung ultrasonographers are often challenged by the lack of visually familiar anatomical correlates that are seen when scanning other organs, such as the heart or kidney, whose boundaries can be well delineated. Many of the lung images are not “intuitive” to the novice, given that the lung is represented most often by artifactual linear echogenic patterns deep to the pleural line. Fortunately, these patterns are few, discrete, and easy to master. The key findings of lung ultrasonography for critical care applications are as follows.
With the transducer in a longitudinal orientation, perpendicular to the skin surface, and centered between two adjacent ribs, a typical lung ultrasound image with the depth adjusted to examine the pleural interface can be displayed (Figure 19-1). The transducer should be situated so that the two rib shadows are located to the sides of the image, with the hyperechoic horizontally orientated pleural line appearing in the center of the image approximately 0.5 cm deep to the edge of the rib shadows. The pleural line represents the apposition of the visceral and parietal pleural surfaces. In the normal examination, the pleural surfaces move against each other during the respiratory cycle. This causes the finding of lung sliding, which is a shimmering mobile pleural line that moves in synchrony with the respiratory cycle (Videos 19-1 and 19-2). A related finding is lung pulse. With lung pulse, the pleural line moves synchronously with cardiac pulsation, as the force of cardiac pulsation is sufficient to cause movement of the lung and overlying visceral pleura (Video 19-3). Sliding lung and lung pulse are dynamic findings that require real-time 2D scanning. For convenience, they may be recorded with M-mode for purposes of easy documentation (Figure 19-2).
(A) The image is obtained using a 3.5 MHz transducer. The transducer is in longitudinal orientation and placed perpendicular to the chest wall to scan through the second intercostal space. The pleural line, the rib shadows, and A lines are identified. (B) The image is obtained using a 7.5 MHz transducer held in an identical fashion as in (A).
The findings of lung sliding and lung pulse have major significance because they exclude the presence of a pneumothorax at the site of transducer application with a high level of certainty.3 Lung sliding and pulse can only be seen when the ultrasound waves propagate to the deeper visceral pleura. When pleural air is interposed between the pleural surfaces, as occurs with pneumothorax, the air acts as a barrier to ultrasound; so lung sliding is lost.
Since air within the pleural space distributes to the anterior thorax in the supine patient, the critically ill patient is ideally positioned for the examination. Multiple anterior rib interspaces sites may be easily examined for sliding lung over both hemithoraces so that the intensivist can promptly and confidently rule out pneumothorax. Several groups have reported on the superiority of ultrasonography to rule out pneumothorax when compared with supine chest radiography.4–7
Although the presence of lung sliding effectively rules out the presence of pneumothorax at the site being examined, the absence of lung sliding is not as useful (Videos 19-4 and 19-5); as loss of lung sliding may occur in conditions other than pneumothorax. Any process that greatly reduces the movement of air into the lung will reduce or eliminate lung sliding. Mainstem bronchial intubation or occlusion (e.g., mucous plug, blood clot, foreign body, and tumor) will ablate lung sliding on the side of the blockage. Similarly, any process that impairs lung inflation, such as severe pneumonia, apnea, or adult respiratory distress syndrome (ARDS), will result in an absence of lung sliding. Processes which lead to pleural symphysis (inflammatory, neoplastic, therapeutic, cicatricial) cause loss of lung sliding. Apnea causes loss of lung sliding, though necessarily of short duration. In summary, the presence of lung sliding is a powerful sign because it rules out the possibility of a pneumothorax being present. The absence of lung sliding is less useful.8
In certain situations, a lung pulse may be observed in the absence of lung sliding. This is a to-and-fro movement along the pleural line caused by transmission of cardiac pulsations. For example, with a unilateral mainstem bronchial block, lung sliding will be lost ipsilateral to the block due to the lack of air entry into the affected lung but a lung pulse can be seen, providing strong alternative evidence of the lack of pneumothorax.9
Although absence of lung sliding is not specific for pneumothorax, the presence of a lung point can provide definitive evidence of a pneumothorax. The lung point is found where partially collapsed lung moves in and out of the pneumothorax space in phase with the respiratory cycle. Some pneumothoraces are total, that is, the lung is completely collapsed; but most are partial with some remaining apposition of the visceral and parietal pleura at some point along the thorax, usually lateral or posterior depending on the size of the pneumothorax. A lung point is described as the sudden appearance of lung sliding from the edge of the screen, arriving in an area where an A line pattern (see below) and lack of lung sliding are initially noted. The lung sliding appears and disappears because the partially collapsed lung inflates to touch the chest wall and deflates away from the chest wall in synchrony with the respiratory cycle. Designated as the lung point, this finding is diagnostic of pneumothorax (Videos 19-6 and 19-7).10 Unfortunately, while 100% specific for pneumothorax, lung point has only a moderate sensitivity for detection of pneumothorax. The low sensitivity is, in part, related to operator experience. The detection of lung sliding is an entry-level skill, while finding a lung point requires more experience. A high-frequency linear vascular transducer is useful in finding a lung point, as it has superior resolution when compared to the phased array transducer that is more commonly used. The search for a lung point is one circumstance where the examiner may routinely use transverse or oblique scanning planes to better examine the long axis of the rib interspaces. The finding of absent lung sliding should cause the examiner to promptly search for a lung point. Absent lung sliding suggests the possibility of pneumothorax; identification of an associated lung point, if present, confirms the diagnosis.